Content:

OS – Ocean Sciences

OS1.0 – The North Atlantic: natural variability and global change

EGU2020-4832 | Displays | OS1.0 | Highlight | Fridtjof Nansen Medal Lecture

The North Atlantic Oscillation and related topics

Richard Greatbatch

We start with the severe European winter of 1962/63, a winter when the North Atlantic Oscillation (NAO) index was strongly negative with persistent easterly wind anomalies across northern Europe and the British Isles. We then note that the NAO is a manifestation of synoptic Rossby wave breaking. The positive feedback with which synoptic eddies act to maintain the atmospheric jet stream against friction turns out to also be the mechanism by which the equatorial deep jets in the ocean are maintained against dissipation. We were fortunate to be able to demonstrate this in both a simple model set-up that supports deep jets and directly from mooring data on, and on either side of, the equator at 23 W in the Atlantic Ocean. The deep jets offer some potential for prediction over the neighbouring African continent on interannual time scales. This then leads to a brief discussion of the importance of the tropics for prediction on both seasonal and decadal time scales and longer, linking back to the winter of 1962/63.  The models we use for prediction not only contain surprisingly large biases but also require the parameterization of unresolved processes and some brief discussion will be given on the representation of mesoscale eddies in ocean models, such as are used in prediction systems and for making future climate projections.

 

 

How to cite: Greatbatch, R.: The North Atlantic Oscillation and related topics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4832, https://doi.org/10.5194/egusphere-egu2020-4832, 2020.

EGU2020-114 | Displays | OS1.0

Random movement of mesoscale eddies in the global ocean

Xiaoming Zhai, Qinbiao Ni, Guihua Wang, and David Marshall

In this study we track and analyze eddy movement in the global ocean using 20 years of altimeter data and show that, in addition to the well-known westward propagation and slight polarity-based meridional deflections, mesoscale eddies also move randomly in all directions at all latitudes as a result of eddy-eddy interaction. The speed of this random eddy movement decreases with latitude and equals the baroclinic Rossby wave speed at about 25° of latitude. The tracked eddies are on average isotropic at mid and high latitudes, but become noticeably more elongated in the zonal direction at low latitudes. Our analyses suggest a critical latitude of approximately 25° that separates the global ocean into a low-latitude anisotropic wavelike regime and a high-latitude isotropic turbulence regime. One important consequence of random eddy movement is that it results in lateral diffusion of eddy energy. The associated eddy energy diffusivity, estimated using two different methods, is found to be a function of latitude. The zonal-mean eddy energy diffusivity varies from over 1500 m2 s-1 at low latitudes to around 500 m2 s-1 at high latitudes, but significantly larger values are found in the eddy energy hotspots at all latitudes, in excess of 5000 m2 s-1. Results from this study have important implications for recently-developed energetically-consistent mesoscale eddy parameterization schemes which require solving the eddy energy budget.  

How to cite: Zhai, X., Ni, Q., Wang, G., and Marshall, D.: Random movement of mesoscale eddies in the global ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-114, https://doi.org/10.5194/egusphere-egu2020-114, 2020.

EGU2020-18490 | Displays | OS1.0

Dissecting the Barotropic Transport in a High-resolution ocean model

Martin Claus, Yuan Wang, Richard Greatbatch, and Jinyu Sheng

We present a method to decompose the time mean vertically averaged transport, as simulated by an high-resolution ocean model, into its four dominant components. These components are driven by the gradient of potential energy per unit area (PE), the divergence of the flux of time mean momentum (MMF) and eddy momentum (EMF), and the wind stress. Since the local vorticity budget and the bathymetry are noisy and dominated by small spatial scales, a barotropic shallow water model is used as a filter to diagnose the respective transports instead of integrating along lines of constant f/H.
Applying this method to the output of a high-resolution model of the North Atlantic we find that PE is the most important driver, including the northwest corner. MMF is an important driver of transport around the Labrador Sea continental slope and, together with the EMF, it drives significant transport along the path of the Gulf Stream and North Atlantic current. Additionally, the circulation patterns driven by the EMF compares well with an estimate based on a satellite product. Hence, the presented method provides insights into the relative importance of the different dynamical processes that may drive barotropic transport in an ocean model. But it may also be used to isolate potential issues if a model misrepresents the barotropic transport.

How to cite: Claus, M., Wang, Y., Greatbatch, R., and Sheng, J.: Dissecting the Barotropic Transport in a High-resolution ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18490, https://doi.org/10.5194/egusphere-egu2020-18490, 2020.

EGU2020-2211 | Displays | OS1.0

The global dominance of the Atlantic circulation, seen through boundary pressures.

Chris W. Hughes, Joanne Williams, Adam Blaker, and Andrew C. Coward

The rapid propagation of boundary waves (or, equivalently, the strong influence of topography on vorticity balance) ensures that bottom pressure along the global continental slope reflects large scale ocean processes, making it possible to see through the fog of the mesoscale, which obscures many observable quantities. This fact is exploited in systems to monitor the Atlantic Meridional Overturning Circulation (AMOC). Here, we use diagnostics from an ocean model with realistic mesoscale variability to demonstrate two things. First: boundary pressures form an efficient method of monitoring AMOC variability. Second: pressures are remarkably constant along isobaths around the global continental slope, varying by less than 5 cm sea-level-equivalent over vast distances below the directly wind-driven circulation. In the latter context, the AMOC stands out as a clear exception, leading to a link between the AMOC and differences in the hydrography of entire ocean basins.

How to cite: Hughes, C. W., Williams, J., Blaker, A., and Coward, A. C.: The global dominance of the Atlantic circulation, seen through boundary pressures., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2211, https://doi.org/10.5194/egusphere-egu2020-2211, 2020.

OS1.1 – Open Session on Ocean Circulation and Climate

EGU2020-13372 | Displays | OS1.1

The Current Feedback to the Atmosphere: Implications for the Ocean Dynamics, Air-Sea Interactions, and Climate.

Lionel Renault, Sebastien Masson, and James C. McWilliams

In the past few years, it has been demonstrated that the regional Ocean-Atmosphere interactions can strongly modulate the variability and the mean physical and biogeochemical state of the ocean. In this presentation, the focus will be on the influence of the surface current on the atmosphere (i.e., current feedback). Based on satellite observations and using a set of regional ocean and atmosphere coupled simulations carried out over different regions encompassing a realistic Tropical Channel, and Eastern and Western boundary current systems, we will illustrate to which extent those interactions can control the exchange of energy between the Ocean and the Atmosphere, the mean, mesoscale, and submesoscale circulations, and the Western Boundary Currents Dynamics. Implications for climate, thermal air-sea interactions and how to force an oceanic model is furthermore discussed.

How to cite: Renault, L., Masson, S., and McWilliams, J. C.: The Current Feedback to the Atmosphere: Implications for the Ocean Dynamics, Air-Sea Interactions, and Climate., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13372, https://doi.org/10.5194/egusphere-egu2020-13372, 2020.

Subtropical western boundary currents play a key role in ocean energy storage and transport and are characterized by elevated mean and eddy kinetic energy. Due to a lack of spatially broad subsurface observations of velocity, most studies of kinetic energy in western boundary currents have relied on satellite-based estimates of surface geostrophic velocity. Since 2015, Spray autonomous underwater gliders have completed more than 175 crossings of the Gulf Stream distributed over more than 1,500 km in along-stream extent between between Miami, FL (~25°N) and Cape Cod, MA (~40°N). The observations include roughly 14,000 absolute ocean velocity profiles in the upper 1000 m. Novel three-dimensional estimates of mean and eddy kinetic energy are constructed along the western margin of the North Atlantic at 10-m vertical resolution. The horizontal and vertical distributions of mean and eddy kinetic energy are analyzed in light of existing independent estimates and theoretical expectations. Observation-based estimates of mean and eddy-kinetic energy such as these serve as important metrics for validation of global circulation models that must adequately represent western boundary currents.

How to cite: Todd, R. E.: Mean and eddy kinetic energy of the Gulf Stream from multiyear underwater glider surveys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3745, https://doi.org/10.5194/egusphere-egu2020-3745, 2020.

EGU2020-72 | Displays | OS1.1

Can seafloor voltage cables be used to study large scale transport? An investigation in the Pacific Ocean.

Neesha Schnepf, Manoj Nair, Jakub Velimsky, and Natalie Thomas

Marine electromagnetic (EM) signals largely depend on three factors: oceanic transport (i.e., depth-integrated flow), the local main magnetic field, and the local seawater conductivity (which depends on the local temperature and salinity). Thus, there is interest in using seafloor telecommunication cables to isolate marine EM signals and study ocean processes because these cables measure voltage differences between their two ends. Data from such cables can provide information on the depth-integrated transport occurring in the water column above the cable. However, these time-varying data are a superposition of all EM fields present at the observatory, no matter what source or process created the field. The main challenge in using such submarine voltage cables to study ocean circulation is properly isolating its signal.

 

Our study utilizes voltage data from retired seaoor telecommunication cables in the Pacific Ocean to examine whether such cables could be used to monitor transport on large-oceanic scales. We process the cable data to isolate the seasonal and monthly variations, and evaluate the correlation between the processed data and numerical predictions of the electric field induced by ocean circulation. We find that the correlation between cable voltage data and numerical predictions strongly depends on both the strength and coherence of the transport owing across the cable. The cable within the Kuroshio Current had the highest correlation between data and predictions, whereas two of the cables in the Eastern Pacific gyre (a region with both low transport values and interfering transport signals across the cable) did not have any clear correlation between data and predictions. Meanwhile, a third cable also located in the Eastern Pacific gyre did have correlation between data and predictions, because although the transport values were low, it was located in a region of coherent transport flow across the cable. While much improvement is needed before utilizing seafloor voltage cables to study and monitor oceanic transport across wide oceanic areas, we believe that the answer to our title's questions is yes: seafloor voltage cables can eventually be used to study large-scale transport.

How to cite: Schnepf, N., Nair, M., Velimsky, J., and Thomas, N.: Can seafloor voltage cables be used to study large scale transport? An investigation in the Pacific Ocean., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-72, https://doi.org/10.5194/egusphere-egu2020-72, 2020.

EGU2020-6214 | Displays | OS1.1

Intense Subsurface Upwelling Associated with Major Western Boundary Currents

Fanglou Liao, Xinfeng Liang, Yun Li, and Andreas Thurnherr

Western boundary currents (WBC), fast flowing currents on the western side of ocean basins, transport a huge amount of warm water poleward, affect the atmospheric conditions along their paths, take up a large amount of carbon dioxide, and regulate the global climate (Minobe et al. 2008; Takahashi et al. 2009; Wu et al. 2012). In contrast to their widely examined horizontal motions, much less attention has been paid to the vertical motions associated with the WBC systems. Here, we examined the spatial and temporal characteristics of vertical motions associated with the major WBC systems by analyzing vertical velocity estimates from five ocean synthesis products and one eddy-permitting ocean simulation over an overlapping period from Jan 1992 to Dec 2009. Robust and intense subsurface upwelling occurs in the five major subtropical WBC systems. These upwelling systems together with the vast downwelling inside subtropical ocean basins form basin-scale zonal overturning circulations and play a crucial role in the vertical transport of ocean properties and tracers inside the global ocean. Also, the vertical motions in the Kuroshio Current and the Eastern Australian Current regions display robust interannual and decadal oscillations, which are well correlated with El Niño–Southern Oscillation and Pacific Decadal Oscillation, respectively. This study unveils an overlooked role of the WBCs in the subsurface oceanic vertical transport and is expected to be a starting point for more in-depth investigations on their dynamics and roles in the climate system.

How to cite: Liao, F., Liang, X., Li, Y., and Thurnherr, A.: Intense Subsurface Upwelling Associated with Major Western Boundary Currents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6214, https://doi.org/10.5194/egusphere-egu2020-6214, 2020.

EGU2020-19271 | Displays | OS1.1

The features of Antarctic Bottom Water in the fracture zones at 7-10 deg. N of the Mid-Atlantic ridge

Valentina Volkova, Alexander Demidov, and Fedor Gippius

Despite the fact that there are numerous estimates of the Antarctic Bottom Water (AABW) formation and transport, its evolution and distribution pathways are still debatable (Morozov E.G. et al., 2010).

The main task of this work was to identify the structure and transport of deep and bottom water mass of the fracture zones (7 40', Vernadsky and Doldrums). The research is based on new data (multibeam bottom relief, temperature, salinity, velocity) obtained during the research cruise on the RV "Akademik Nikolaj Strakhov" in October-November 2019 and WODB18 historical data.

The main result of the research is proper estimation of the AABW and LNADW transport, which takes into consideration the influence of fracture zone morphometry. Accordingly, the preliminary circulation scheme of water masses is obtained.

How to cite: Volkova, V., Demidov, A., and Gippius, F.: The features of Antarctic Bottom Water in the fracture zones at 7-10 deg. N of the Mid-Atlantic ridge , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19271, https://doi.org/10.5194/egusphere-egu2020-19271, 2020.

EGU2020-13823 | Displays | OS1.1

The impact of thermohaline staircases: estimates from a global analysis of Argo floats

Carine van der Boog, Julie D Pietrzak, Henk A Dijkstra, and Caroline A Katsman

Thermohaline staircases are stepped structures in the temperature and salinity stratification that result from double diffusive processes. In the open ocean, double diffusive processes enhance the downgradient diapycnal heat transfer compared to turbulent mixing. However, in combination with salinity effects, the resulting buoyancy flux within the thermohaline staircases is counter gradient. This vertical density transport strengthens the stratification and, consequently, affects the density of the water masses above and below the staircase layer. Although 44 percent of the world’s oceans is susceptible to double diffusion and thermohaline staircases are ubiquitous in these regions, the impact of double diffusion on diapycnal heat transfer and on water mass transformation has not been quantified yet. Here, we analyse a dataset of Argo float profiles to obtain a global overview of the occurrence of thermohaline staircases and to estimate their impact on diapycnal heat transfer and water mass transformation. Several regions with a high staircase occurrence are identified. Besides the well-known regions in the Caribbean Sea, the Mediterranean Sea and the subtropical Atlantic Ocean, our analysis reveals a new staircase region in the Indian Ocean. Using this global overview, we estimate, for the first time, the contribution of downgradient diapycnal heat transfer by the staircases. It appears that this contribution is very low compared to the dissipation required to maintain the observed temperature stratification. However, each staircase region can potentially impact the global circulation by affecting the density of the water masses above and below. In particular, the staircase region in the Indian Ocean overlies the waters of the Tasman Leakage. These waters flow westward from Australia towards the Agulhas region and affect the properties of waters entering the Atlantic Ocean. This implies that the vertical flux of salt into the Tasman Leakage waters induced by the presence of thermohaline staircases above can impact the salt transport into the Atlantic Ocean, which in turn is expected to impact the Atlantic Meridional Overturning Circulation. 

How to cite: van der Boog, C., Pietrzak, J. D., Dijkstra, H. A., and Katsman, C. A.: The impact of thermohaline staircases: estimates from a global analysis of Argo floats, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13823, https://doi.org/10.5194/egusphere-egu2020-13823, 2020.

EGU2020-11565 | Displays | OS1.1

Observational evidence of diapycnal upwelling in a bottom enhanced mixing environment

Marcus Dengler, Martin Visbeck, Toste Tanhua, Jan Lüdke, and Madelaine Freund

In the framework of the Peruvian Oxygen minimum zone System Tracer Release Experiment (POSTRE) about 70 kg of trifluoromethyl sulfur pentafluoride (SF5CF3) was injected into the bottom boundary layer of the upper Peruvian continental slope at 250m depth in October 2015. Three different injection sites, at 10°45’S, 12°20’S and 14°S were selected. At the tracer release sites and due to tide-topography interaction, mixing above the upper continental slope of Peru was intensified. Turbulent dissipation rates increase by about an order of magnitude in lower 50 to 100m above the bottom. During previous tracer release experiments, where tracer was injected into the stratified mixing layer above the bottom boundary layer, a change of the center of mass toward higher densities resulted. Newer theories suggest that this diapycnal downwelling is balanced by a diapycnal upwelling within the bottom boundary layer. Indeed, during the tracer survey it was found that the density of tracer’s center of mass had decreased by 0.13 kg m-3. This corresponds to an upward displacement of 70-100m. Using microsctructure shear data from 8 cruises, we obtain a diapycnal velocity of about 0.5 m day-1 within the bottom boundary layer. This suggests that on average, the tracer was trapped within the bottom boundary layer for a period between 1.5 and 3 month. Overall, our tracer study provides the first observational evidence of diapycnal upwelling occurring within the bottom boundary layer of a bottom enhanced mixing environment and supports recent ideas of a vigorous global overturning circulation.

How to cite: Dengler, M., Visbeck, M., Tanhua, T., Lüdke, J., and Freund, M.: Observational evidence of diapycnal upwelling in a bottom enhanced mixing environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11565, https://doi.org/10.5194/egusphere-egu2020-11565, 2020.

EGU2020-5207 | Displays | OS1.1

Ocean-atmosphere fluxes of carbon dioxide and heat in response to phytoplankton light absorption

Rémy Asselot, Frank Lunkeit, Phil Holden, and Inga Hense

Oceanic phytoplankton absorbing solar radiation can influence the upper ocean physics. This process is called phytoplankton light absorption. Previous studies indicate that phytoplankton light absorption significantly impacts the oceanic heat distribution and, if taken into account in an Earth System model, can lead to different climates under similar primary production. However, the dominant processes responsible for these drastic changes in atmospheric temperature have not been yet identified. Phytoplankton light absorption increases the sea surface temperature, therefore altering the exchange of heat between the ocean and the atmosphere. Additionally, phytoplankton light absorption indirectly modifies the ocean carbon cycle and thus the CO2 flux into the atmosphere. To shed light on these aspects, we use an Earth System model of intermediate complexity coupled to an ecosystem model (EcoGENIE). By running a suite of experiements, we determine which fluxes are most important in controlling atmospheric temperature. Here, we present first results of our study.

How to cite: Asselot, R., Lunkeit, F., Holden, P., and Hense, I.: Ocean-atmosphere fluxes of carbon dioxide and heat in response to phytoplankton light absorption, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5207, https://doi.org/10.5194/egusphere-egu2020-5207, 2020.

EGU2020-11076 | Displays | OS1.1

Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

Svein Østerhus, Rebecca Woodgate, Héðinn Valdimarsson, Bill Turrell, Laura de Steur, Detlef Quadfasel, Steffen M. Olsen, Martin Moritz, Craig M. Lee, Karin Margretha Larsen, Steingrímur Jónsson, Clare Johnson, Kerstin Jochumsen, Bogi Hansen, Beth Curry, Stuart Cunningham, and Barbara Berx

Conditions in the Arctic are in part driven by the ocean state in the Arctic Mediterranean (AM), the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Exchange between the lower latitude ocean basins and this region occurs through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow). These waters are subsequently modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water (surface outflow) on both sides of Greenland. These exchanges transport heat and salt into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, the observational coverage has become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations.

In this EGU contribution, we focus on the observations (incl. volume transport time series) of all the main AM-exchange branches collected in the last 20 to 30 years.

How to cite: Østerhus, S., Woodgate, R., Valdimarsson, H., Turrell, B., de Steur, L., Quadfasel, D., Olsen, S. M., Moritz, M., Lee, C. M., Larsen, K. M., Jónsson, S., Johnson, C., Jochumsen, K., Hansen, B., Curry, B., Cunningham, S., and Berx, B.: Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11076, https://doi.org/10.5194/egusphere-egu2020-11076, 2020.

The Lofoten Basin is one of the most dynamically unstable regions of the North Atlantic and represents a ‘hot spot’ of the mesoscale eddy activity in the Nordic Seas.  A quasi-stationary, deep, anticyclonic eddy is located in the central part of the basin. One of the key features of the Lofoten Basin circulation is a separation of eddies from the main branch of Norwegian current and their westward propagation towards the central part of the basin. Because of these processes, warm and saline Atlantic waters are transported to the deeper part of the basin. Understanding the physical processes responsible for the water mass transformations in this area is of particular interest in order to apprehend the climate of the region.

In this study we obtain three-dimensional structures of cyclonic and anticyclonic eddies for the LB region by combining the observational data set covering the 2000-2017 period with satellite altimetry data. The results reveal that significant eddy-induced anomalies are concentrated within a distance of 1 radius of the composite AE and CE and extend vertically to the depth of 1000 m. The core of the composite AE is located in the 200-400 m while the composite CE has a double-core structure with the maximum anomalies centered in the upper layer above 100 m and a negative peak located at 700 m. The difference in the structure of AE and CE is referred to the upwelling and downwelling processes in the AEs and CEs respectively.

The study also provides an estimation of the depth-integrated heat and salt transport as well as zonal volume eddy-induced transport. Each AE (CE) generates volume transport of 1.98 Sv (1.87 Sv), heat transport of 2.9*1014 W (-8.3*104 W) and salt transport of 2.3*106 kg/s (-1.6*1013 kg/s).  Zonal eddy-induced transport has a general westward propagation direction reaching maximum of 0.6 Sv in the north-eastern part of the study area. The northward transport takes place predominantly in the southern and eastern parts of the study region and has significantly smaller magnitude. 

 

This work was supported by Russian Science Foundation [project № 18-17-00027];

How to cite: Sandalyuk, N.: Thermohaline structure and transport of mesoscale eddies in the Lofoten Basin from in situ and altimetry data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11322, https://doi.org/10.5194/egusphere-egu2020-11322, 2020.

EGU2020-267 | Displays | OS1.1

Glider observations of the Northwestern Iberian Margin during an exceptional summer upwelling season

Callum Rollo, Karen Heywood, Rob Hall, Eric Desmond Barton, and Jan Kaiser

We present results from a 2 month deployment of an ocean glider over the Northwestern Iberian Margin. Glider observations during the exceptionally strong 2010 summer upwelling season resolved the evolution of physical and biogeochemical variables during two upwelling events. Upwelling brought low oxygen Eastern North Atlantic Central Water from 190 m depth onto the shelf up to a depth of 50 m. During the two observed periods of upwelling,
equatorward transport over the shelf increased from 0.13 (± 0.04) Sv to 0.18 (± 0.08) Sv and a poleward jet developed over the shelf-break. The persistent upwelling favourable winds maintained equatorward flow on the outer shelf for two months with no reversals during relaxation periods, a phenomenon not previously observed. During upwelling, near surface chlorophyll a concentration increased by more than 6 mg m-3 . Dissolved oxygen concentration in the near surface increased by more than 40 μmol kg-1 , 6 days after the chlorophyll a maximum.


This was the first and, to date, only deployment of a glider over the North West Iberian Margin. A single glider was able to occupy a cross shelf section for two months, without the need for a costly ship based campaign. This presentation highlights some of the challenges of using gliders to study shelf break regions.

How to cite: Rollo, C., Heywood, K., Hall, R., Barton, E. D., and Kaiser, J.: Glider observations of the Northwestern Iberian Margin during an exceptional summer upwelling season, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-267, https://doi.org/10.5194/egusphere-egu2020-267, 2020.

EGU2020-7494 | Displays | OS1.1

Does lateral stirring really take place along neutral surfaces in double-diffusive regions of the oceans?

Gabriel Wolf, Tailleux Remi, Ferreira David, and Kuhlbrodt Till

Potential temperature/salinity (theta/S) characteristics of water masses in the ocean interior can often be traced back over long distances to their source regions. In practice, understanding how water masses are altered by interior mixing and stirring requires a detailed understanding of the interior pathways linking fluid parcels to their source regions. So far, oceanographers have generally assumed that these pathways are strongly constrained to take place on potential density surfaces of some kind, of which the most commonly employed have been the Jackett and McDougall neutral density variable and sigma2, the potential density referenced to 2000 dbar. Because sigma2 is a somewhat ad-hoc and artificial construct, the more physically-based neutral density variable has been widely assumed to represent the most accurate variable to describe interior pathways, but the analysis of van Sebille et al. (2011) intriguingly suggests otherwise. In order to shed light on the issue, this work hypothesizes that if neutral surfaces were optimal to describe lateral stirring in the ocean, they should be the surfaces along which the observed spread in potential temperature and salinity anomalies should be minimum, since lateral stirring is about 7 orders of magnitude more vigorous in the lateral directions than perpendicular to them. Surprisingly, it is found that this is actually never the case in ocean regions with positive density ratios, traditionally associated with double-diffusive regimes. In those regions, indeed, it is always possible to find material surfaces, not necessarily definable in terms of potential density, along which the spread is reduced for both potential temperature and salinity compared to that over neutral surfaces. In doubly-stable regions, on the other hand, it is not possible to find material variables able to simultaneously reduce both the spread in potential temperature and salinity compared to that over neutral surfaces. Given the widespread nature of double-diffusive regimes in the world oceans, especially in the Atlantic Ocean, these results have strong implications for the ability of ocean climate models to accurately simulate water masses, as it is unclear how to maintain water masses properties by mixing vigorously along directions along which the spread in theta/S is far from its minimum.

How to cite: Wolf, G., Remi, T., David, F., and Till, K.: Does lateral stirring really take place along neutral surfaces in double-diffusive regions of the oceans?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7494, https://doi.org/10.5194/egusphere-egu2020-7494, 2020.

 The input of mechanical power to the ocean due to the surface wind-stress, in regions which correspond to different regimes of ocean dynamics, is considered using data from satellites observations. Its dependence on the coarse-graining range of the atmospheric and oceanic velocity in space from 0.5° to 10° and time from 6h to 40 days is determined.  In the area of the Gulf Stream and the Kuroshio extensions the dependence of the power-input on space-time coarse-graining  varies over tenfold for the coarse-graining considered. It decreases over twofold for the Gulf Stream extension and threefold for the Kuroshio extension, when the coarse-graining length-scale passes from a few degrees to 0.5° at a temporal coarse-graining scale of a few days. It increases over threefold in the Gulf Stream and the Kuroshio extensions when the coarse-graining passes from several days to 6h at a spatial coarse graining of a few degrees. The power input is found to increase monotonically with shorter coarse-graining in time. Its variation with coarse graining in space has no definite sign. Results show that including the dynamics at scales below a few degrees reduces considerably the power input by air-sea interaction in regions of strongly non-linear ocean currents.
  When the ocean velocities are not considered in the shear calculation the power-input is considerably (up to threefold) increased. The dependence of the power input on coarse graining in space and time is close to being multiplicatively separable in all regions and for most of the coarse-graining domain considered.

How to cite: Wirth, A.: On the length and time scales of the power supply to the ocean between the meso-scale and the synoptic-scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3409, https://doi.org/10.5194/egusphere-egu2020-3409, 2020.

EGU2020-5657 | Displays | OS1.1

Kinetic Energy Conversion in A Wind-forced Submesoscale Flow

Song Li, Nuno Serra, and Detlef Stammer

Despite recent progress in measuring the ocean eddy field with satellite missions at the mesoscale (order of 100 km), containing the major fraction of ocean kinetic energy, many questions still remain regarding the generation, conversion and dissipation mechanisms of eddy kinetic energy (Ke). In this work, we use the output from an idealized 500-m resolution ocean numerical simulation to study the conversion of Ke in the absence and presence of wind stress forcing. In contrast to the result of the unforced run, Ke increased approximately nine times in the mixed layer and considerably in the pycnocline in the forced run. Eddies and filaments were seen to re-stratify the mixed layer and wind-induced turbulence at the base of the mixed layer promoted its deepening and therefore dramatically enhanced the exchange between Ke and eddy available potential energy (Pe). The wind stress forcing additionally affected the conversion processes between Pe and mean kinetic energy (Km). The wind also excited inertial and superinertial motions throughout almost the whole water column. Although those motions played a major role in the conversion between Pe and Ke, the net effect by inertial and superinertial flows was almost null. In addition, we found an asymmetric character in kinetic energy conversion in eddies. Cyclonic and anti-cyclonic eddies showed different behaviour regarding conversion from Pe and Ke, which was positive on the high Ke part in the anti-cyclonic eddy but negative in the cyclonic eddy.

How to cite: Li, S., Serra, N., and Stammer, D.: Kinetic Energy Conversion in A Wind-forced Submesoscale Flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5657, https://doi.org/10.5194/egusphere-egu2020-5657, 2020.

EGU2020-13466 | Displays | OS1.1

Towards new online access services for the EMSO ERIC temperature and salinity data

Alice Novello, Dominique Lefevre, Nadia LoBue, Ivan Rodero, Raul Bardaji, and Mathilde Cannat

The European Multidisciplinary Seafloor and water column Observatory (EMSO) consists, to date, of 11 regional multiple sensor-equipped platforms distributed around Europe from the Atlantic Ocean to the Mediterranean, and the Black Sea. Each system collects multidisciplinary measurements in the water column as well as at the seafloor addressing several critical questions related to ocean health, climate change, marine ecosystems and natural hazards. EMSO is a European Research Infrastructure Consortium (ERIC) since 2016, and one of the many challenges has been to design new online services promoting marine data produced by the whole network. Here, we report on an on-going activity to compile, control and deliver quality controlled temperature and salinity data and metadata gathered through the EMSO network from the sea surface down to 4000m. As part of this effort, we work on the development of online tools for temperature and salinity data visualization and knowledge discovery based on widely used software components such as dashboards. These services aim to support the stakeholders' needs (from scientists and industries to institutions and policymakers) by providing relevant information on multidisciplinary oceanographic data. They also highlight the importance of filling the knowledge gap on the abyssal ocean by delivering useful deep long-term series necessary to assess the impact of key processes on global issues such as climate change and marine ecosystem sustainability.

How to cite: Novello, A., Lefevre, D., LoBue, N., Rodero, I., Bardaji, R., and Cannat, M.: Towards new online access services for the EMSO ERIC temperature and salinity data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13466, https://doi.org/10.5194/egusphere-egu2020-13466, 2020.

EGU2020-18017 | Displays | OS1.1

A new project to monitor the Ocean Heat Content and the Earth Energy imbalance from space: MOHeaCAN

Michaël Ablain, Benoit Meyssignac, Alejandro Blazquez, Marti Florence, Rémi Jugier, and Jérôme Benveniste

The Earth Energy Imbalance (EEI) is a key indicator to understand the Earth’s changing. However, measuring this indicator is challenging since it is a globally integrated variable whose variations are small, of the order of several tenth of W.m-2, compared to the amount of energy entering and leaving the climate system of ~340 W.m-2. Recent studies suggest that the EEI response to anthropogenic GHG and aerosols emissions is 0.5-1 W.m-2. It implies that an accuracy of <0.3 W.m-2 at decadal time scales is necessary to evaluate the long term mean EEI associated with anthropogenic forcing. Ideally an accuracy of <0.1 W.m-2 at decadal time scales is desirable if we want to monitor future changes in EEI. The ocean heat content (OHC) is a very good proxy to estimate EEI as ocean concentrates the vast majority of the excess of energy (~93%) associated with EEI. Several methods exist to estimate OHC:

  • the direct measurement of in situ temperature based on temperature/Salinity profiles (e.g. ARGO floats),
  • the measurement of the net ocean surface heat fluxes from space (CERES),
  • the estimate from ocean reanalyses that assimilate observations from both satellite and in situ instruments,
  • the measurement of the thermal expansion of the ocean from space based on differences between the total sea-level content derived from altimetry measurements and the mass content derived from GRACE data (noted “Altimetry-GRACE”).

To date, the best results are given by the first method based on ARGO network. However ARGO measurements do no sample deep ocean below 2000 m depth and marginal seas as well as the ocean below sea ice. Re-analysis provides a more complete estimation but large biases in the polar oceans and spurious drifts in the deep ocean mask a significant part of the OHC signal related to EEI. The method based on estimation of ocean net heat fluxes (CERES) is not appropriate for OHC calculation due to a too strong uncertainty (±15 W.m-2). 

In the MOHeaCAN project supported by ESA, we are being developed the “Altimetry-GRACE” approach  which is promising since it provides consistent spatial and temporal sampling of the ocean, it samples the entire global ocean, except for polar regions, and it provides estimates of the OHC over the ocean’s entire depth. Consequently, it complements the OHC estimation from ARGO.  However, to date the uncertainty in OHC from this method is close to 0.5 W.m-2, and thus greater than the requirement of 0.3 W.m-2 needed to a good EEI estimation. Therefore the scientific objective of the MOHeaCan project is  to improve these estimates :

How to cite: Ablain, M., Meyssignac, B., Blazquez, A., Florence, M., Jugier, R., and Benveniste, J.: A new project to monitor the Ocean Heat Content and the Earth Energy imbalance from space: MOHeaCAN, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18017, https://doi.org/10.5194/egusphere-egu2020-18017, 2020.

EGU2020-6504 | Displays | OS1.1

GOANA, a Global Ocean Atlas, Neutrally Averaged

Paul Barker and Trevor McDougall

Isopycnally averaged hydrographic data gives results that are significantly different to the standard method of averaging at constant depth. The act of averaging isopycnally ensures that water masses are neither created or destroyed.  We average using the weighted least squares quadratic (or LOESS) fitting method of Chelton and Schlax (1994) and Ridgway et al. (2002) along appropriately defined density surfaces.  This produces an gridded oceanographic atlas that is composed of the Fourier coefficients of the mean temporal trend, the strength of the semi-annual and seasonal cycle allowing the user to reconstruct a climatology at any temporal resolution. Initially we are producing an atlas consisting of Absolute Salinty and Conservative Temperature but in the future we aim to include nutrient data.

How to cite: Barker, P. and McDougall, T.: GOANA, a Global Ocean Atlas, Neutrally Averaged, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6504, https://doi.org/10.5194/egusphere-egu2020-6504, 2020.

EGU2020-13483 | Displays | OS1.1

Variability of seawater property after typhoon passage in the Philippine sea of the western North Pacific

Kyung-Hee Oh, Seok Lee, Hong Sik Min, and Sok-Kuh Kang

Sea water temperature and salinity measurements have been collected onboard in September in the Philippine seas of the western North Pacific. This area is close to typhoon occurrence area and is the path through which developed typhoons pass, and also large and small eddies are developed. Therefore variability of sea water property is large.  As a result of analysis, the seawater properties of the upper water showed a big difference before and after the typhoon. After the typhoon, surface water temperature dropped by about 1 degree C and salinity by 1 psu.  Mixed layer became deeper, and changes in seawater properties occurred throughout the upper layers. The depth of the mixed layer was largely different by more than 30-50m, especially the water temperature was changed more than 3 degree C at the depth below thermocline. Real-time sea surface water temperature and salinity measurements show more easily identify the physical property change of sea surface water before and after typhoon.

How to cite: Oh, K.-H., Lee, S., Min, H. S., and Kang, S.-K.: Variability of seawater property after typhoon passage in the Philippine sea of the western North Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13483, https://doi.org/10.5194/egusphere-egu2020-13483, 2020.

EGU2020-13815 | Displays | OS1.1

Subthermocline eddies in the Philippine Sea in 2017 to 2018

Chang-Woong Shin and Jae Hak Lee

EGU2020-1879 | Displays | OS1.1

Eddies and their energetics in the Bay of Bengal

Navin Chandra and Vimlesh Pant

Eddies are integral part of ocean circulation. They play an important role in energy transfer. The surface kinetic energy in eddies can be ten times higher than the energy of the current through which these are generated. Eddies influence the thermodynamic characteristics of the upper-ocean. Oceanic eddies trap and transport hot (cold) water in the core of an anticyclonic (cyclonic) eddy. Therefore, these eddies can modify the thermal structure by the advection of temperature anomalies and its subsequent mixing. Generation of eddies takes place mainly due to the baroclinic instability of the ocean. However, some of the eddies may form due to coastal and bathymetrical geometry. The Bay of Bengal (BoB) is an enclosed basin in the northern Indian Ocean (IO). The BoB exhibits unique physical and dynamical properties due to surplus low-saline waters and shallow mixed layer. It observes seasonal variation of wind and changes in the surface current pattern. Major rivers originating from the Himalayan glaciers drain into the BoB throughout the year with a peak in July-October. The riverine freshwater together with strong post-monsoon (October-November) coastal current generate complex and turbulent surface current pattern with a large number of eddies in the BoB. The wind forcing, coastal currents, and bathymetry make favorable conditions for the generation of eddies in the BoB. In the present study, a numerical ocean model Regional Ocean Modelling System (ROMS) used to simulate the mesoscale eddies in the BoB. The ROMS model uses sigma vertical coordinates which helps in taking account of the effects of coastal and bathymetrical structures on surface circulation and eddy generation. The model results are verified with the available observations. For the detection and tracking of eddies at the surface, both the geometrical and dynamical methods are used. The geometrical method is based on the identification of local minima and maxima of dynamic sea surface height. Whereas, the dynamical method utilizes current turbulences arising from strain or vorticity to identify eddies. Using model simulations, the cyclonic and anticyclonic eddies are identified in the BoB. The life span (time period) and the kinetic energy of individual eddies are calculated. The spatial and temporal distribution of eddies and their energetics in the BoB are discussed. Further, the propagation tracks of individual eddies are estimated.

How to cite: Chandra, N. and Pant, V.: Eddies and their energetics in the Bay of Bengal , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1879, https://doi.org/10.5194/egusphere-egu2020-1879, 2020.

EGU2020-702 | Displays | OS1.1

Representation of the Seasonal Cycle of sea surface temperature in CMIP6 models

Yanxin Wang, Karen Heywood, David Stevens, and Gillian Damerell

Sea surface temperature (SST) seasonal extrema are important for water mass formation, intensification of tropical cyclones and coral bleaching, so should be well-represented in models used for future climate projections. Typically, climate model evaluations focus on annual or longer-term mean SST. However, accurate mean SST does not guarantee accurate seasonal extrema or annual cycles. Models that have no biases in mean SST can have biases in seasonal extrema and annual cycles, and vice versa.

Here we assess seasonal extrema in a selection of CMIP6 model historical runs (including BCC-CSM2-MR, CanESM5, CESM2, GFDL-CM4 and GISS-E2-1-G), averaged over 1981-2010, against the World Ocean Atlas (WOA18) observational climatology.  The magnitude and pattern of SST biases for seasonal extrema vary from model to model. GFDL-CM4 and CESM2 simulate SST extrema reasonably well, while BCC-CSM2-MR and GISS-E2-1-G have obvious deficiencies. The global area-weighted root mean square (RMS) difference from WOA18 is larger than 2oC in BCC-CSM2-MR and GISS-E2-1-G, and their common maximum bias (larger than 5oC) is the cold bias located in the subpolar North Pacific, Greenland Sea and Norwegian Sea. The model biases of maximum SST (summer SST) and minimum SST (winter SST) are in some cases different, leading to biased SST annual cycles. The SST biases are typically smaller for summer, except for models with significant winter cold bias in the high latitudes of the Northern Hemisphere (BCC-CSM2-MR and GISS-E2-1-G). Generally speaking, the bias of the SST annual cycle is smaller than that of seasonal extrema; models that are too cold in winter are typically also too cold in summer. In eastern boundary regions, the models have too small annual cycles. In these regions, the warm bias of winter SST is less than the warm bias of summer SST. This is because the warm bias in models due to poorly captured stratocumulus can be compensated by coastal upwelling, which cools the sea surface more in summer than in winter.

We note that extra attention should be paid when evaluating SST extrema in some polar areas as the observational climatology there can be unrealistic, particularly in winter.

How to cite: Wang, Y., Heywood, K., Stevens, D., and Damerell, G.: Representation of the Seasonal Cycle of sea surface temperature in CMIP6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-702, https://doi.org/10.5194/egusphere-egu2020-702, 2020.

EGU2020-9726 | Displays | OS1.1

The improvements to the numerical model of South China Sea Ocean Circulations

Xueming Zhu, Hui Wang, and Ziqing Zu

The South China Sea (SCS) ocean circulations numerical model has been build up based on ROMS with high horizontal resolution. It had been operated in NMEFC to provide daily updated the hydrodynamic forecasting in SCS for the future 5 days since 2013, and named as the SCS operational Oceanography Forecasting System (SCSOFS). Recently, a few systematic optimizations have been carried out to the configuration of the physical model to improve SCSOFS forecast skill. For example, the differential schemes of horizontal and vertical advection of tracers are changed from 4th-order centered to 4th-ordered Akima, the schemes of horizontal mixing of tracers are changed from along epineutral surfaces to along geopotential surfaces, in order to correct for the spurious diapycnal diffusion of the advection operator in terrain-following coordinates, which could cause anomaly temperature increasing about 1 centigrade in deep layer. The method of sea surface atmospheric forcing is changed from direct forcing to bulk formula, by introducing the negative feedback effects between ocean and atmosphere, in order to improve forecast skill of sea surface temperature.

How to cite: Zhu, X., Wang, H., and Zu, Z.: The improvements to the numerical model of South China Sea Ocean Circulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9726, https://doi.org/10.5194/egusphere-egu2020-9726, 2020.

EGU2020-7896 | Displays | OS1.1

Transient Response of Atlantic Heat and Freshwater Transports in Future Climate Scenarios

Jennifer Mecking and Sybren Drijfhout

Ocean heat and freshwater transports play an important role in today’s climate system.  The Atlantic meridional heat transport transports 1.2 PW of heat northward leading to the warm climate we experience in Europe today, while the freshwater transport due to the Atlantic Meridional Overturning Circulation (AMOC) is often used as an indicator for the stability of the AMOC.  Future climate projections show that the AMOC is expected to weaken over the next several decades.  These changes to the AMOC as well as other circulations changes will not only impact the heat and freshwater transports in the Atlantic but also the temperature and salinity structure.  Using both CMIP5 and CMIP6 data this study untangles the impacts of velocity changes versus temperature/ salinity in future climate projections on Atlantic heat and freshwater transports.  Initial results show that changes in velocity dominate heat transport changes while the changes in salinity structure play a large role in freshwater transports with the impact of velocity changes being latitude and model dependent.

How to cite: Mecking, J. and Drijfhout, S.: Transient Response of Atlantic Heat and Freshwater Transports in Future Climate Scenarios, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7896, https://doi.org/10.5194/egusphere-egu2020-7896, 2020.

As an important branch of the global overturning circulation, the deep western boundary current (DWBC) in the Pacific was poorly understood due to sparse observations. Six state-of-the-art global ocean model outputs were used herein to evaluate their performance for simulating the DWBC in the Melanesian Basin (MB) and Central Pacific Basin (CPB). These model outputs were compared to the historical observations, in aspects of water-mass characteristics, spatial structure and meridional volume transport of the DWBC, and seasonal variation. The results showed that most of the models reproduced the DWBC in the two basins well. Besides OFES with obvious cold and salty biases, the other models had minor deviations of the temperature and salinity in the deep layer. These models can reconstruct the spatial structure of the DWBC in detail and simulate appropriate transports of the eastern branch DWBC, ranging from 6.36 Sv to 8.55 Sv. But the western branch DWBC was underestimated in the models except HYCOM (4.48 Sv). HYCOM performed best for the DWBC with a whole transport of 12.84 Sv. Analysis of the temperature and salinity from Levitus data demonstrates the existence of annual and semi-annual cycles in the deep water of the MB and CPB, respectively, with warmer and saltier water mass in summer and autumn. Overall, the six models have good abilities to simulate the seasonal variations of temperature and volume transport of the DWBC in the Pacific. The seasonal signals probably originated from the DWBC upstream and propagated along its pathway.

How to cite: Jiang, H. and Xu, H.: Evaluation of global ocean model on simulating deep western boundary current in the Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8152, https://doi.org/10.5194/egusphere-egu2020-8152, 2020.

OS1.5 – Chaotic variability and modelling uncertainties in the ocean: towards probabilistic oceanography.

EGU2020-7226 | Displays | OS1.5 | Highlight

Quantifying uncertainty in decadal ocean heat uptake due to intrinsic ocean variability

Bablu Sinha, Alex Megann, Thierry Penduff, Jean-Marc Molines, and Sybren Drijfhout

Remarkably, global surface warming since 1850 has not proceeded monotonically, but has consisted of a series of decadal timescale slowdowns (hiatus periods) followed by surges. Knowledge of a mechanism to explain these fluctuations would greatly aid development and testing of near term climate forecasts. Here we evaluate the influence of ocean intrinsic variability on global ocean heat uptake and hence the rate of global surface warming, using a 50-member ensemble of eddy-permitting ocean general circulation model simulations (OCCIPUT ensemble) forced with identical surface atmospheric condition for the period 1960-2015. Air-sea heat flux, integrated zonally and accumulated with latitude provides a useful measure of ocean heat uptake. We plot the ensemble mean difference of this quantity between 2000-2009 (hiatus) and 1990-1999 (surge). OCCIPUT suggests that the 2000s saw increased ocean heat uptake of ~0.32 W m-2compared to the 1990s and that the increased uptake was shared between the tropics and the high latitudes. OCCIPUT shows that intrinsic ocean variability modifies the mean ocean heat uptake change by up to 0.05 W m-2or ±15%. Moreover composite analysis of the ensemble members with the most extreme individual decadal heat uptake changes pinpoints the southern and northern high latitudes as the regions where intrinsic variability plays a large role: tropical heat uptake change is largely fixed by the surface forcing. The western boundary currents and the Antarctic Circumpolar Current (i.e. eddy rich regions) are responsible for the range of simulated ocean heat uptake, with the North Pacific exhibiting a particularly strong signal. The origin of this North Pacific signal is traced to decadal timescale latitudinal excursions of the Kuroshio western boundary current.

How to cite: Sinha, B., Megann, A., Penduff, T., Molines, J.-M., and Drijfhout, S.: Quantifying uncertainty in decadal ocean heat uptake due to intrinsic ocean variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7226, https://doi.org/10.5194/egusphere-egu2020-7226, 2020.

EGU2020-5689 | Displays | OS1.5

Forced and chaotic variability of interannual variability of regional sea level and its causes scale over 1993-2015

Alice Carret, William Llovel, Thierry Penduff, Jean-Marc Molines, and Benoît Meyssignac

Since the early 1990s, satellite altimetry has become the main observing system for continuously measuring the sea level variations with a near global coverage. Satellite altimetry has revealed a global mean sea level rise of 3.3 mm/yr since 1993 with large regional sea level variability that differs from the mean estimate. These measurements highlight complex structures especially for the western boundary currents or the Antarctic Circumpolar Current. A recent study shows that the chaotic ocean variability may mask atmospherically-forced regional sea level trends over 38% of the global ocean area from 1993 to 2015. The chaotic variability is large for the western boundary currents and in the Southern Ocean. The present study aims to complement this previous work in focusing on the interannual variability of regional sea level. A global ¼° ocean/sea-ice 50-member ensemble simulation is considered to disentangle the imprints of the atmospheric forcing and the chaotic ocean variability on the interannual variability of regional sea level over 1993-2015. We investigate the forced (i.e., ensemble mean) versus the chaotic variability (i.e., ensemble standard deviation) for the interannual variability of regional sea level and its causes (i.e., steric sea level and manometric sea level contribution). We complement our investigations by partitioning the steric component into thermosteric sea level (i.e., temperature change only) and halosteric sea level (i.e., salinity change only). One of the goals of the study is to highlight the hot spots region of large chaotic variability for regional sea level and its different components.

How to cite: Carret, A., Llovel, W., Penduff, T., Molines, J.-M., and Meyssignac, B.: Forced and chaotic variability of interannual variability of regional sea level and its causes scale over 1993-2015, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5689, https://doi.org/10.5194/egusphere-egu2020-5689, 2020.

EGU2020-19875 | Displays | OS1.5

Year-to-year meridional shifts of the Great Whirl driven by oceanic internal instabilities

Kwatra Sadhvi, Iyyappan Suresh, Izumo Takeshi, Jerome Vialard, Matthieu Lengaigne, Thierry Penduff, and Jean Marc Molines

The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that forms off the horn of Africa in the western Arabian Sea. It generally appears in June, peaks in July-August, and dissipates in September. While the annual cycle of the GW has been described by past literature, its year-to-year variability has not yet been thoroughly explored. Satellite sea-level observations reveal that the leading mode of interannual variability (half of the interannual summer variance in the GW region) is associated with a typically ~100-km GW northward or southward shift. This meridional shift is associated with coherent sea surface temperature (SST) and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies (and vice versa). Eddy-resolving (~10-km resolution) simulations with an ocean general circulation model capture those observed patterns reasonably well, even in the absence of interannual variations in the surface forcing. Interannual surface forcing variations enhance the GW interannual variability, but do not constrain its phase. Our results hence indicate that year-to-year variations in the Somalia upwelling SST and productivity associated with the GW are thus not a deterministic response to surface forcing, but largely arise from oceanic internal instabilities.

How to cite: Sadhvi, K., Suresh, I., Takeshi, I., Vialard, J., Lengaigne, M., Penduff, T., and Molines, J. M.: Year-to-year meridional shifts of the Great Whirl driven by oceanic internal instabilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19875, https://doi.org/10.5194/egusphere-egu2020-19875, 2020.

EGU2020-20309 | Displays | OS1.5

Deconstructing the subtropical AMOC variability

Quentin Jamet, William Dewar, Nicolas Wienders, Bruno Deremble, Sally Close, and Thierry Penduff

Mechanisms driving the North Atlantic Meridional Overturning Circulation (AMOC) variability at low-frequency are of central interest for accurate climate predictions. However, the origin of this variability remains under debate, complicating for instance the interpretation of the observed time series provided by the RAPID-MOCHA-WBTS program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing, the signal of remote origin and the ocean intrinsic dynamics for the subtropical low-frequency AMOC variability. We analyse for this a set of four ensembles of a regional (20oS - 55oN), eddy-resolving (1/12o) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals.

The analysis of the four ensemble mean AMOCs reveals predominance of local, atmospherically forced signal at interannual time scales (2-10 years), while signals imposed by the boundaries imprint at decadal (10-30 years) time scales. Due to this marked time scale separation, we show that most of the subtropical AMOC forced variability can be understood as a linear superposition of these two signals. Analyzing the ensemble spread of the four ensembles, we then show that the subtropical AMOC is also characterized by an intrinsic variability, which organizes as a basin scale mode peaking at interannual time scales. This basin scale mode is found to be weakly sensitive to the surrounding forced signals, suggesting no causal relationship between the two. Its spatio-temporal pattern shares however similarities with the atmospherically forced signal, which is likely to make the attribution from a single eddy-resolving simulation, or from observations, more difficult.

How to cite: Jamet, Q., Dewar, W., Wienders, N., Deremble, B., Close, S., and Penduff, T.: Deconstructing the subtropical AMOC variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20309, https://doi.org/10.5194/egusphere-egu2020-20309, 2020.

EGU2020-22418 | Displays | OS1.5

On wind-driven energetics of subtropical gyres

William K. Dewar, Quentin Jamet, Bruno Deremble, and Nicolas Wienders

The flow of energy in the wind-driven circulation is examined in a 
combined theoretical and numerical study. Based on a multiple scales 
analysis of the ocean interior, we find the mesoscale field is strongly 
affected by the ventilated thermocline, but no feed back from the eddies 
to the large scale is found.  We then analyze the western boundary 
region arguing that the associated currents divide between coastal jets, 
which conserve mean energy, and open ocean, separated jet extensions
where the mesoscale is energized by the mean field.   It is the 
separated jet zone where the primary loss of general circulation energy 
to the mesoscale occurs.  Connections to the `Thickness Weighted 
Average' form of the primitive equations are made which support the 
differing roles of the eddies in these regions.  These ideas are then 
tested by an analysis of a regional primitive equation 1/12-degree 
numerical model of the North Atlantic. The predictions of the theory are 
generally supported by the numerical results.  The one exception is that 
topographic irregularities in the coastal jet spawn eddies, although 
they contribute modestly to the energy budget.  We therefore conclude 
the primary sink of wind input into the mean circulation is in the 
separated jet, and not the interior.  The analysis also shows
wind forcing is much smaller than the interior energy fluxes. Thus, the 
general circulation is characterized as recirculating energy in the 
manner of a Fofonoff gyre.

How to cite: Dewar, W. K., Jamet, Q., Deremble, B., and Wienders, N.: On wind-driven energetics of subtropical gyres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22418, https://doi.org/10.5194/egusphere-egu2020-22418, 2020.

EGU2020-21330 | Displays | OS1.5

Eddy-Mean flow oscillations in the Southern Ocean

Sebastiano Roncoroni and David Ferreira

Geostrophic eddies have a leading order effect on the dynamics of the Southern Ocean (SO), and numerous studies have shown that they are also key to the response of both the zonal transport and the meridional overturning circulation to wind stress changes. The role played by eddies in setting the intrinsic variability of the SO, however, is less well-understood. Here, inspired by recent work on the atmospheric jet, we investigate whether the eddy-mean flow interaction in the Antarctic Circumpolar Current can be described by a prey-predator nonlinear model.

 

To this end, we analyse data from a high-resolution eddy-resolving configuration of the MIT general circulation model: an idealised “channel” model with mechanical and thermodynamical forcing at the surface, and plausible zonal and meridional circulations.

 

Here, we show that a mechanism of eddy-mean flow interaction driving the intrinsic variability of the SO-like model is well described by a stochastic non-linear oscillator with damping. This model is a generalisation of the Ambaum-Novak oscillator, which has been successfully employed to describe the atmospheric storm track variability.

 

We find that, on length scales similar to that of individual zonal jets, the eddy-mean flow interaction is characterised by a high-frequency oscillatory mode, and that the characteristic time scale of the oscillation is comparable with classical estimates of the baroclinic life-cycle. A Gaussian smoothing of the phase space diagram also reveals the damped oscillatory character of the oscillation: this is in contrast with the atmospheric case, where damping is negligible and orbits are confined to energy surfaces.

 

This result may help inform the interpretation of the SO intrinsic and forced variability (such as, for example, the response to wind stress changes), and pave the way to further studies featuring more realistic model configurations.

How to cite: Roncoroni, S. and Ferreira, D.: Eddy-Mean flow oscillations in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21330, https://doi.org/10.5194/egusphere-egu2020-21330, 2020.

A major challenge of modern ocean modelling is how to represent in ocean models small-scale features with length scales smaller than the grid spacing. It is known that small scale eddies are important for maintaining Western boundary currents such as the Gulf Stream and Kuroshio; it is therefore of great importance that these are well represented in any global ocean model. The small scales are often parameterised by viscosity closures or GM parameterisations. The Stochastic Advection by Lie Transport (SALT) method is a propsed alternative which is defined so as to preserve important physical properties of the flow solution. Stochasticity is introduced into the fluid dynamical variational principle so that the resulting Euler-Poincaré equations give a stochastic version of the fluid equations which maintain a Kelvin circulation theorem and conservation of potential vorticity. The stochastic terms are then tuned using empirical orthogonal functions obtained from fine-grid model runs in order to capture the small-scale effects. This method has been shown to be effective for quasigeostrophic models and the 2D Euler equations. Here we present an application to the Finite volumE Sea-ice Ocean Model (FESOM2.0), a primitive equation model; we show preliminary results from this implementation.

How to cite: Patching, S.: Stochastic Advection for eddy parameterisation in Primitive Equation Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11312, https://doi.org/10.5194/egusphere-egu2020-11312, 2020.

Oceanic turbulent flows consist of complex motions (fronts, eddies and waves) that co-exist on many different spatio-temporal scales and nonlinearly interacting with each other. In this study data-adaptive harmonic decomposition (DAHD) has been applied to high-dimensional datasets of complex turbulent flows simulated by ocean models of different complexity. DAHD allows a low-rank description of multiscale and chaotic dynamics by a small subset of data-adaptive patterns oscillating harmonically at given temporal frequency. The shape and scaling laws of temporal energy spectrum of the extracted patterns reveal global fingerprint of underlying dynamics, providing new opportunities to characterize and compare oceanic datasets and models. 


1. Ryzhov, E.A., D. Kondrashov, N. Agarwal, and P.S. Berloff, 2019: 
On data-driven augmentation of low-resolution ocean model dynamics, 
Ocean Modelling, 142, doi:10.1016/j.ocemod.2019.101464. 

2. Kondrashov, D., M. D. Chekroun and P. Berloff, 2018: 
Multiscale Stuart-Landau Emulators: Application to Wind-Driven Ocean Gyres,
Fluids, 3(1), 21, doi:10.3390/fluids3010021.

How to cite: Kondrashov, D.: Data-adaptive harmonic analysis of high-dimensional oceanic turbulent flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12290, https://doi.org/10.5194/egusphere-egu2020-12290, 2020.

EGU2020-11127 | Displays | OS1.5

Ensemble quantification of short-term predictability of the ocean fine-scale dynamics: a western mediterranean test case at kilometric-scale resolution.

Stéphanie Leroux, Jean-Michel Brankart, Aurélie Albert, Pierre Brasseur, Laurent Brodeau, Julien Le Sommer, Jean-Marc Molines, and Thierry Penduff

“Predictability” in operational forecasting systems can be viewed as the ability to meet the forecast accuracy that is required for a given application. In the literature, the most usual approach is to assume that predictability is mainly limited by model instability (i.e. the chaotic behaviour of the system), which means assuming that initial and model errors are small. But, in operational systems, initial and model errors cannot usually be assumed small, because of the complexity of the system and because observations and model resources are limited. In this study, we propose  a practical approach to take into account such model and initial condition errors, in the aim to evaluate the predictability of the fine-scale dynamics in a CMEMS-like operational system, based on ensemble experiments with the ocean numerical model NEMO.

    To do so, we set up a regional model configuration MEDWEST60 with NEMO v3.6,  212 vertical levels and a kilometric-scale horizontal resolution (1/60º). Such a resolution allows to simulate the fine-scale dynamics up to an effective resolution of  ~10 km. The domain covers the Western Mediterranean sea from Gibraltar to Corsica-Sardinia. The configuration includes tides and is forced at the western and eastern boundaries with hourly outputs from a reference simulation on a larger domain, also including tides, and based on the exact same horizontal and vertical grid.

    The practical approach we follow consists first in performing a set of several  short (~1month) ensemble forecast experiments to study the growth of forecast errors for different levels of  model error and initial condition error. In practice, we need to implement a tunable source of model error in MEDWEST60, that might represent e.g. numerical errors, forcing errors, missing or uncertain physics via stochastic parameterization (in this presentation, we will focus on a first set of ensemble experiments where stochastic perturbations are added on the model vertical grid). It is then used to generate different levels of error on the initial conditions. 

    In a second step, by inverting the dependence between forecast error on the one hand and initial and model error on the other hand, we aim to diagnose the level of initial and model accuracy needed for a given targeted accuracy of the forecasting system. 

Practical questions addressed by such experiments relate to the relative importance of model accuracy vs initial condition accuracy for the  forecast of the finest scales in a CMEMS system. From this we can infer information about (a) predictability - for instance, the time along which a forecast remains meaningful for the fine scales. And information about (b) controllability by the observations, for instance, the minimal time to consider between two passes of a future satellite to be able to follow a given observed fine-scale structure - front, eddy, etc

How to cite: Leroux, S., Brankart, J.-M., Albert, A., Brasseur, P., Brodeau, L., Le Sommer, J., Molines, J.-M., and Penduff, T.: Ensemble quantification of short-term predictability of the ocean fine-scale dynamics: a western mediterranean test case at kilometric-scale resolution., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11127, https://doi.org/10.5194/egusphere-egu2020-11127, 2020.

EGU2020-6489 | Displays | OS1.5

Predictability of estuarine model using Information Theory: ROMS Ocean State Ocean Model

Aakash Sane, Baylor Fox-Kemper, David Ullman, Christopher Kincaid, and Lewis Rothstein

With a focus on modelling physical aspects of estuaries covering Rhode Island, USA, the Ocean State Ocean Model (OSOM) has been implemented using the Regional Ocean Modeling System. The estuary includes Narragansett Bay, Mt. Hope Bay, and nearby regions including the shelf circulation from Long Island to Nantucket. Our goal is to find predictability and estuarine time scales in order to build a forecasting system 

 

Perturbed ensemble simulations with altered initial condition parameters (temperature, salinity) are combined with concepts from Information Theory to quantify the predictability of the OSOM forecast system. Predictability provides a theoretical estimate of the potential forecasting capabilities of the model in the form of prediction time scales and enhances readily estimable timescales such as the freshwater/ saline water flushing timescale. The predictability of the OSOM model is around 10-40 days, varying by perturbation parameters and season. Internal variability is low when compared to forced variability for the current resolution of OSOM suggesting modest chaos at this resolution.

 

Freshwater flushing time scale and total exchange flow was calculated for the OSOM model. The freshwater flushing time scale was found to be ~20 days and varies with the choice of the estuary boundary. The predictability time scales and flushing time scales reveal important dynamics of the tracers involved and elucidate their role in driving the estuary.  

How to cite: Sane, A., Fox-Kemper, B., Ullman, D., Kincaid, C., and Rothstein, L.: Predictability of estuarine model using Information Theory: ROMS Ocean State Ocean Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6489, https://doi.org/10.5194/egusphere-egu2020-6489, 2020.

EGU2020-6000 | Displays | OS1.5

Impact of Atmospheric and Model Physics Perturbations On a High-Resolution Ensemble Data Assimilation System of the Red Sea

siva reddy sanikommu, Habib Toye, Peng Zhan, Sabique Langodan, George Krokos, Omar Knio, and Ibrahim Hoteit

The Ensemble Adjustment Kalman Filter of the Data Assimilation Research Testbed is implemented to assimilate observations of satellite sea surface temperature, altimeter sea surface height and in-situocean temperature and salinity profiles into an eddy-resolving 4km-Massachusetts Institute of Technology general circulation model (MITgcm) of the Red Sea. We investigate the impact of three different assimilation strategies (1) Iexp– inflates filter error covariance by 10%, (2) IAexp– adds ensemble of atmospheric forcing to Iexp, and (3) IAPexp– adds perturbed model physics toIAexp. The assimilation experiments are run for one year, starting from the same initial ensemble on 1stJanuary, 2011 and the data are assimilated every three days.

Results demonstrate that the Iexp mainly improved the model outputs with respect to assimilation-free MITgcm run in the first few months, before showing signs of dynamical imbalances in the ocean estimates, particularly in the data-sparse subsurface layers. The IAexp yielded substantial improvements throughout the assimilation period with almost no signs of imbalances, including the subsurface layers. It further well preserved the model mesoscales features resulting in an improved forecasts for eddies, both in terms of intensity and location. Perturbing model physics in IAPexp slightly improved the forecast statistics. It further increased smoothness in the ocean forecasts and improved the placement of basin-scale eddies, but caused loss of some high-resolution features. Increasing hydrographic coverage helps recovering the losses and yields more improvements in IAPexp compared to IAexp. Switching off inflation in IAexp and IAPexp leads to further improvements, especially in the subsurface layers.

How to cite: sanikommu, S. R., Toye, H., Zhan, P., Langodan, S., Krokos, G., Knio, O., and Hoteit, I.: Impact of Atmospheric and Model Physics Perturbations On a High-Resolution Ensemble Data Assimilation System of the Red Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6000, https://doi.org/10.5194/egusphere-egu2020-6000, 2020.

EGU2020-2737 | Displays | OS1.5

Forced and chaotic variability of basin-scale heat budgets in the global ocean: focus on the South Atlantic crossroads.

Thierry Penduff, Fei-Er Yan, Imane Benabicha, Jean-Marc Molines, and Bernard Barnier

The OCCIPUT eddy-permitting (1/4°) global ocean/sea-ice 50-member ensemble simulation is analyzed over the period 1980-2015 to identify how the atmosphere and the intrinsic/chaotic ocean variability modulate the basin-scale Ocean Heat Content (OHC) at various timescales. In all regions of the simulated world ocean, the atmospherically-forced interannual OHC variability is driven by both air-sea heat fluxes (Qnet) and advective heat transport convergences (Conv), while the intrinsic component is driven by Conv, and damped by Qnet. 

We focus on the Atlantic sector of the Southern Ocean (SA), where the oceanic “chaos” explains 36 to 90% of the interannual and decadal heat transport variability across the limits of the basin, and 22% of this huge basin’s OHC variability at interannual and decadal timescales.

The model also simulates the Antarctic Circumpolar Wave (ACW) that was observed in the 80-90’s, with large impacts on OHC and heat transports in the Southern Ocean. This forced signal appears south of Australia, propagates eastward around Antarctica and northward into the Tropical Atlantic and the Tropical Indian Ocean. 

These results highlight the substantial contribution of large-scale low-frequency chaotic heat advection in eddy-active regions, and its major impact on decadal OHC variations over key basins. They suggest that climate simulations using eddying ocean models include an oceanic and random source of large-scale low-frequency variability whose atmospheric impacts remain to be assessed.

How to cite: Penduff, T., Yan, F.-E., Benabicha, I., Molines, J.-M., and Barnier, B.: Forced and chaotic variability of basin-scale heat budgets in the global ocean: focus on the South Atlantic crossroads., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2737, https://doi.org/10.5194/egusphere-egu2020-2737, 2020.

OS1.6 – Improved Understanding of Ocean Variability and Climate

EGU2020-5978 | Displays | OS1.6 | Highlight

Argo Beyond 2020: Towards a global, full-depth multidisciplinary array

Susan Wijffels, Toshio Suga, and Dean Roemmich and the Argo Steering Team

Starting in 2000, Argo reached global coverage in 2007 and has sustained a globally distributed array of ~ 3000 profiling floats for almost two decades. This Argo array delivers ocean temperature and salinity profiles from the sea surface to 2000 dbar roughly 300km apart every 10 days in realtime. Just as the present Argo array originated from an opportunistic mix of developments in both technology and data management, a new step-change in global ocean observing is now possible. Advances in platform and sensor technologies presents a new opportunity to (i) improve Argo’s global reach and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems – all within the context of a comprehensive Argo data system. Each of these enhancements are evolving along a path from experimental deployments to regional pilot arrays to global implementation.The ultimate objective is to implement a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System. The integrated system will deliver enhanced operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

How to cite: Wijffels, S., Suga, T., and Roemmich, D. and the Argo Steering Team: Argo Beyond 2020: Towards a global, full-depth multidisciplinary array, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5978, https://doi.org/10.5194/egusphere-egu2020-5978, 2020.

We consider here the potential of Voluntary Observing Ship (VOS) observations available form the ICOADS for estimating ocean surface heat budget at centennial time scales. VOS provide the longest coverage of the World Ocean by in-situ meteorological observations in time going back to the mid 18th century. We concentrate here on the shortwave and longwave radiative fluxes, largely relying on cloud cover. Visually observed cloud cover reports from Voluntary Observing Ships (VOS) and assimilated in ICOADS are were used to build long-term time series of cloud cover and short-wave radiation characteristics over the ocean for the last century. Cloud cover reports from VOS are subject for a number of inhomogeneities and uncertainties. Considering the centennial perspective, in 1949, WMO changed the practice of reporting cloud cover from tenths to octas. Moreover, some additional uncertainties were inherent in the early 20th century reports. This resulted in a definite break in cloud cover time series which further propagate to the inhomogeneity of the reconstructed time series of shortwave and longwave radiative fluxes. This inhomogeneity was associated with (while not limited to) the biased convertionconversion of tens to octas when developing ICOADS records using IMMA (and earlier generation formats). In this convertionconversion octa values “2” and “6” consolidated values corresponding to 2 and 3 tens and 7 and 8 tens respectively, thus making the fractional cloud cover distribution peaked to 2 and 6 octas. In order to remove correct this bias and to homogenize cloud cover time series we developed a new method based upon a discrete probability distribution for fractional cloud cover. Applying analytical distribution, we provide the correction of cloud cover reports and arrive to homogeneous time series of cloud cover. Further homogenized times series of cloud cover were used for computing radiative fluxes over the global ocean for the period from 1900 onwards.

How to cite: Gulev, S. and Aleksandrova, M.: Homogenizing visually observed cloud cover over global oceans with implications for reconstructions of radiative fluxes at sea surface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12826, https://doi.org/10.5194/egusphere-egu2020-12826, 2020.

EGU2020-13069 | Displays | OS1.6 | Highlight

A global Biogeochemical Argo pilot array: Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) profiling floats and results

Lynne Talley, Kenneth Johnson, Stephen Riser, Jorge Sarmiento, Joellen Russell, Emmanuel Boss, Matthew Mazloff, and Susan Wijffels

The ocean provides critical services to life on the planet, absorbing 93% of the heat from anthropogenic warming and a quarter of human carbon dioxide (CO2) emissions each year. However, rising ocean temperatures and CO2 levels also change the marine environment: pH and oxygen levels fall, ocean currents change, and nutrient fluxes and concentrations are shifting, all with large effects on ecosystems and the cycles of oxygen, nitrogen, and carbon throughout the ocean and atmosphere. Observing these biogeochemical (BGC) processes across remote ocean areas with seasonal to interannual resolution has been impractical due to the prohibitive costs associated with ship observations. Yet such observations are essential to understand the natural and perturbed systems.

Profiling floats, proven in the Argo program, with BGC sensors (oxygen, nitrate, pH, bio-optical) provide a transformative solution to this need.  BGC profiling floats are capable of observing chemical and biological properties from 2000 m depth to the surface every 10 days for many years. Based on various OSSE and sampling approaches, global coverage can be achieved with 1000 BGC floats contributing to the core T/S Argo array of about 4000.

The U.S. Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program serves as a major basin-scale pilot for such a global array. Its 141 operating BGC floats, building towards an ultimate 200 floats, demonstrate that the major challenges associated with operating a large-scale, robotic network have been overcome, and that there is a substantial user base for the data. Data have been publicly available in near real-time since the start of SOCCOM. Robust protocols for QC, calibration and validation of BGC float data have been developed, based on GLODAPv2 climatologies and relationships between the observed float variables. Data are being incorporated in BGC state estimation and are being used for comparison/validation of ocean models used for climate. Initial SOCCOM results are already transforming understanding of Southern Ocean biogeochemistry. Annual cycles of air-sea carbon flux are revealing major surprises, including strong outgassing within the Antarctic Circumpolar Current.  Annual net community production in all major regimes of the Southern Ocean has been quantified.  The broad-scale float profiling has validated NASA's satellite algorithms for POC and chlorophyll in the Southern Ocean. As the international community moves forward towards sustained BGC-Argo deployments, SOCCOM can provide its experience in sensors, floats, deployments, calibration, and data management. 

How to cite: Talley, L., Johnson, K., Riser, S., Sarmiento, J., Russell, J., Boss, E., Mazloff, M., and Wijffels, S.: A global Biogeochemical Argo pilot array: Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) profiling floats and results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13069, https://doi.org/10.5194/egusphere-egu2020-13069, 2020.

To understand the role of the ocean in the climate system, it is no longer sufficient to study either physics or biogeochemistry. Future efforts need to combine these disciplines to truly understand our future climate. The water mass transformation (WMT) weaves together circulation, thermodynamics, and biogeochemistry into a description of the ocean that complements traditional Eulerian and Lagrangian methods. Here we present a derivation of a WMT framework that offers an analysis that renders novel insights and predictive capabilities for studies of ocean physics and biogeochemistry that determine ocean tracer uptake, circulation and storage. We will discuss application for this framework for biogeochemical studies and its potential for inferring unmeasurable biogeochemical processes from estimates of the measurable physical processes.

How to cite: Groeskamp, S.: The Water Mass Transformation Framework for Ocean Physics and Biogeochemistry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5476, https://doi.org/10.5194/egusphere-egu2020-5476, 2020.

EGU2020-3628 | Displays | OS1.6 | Highlight

Pathways and time scales of ocean heat uptake and redistribution in a global ocean-ice model

Alice Marzocchi, George Nurser, Louis Clement, and Elaine McDonagh

Changes in regional ocean heat content are not only sensitive to anthropogenic and natural influences, but also substantially impacted by the redistribution of heat, which is in turn driven by changes in ocean circulation and air-sea fluxes. Using a set of numerical simulations with an ocean-sea-ice model of the NEMO framework, we assess where the ocean takes up heat from the atmosphere and how ocean currents transport and redistribute that heat. Here, the strength and patterns of the net uptake of heat by the ocean are treated like a passive tracer, by including simulated sea water vintage dyes, which are released annually between 1958 and 2017. An additional tracer released in year 1800 is also used to investigate longer-term variability. All dye tracers are released from 29 surface patches, representing different water mass production sites, allowing us to identify when and where water masses were last ventilated. The tracers’ distribution and fluxes are shown to capture years of strong and weak convection at deep and mode water formation sites in both hemispheres, when compared to the available observations. Using this approach, which can be applied to any passive tracer in the ocean, we can: (1) assess the relative role of each of the water mass production sites, (2) evaluate the regional and depth distribution of the tracers, and (3) determine their variability on interannual, multidecadal and centennial time scales.

How to cite: Marzocchi, A., Nurser, G., Clement, L., and McDonagh, E.: Pathways and time scales of ocean heat uptake and redistribution in a global ocean-ice model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3628, https://doi.org/10.5194/egusphere-egu2020-3628, 2020.

EGU2020-4696 | Displays | OS1.6

Historical ocean heat uptake in CMIP6 Earth System models: global and regional perspectives

Till Kuhlbrodt, Aurore Voldoire, Matthew Palmer, Rachel Killick, and Colin Jones

Ocean heat content is arguably one of the most relevant metrics for tracking global climate change and in particular the current global heating. Because of its enormous heat capacity, the global ocean stores about 93 percent of the excess heat in the Earth System. Time series of global ocean heat content (OHC) closely track Earth’s energy imbalance as observed as the net radiative balance at the top of the atmosphere. For these reasons simulated OHC time series are a cornerstone for assessing the scientific performance of Earth System models (ESM) and global climate models. Here we present a detailed analysis of the OHC change in simulations of the historical climate (20th century up to 2014) performed with four of the current, state-of-the art generation of ESMs and climate models. These four models are UKESM1, HadGEM3-GC3.1-LL, CNRM-ESM2-1 and CNRM-CM6-1. All four share the same ocean component, NEMO3.6 in the shaconemo eORCA1 configuration, and they all take part in CMIP6, the current Phase 6 of the Coupled Model Intercomparison Project. Analysing a small number of models gives us the opportunity to analyse OHC change for the global ocean as well as for individual ocean basins. In addition to the ensemble means, we focus on some individual ensemble members for a more detailed process understanding. For the global ocean, the two CNRM models reproduce the observed OHC change since the 1960s closely, especially in the top 700 m of the ocean. The two UK models (UKESM1 and HadGEM3-GC3.1-LL) do not simulate the observed global ocean warming in the 1970s and 1980s, and they warm too fast after 1991. We analyse how this varied performance across the models relates to the simulated radiative forcing of the atmosphere. All four models show a smaller ocean heat uptake since 1971, and a larger transient climate response (TCR), than the CMIP5 ensemble mean. Close analysis of a few individual ensemble members indicates a dominant role of heat uptake and deep-water formation processes in the Southern Ocean for variability and change in global OHC. Evaluating OHC change in individual ocean basins reveals that the lack of warming in the UK models stems from the Pacific and Indian basins, while in the Atlantic the OHC change 1971-2014 is close to the observed value. Resolving the ocean warming in depth and time shows that regional ocean heat uptake in the North Atlantic plays a substantial role in compensating small warming rates elsewhere. An opposite picture emerges from the CNRM models. Here the simulated OHC change is close to observations in the Pacific and Indian basins, while tending to be too small in the Atlantic, indicating a markedly different role for the Atlantic meridional overturning circulation (AMOC) and cross-equatorial heat transport in these models.

How to cite: Kuhlbrodt, T., Voldoire, A., Palmer, M., Killick, R., and Jones, C.: Historical ocean heat uptake in CMIP6 Earth System models: global and regional perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4696, https://doi.org/10.5194/egusphere-egu2020-4696, 2020.

EGU2020-3703 | Displays | OS1.6

Human-induced changes to the global ocean water masses and their time of emergence

Yona Silvy, Eric Guilyardi, Jean-Baptiste Sallée, and Paul Durack

The World Ocean is rapidly changing, with global and regional modification of temperature and salinity evident at the surface and depth. These changes have widespread and irreversible impacts including sea-level rise, changes to the oxygen and carbon contents of the ocean interior, or changing habitats, diversity and resilience of ecosystems. While the most pronounced temperature and salinity changes are located in the upper few hundred metres, changes in water-masses at depth are already observed and will likely strengthen and persist in the future as water-masses form at the surface and propagate in the deep ocean along density surfaces, storing the anthropogenic signal away from the atmosphere for decades to millennia. Here, using 11 climate models, we define when anthropogenic temperature and salinity changes are expected to emerge from natural background variability in the ocean interior. On a basin-scale zonal average, the model simulations predict that in 2020, 20–55% of the Atlantic, Pacific and Indian basins have an emergent anthropogenic signal; reaching 40–65% in 2050, and 55–80% in 2080. The well-ventilated Southern Ocean water-masses emerge very rapidly, as early as the 1980s-1990s, while the Northern Hemisphere emerges in the 2010s to 2030s. Additionally, dedicated idealized simulations of the IPSL coupled climate model are examined to study the role of each separate surface forcing on the time scales associated with the patterns of temperature and salinity change under a global warming scenario, and the influence of excess versus redistributed heat and salt. Our results highlight the importance of maintaining and augmenting an ocean observing system capable of detecting and monitoring anthropogenic changes. 

How to cite: Silvy, Y., Guilyardi, E., Sallée, J.-B., and Durack, P.: Human-induced changes to the global ocean water masses and their time of emergence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3703, https://doi.org/10.5194/egusphere-egu2020-3703, 2020.

EGU2020-18438 | Displays | OS1.6 | Highlight

Intercomparison of anthropogenic ocean heat uptake processes in AOGCMs

Matthew Couldrey and Jonathan Gregory

Thermosteric sea level change, resulting from ocean heat uptake, is a key component of recent and future sea level rise. The various atmosphere-ocean general circulation models (AOGCMs) used to predict future climate produce diverse spatial patterns of future thermosteric sea level rise. Most of this model spread occurs because the representation of ocean circulation and heat transport is different across models. These effects can be analysed through new simulations carried out as part of the Flux Anomaly Forced Intercomparison Project (FAFMIP), in which the exchanges of heat and salt are attributed to specific ocean circulation processes, namely the vertical dianeutral processes (convection, boundary layer mixing, shear instability mixing etc), isopycnal diffusion and residual-mean advection. Here, we present an intercomparison of ocean heat content change in FAFMIP experiments from a water-mass following perspective, to distinguish oceanic heat redistribution and uptake. We find that the redistribution of heat is a key difference across AOGCMs.

How to cite: Couldrey, M. and Gregory, J.: Intercomparison of anthropogenic ocean heat uptake processes in AOGCMs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18438, https://doi.org/10.5194/egusphere-egu2020-18438, 2020.

EGU2020-5575 | Displays | OS1.6 | Highlight

The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses

Laura Jackson, Clotilde Dubois, Gael Forget, Keith Haines, Matt Harrison, Dorotea Iovino, Armin Kohl, Davi Mignac, Ssimona Masina, Drew Peterson, Christopher Piecuch, Chris Roberts, Jon Robson, Andrea Storto, Takahiro Toyoda, Maria Valdivieso, Chris Wilson, Yiguo Wang, and Hao Zuo

The observational network around the North Atlantic has improved significantly over the last few decades with the advent of Argo and satellite observations, and the more recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC) using arrays such as RAPID and OSNAP. These have shown decadal timescale changes across the North Atlantic including in heat content, heat transport and the circulation. 

However there are still significant gaps in the observational coverage, and significant uncertainties around some observational products. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and are potentially tools that can be used to understand the observed changes. However the suitability of the reanalyses for the task must also be assessed.
We use an ensemble of global ocean reanalyses in comparison with observations in order to examine the mean state and interannual-decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture different processes and whether any understanding can be inferred. In particular we look at ocean heat content, transports, the AMOC and gyre strengths, water masses and convection. 

 

How to cite: Jackson, L., Dubois, C., Forget, G., Haines, K., Harrison, M., Iovino, D., Kohl, A., Mignac, D., Masina, S., Peterson, D., Piecuch, C., Roberts, C., Robson, J., Storto, A., Toyoda, T., Valdivieso, M., Wilson, C., Wang, Y., and Zuo, H.: The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5575, https://doi.org/10.5194/egusphere-egu2020-5575, 2020.

EGU2020-8192 | Displays | OS1.6 | Highlight

Long-term observations of the strongest inflow branch of warm water to the Arctic Mediterranean

Bogi Hansen, Karin M. H. Larsen, Hjálmar Hátún, and Svein Østerhus

Warm and saline water from the North Atlantic enters the Arctic Mediterranean through three gaps. The strongest of these three flows is the inflow between Iceland and Faroes, which is focused into a narrow boundary current north of the Faroes. This boundary current, the Faroe Current, has been observed with regular CTD cruises since 1988 and with moored ADCPs since 1997, as well as satellite altimetry since 1993. Once calibrated by the long-term ADCP measurements, the satellite altimetry is found to yield high-accuracy determination of the velocity field and volume transport down to fixed depth. Due to geostrophic adjustment, satellite altimetry combined with CTD data also allow fairly accurate determination of the depth of the Atlantic layer. From the combined data set, monthly transport time series have been generated for the period Jan 1993 to April 2019. Over the period, the annually averaged volume transport of Atlantic water in the Faroe Current seems to have increased slightly, while the heat transport relative to an outflow temperature of 0°C increased by 13%, significant at the 95% level. The salinity increased from the mid-1990s to around 2010, after which it has decreased, especially after 2016, leading to the lowest salinities in the whole period since 1988. To stay updated on a possible inflow reduction due to reduced thermohaline ventilation caused by this freshening, the future monitoring system of the Faroe Current is planned to be expanded with moored PIES (Pressure Inverted Echo Sounders). An experiment with two PIES in 2017-2019 has documented that these instruments allow high-accuracy monitoring of the depth of the Atlantic layer on the section, which combined with satellite altimetry and CTD observations should give more accurate transport estimates.

How to cite: Hansen, B., Larsen, K. M. H., Hátún, H., and Østerhus, S.: Long-term observations of the strongest inflow branch of warm water to the Arctic Mediterranean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8192, https://doi.org/10.5194/egusphere-egu2020-8192, 2020.

EGU2020-18974 | Displays | OS1.6

New Insight Into the Formation and Evolution of the East Reykjanes Ridge Current and Irminger Current

Virginie Thierry, Tillys Petit, and Herlé Mercier

The Reykjanes Ridge strongly influences the circulation of the North Atlantic Subpolar Gyre as it flows to the Irminger Sea from the Iceland Basin. The circulation is composed of two main along‐ridge currents: the southwestward East Reykjanes Ridge Current (ERRC) in the Iceland Basin and the northeastward Irminger Current (IC) in the Irminger Sea. To study their interconnection through the ridge, as well as their connections with the interior of each basin, velocity and hydrological measurements were carried out along and perpendicular to the crest of the Reykjanes Ridge in June–July 2015 as part of the Reykjanes Ridge Experiment project. This new data set changes our view of the ERRC and IC as it reveals undocumented along‐stream evolutions of their hydrological properties, structures, and transports. These evolutions are due to flows connecting the ERRC and IC branches at specific locations set by the bathymetry of the ridge and to significant connections with the interiors of the basins. Overall, the ERRC transport increases by 3.2 Sv between 63°N and 59.5°N and remains almost constantly southward. In the Irminger Sea, the increase in IC transport of 13.7 Sv between 56°N and 59.5°N, and the evolution of its properties are explained by both cross‐ridge flows and inflows from the Irminger Sea. Further north, bathymetry steers the IC northwestward into the Irminger Sea. At 63°N, the IC water masses are mostly issued from the cross-ridge flow.

How to cite: Thierry, V., Petit, T., and Mercier, H.: New Insight Into the Formation and Evolution of the East Reykjanes Ridge Current and Irminger Current, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18974, https://doi.org/10.5194/egusphere-egu2020-18974, 2020.

EGU2020-9280 | Displays | OS1.6

Along-stream evolution of Gulf Stream volume transport and water properties from underwater glider observations
not presented

Joleen Heiderich and Robert E. Todd

The Gulf Stream is the western boundary current in the subtropical North Atlantic and a principal component of the upper limb of the Atlantic Meridional Overturning Circulation. Thus, it plays an important role in poleward heat and volume transport, as well as in the redistribution and modification of various water masses. Despite its importance in the climate system, many details of the Gulf Stream’s increase in volume along the US East Coast and the associated entrainment of various water masses are not well known due to a paucity of sustained subsurface measurements within and near the Gulf Stream. Observations from more than 30 Spray autonomous underwater glider missions comprising over 22,000 profiles and more than 180 distinct cross-Gulf Stream transects collected between 2004 and the present fill a 1,500-km-long gap in sustained subsurface measurements; they provide concurrent measurements of hydrography and velocity in and near the Gulf Stream over more than 15 degrees of latitude between Florida and New England. These observations are used to characterize the along-stream evolution of Gulf Stream volume transport including classification by water properties. Remotely formed intermediate waters (i.e., Antarctic Intermediate Water (AAIW) and upper Labrador Sea Water (uLSW)) are significant components of Gulf Stream transport. AAIW is formed at high southern latitudes and enters the Gulf Stream through the Florida Strait, while uLSW is formed through deep convection in the Labrador Sea and encounters the Gulf Stream at Cape Hatteras as the uppermost layer of the Deep Western Boundary Current. Though it is well known where AAIW and uLSW initially encounter the Gulf Stream, their distribution, advection, and modification within the Gulf Stream remain poorly resolved. The extensive glider observations are used to characterize the evolution and intermittency of AAIW and uLSW pathways within and near the Gulf Stream, including effects of near-bottom mixing and the mechanisms by which uLSW crosses isobaths to arrive over the O(1000)-m-deep Blake Plateau south of Cape Hatteras. This first look at Gulf Stream transport by water class and the three-dimensional pathways followed by intermediate water masses within the Gulf Stream provides a reference for global circulation models to replicate.

How to cite: Heiderich, J. and Todd, R. E.: Along-stream evolution of Gulf Stream volume transport and water properties from underwater glider observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9280, https://doi.org/10.5194/egusphere-egu2020-9280, 2020.

EGU2020-10226 | Displays | OS1.6 | Highlight

Historical Reconstruction of Anthropogenic Carbon and Excess Heat Content in the Subtropical North Atlantic Ocean

Herle Mercier and Marie-Jose Messias

The oceans have mitigated global warming by the absorption of 90% of the excess heat resulting from anthropogenic radiative forcing and of 1/3 of the anthropogenic carbon (Cant). There are still major uncertainties concerning their regional rates of uptake (or loss), transport and storage by the oceans, knowledge of which is key to the heat and carbon balances, and essential to reduce the uncertainties in global warming prediction. Here, we used tracers observations (transient and passive CFC-11, CFC-12, SF6, natural C14, the conservative PO4* and NO3*, salinity and temperature) and a maximum entropy inverse method to compute Green’s functions (G), which contain intrinsically information on ocean dynamics and transit times from the source regions. From G, we propagated surface history of temperature and Cant to reconstruct their fields in the ocean for the industrial era and to quantify their source regions. We present reconstructions of Cant and excess heat (taken as the temperature anomaly from 1850) along the 24°N trans-Atlantic section, at the crossroads of the main contributors of the AMOC and an hot spot of heat and carbon storage, from 5 repeats spanning 1992 to 2015. We show that Cant reconstructions, dominated by the strong increase of Cant in the atmosphere, compare well with a previous global historical reconstruction as well as Cant estimates in the water masses at 24°N. The excess heat reconstructions are tempered by the natural variability that can exceed the anthropogenic trend. They show a net invasion and warming of the top 800m from the 1920’s (0.01°C/y). The trend slightly weakens in the late 1970’s followed by an acceleration from the 2000’s (0.02°C/y). For the well–ventilated deeper waters of the DWBC around 1500m, after a notable cooling period, a weak warming departs in the 1950’s with a trend of 0.001°C/y up to the 2000’s and of 0.006°C/y afterwards. The waters below 2000m suggest a continuous warming from the 1930’s, with a more pronounced trend centered at 3000m of 0.001°C/y up to the 2000’s and of 0.003°C/y afterwards. This excess heat evolution in the DWBC contrasts with the Cant evolution which shows continuous increase in Cant content in the upper NADW. Our results highlight the difference of drowning up of Cant ant heat into the deeper ocean, reflecting their different surface histories in the formation regions.

How to cite: Mercier, H. and Messias, M.-J.: Historical Reconstruction of Anthropogenic Carbon and Excess Heat Content in the Subtropical North Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10226, https://doi.org/10.5194/egusphere-egu2020-10226, 2020.

EGU2020-6123 | Displays | OS1.6

Temporal Variability of the Meridional Overturning Cells in the South Atlantic

Marion Kersalé, Christopher Meinen, Renellys Perez, Matthieu Le Hénaff, Daniel Valla, Tarron Lamont, Olga Sato, Shenfu Dong, Thierry Terre, Mathias van Caspel, Maria Paz Chidichimo, Marcel van den Berg, Sabrina Speich, Alberto Piola, Edmo Campos, Isabelle Ansorge, Denis Volkov, Rick Lumpkin, and Silvia Garzoli

Variations in the Meridional Overturning Circulation (MOC) are known to have important impacts on global scale climate phenomena including precipitation patterns, surface air temperatures, coastal sea level, and extreme weather. The MOC flow structure in the South Atlantic is thought to control the stability of the entire global MOC system. Given this importance, significant resources have been invested on observing the MOC in the South Atlantic over the past decade. Multiple years of full-depth daily observations from moored instruments at 34.5°S are used to calculate the meridional transports near the western and eastern boundaries, as well as the basin-wide interior transports, via geostrophic methods. These transport estimates are combined with Ekman transports derived from satellite wind products to yield daily estimates of the total meridional transports. Analysis of the MOC volume transport using all available moored instruments from 2013 to 2017 allows us to quantify for the first time the daily volume transport of both the upper and abyssal overturning cells at 34.5°S. The structure of these flows is characterized in unprecedented detail; no statistically significant trend is detectable in either cell. Abyssal-cell transport variability is largely independent of the transport variability in the upper-cell. Analysis of this new data set is crucial for improving our understanding of the temporal and spatial scales of variability that governs MOC related flows, and for disentangling their respective roles in modulating its overall variability.

How to cite: Kersalé, M., Meinen, C., Perez, R., Le Hénaff, M., Valla, D., Lamont, T., Sato, O., Dong, S., Terre, T., van Caspel, M., Chidichimo, M. P., van den Berg, M., Speich, S., Piola, A., Campos, E., Ansorge, I., Volkov, D., Lumpkin, R., and Garzoli, S.: Temporal Variability of the Meridional Overturning Cells in the South Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6123, https://doi.org/10.5194/egusphere-egu2020-6123, 2020.

EGU2020-4202 | Displays | OS1.6

Infrastructure for Sustainable Development of Marine Research, Including the Participation of Bulgaria in the European Infrastructure Euro-Argo

Atanas Palazov, Snejana Moncheva, Elisaveta Peneva, Ivan Ivanov, Rumen Kishev, Elitca Petrova, Peycho Kaloyanchev, Christo Pirovsky, and Dimitar Stavrev

MASRI – Infrastructure for Sustainable Development of Marine Research Including the Participation of Bulgaria in the European Infrastructure Euro-Argo is a project of the National roadmap for scientific Infrastructure (2017-2023) of Bulgaria. The mission of MASRI is to build and utilize a modern research infrastructure which will provide the basis for highly efficient marine and maritime research to expand our knowledge of the marine environment and to support blue growth and implementation of marine policy and maritime spatial planning in order to achieve UN Sustainable Development Goal 14: Conservation and sustainable use of oceans, seas and marine resources for sustainable development.

MASRI activities include the modernization of existing unique resources and equipment and the establishment of new facilities. The research infrastructure consists of four main modules: Research fleet; National Operational Marine Observing System – NOMOS; Data and information center and Research laboratory complex, each representing a distinct on functional basis part of the scientific infrastructure, and consists of separate components distributed physically in different scientific organizations, in the city of Varna. Thus, MASRI is intended to be a large-scale, interdisciplinary multifunctional (physics, chemistry, biology, geology, aquacultures, medicine, energy, underwater, and offshore technologies) marine research infrastructure of scientific significance and will provide unique facilities (including databases and computer network) which will be widely accessible on national, regional and international level for multidisciplinary researches.

Research vessels are intended to provide access to the investigated medium – the sea and they are providing a working platform for conducting research. NOMOS is a system of systems to measure in situ parameters of the marine environment and the surrounding atmosphere. It is designed to provide information on the state of the marine environment for scientific research, forecasting and marine industry. Data and information center provide a computing environment, communication environment and environment for quality control and reliable storage of data and information within the scientific infrastructure. Research laboratory Complex represents a system of research laboratories for chemical, biological and geological analyzes and for relevant research on marine medicine as well as of laboratories for marine resources and technologies research.

As an important module of MASRI, NOMOS includes several components: BulArgo – a system of profiling floats to measure the profiles of the characteristics of the marine environment in the depth up to 2000m; waves and currents monitoring system; national sea level observing system; moorings network; coast research bases and metrological control laboratory.

MASRI is also intended to support the participation of Bulgaria in European research infrastructure consortia Euro-Argo ERIC. Al least three floats are provided and launched in the Black sea every year in the frame of the BulArgo project. Thus, BulArgo gives an important contribution to the Argo program in particular in the Black sea, providing a significant volume of very important in-situ data both for climatic research, for assimilation into the models and verification of the forecasts.

How to cite: Palazov, A., Moncheva, S., Peneva, E., Ivanov, I., Kishev, R., Petrova, E., Kaloyanchev, P., Pirovsky, C., and Stavrev, D.: Infrastructure for Sustainable Development of Marine Research, Including the Participation of Bulgaria in the European Infrastructure Euro-Argo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4202, https://doi.org/10.5194/egusphere-egu2020-4202, 2020.

EGU2020-9044 | Displays | OS1.6

Coordinating sustained coastal and ocean observing efforts in Germany

Kerstin Jochumsen, Ralf Bachmayer, Burkard Baschek, Angelika Brandt, Jan-Stefan Fritz, Birgit Gaye, Felix Janssen, Johannes Karstensen, Alexandra Kraberg, Pedro Martinez, Annemiek Vink, and Oliver Zielinski

Germany’s national ocean observing activities are carried out by multiple actors including governmental bodies, research institutions, and universities, and miss central coordination and governance. A particular strategic approach to coordinate and facilitate ocean research has formed in Germany under the umbrella of the German Marine Research Consortium (KDM). KDM aims at bringing together the marine science expertise of its member institutions and collectively presents them to policy makers, research funding organizations, and to the general public. Within KDM, several strategic groups (SGs), composed of national experts, have been established in order to strengthen different scientific and technological aspects of German Marine Research. Here we present the SG for sustained open ocean observing and the SG for sustained coastal observing. The coordination effort of the SG’s include (1) Representing German efforts in ocean observations, providing information about past, ongoing and planned activities and forwarding meta-information to data centers (e.g., JCOMMOPS), (2) Facilitating the integration of national observations into European and international observing programs (e.g. GCOS, GOOS, BluePlanet, GEOSS), (3) Supporting innovation in observing techniques and the development of scientific topics on observing strategies, (4) Developing strategies to expand and optimize national observing systems in consideration of the needs of stakeholders and conventions, (5) Contributing to agenda processes and roadmaps in science strategy and funding, and (6) Compiling recommendations for improved data collection and data handling, to better connect to the global data centers adhering to quality standards.

How to cite: Jochumsen, K., Bachmayer, R., Baschek, B., Brandt, A., Fritz, J.-S., Gaye, B., Janssen, F., Karstensen, J., Kraberg, A., Martinez, P., Vink, A., and Zielinski, O.: Coordinating sustained coastal and ocean observing efforts in Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9044, https://doi.org/10.5194/egusphere-egu2020-9044, 2020.

EGU2020-11585 | Displays | OS1.6

CLASSnmat: a new dataset of Night Marine Air Temperature back to 1880

Richard Cornes, Elizabeth Kent, David Berry, and John Kennedy

We describe the construction of a new global dataset of Night Marine Air Temperature (NMAT), which provides monthly 5-degree values of NMAT back to 1880 with associated uncertainty estimates. The new dataset (CLASSnmat) builds on the HadNMAT2 dataset, which was released in 2013. CLASSnmat uses the ship-based NMAT values from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS Release 3). However, a new method is used in CLASSnmat to remove duplicated values from the observations, and to infill missing ship identifiers. In addition, a revised method of correcting the warm-bias that occurs in the data during World 2 is applied, which allows the retention of more data than in HadNMAT2. As with its predecessor, the NMAT data in CLASSnmat are not interpolated to grid-cells devoid of observations, but a revised gridding method is used which improves the propagation of uncertainty from the individual measurements through to the gridded values. CLASSnmat is released with NMAT values corrected to 2, 10 and 20m height to allow direct comparison against other measures of temperature, e.g. land-based observations or reanalysis temperature values.

How to cite: Cornes, R., Kent, E., Berry, D., and Kennedy, J.: CLASSnmat: a new dataset of Night Marine Air Temperature back to 1880, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11585, https://doi.org/10.5194/egusphere-egu2020-11585, 2020.

Accurate estimates and forecasts of physical and biogeochemical processes at the air-sea interface must rely on integrated in-situ and satellite surface observations of essential Ocean/Climate Variables (EOVs /ECVs). Such observations, when sustained over appropriate temporal and spatial scales, are particularly powerful in constraining and improving the skills, impact and value of weather, ocean and climate forecast models. The calibration and validation of satellite ocean products also rely on in-situ observations, thus creating further positive high-impact applications of observing systems designed for global sustained observations of EOV and ECVs.

The Global Drifter Program has operated uninterrupted for several decades and constitutes a particular successful example of a network of multiparametric platforms providing observations of climate, weather and oceanographic relevance (e.g. air-pressure, sea surface temperature, ocean currents). This presentation will review the requirements of sustainability of an observing system such as the GDP (i.e. cost effectiveness, peer-review of the observing methodology and of the technology, free data access and international cooperation), will present some key metrics recently used to quantify the impact of drifter observations, and will discuss two prominent examples of GDP regional observations and the transition to operations of novel platforms, such us wind and directional wave spectra drifters, in sparsely sampled regions of the Arabian Sea and of the North Atlantic Ocean.

How to cite: Centurioni, L. and Hormann, V.: Global In-Situ Observations of Essential Climate and Ocean Variables by the Global Drifter Program. Applications and Impacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20408, https://doi.org/10.5194/egusphere-egu2020-20408, 2020.

EGU2020-7828 | Displays | OS1.6

Heat and carbon changes in the ocean as a transient response and tool for decomposing heat uptake

Charles Turner, Kevin Oliver, Peter Brown, and Elaine McDonagh

Whilst anthropogenic activities are significantly altering the climate, both warming the atmosphere and increasing CO2, the ocean is

significantly ameliorating both effects. This effect is so important that the transient climate response to carbon emissions (TCRE), can be

formulated primarily in terms of the ocean. We show that in direct analogy to the TCRE, Anthropogenic Carbon (Canth) and temperature increases in the ocean are

linearly related, both globally and integrated over a range of scales. These ocean responses are typically of order 0.02K/mumol/kg,

(equivalently ~80MJ/mol). This linear relation allows for direct translation between temperature and carbon inventory increases. Furthermore,

we are far better able to decompose DIC changes into Canth increases and that of other carbon pools, than we are decomposing heat

inventory changes into added and redistributed heat. By separating total DIC change into Canth and that of other carbon pools, we can therefore remove the effect

of the transient response relationship between heat and carbon. This allows the production of estimates of added and redistributed heat in the

ocean from remaining DIC changes. Our results suggest that the variability of the transient response is predominately set by heat uptake, not carbon, and that this

variability may be traced to individual water masses. Therefore, it may be necessary to separate this transient response regionally in order

to obtain accurate estimates of added and redistributed heat at a global scale using this technique. The Eulerian transient response is set

predominantly by isotherm heave. The part of the transient response set by climate sensitivity, analogous to a semi-Lagrangian approach, is

set largely by patterns of regional heat uptake.

How to cite: Turner, C., Oliver, K., Brown, P., and McDonagh, E.: Heat and carbon changes in the ocean as a transient response and tool for decomposing heat uptake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7828, https://doi.org/10.5194/egusphere-egu2020-7828, 2020.

EGU2020-20822 | Displays | OS1.6

Ocean climate response to anomalous surface buoyancy and momentum fluxes

Rene Navarro-Labastida and Riccardo Farneti

The aim of the project is to evaluate the response of the global ocean climate to anomalous surface fluxes in terms of ocean heat uptake and circulation changes. All simulations have been performed with the NOAA-GFDL Modular Ocean Model (MOM) version 5. Ocean-only MOM has been integrated toward a near-equilibrium state using as multicentinal initial conditions derivated from a former CORE-I protocol implementation (Griffies et al., 2009). After equilibrium, a restored control simulation has been obtained by a further 70 years of integration while effective total air-sea heat fluxes and freshwater fluxes were stored at daily intervals. A second control simulation has been obtained by the prescription of these storage fluxes. Differences between the restored and prescribed fluxes controls are rather small. Explicit flux sensitivity experiments are proposed by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) in which prescribed surface flux perturbations are applied to the ocean in separated simulations (Gregory et al., 2016). Experiments are 70 years long and branch from piControl conditions. Both wind stress and freshwater anomalies implies nearly-to-zero temperature changes in volume mean temperature. Only the last implies a rather small cooling effect after year 50 of integration. In contrast, anomalous heat flux causes significant volume mean temperature changes. Observed total temperature changes are solely determined by the local addition of heat implying vanishing of the redistribution effect in the entire ocean by inter-basin exchanges and vertical mixing. So far, surface heat anomalies produce the most notable zonal-mean change in ocean temperature. Strong positive temperature change is observed along the top ocean while deepening of temperature anomalies occurs at high latitudes in both hemispheres. Both added and redistributed temperature tracers show maxima in the same area. In most cases, both processes are proportionally inverse. Except for the northern ocean, added temperature tracer is roughly limited to the first 1000 m deep. In contrast, redistributed temperature tracer shows the cooling of subtropical areas and the warming of both the tropical and southern ocean. Maximum at the North Atlantic is possibly due to atmosphere-sea feedbacks, while near-surface tropical and subtropical changes are due to redistribution processes. Heat is mainly taken as a passive tracer in the North Atlantic Ocean and along the entire Southern Ocean. Warming up of mid and low latitudes by redistribution processes is due to the weakening of the Atlantic Meridional Overturning Circulation (AMOC). In turn, changes in AMOC are dominated by surface heat flux changes. The reduction of northward heat transport cools down high latitudes near the surface causing low latitudes to warm up.

 

How to cite: Navarro-Labastida, R. and Farneti, R.: Ocean climate response to anomalous surface buoyancy and momentum fluxes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20822, https://doi.org/10.5194/egusphere-egu2020-20822, 2020.

EGU2020-10120 | Displays | OS1.6

On the suitability of two-layer energy-balance models for representing deep ocean heat uptake

Peter Shatwell, Arnaud Czaja, and David Ferreira

Over 90% of the excess heat energy due to global warming is taken up by the oceans. Because of this, ocean heat uptake and planetary heat uptake can be considered equivalent. This heat energy is readily taken up by the oceanic mixed-layer on decadal timescales and subsequently transferred to the thermocline and deep ocean below on longer, centennial timescales by different ventilation mechanisms. The ventilation rate is affected by many things including the mixed-layer depth, the strength of the overturning, and mode-water formation. In current two-layer energy-balance models (EBMs), all ventilation mechanisms are reduced and parameterised by a simple linear vertical heat-exchange term that depends on the temperature difference between the upper and lower layers (representing the mixed-layer and deep ocean, respectively). 

Two-layer EBMs have been used successfully to reproduce the global mean surface temperature responses for CMIP5 models in abrupt CO2-quadrupling experiments. Little attention has been paid to the EBM-predicted deep ocean response, however. We perform an abrupt CO2-doubling experiment using an idealised aquaplanet model with a simple geometry that splits the ocean into small, large, and southern ocean basins. By fitting a two-layer EBM regionally to each basin's deep temperature response, we find that it provides a good fit only for the small basin. We suggest this is due to the small basin exhibiting a deep overturning circulation — not seen in the other model basins — which connects the ocean surface to its interior; only this ventilation mechanism can be successfully parameterised by a linear vertical heat-exchange. By considering the wind-driven circulation theory of Rhines and Young, we suggest a new parameterisation for the two-layer EBM deep ocean heat uptake that may be more suitable for basins without deep overturning.

How to cite: Shatwell, P., Czaja, A., and Ferreira, D.: On the suitability of two-layer energy-balance models for representing deep ocean heat uptake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10120, https://doi.org/10.5194/egusphere-egu2020-10120, 2020.

EGU2020-6157 | Displays | OS1.6

Effect of mesoscale eddies on subtropical mode water formation and ocean heat storage

Yanxu Chen, Sabrina Speich, and Laurent Bopp

Mode water formation results from air-sea exchange processes in association with the dynamics and thermodynamics of ocean currents or fronts in every ocean basin. Here, a new algorithm is applied to the Argo global array to define surface mixed layer depths and to detect mode waters with homogeneous properties underneath. Specifically, we revisit the spatial and temporal evolution of South Atlantic subtropical mode water (SASTMW) using this new algorithm and find that our set of criteria is more precise than previous detections of mode water. With satellite altimetry measurements and eddy tracking algorithms (Laxenaire et al., 2018), the colocalization between mesoscale eddies and mode waters can be achieved. We then test how much the profiles indicative of mode water are matched with locations of mesoscale eddies and to what extent these eddies influence mode water variability. In addition, we investigate the relationship between the temporal integral of surface heat flux with the heat stored within the layers of the SASTMWs during the formation periods. Nearly all Argo profiles indicate that mode water formation occurs at the time and within the region where loss of latent heat flux from ocean to the atmosphere is significant. Anticyclonic eddies, specifically, play a crucial role in heat redistribution associated with mode waters advected by the subtropical gyre.

How to cite: Chen, Y., Speich, S., and Bopp, L.: Effect of mesoscale eddies on subtropical mode water formation and ocean heat storage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6157, https://doi.org/10.5194/egusphere-egu2020-6157, 2020.

EGU2020-11409 | Displays | OS1.6

Atlantic- and Arctic Water transport across the Yermak Plateau

Frank Nilsen, Eli Anne Ersdal, and Ragnheid Skogseth

The pathway by which Atlantic Water ultimately inflows to the Arctic Ocean via the Yermak Plateau are of great interest for improving the current understanding of the evolving state of the European Arctic. The Arctic branches of the West Spitsbergen Current (WSC), i.e. the Svalbard Branch (SB), the Yermak Pass Branch (YPB) and the Yermak Branch (YB), are the primary routes through which warm AW enters the Arctic Ocean (AO). These branches either flow around (YB) or passes (SB, YPB) over the Yermak Plateau, the Arctic Sill, which is a topographic obstacle for warm water intrusion to the Arctic and possible melting of sea ice. In addition, The Spitsbergen Polar Current (SPC), carrying fresh costal and Arctic type water from the Barents Sea has to cross the Yermak Platea along the northwestern corner of the Spitsbergen coastline. In order to reveal the dynamics across the YP and the roles of the different AW branches in heat flux variability across this arctic sill, a set of in situ ocean data, ocean climatology (UNIS HD), reanalyzed atmospheric data (NORA10) and altimetry data products from Ssalto/Duacs (CMEMS), where synthesized in order to study the seasonal and year-to-year variability in ocean currents across the YP. In situ data from the Remote Sensing of Ocean Circulation and Environmental Mass Changes (REOCIRC) project consist of water time series of temperature, salinity, ocean current and Ocean Bottom Pressure (OBP), which covered the SB and the SPC. Air-ocean interaction mechanisms for controlling volume transport and heat fluxes in the SB and SPC are presented, and further linked to the variability of the other primary AW routes towards the AO. Moreover, surface geostrophic currents from Absolute Dynamic Topography (ADT) are calibrated against the geostrophic bottom current calculated from in situ OBP recorders. Estimates of winter volume- and heat transports across the YP for the time period 1993-2019 are presented, and interannual variability in the SB linked to the WSC and other AW branches are discussed together with consequences for sea ice melting north of Svalbard.

How to cite: Nilsen, F., Ersdal, E. A., and Skogseth, R.: Atlantic- and Arctic Water transport across the Yermak Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11409, https://doi.org/10.5194/egusphere-egu2020-11409, 2020.

EGU2020-17494 | Displays | OS1.6

Topographically trapped waves along the continental slope north of Svalbard

Kjersti Kalhagen, Frank Nilsen, Ragnheid Skogseth, Ilker Fer, Zoé Koenig, and Eivind Kolås

On the continental slope north of Svalbard, Atlantic Water is transported eastward as a part of the Arctic Circumpolar Boundary Current. As inflow of Atlantic Water through the Fram Strait is the largest oceanic heat source to the Arctic Ocean, it is important to improve our knowledge about the dynamics and processes that govern the heat exchange between Atlantic Water and water masses of Arctic origin. This includes processes that enable lateral exchange across the shelf break or into the interior of the deep basin. Here, we study the vorticity dynamics on the slope and its contribution to the water mass modifications and heat exchange. Focusing on topographically trapped waves – sub-inertial oscillations trapped to follow the continental slope – we establish their existence and properties on the northern slope of Svalbard using a free baroclinic wave model. Their dependence on background stratification and current properties is explored in sensitivity analysis. Next, we discuss their contribution to lateral exchange from the boundary current on the slope to the continental shelf, troughs, and the deep Nansen Basin in the Arctic Ocean, including exchange associated with instabilities and resulting eddy shedding off the vorticity waves. Hydrographic and current time series from 2018-19 at two mooring arrays crossing the slope north of Svalbard (The Nansen Legacy project) are used to associate the observed physical environment with model-predicted topographic waves. Analysis of the in-situ data will determine which wave mode that can exist over the sloping seafloor and the observed hydrography and flow, and the model will give the corresponding spatial characteristics for the given frequencies and wave numbers. Energetic oscillations present in the observations are analyzed in light of the model results. Of special interest are the seasonal variability in hydrography and current strength and the resulting modification of the wave characteristics. Moreover, the interaction between the vorticity waves and tidal oscillations in the diurnal band is emphasized.

How to cite: Kalhagen, K., Nilsen, F., Skogseth, R., Fer, I., Koenig, Z., and Kolås, E.: Topographically trapped waves along the continental slope north of Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17494, https://doi.org/10.5194/egusphere-egu2020-17494, 2020.

EGU2020-10620 | Displays | OS1.6

Decadal physical-biogeochemical changes in the Newfoundland and Labrador ecosystem

Frédéric Cyr, Olivia Gibb, David Bélanger, Guoqi Han, Gary Maillet, and Pierre Pepin

Located on a crossroads of some of the main currents associated to the Atlantic meridional overturning circulation (AMOC), Newfoundland and Labrador (NL) shelves are specially affected by changes in large-scale ocean circulation. Such circulation changes impact not only the regional climate, but also the overall water masses composition, with consequences on physical conditions, nutrient availability, oxygen content, pH, etc. Systematic hydrographic observations of this system have been carried out by Canada and other countries since 1948. The observational program was reinforced in 1999 with the creation of the Atlantic Zone Monitoring Program (AZMP), ensuring enhanced seasonal coverage and new biogeochemical observations. In 2014, this monitoring was augmented with the monitoring of ocean acidification parameters. Here we review historical physical-biogeochemical changes on the NL shelves, with an emphasis on low frequency variability and cycles. Results suggest, for example, that the cold intermediate layer (CIL), a cold mid-depth layer that is a key feature of the NL ecosystem, exhibited profound changes during the last 70 years. In the mid 60's, the CIL was anomalously warm compared to the rest of the time series. This warm period was followed by a cold period centered in the early 90's. Historical salinity records also suggest that fresher waters are found during warmer years, and vice-versa. Nitrate/Phosphate ratios suggest recent changes in water masses composition towards less Arctic waters flowing on the shelves. This is concurrent with a reduction in nutrients concentration on the NL shelves since about 2012, together with changes in the strength of the Labrador Current along the shelf.

How to cite: Cyr, F., Gibb, O., Bélanger, D., Han, G., Maillet, G., and Pepin, P.: Decadal physical-biogeochemical changes in the Newfoundland and Labrador ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10620, https://doi.org/10.5194/egusphere-egu2020-10620, 2020.

EGU2020-22652 | Displays | OS1.6

Transport of Excess Heat at 24.5°N

Marie-José Messias and Herlé Mercier

Repeated hydrographic surveys have allowed for the monitoring of the 24.5°N trans-Atlantic transect of volume and heat transports  since the middle of the last century. However, identifying the geographic origins and the temporal characteristics of full depth ocean heat content (OHC) anomalies is still at the frontier of  global ocean warming research albeit it is critical to the  understanding of  the current warming of the ocean and its future evolution. To address this gap,  we  combine volume transports at 24.5°N  with an historical reconstruction of  excess heat, which we define as the heat gained across the section since the year 1850 to  present. The  reconstruction is based on  a maximum entropy approach  that links the  location and time of the last entry into the ocean of a series of transient and geochemical tracers to their full depth in situ measurements in the interior. Here, we apply it to tracers measured on the hydrographic sections at  24.5°N since 1992. This methodology is a step forward in exploring the coherence of the OHC distributions at 24.5°N over time with the variability of the SST in  the source regions and the role of the AMOC, all genuinely based on observations. We find that the AMOC ranges from 16 to 19 Sv, heat transport from 0.9 to 1.5 PW and excess heat transport from 19 to 31 TW. The excess heat is transported northward across 24.5°N thus reinforcing the warming of the North Atlantic Ocean.

How to cite: Messias, M.-J. and Mercier, H.: Transport of Excess Heat at 24.5°N, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22652, https://doi.org/10.5194/egusphere-egu2020-22652, 2020.

EGU2020-10929 | Displays | OS1.6

Absolute Brazil Current transport variability at 34.5°S from a long-term moored array

Maria Paz Chidichimo, Alberto R. Piola, Christopher S. Meinen, Edmo J. Campos, Renellys Perez, Daniel Valla, Shenfu Dong, Rick Lumpkin, and Silvia L. Garzoli

OS1.7 – The North Atlantic : natural variability and global change

EGU2020-10546 | Displays | OS1.7

Transient Tracers and Anthropogenic Carbon in Central Labrador Sea: a Multi-Decadal Study

Lorenza Raimondi, Kumiko Azetsu-Scott, Toste Tanhua, Igor Yashayaev, and Doug Wallace

Over the last thirty years the Bedford Institute of Oceanography (BIO) has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP), which includes annual occupation of several sections and stations in the Northwest Atlantic Ocean. Among these, the AR7W line across the Labrador Sea has one of the longest time-series where both transient tracers and dissolved inorganic carbon (DIC) have been collected since the early 1990s.

Among multiple transient tracers that have been measured along this transect (CFC-11, CFC-113, CCl4 and SF6), only measurement of CFC-12 extends over the full time-series from 1992 to 2018, overlapping with DIC observations. Measurements of CFC-12 were also available for a previous cruise in 1986, extending the time-series to three decades.

In this work we present the temporal variability of CFC-12 (1986-2016) and DIC (1992-2016) concentrations as well as their distribution in the major water masses of the region.

The CFC-12 data are used to reconstruct the time-history of the tracer’s saturation at the time of convection based on multiple regression with the atmospheric input function of CFC-12 and the annual maximum mixed layer depth. The so-modelled time-varying saturation is employed to relax the constant saturation assumption of the Transit Time Distribution (TTD) method, allowing for a better estimate of anthropogenic carbon (Cant) in the region.

We present the column inventories and storage rate of Cant in central Labrador Sea between 1986 and 2016 obtained using the TTD method with time-varying saturation. We compare these estimates with a classical TTD approach that assumes constant saturation, and we highlight the differences in trends and magnitudes obtained with the two approaches.    

Finally, our work shows the multi-decadal dataset of DIC in the Labrador Sea which enables a comparison between the TTD-based Cant estimates and the measured DIC trends, providing insights into temporal variability of natural carbon in the region.

How to cite: Raimondi, L., Azetsu-Scott, K., Tanhua, T., Yashayaev, I., and Wallace, D.: Transient Tracers and Anthropogenic Carbon in Central Labrador Sea: a Multi-Decadal Study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10546, https://doi.org/10.5194/egusphere-egu2020-10546, 2020.

EGU2020-13682 | Displays | OS1.7

More than two decades of Faroe Bank Channel overflow: Stable, but warming

Karin Margretha Húsgarð Larsen, Bogi Hansen, Hjálmar Hátún, and Svein Østerhus

Since November 1995, we have monitored the volume transport of Faroe Bank Channel overflow (FBC-overflow) and since 2001, the bottom temperature at the sill of the channel. The FBC-overflow is the coldest and densest overflow component and contributes approximately one third of the total overflow. Together with water that it entrains en route, it is therefore one of the main sources for North Atlantic Deep Water and the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In spite of reported AMOC weakening, the FBC-overflow has shown no signs of reduced volume transport. In contrast, a linear trend analysis indicated a weak (but non-significant) positive trend in volume transport of +5% from 1996 to 2018. The bottom water at the sill of the channel is the coldest component of the FBC-overflow and the densest overflow component overall. Since high-quality monitoring of the bottom water temperature began in summer 2001, the bottom water has warmed by approximately 0.2 °C with most of the warming occurring in two periods: 2004-2007 and 2015-2019. During the period, salinity has also been changing and the combined temperature/salinity effect on the density of the FBC-overflow will be discussed.

How to cite: Larsen, K. M. H., Hansen, B., Hátún, H., and Østerhus, S.: More than two decades of Faroe Bank Channel overflow: Stable, but warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13682, https://doi.org/10.5194/egusphere-egu2020-13682, 2020.

EGU2020-19663 | Displays | OS1.7

The recent AMOC variability in the Subpolar Gyre: results across the OVIDE section

Pascale Lherminier, Herlé Mercier, Fiz F. Perez, and Marcos Fontela

According to the subpolar AMOC index built from ARGO and altimetry, the AMOC amplitude across the OVIDE section (from Greenland to Portugal) was similar to that of the mid-1990s between 2014 and 2017, i.e. 4-5 Sv above the level of the 2000s. It then returned to average values in 2018. The same index computed independently from the biennial summer cruises over 2002-2018 confirms this statement. Interestingly, despite the concomitant cold and fresh anomaly in the subpolar Atlantic, the heat flux across OVIDE remains correlated with the AMOC amplitude. This can be explained by the paths taken by the North Atlantic Current and the transport anomalies in the subarctic front. In 2014, the OVIDE section was complemented by a section from Greenland to Newfoundland (GA01), showing how the water of the lower limb of the AMOC was densified by deep convection in the Labrador Sea. The spatial patterns of volume, heat, salt and oxygen transport anomalies after 2014 will be discussed at the light of the 2000s average.

How to cite: Lherminier, P., Mercier, H., Perez, F. F., and Fontela, M.: The recent AMOC variability in the Subpolar Gyre: results across the OVIDE section, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19663, https://doi.org/10.5194/egusphere-egu2020-19663, 2020.

EGU2020-4416 | Displays | OS1.7

The North Atlantic Eastern Boundary: Observations from Moorings at Goban Spur 2016-2019

Martin Moritz, Kerstin Jochumsen, Dagmar Kieke, Birgit Klein, Holger Klein, Manuel Köllner, and Monika Rhein

Since 2016 a moored observatory is operated at the eastern extension of the “North Atlantic Changes (NOAC)” array at 47°/48°N. This observatory is installed across the shelf break at Goban Spur and consists of two deep-sea moorings that are separated by about 60 km.  

The aim of this ongoing monitoring program is to quantify the variability and trends in the properties and transport rates of water masses that are advected northwards along the North Atlantic Eastern Boundary and modify the adjacent regions, i.e. the Northwest European Shelf, North Sea, Nordic Seas and Arctic Ocean. Furthermore, the continuous long term time series are essential for a thorough understanding of the circulation system in the eastern North Atlantic and the underlying physical mechanisms that govern its variability.

Here, we present results of the analysis of temperature, salinity and current velocity time series from 2016 to 2019. These provide a descriptive view of the complex current structure and variability of water masses on daily to intra- and inter-annual time scales.

The most pronounced signal in the variability of temperature and salinity is caused by the presence of Mediterranean Outflow Water located at about 1000 m depth. During the observation period we find significant positive trends in temperature and salinity in the depth range of 500 to 1500 m. The velocity measurements of the onshore mooring show a northeastward directed mean flow following the topography with along-slope variations, while the flow at the offshore mooring position is more unstable with predominantly cross-slope variations. 

The combination of our observations with float and altimeter data indicates that the presence of eddies and the interaction with the topography seems to play a crucial role for setting the variability of the flow in this region.

Finally, we present an approach to evaluate the volume fluxes at the eastern boundary that will add toward an integrated estimate of the strength of the Atlantic Meridional Overturning Circulation at 47°/48°N.

How to cite: Moritz, M., Jochumsen, K., Kieke, D., Klein, B., Klein, H., Köllner, M., and Rhein, M.: The North Atlantic Eastern Boundary: Observations from Moorings at Goban Spur 2016-2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4416, https://doi.org/10.5194/egusphere-egu2020-4416, 2020.

EGU2020-3615 | Displays | OS1.7

Link between transformation rate and overturning in the Iceland Basin and Irminger Sea

Tillys Petit, Susan Lozier, Simon A. Josey, and Stuart A. Cunningham

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, transforms warm and salty waters from the subtropical gyre into colder and fresher waters in the subpolar gyre and Nordic Seas. To measure the mean AMOC and its variability at subpolar latitudes, the Overturning in the Subpolar North Atlantic Program (OSNAP) array was deployed in the summer of 2014. Based on observations through May 2016, the majority of the light‐to‐dense water conversion takes place north of the OSNAP East line, which runs from the southeast tip of Greenland to the Scottish shelf. In this study, we assess the transformation of dense waters in the area located between the Greenland-Scotland Ridge and the OSNAP East section. From 2014 to 2016, the mean overturning within this area is estimated at 6.9 ± 1.3 Sv across σ0 = 27.55 kg m-3, the isopycnal that separates the northward and southward flows. This mean overturning estimate is in close agreement with the value (6.5 ± 1 Sv) derived by applying water mass transformation theory to air-sea buoyancy fluxes from atmospheric reanalysis. However, the large monthly variability of the overturning (standard deviation of 4.1 Sv) cannot easily be attributed to the buoyancy forcing or to variability in the overflow through the Greenland-Scotland Ridge. We explore possible mechanisms that can account for this variability.  

How to cite: Petit, T., Lozier, S., Josey, S. A., and Cunningham, S. A.: Link between transformation rate and overturning in the Iceland Basin and Irminger Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3615, https://doi.org/10.5194/egusphere-egu2020-3615, 2020.

The temperature in the Atlantic waters south of Iceland has increased by about 1°C since 1995 with most of the rise occurring before 2000. A similar rise in air temperature in Iceland was observed simultaneously and the rise in temperature is often interpreted as being caused by global warming. Many effects of this in the ocean and on land such as changed distribution of marine species in the area as well as melting of glaciers in Iceland have been attributed to this rising temperature. However, it is unlikely that this rapid increase in temperature was solely due to global warming, especially since it was accompanied by an increase in salinity. It is more likely that there was a change in the ocean circulation in the area leading to more sub-tropical water entering the sub-polar gyre causing a shift in temperature and salinity. A similar increase in temperature and salinity was observed earlier during 1930-1964 in this area. Between the two warm periods the waters were dominated by lower temperature and salinity. These changes have been related to the Atlantic Multidecadal Oscillation. By comparing the water mass properties in the two warm periods it is possible to estimate the relative contribution from natural variability and global warming for the recent warm period. It will be shown how the retreat and advancing of glaciers in Iceland are in harmony with the changes in water mass properties in the waters south of Iceland. It is important that decisions about how to adapt to coming climate change are based on how much of the observed change is due to natural variability and global warming respectively. This is a method that can be used in other areas of the northern North Atlantic.

How to cite: Jónsson, S.: Estimating global warming and natural variability signals in the ocean south of Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7488, https://doi.org/10.5194/egusphere-egu2020-7488, 2020.

EGU2020-2645 | Displays | OS1.7

Dynamical constraints on the choice of the North Atlantic subpolar gyre index

Vimal Koul, Jan-Erk Tesdal, Manfred Bersch, Sebastian Brune, Hjálmar Hátún, Helmuth Haak, Leonard Borchert, Corinna Schrum, and Johanna Baehr

The North Atlantic Subpolar Gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between different indices of SPG strength based on Sea Surface Height (SSH) observations have raised questions about the active role SPG strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing SSH-based and various other SPG-strength indices derived from observations and a global coupled model, we show that the interpretation of SPG strength-salinity relationship is dictated by the choice of the SPG index. Our results emphasize that SPG indices should be interpreted cautiously because they represent variability in different regions of the subpolar North Atlantic. Idealized Lagrangian trajectory experiments illustrate that zonal shifts of main current pathways in the ENA and meridional shifts of the North Atlantic Current (NAC) in the western intergyre region during strong and weak SPG circulation regimes are manifestations of variability in the size and strength of the SPG. Such shifts in advective pathways modulate the proportions of subpolar and subtropical water reaching the ENA, and thus impact salinity. Inconsistency among SPG indices arises due to the inability of some indices to capture the meridional shifts of the NAC in the western intergyre region. Overall, our results imply that salinity variability in the ENA is not exclusively sourced from the subtropics, instead the establishment of a dominant subpolar pathway also points to redistribution within the SPG.

How to cite: Koul, V., Tesdal, J.-E., Bersch, M., Brune, S., Hátún, H., Haak, H., Borchert, L., Schrum, C., and Baehr, J.: Dynamical constraints on the choice of the North Atlantic subpolar gyre index, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2645, https://doi.org/10.5194/egusphere-egu2020-2645, 2020.

EGU2020-5644 | Displays | OS1.7

From small whirls to the global ocean: how eddies affect the Atlantic Meridional Overturning Circulation

Caroline Katsman, Nils Brüggemann, Sotiria Georgiou, Juan-Manuel Sayol Espana, Stefanie Ypma, Carine van der Boog, and Julie Pietrzak

In the North Atlantic Ocean, intense downward motions connect the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC). In addition, the AMOC also displays a pronounced signature in density space, with lighter waters moving northward and denser waters returning southward.

While at first glance it is appealing to associate this sinking of water masses in the North Atlantic Ocean with the occurrence of the formation of dense water masses by deep convection, this is not correct: the net vertical motion over convection areas is small. The downward flow required to connect the upper and lower branches of the AMOC thus has to occur outside the deep convection areas. Indeed, earlier studies have pointed out theoretically that strong sinking can only occur close to continental boundaries, where ageostrophic processes play a role. However, observations clearly indicate that convected water masses formed in marginals seas constitute an important component of the lower limb of the AMOC.

This apparent contradiction is explored in this presentation, by studying the overturning in the AMOC from a perspective in depth space (Eulerian downwelling) and density space (downwelling across isopycnals). Based on analyses of both a high-resolution global ocean model and dedicated process studies using idealized models we analyze the characteristics of the sinking, of diapycnal mixing, and investigate how these are linked. 

It appears that eddies play a crucial role for the overturning, both in depth space and density space. They control the characteristics of the yearly cycle of convection and restratification, the magnitude of the Eulerian sinking near continental boundaries, and steer the export of dense waters formed in the interior of the marginal seas via the boundary current system.

These studies thus reveal a complex three-dimensional view on sinking, diapycnal water mass transformation and overturning in the North Atlantic Ocean, involving the boundary current, the interior and interactions with the eddy field.  This implies that it is essential to resolve these eddies to be able to properly represent the overturning in depth and density space in the North Atlantic Ocean and its response to changing conditions in a future climate.

How to cite: Katsman, C., Brüggemann, N., Georgiou, S., Sayol Espana, J.-M., Ypma, S., van der Boog, C., and Pietrzak, J.: From small whirls to the global ocean: how eddies affect the Atlantic Meridional Overturning Circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5644, https://doi.org/10.5194/egusphere-egu2020-5644, 2020.

EGU2020-13859 | Displays | OS1.7

The Atlantic Overturning Circulation: At its Weakest in a Millennium?

Stefan Rahmstorf and Levke Caesar

The Atlantic Meridional Overturning Circulation (AMOC) is a major mechanism for northward heat transport on our planet and the prime reason why the Northern Hemisphere is warmer than the Southern Hemisphere (Feulner et al. 2013). The AMOC is a sensitive non-linear system dependent on subtle thermohaline density differences in ocean water, and major AMOC transitions have been implicated e.g. in millennial climate events during the last glacial (Rahmstorf 2002).

There is evidence that the AMOC is slowing down in response to modern global warming, as predicted by climate models (Caesar et al. 2018). We will review and compile proxy evidence for AMOC changes during the past 1-2 millennia, including e.g. Sherwood et al. 2011, Thibodeau et al. 2018, Thornalley et al. 2018, Rahmstorf et al. 2015, Zanna et al. 2019. We conclude that there now is substantial and consistent evidence from multiple independent sources for a modern AMOC slowdown that is unprecedented in at least a millennium.

References

Caesar, L., S. Rahmstorf, A. Robinson, G. Feulner, and V. Saba. 2018. Nature, 556: 191-96.

Feulner, G, S Rahmstorf, A Levermann, and S Volkwardt. 2013. Journal of Climate, 26: 7136-50.

Rahmstorf, S. 2002. Nature, 419: 207-14.

Rahmstorf, S., Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford, and Erik J. Schaffernicht. 2015. Nature Climate Change, 5: 475-80.

Sherwood, O. A., M. F. Lehmann, C. J. Schubert, D. B. Scott, and M. D. McCarthy. 2011. Proc Natl Acad Sci U S A, 108: 1011-5.

Thibodeau, Benoit, Christelle Not, Jiang Hu, Andreas Schmittner, David Noone, Clay Tabor, Jiaxu Zhang, and Zhengyu Liu. 2018. Geophysical Research Letters, 45: 12,376-12,85.

Thornalley, D. J. R., D. W. Oppo, P. Ortega, J. I. Robson, C. M. Brierley, R. Davis, I. R. Hall, P. Moffa-Sanchez, N. L. Rose, P. T. Spooner, I. Yashayaev, and L. D. Keigwin. 2018. Nature, 556: 227-30.

Zanna, L., S. Khatiwala, J. M. Gregory, J. Ison, and P. Heimbach. 2019. Proc Natl Acad Sci U S A, 116: 1126-31.

How to cite: Rahmstorf, S. and Caesar, L.: The Atlantic Overturning Circulation: At its Weakest in a Millennium?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13859, https://doi.org/10.5194/egusphere-egu2020-13859, 2020.

EGU2020-1384 | Displays | OS1.7

Mechanisms for AMOC decline in the late 20th Century

Alex Megann, Adam Blaker, Simon Josey, Adrian New, and Bablu Sinha

The recent decline in the Atlantic meridional overturning circulation (AMOC) has attracted more than a little interest. The strongest AMOC recorded by the RAPID campaign at 26°N was at the start (2004/5), after which it declined about 3 Sv with a pronounced minimum in 2010. Proxies based on temperature and surface elevation have been used to extrapolate the AMOC strength before the RAPID era, and point reasonably reliably to a maximum strength in the mid 1990s, followed by a rise to a maximum at the start of the RAPID campaign in around 2005. Further back, less robust proxy data suggest that the AMOC gradually rose from the 1970s to the peak in 1990. This raises two questions: firstly, what drove these decadal variations in the overturning circulation (and hence of the ocean heat transport); and secondly whether there are observations that lead to useful predictive skill for changes in the AMOC. The surface-forced streamfunction, estimated from modelled/observed buoyancy fluxes, has been shown to be a reasonably good predictor of decadal changes in the overturning strength, preceding the latter with a lead time of about 5 years. although the reliability of the correlations before 2000 is limited by data sparsity, and especially so in the pre-satellite era.

To verify a causal link between surface forcing and decadal variations in the AMOC over longer timescales, numerical simulations present a powerful tool. A set of hindcast integrations of a global 0.25° NEMO ocean configuration has been carried out from 1958 until nearly the present day, with a selection of standard surface forcing datasets (CORE2, DFS5.2 and JRA55). These show an evolution of the AMOC strength from 1970 onwards which is consistent, both between forcing datasets and with that inferred from observations. The surface-forced streamfunction is evaluated for these experiments and is used to relate the time evolution of the AMOC to changes in the individual components of the buoyancy flux, and the surface heat loss from the Labrador and Irminger Seas is found to be the dominant predictor of AMOC changes.

How to cite: Megann, A., Blaker, A., Josey, S., New, A., and Sinha, B.: Mechanisms for AMOC decline in the late 20th Century, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1384, https://doi.org/10.5194/egusphere-egu2020-1384, 2020.

EGU2020-5785 | Displays | OS1.7

Pending recovery in the strength of the meridional overturning circulation at 26°N

Ben Moat, David Smeed, Eleanor Frajka-Williams, Damien Desbruyeres, Claudie Beaulieu, William Johns, Darren Rayner, Alejandra Sanchez-Franks, Molly Baringer, Denis Volkov, and Harry Bryden

The strength of the Atlantic meridional overturning circulation (AMOC) at 26°N has now been continuously measured by the RAPID array over the period April 2004 - Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008-2010. In 2013-2015, a period of strong buoyancy- forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing.

Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3-9 years. At 45°N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26°N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26°N is higher than its previous low. Extending the record at 26°N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45°N by 0-2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

How to cite: Moat, B., Smeed, D., Frajka-Williams, E., Desbruyeres, D., Beaulieu, C., Johns, W., Rayner, D., Sanchez-Franks, A., Baringer, M., Volkov, D., and Bryden, H.: Pending recovery in the strength of the meridional overturning circulation at 26°N, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5785, https://doi.org/10.5194/egusphere-egu2020-5785, 2020.

EGU2020-8758 | Displays | OS1.7

Observed Evidence of a Stable Atlantic Meridional Overturning Circulation since the 1990s

Yao Fu, Feili Li, Johannes Karstensen, N. Penny Holliday, and Chunzai Wang

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important in the global climate system due to its role in the meridional heat and freshwater distribution. Model simulations and constructed AMOC indices suggest that the AMOC may have been weakening for decades. However, direct AMOC observations, introduced in 2004 in the subtropics (the RAPID program) and in 2014 in the subpolar North Atlantic (the OSNAP program), are not sufficiently long to capture changes dating back to previous periods. Here we use repeated hydrographic sections in the subtropical and subpolar North Atlantic through the early 1990s to the mid-2010s, combined with a box inverse model that is constrained using satellite altimetry, to analyze hydrographic changes and the AMOC. In combination with a state-of-the-art ocean state estimate, GECCO2, we show that despite dramatic hydrographic changes in the subtropical and subpolar North Atlantic over the past two and half decades, the AMOC has not significantly weakened over the same period. Our hydrography-based estimates also illustrate a remarkably stable partition of the subpolar overturning between the Labrador basin and the eastern subpolar basins on decadal timescales since the 1990s.

How to cite: Fu, Y., Li, F., Karstensen, J., Holliday, N. P., and Wang, C.: Observed Evidence of a Stable Atlantic Meridional Overturning Circulation since the 1990s, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8758, https://doi.org/10.5194/egusphere-egu2020-8758, 2020.

EGU2020-5894 | Displays | OS1.7

Contrasting sources of variability in subtropical and subpolar Atlantic overturning

Yavor Kostov, Helen L. Johnson, David P. Marshall, Gael Forget, Patrick Heimbach, N. Penny Holliday, Feili Li, M. Susan Lozier, Helen R. Pillar, and Timothy Smith

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can tell us how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the circulation is influenced by multiple factors which co-vary and whose overlapping impacts can persist for years.  Here we reconstruct and unambiguously attribute variability in the AMOC at the latitudes of two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While on inter-annual timescales, AMOC variability at 26°N is overwhelmingly dominated by a linear response to local wind stress, in contrast, AMOC variability at subpolar latitudes is generated by both wind stress and surface temperature and salinity anomalies. Our analysis allows us to obtain the first-ever reconstruction of subpolar AMOC from forcing anomalies at the ocean surface.

How to cite: Kostov, Y., Johnson, H. L., Marshall, D. P., Forget, G., Heimbach, P., Holliday, N. P., Li, F., Lozier, M. S., Pillar, H. R., and Smith, T.: Contrasting sources of variability in subtropical and subpolar Atlantic overturning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5894, https://doi.org/10.5194/egusphere-egu2020-5894, 2020.

The Atlantic meridional overturning circulation (AMOC) is a large-scale oceanic circulation comprising a 2-layer flow: the net northward flow in the upper 1000 m of the Atlantic and net southward flow below. Variations in the AMOC have significant repercussions for the climate system hence there is a need for proxies that can measure changes in the AMOC on larger spatial scales. Here we show a direct calculation of ocean circulation at 26°N from satellites compares well with transport estimates from the RAPID mooring array. In the surface layer (1000 m), transport is estimated from satellite altimetry and has a correlation of r=0.79 (significant at 95% level) with the MOC transport estimates from RAPID. We find that the relationship between sea level anomaly and dynamic height from the western boundary RAPID moorings is robust in the surface layer, with poor agreement occurring largely below 1000 m. Below 1000 m, the return flow of the AMOC is estimated using ocean bottom pressure from satellite gravimetry. This has a correlation of r=0.75 (significant at the 95% level) when compared to the deeper (1000-5000 m) RAPID transports. Combining the results from satellite altimetry and gravimetry, estimates of full-depth 2-layer circulation at 26°N are demonstrated. Finally, empirical orthogonal function analysis reveals that the barotropic and baroclinic streamfunctions are linked to wind stress curl and buoyancy forcing, respectively.

How to cite: Sanchez-Franks, A., Frajka-Williams, E., and Moat, B.: Can satellites replace mooring arrays? Satellite altimetry transport estimates of the Atlantic overturning meridional circulation along the RAPID 26°N mooring array, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11549, https://doi.org/10.5194/egusphere-egu2020-11549, 2020.

The atmosphere over the North Atlantic sector exhibits significant interannual and interdecadal variability, as well as long-term trends due to global change. This variability is accompanied by changes in predictability. The origins of North Atlantic variability can to a large extent be traced back to the ocean and the land surface, the upper atmosphere, the tropics, as well as circum-global patterns. In particular, the tropical Pacific and the upper atmosphere have a strong influence on interannual and decadal variability in the North Atlantic region. As an example, the tropical Pacific affects the North Atlantic both through a tropospheric pathway across North America and through an indirect pathway through the stratosphere. Hence, due to the large number of factors influencing the North Atlantic region, their inter-dependence and their non-stationarity, the influence of these different factors is difficult to disentangle. Furthermore, models are often not able to capture the inter-dependence and superposition of these factors, which affects to what extent models are able to predict the North Atlantic region. This submission will explore the contribution to variability and predictability for several of these remote influences.

 

How to cite: Domeisen, D.: Origins of variability and predictability in the North Atlantic region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7815, https://doi.org/10.5194/egusphere-egu2020-7815, 2020.

EGU2020-11083 | Displays | OS1.7

North Atlantic decadal variability in a coupled global model and relevance to observations

Yochanan Kushnir, Dog Run (Donna) Lee, and Mingfang Ting

This study focuses on the decadal time scale variability of the North Atlantic Ocean-Atmosphere system. This time scale is relevant to preparedness and adaptation as society becomes increasingly threatened by the adverse impact of anthropogenic climate change. North Atlantic decadal climate variability has been related to interaction between the subpolar and subtropical gyre and manifested in persistent multi-year SST and heat content anomalies and shifts in the latitude of the Gulf Stream/North Atlantic Current (GS/NAC). We apply a space-time analysis to annual, North Atlantic, upper ocean heat content (OHC) anomalies from the National Center for Atmospheric Research (NCAR), Community Earth System Model (CESM) long pre-industrial control run. The analysis reveals decadal anomalies associated with two patterns: a dipole centered on the GS/NAC, in the western side of the Basin that oscillates quasi-regularly, reversing its sign every of 6 to 7 years. The second pattern is centered in the eastern side of the basin and lags the first by about 5 years, implying that heat is transported between the subtropical and subpolar gyres. Analysis of surface windstress anomalies connected with these OHC fluctuations implies that the latter are forced by stochastic atmospheric variability. Further analysis compares the model patterns with observations to determine their relevance and predictability and assesses their response to climate change.

How to cite: Kushnir, Y., Lee, D. R. (., and Ting, M.: North Atlantic decadal variability in a coupled global model and relevance to observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11083, https://doi.org/10.5194/egusphere-egu2020-11083, 2020.

EGU2020-18650 | Displays | OS1.7

Extratropical cyclone induced sea surface temperature anomalies in the 2013/14 winter

Helen Dacre, Simon Josey, and Alan Grant

The 2013/14 winter averaged sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region.  This season was also unusually stormy with extratropical cyclones passing over the mid-North Atlantic every 3 days.  However, the processes by which cyclones contribute towards seasonal SST anomalies are not fully quantified. In this paper a cyclone identification and tracking method is combined with ECMWF atmosphere and ocean reanalysis fields to calculate cyclone-relative net surface heat flux anomalies and resulting SST changes.  Anomalously large negative heat flux is located behind the cyclones cold front resulting in anomalous cooling up to 0.2K/day when the cyclones are at maximum intensity.  This extratropical cyclone induced 'cold wake' extends along the cyclones cold front but is small compared to climatological variability in the SST's.  To investigate the potential cumulative effect of the passage of multiple cyclone induced SST cooling in the same location we calculate Earth-relative net surface heat flux anomalies and resulting SST changes for the 2013/2014 winter period.  Anomalously large winter averaged negative heat flux occurs in a zonally orientated band extending across the North Atlantic between 40-60 oN. The 2013/2014 winter SST cooling anomaly associated with air-sea interactions (anomalous heat flux, mixed layer depth and entrainment at the base of the ocean mixed layer) is estimated to be -0.67 K in the mid-North Atlantic (68% of the total cooling anomaly).  The role of cyclones is estimated using a cyclone masking technique which encompasses each cyclone centre and its trailing cold front. The environmental flow anomaly in 2013/2014 sets the overall tripole pattern of heat flux anomalies over the North Atlantic.  However, the presence of cyclones doubles the magnitude of the negative heat flux anomaly in the mid-North Atlantic.  Similarly, the environmental flow anomaly determines the location of the SST cooling anomaly but the presence of cyclones enhances the SST cooling anomaly.  Thus air-sea interactions play a major part in determining the extreme 2013/2014 winter season SST cooling anomaly. The environmental flow anomaly determines where anomalous heat flux and associated SST changes occur and the presence of cyclones influences the magnitude of those anomalies.

How to cite: Dacre, H., Josey, S., and Grant, A.: Extratropical cyclone induced sea surface temperature anomalies in the 2013/14 winter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18650, https://doi.org/10.5194/egusphere-egu2020-18650, 2020.

EGU2020-6802 | Displays | OS1.7

Predicting the 2015 North Atlantic Cold Blob

Sybren Drijfhout, Jenny Mecking, Joel Hirschi, and Alex Megann

Leading up to and during the summer of 2015 sea surface temperatures (SSTs) in the eastern North Atlantic Subpolar Gyre reached anomalously low values while in the subtropical gyre just to the SSTs were anomalously warm. Recent observation and modelling studies have found evidence showing that these SST anomalies can be linked to the heat wave experienced over Europe that summer.  The latest observation based data still shows anomalously cold temperatures, as well as the anomalously fresh conditions that went along the 2015 cold blob in the upper layers of the eastern North Atlantic Subpolar gyre.  A second heat wave over Europe occurred in the summer of 2018 where the SSTs reached another minimum value.  Therefore, being able to predict the development, enhancement and persistence of such an anomaly is essential for good seasonal and longer predictions.  At present several modelling systems have had difficulties in simulating/maintaining the 2015 cold blob. In this work we apply a novel initialization technique using anomalous initialization from a forced ocean simulation to simulate the 2015 cold blob.  Initial results show that the model is able to maintain the cold blob as well as have a strengthening of the cold blob, however, it has difficulties capturing the timing of this strengthening.

How to cite: Drijfhout, S., Mecking, J., Hirschi, J., and Megann, A.: Predicting the 2015 North Atlantic Cold Blob, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6802, https://doi.org/10.5194/egusphere-egu2020-6802, 2020.

EGU2020-19025 | Displays | OS1.7

Impact of the North Atlantic Warming Hole on Sensible Weather

Melissa Gervais, Jeffrey Shaman, and Yochanan Kushnir

In future climate projections there is a notable lack of warming in the North Atlantic subpolar gyre, known as the North Atlantic warming hole (NAWH). The NAWH has been previously shown to contribute to a poleward shift and eastward elongation of the North Atlantic jet that constitutes an additional important driver of future changes in the North Atlantic jet using a set of large-ensemble atmosphere simulations with the Community Earth System model.  The current study investigates the impact of the warming hole on sensible weather, particularly over Europe using the same simulations. North Atlantic jet regimes are classified within the model simulations by applying self-organizing maps to winter daily wind speeds on the dynamic tropopause. The NAWH is found to increase the prevalence of jet regimes with stronger and more poleward jets.  A previously identified transient eddy-mean response to the NAWH that leads to downstream enhancements of wind speeds is found to be dependent on the jet regimes. These localized regime-specific changes vary by latitude and strength, combining to form the broad increase in seasonal mean wind speeds over Eurasia. Impacts on surface temperature and precipitation within the various North Atlantic jet regimes are also investigated. A large decrease in surface temperature over Eurasia is found to be associated with the NAWH in regimes where air masses are advected over the subpolar gyre.  Precipitation is found to be locally suppressed over the warming hole region and increased directly downstream. The impact of this downstream response on coastal European precipitation is dependent on the strength of the NAWH.

How to cite: Gervais, M., Shaman, J., and Kushnir, Y.: Impact of the North Atlantic Warming Hole on Sensible Weather, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19025, https://doi.org/10.5194/egusphere-egu2020-19025, 2020.

Research to date has shown strong multi-decadal variability of the North Atlantic Oscillation (NAO) in late winter, particularly in March when correlations to North Atlantic (NA) ocean variability (Atlantic multi-decadal variability (AMV)) are particularly strong. This late-winter low-frequency atmospheric variability appears too weak in the majority of climate models across a range of indices of North Atlantic large-scale atmospheric circulation. It appears that models do not successfully reproduce responses to either (or both) proximal sea-surface temperature (SST) variability at mid-latitudes or teleconnections to SST variability in the sub tropics. 

Here, an in-depth analysis of the winter evolution of multiple indices of North Atlantic mid-latitude atmospheric circulation will be presented based on both re-analysis data and historical simulations of coupled climate models (CMIP5 and CMIP6). The atmospheric indices assessed will include the NAO, speed and latitude of the NA eddy driven jet and lower-tropospheric westerly wind strength in a region of maximum variability to the west of the UK. Results so far indicate that the CMIP6 models do not exhibit a clear change from CMIP5 in terms of the representation of low-frequency late-winter atmospheric variability. To diagnose in more detail possible origins of differences between observed and simulated variability, a detailed evaluation of early- to late-winter evolution in variability of the above indices will be presented, with an initial focus on observations (re-analysis and SST re-constructions) and incorporating the following questions:  
- Are there significant differences in the relative strength of linkages to tropical and extra-tropical SST variability across the different atmospheric indices? 
- Is the observed late-winter maximum in correlations between NA atmospheric indices and North Atlantic SSTs still apparent at sub-decadal timescales?
Initial results indicate that there are stronger tropical linkages for jet speed and that at sub-decadal timescales late winter is does not dominate in terms of correlations between atmospheric and SST variability. Updates on these early results will be presented along with implications of the results for differences between observed and simulated variability. 

How to cite: Bracegirdle, T.: Observed and simulated (CMIP5 and CMIP6) early- to late-winter evolution of North Atlantic atmospheric variability and links to the ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18861, https://doi.org/10.5194/egusphere-egu2020-18861, 2020.

EGU2020-2414 | Displays | OS1.7

Response of South and East Asian summer climate to North Atlantic SST anomalies: sensitivity to SST patterns

Satyaban Bishoyi Ratna, Timothy Osborn, Manoj Joshi, and Juerg Luterbacher

We simulate the response of Asian summer climate to AMO-like (Atlantic Multidecadal Oscillation) sea surface temperature (SST) anomalies using the Intermediate General Circulation Model version 4 (IGCM4). Separate AMO SST patterns are obtained from seven Coupled Model Intercomparison Project phase 5 (CMIP5)/Paleoclimate Model Intercomparison Project phase 3 (PMIP3) global climate models, to explore the sensitivity of the atmospheric response to the SST pattern. Experiments are performed with seven individual and composited AMO SST anomalies globally, and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive AMO phase, a Rossby wave train propagates eastward, causing a high pressure anomaly over eastern China/Japan region, associated with a low level anomalous anticyclonic circulation along with warm and dry anomalies. In contrast, the mid-latitude Rossby wave train is less robust in response to the cold phase of the AMO. The circulation response and the associated temperature and precipitation anomalies are sensitive to the AMO SST anomaly patterns. The comparison between global SST and N Atlantic SST experiments indicates that midlatitude East Asian climate anomalies are forced from the North Atlantic region. However, global SST anomaly experiments show that the SST anomalies outside the North Atlantic region, but still associated with AMO, strongly influence South Asian climate as they either strengthen or reduce the precipitation. Experiments with different amplitudes of negative and positive AMO anomalies test the linearity of the response. Over a large region of South and East Asia, temperature has a linear response to the amplitude of North Atlantic SST anomaly associated with both positive and negative AMO conditions, but the precipitation response is nonlinear.

How to cite: Bishoyi Ratna, S., Osborn, T., Joshi, M., and Luterbacher, J.: Response of South and East Asian summer climate to North Atlantic SST anomalies: sensitivity to SST patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2414, https://doi.org/10.5194/egusphere-egu2020-2414, 2020.

Interannual variations in the upper ocean heat and freshwater contents in the subpolar North Atlantic has important climatic effect. It affects the intensity of deep convection, which, in turn, forms the link between upper and deep ocean circulation of the global ocean Conveyor Belt.

The upper ocean heat content is primarily affected by two main process: by the ocean-atmosphere heat exchange and by oceanic heat advection. The intensity of both fluxes in the subpolar gyre is linked to the character of atmospheric circulation, largely determined by the phase of the North Atlantic Oscillation (NAO).

To study the interannual variability of the oceanic heat advection (in the upper 500th meters layer) we compare the results from four different data-sets: ARMOR-3D (1993-2018), SODA3.4.2 and SODA3.12.2 (1980-2017), and ORAS5 (1958-2017). The ocean-atmosphere heat exchange is accessed as the sum of the latent and the sensible heat fluxes, obtained from OAFlux data-set (1958-2016).

The oceanic heat advection to the Labrador and to the Irminger seas has high negative correlation (-0.79) with that into the Nordic Seas. During the years with high winter NAO Index (NAOI) the oceanic heat advection into the Subpolar Gyre decreases, while to the Nordic Seas – increases. These variations go in parallel with the intensification of the Norwegian, the West Spitsbergen and the slope East Greenland currents and weakening of the West Greenland and the Irminger Currents. During the years with high NAOI, the ocean heat release (both sensible and latent) over the Labrador and Irminger seas increases, but over the Norwegian Sea it decreases.

In summary, the results show that, during the positive NAO phase, the observed decrease of the heat content in the upper Labrador and Irminger seas is linked to both, a higher oceanic het release and a lower intensity of advection of warm water from the south. In the Norwegian Sea, the opposite sign of variations of the fluxes above leads to a simultaneous warming of the upper ocean.

The investigation is supported by the Russian Scientific Foundation (RSF), number of project 17-17-01151.

 

 

How to cite: Iakovleva, D. and Bashmachnikov, I.: Variations of oceanic and atmospheric heat fluxes in the North Atlantic and their link to the North Atlantic Oscillation Index, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-594, https://doi.org/10.5194/egusphere-egu2020-594, 2020.

EGU2020-614 | Displays | OS1.7

Timescale-dependent AMOC-AMO relationship
not presented

Hyojeong Kim and Soon-Il An

Previous studies showed that both AMOC and AMO work in different ways in interdecadal and multidecadal timescales. Although their relationship has also been covered in many studies, the possibility that overlapping between multiple timescales may have diluted their inherent relation has not been considered. To understand their physical relation correctly, it is necessary to consider interdecadal and multidecadal timescales, separately.

Here, we apply a band-pass filter to the AMO and AMOC indices obtained from a present-day climate simulation, to separate interdecadal and multidecadal variability. The results show that strong AMOC induces a warm phase of AMO by the northward heat transport in both timescales, but with a different time lag. This is because, in the interdecadal timescale, the southward propagation of AMOC anomaly gradually warms up the Atlantic basin from the high to low latitudes, resulting in a lag of seven years. As the delayed AMO peak provides negative feedback to AMOC by surface density modulation, the AMOC-AMO relationship can be described as an oscillatory system. On the other hand, AMOC in the multidecadal timescale matures at once in the entire basin, simultaneously warming the surface. The synchronous maturity of AMOC and AMO indicates that AMO-related density changes cannot account for the AMOC phase transition, and AMO remains a relatively passive component in their relationship. This study implies that overlooking timescale-dependency in physical processes may obscure our understanding of interactions between climate components.

How to cite: Kim, H. and An, S.-I.: Timescale-dependent AMOC-AMO relationship, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-614, https://doi.org/10.5194/egusphere-egu2020-614, 2020.

Atlantic Meridional Overturning Circulation (AMOC) contribute to long-term climate variability of Northern Hemisphere. The North Atlantic Ocean carries 25% of global heat transferred tropics to polar latitudes of the Northern Hemisphere (Srokosz, 2012). In the subpolar seas of the North Atlantic water goes down in few localized areas to deep convection, where all Atlantic deep water masses are formed. This process pumps a huge amount of CO2 to the deep ocean, which have strong consequence for global climate (Buckley and Marshall, 2016; Kuhlbrodt, 2007). The water comes back to the surface mainly in upwelling regions of the Southern Ocean (Toggweiler and Samuels 1998, Delworth and Zeng, 2008), as well as in the tropics due to vertical mixing.

In this study we try to link the long-term variability of the AMOC to it’s main driving mechanisms: the deep ocean convection in the Greenland, the Labrador and the Irminger seas, and to wind driven upwelling in the Southern Ocean.

As a reference for the AMOC intensity on the decadal and longer time scales, we use AMOC indexes from several studies (Caesar, 2018; Chen and Tung, 2018), which extend the time series back to the 1950s. The intensity of deep convection (IC) over the same time period is computed using convection index (Bashmacnikov et al., 2019). Wind-driving upwelling in the Southern Ocean is computed through evaluation of the divergence of Ekman fluxes (ED), using the wind velocity from atmospheric reanalysis (ERA 40 1957-1979 and ERA-Interim 1980-2016).

To estimate contribution of each of the forcing factors to the temporal variability of the AMOC, were used cross-correlation and regression analyses with varying time lags. The biggest cross-correlation coefficient was found with the IC in the Greenland Sea, the negative lags indicate that it is the AMOC, which affects the variability of convection intensity. The second largest cross-correlation coefficient was found with the IC in the Labrador Sea (0.7) with the lag of 13 years. The maximum cross-correlation with the IC in the Irminger Sea was 0.6 on a narrow interval of the time lags. The ED in Southern ocean demonstrate a significant correlation with the AMOC, with the correlation coefficient of 0.5 at the time lag of 15 years.

The contributions of each of the control mechanisms to temporal variability of the AMOC were investigated by the regression analysis for the time lags at which the maximum cross-correlations of each of the parameters are obtained. As a result the maximum regression coefficient was obtained for the IC in the Irminger Sea (0.65), the second one for the ED (0.35) using the time lags of 9 and 25 years, respectively. The regression coefficient for the IC in the Labrador Sea did not exceed 0.2 for all tested time lags. The physical mechanism, connecting the variability of the AMOC intensity to these two control mechanisms is a subject of our further research.

The work was supported by a grant from the Russian science Foundation (project No. 17-17-01151)

How to cite: Kuznetcova, D. and Mamadzhanian, A.: Do deep convection control the long-term variability of the Atlantic Meridional Overturning Circulation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-754, https://doi.org/10.5194/egusphere-egu2020-754, 2020.

EGU2020-844 | Displays | OS1.7

Centennial variability driven by salinity exchanges between the Atlantic and Arctic in a coupled climate model

Weimin Jiang, Guillaume Gastineau, and Francis Codron

The centennial to multi-centennial variability of the Atlantic Meridional Overturning Circulation (AMOC) is studied in a 1200-yr pre-industrial control simulation of the IPSL-CM6-LR atmosphere-ocean coupled model. In this run, a spectrum analysis finds a periodicity of the low-frequency variability of AMOC, with a period of about 200-year. This variability alters the Northern Hemisphere climate over the land and modulates the Arctic sea ice extent and volume. The associated density variations show large positive (negative) salinity-driven density anomalies in the Nordic Seas and subpolar gyre associated with a strong (week) AMOC state. The positive salinity anomalies in the Greenland Sea are found to be generated by anomalous southward salinity transport in the Fram Straits. The gradual AMOC increase and the associated oceanic northward heat transport melt the sea ice in the Arctic and build shallow negative salinity anomalies in the central Arctic. In parallel, the AMOC is also associated with a northward salt transport into the Eastern Arctic, by an inflow of Atlantic water from the Barents Sea to the East Siberian Ocean. The accumulated surface freshwater in the central Arctic is ultimately exported into the Atlantic mainly through the Fram Strait via intensified East Greenland Current, lowering the upper ocean density and enhancing the stratification at the regions where the cold deep limb of AMOC is formed. The positive salinity anomalies at subsurface then slowly reach the surface though diffusion, increasing the surface salinity. The oscillation then turns into the opposite phase.

How to cite: Jiang, W., Gastineau, G., and Codron, F.: Centennial variability driven by salinity exchanges between the Atlantic and Arctic in a coupled climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-844, https://doi.org/10.5194/egusphere-egu2020-844, 2020.

EGU2020-2379 | Displays | OS1.7

Aerosol-forced AMOC changes in CMIP6 historical simulations.

Matthew Menary, Jon Robson, Richard Allan, Ben Booth, Christophe Cassou, Guillaume Gastineau, Jonathan Gregory, Dan Hodson, Colin Jones, Juliette Mignot, Mark Ringer, Rowan Sutton, Laura Wilcox, and Rong Zhang

The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. Reliable simulations of its decadal to century-timescale evolution are key to providing skilful predictions of future regional climate, and to understanding the likelihood of a potential AMOC collapse. However, there remains considerable uncertainty even in past AMOC evolution. Here, we show that the multi-model mean AMOC strengthened by approximately 10% to 1985 in new historical simulations for the 6th Coupled Model Inter-comparison Project (CMIP6), contrary to results obtained from CMIP5. The simulated strengthening is due to a stronger anthropogenic aerosol forcing, in particular due to aerosol-cloud interactions. However, comparison with an observed sea surface temperature fingerprint of AMOC evolution during 1850-1985, and the shortwave forcing during 1985-2014, suggest that anthropogenic forcing and the subsequent AMOC response may be overestimated in some CMIP6 models.

How to cite: Menary, M., Robson, J., Allan, R., Booth, B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L., and Zhang, R.: Aerosol-forced AMOC changes in CMIP6 historical simulations., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2379, https://doi.org/10.5194/egusphere-egu2020-2379, 2020.

EGU2020-2981 | Displays | OS1.7

Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models

Susan Lozier, Matthew Menary, and Laura Jackson

The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.

How to cite: Lozier, S., Menary, M., and Jackson, L.: Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2981, https://doi.org/10.5194/egusphere-egu2020-2981, 2020.

EGU2020-3036 | Displays | OS1.7

Deep convection in the Lofoten Basin: ARGO vs MITgcm

Aleksandr Fedorov and Belonenko Tatyana

The Lofoten basin (the LB) contains relatively warm and salty waters regarding border basins such as Greenland and Barents Seas. Variability of the processes inside occurring in the basin reflects on the climate as on the mesoscales as on the interannual scales. We use a term mixed layer depth (MLD) as a border of the pycnocline in the water column, this parameter lets us evaluate the intensity of the convection in the region. Several methods of MLD calculations are tested in the current study: Kara, Montegut, and Dukhovskoy. The convection in the basin destructs stratification and forms massive intermediate water mass. The MITgcm data for 1993-2012 and over 5000 in-situ Argo T, S profiles for 2001-2017 were used in the calculations of the MLD.

We consider the maximum MLD (mMLD) in the region and its spatial distribution. The mMLD is higher in the central part of the LB and corresponds to the location of the Lofoten basin eddy (the LBE). Here the mMLD reaches 1000 meters, the medium maximum is 400 meters based both on the in-situ and model data. The maximum mixed layer depth ​​varies in the range of 400-1000 meters according to both datasets were used. The MLD over 400 meters is observed from January to May every year.

Acknowledgments: The authors acknowledge the support of the Russian Science Foundation (project No. 18-17-00027). The results of the MITgcm were provided by D.L. Volkov, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, USA.

How to cite: Fedorov, A. and Tatyana, B.: Deep convection in the Lofoten Basin: ARGO vs MITgcm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3036, https://doi.org/10.5194/egusphere-egu2020-3036, 2020.

EGU2020-4992 | Displays | OS1.7

Direct temporal cascade of temperature variance in eddy-permitting simulations of multidecadal variability

Antoine Hochet, Thierry Huck, Olivier Arzel, Florian Sevellec, Alain Colin de Verdiere, Matthew Mazloff, and Bruce Cornuelle

The North Atlantic is characterized by basin-scale multidecadal fluctuations of the sea surface temperature with periods ranging from 20 to 70 years.
One candidate for such a variability is a large-scale baroclinic instability of the North Atlantic Current. Because of the long time scales involved, most of the studies devoted to this problem are based on low resolution numerical models leaving aside the effect of explicit meso-scale eddies.   
How high-frequency motions associated with the meso-scale eddy field affect the basin-scale low-frequency variabiliy is the central question of this study.

This issue is addressed using an idealized configuration of an Ocean General Circulation Model at eddy-permitting resolution (20 km). A new diagnostic allowing the calculation of nonlinear fluxes of temperature variance in frequency space is presented. Using this diagnostic, we show that the primary effect of meso-scale eddies is to damp low frequency  temperature variance and to transfer it to high frequencies.

How to cite: Hochet, A., Huck, T., Arzel, O., Sevellec, F., Colin de Verdiere, A., Mazloff, M., and Cornuelle, B.: Direct temporal cascade of temperature variance in eddy-permitting simulations of multidecadal variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4992, https://doi.org/10.5194/egusphere-egu2020-4992, 2020.

EGU2020-5498 | Displays | OS1.7

AMOC hysteresis in an eddy-permitting GCM and monitoring indicators

Laura Jackson and Richard Wood

We conduct idealised experiments with HadGEM3-GC2, which is a pre-CMIP6 eddy-permitting GCM, to test for the presence of thresholds in the AMOC. We add fresh water to the North Atlantic for different rates and lengths of time, and then examine the AMOC recovery. In some cases the AMOC recovers to its original strength, however if the AMOC weakens sufficiently it does not recover and stays in a weak state for up to 300 years.

We test various indictors that have been proposed for monitoring the AMOC with this ensemble of experiments (and other scenarios). In particular we ask whether fingerprints can provide early warning or faster detection of weakening or recovery, or indications of crossing the threshold. We find metrics that perform best are the temperature metrics based on large scale differences, the large scale meridional density gradient, and the vertical density difference in the Labrador Sea. Mixed layer depth is also useful for indicating whether the AMOC recovers after weakening. 

How to cite: Jackson, L. and Wood, R.: AMOC hysteresis in an eddy-permitting GCM and monitoring indicators, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5498, https://doi.org/10.5194/egusphere-egu2020-5498, 2020.

EGU2020-6915 | Displays | OS1.7

Drivers of deep heat uptake in the North Atlantic Subpolar Gyre

Damien Desbruyeres, Bablu Sinha, Elaine McDonagh, Simon Josey, Alexis Megann, David Smeed, Penny Holliday, Adrian New, and Ben Moat

The decadal to multi-decadal temperature variability of the intermediate (700 – 2000 m) North Atlantic Subpolar Gyre (SPG) significantly imprints the global pattern of ocean heat uptake. Here, the origins and dominant pathways of this variability are investigating with an ocean analysis product (EN4), an ocean state estimate (ECCOv4), and idealized modeling approaches. Sustained increases and decreases of intermediate temperature in the SPG correlate with long-lasting warm and cold states of the upper ocean – the Atlantic Multidecadal Variability – with the largest anomalous vertical heat exchanges found in the vicinity of continental boundaries and strong ocean currents. In particular, vertical diffusion along the boundaries of the Labrador and Irminger Seas and advection in the region surrounding Flemish Cap stand as important drivers of the recent warming trend observed during 1996-2014. The impact of those processes is well captured by a 1-dimensional diffusive model with appropriate boundary-like parametrization and illustrated through the continuous downward propagation of a passive tracer in an eddy-permitting numerical simulation. Our results imply that the slow and quasi-periodic variability of intermediate thermohaline properties in the SPG are not strictly driven by the well-known convection-restratification events in the open seas but also receives a key contribution from boundary sinking and mixing. Increased skill for modelling and predicting intermediate-depth ocean properties in the North Atlantic will hence require the appropriate representation of surface-deep dynamical connections within the boundary currents encircling Greenland and Newfoundland.

How to cite: Desbruyeres, D., Sinha, B., McDonagh, E., Josey, S., Megann, A., Smeed, D., Holliday, P., New, A., and Moat, B.: Drivers of deep heat uptake in the North Atlantic Subpolar Gyre, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6915, https://doi.org/10.5194/egusphere-egu2020-6915, 2020.

EGU2020-7755 | Displays | OS1.7

Extreme sea levels in the context of climate change

Lucia Pineau-Guillou, Pascal Lazure, and Guy Wöppelmann

EGU2020-8399 | Displays | OS1.7

Decomposing barotropic transport variability in a high-resolution ocean model of the North Atlantic Ocean

Yuan Wang, Richard Greatbatch, Martin Claus, and Jinyu Sheng

Temporal variability of the annual mean barotropic streamfunction in a high-resolution model configuration (VIKING20) for the northern North Atlantic is analyzed using a decomposition technique based on the vertically-averaged momentum equation. The method is illustrated by examining how the Gulf Stream transport in the recirculation region responds to the winter North Atlantic Oscillation (NAO). While no significant response is found in the year overlapping with the winter NAO index, a tendency is found for the Gulf Stream transport to increase as the NAO becomes more positive, starting in lead years 1 and 2 when the mean flow advection (MFA) and eddy momentum flux (EMF) terms associated with the nonlinear terms dominate in the momentum equations. Only after 2 years, the potential energy (PE) term, associated with the density field, starts to play a role and it is only after 5 years that the transport dependence on the NAO ceases to be significant. The PE contribution to the transport streamfunction has significant memory of up to 5 years in the Labrador and Irminger Seas. However, it is only around the northern rim of these seas that VIKING20 and the transport reconstruction exhibit similar memory. This is due to masking by the nonlinear MFA and EMF contributions.

How to cite: Wang, Y., Greatbatch, R., Claus, M., and Sheng, J.: Decomposing barotropic transport variability in a high-resolution ocean model of the North Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8399, https://doi.org/10.5194/egusphere-egu2020-8399, 2020.

EGU2020-8602 | Displays | OS1.7

Spreading dynamics of central Labrador and Irminger Sea Waters

Patricia Handmann, Martin Visbeck, and Arne Biastoch

Water mass formation in the Subpolar North Atlantic and successive southward export, connects high latitudes with lower latitudes, as a part of the lower Atlantic meridional overturning (AMOC) limb. The role of regional importance, in particular the respective roles of the Labrador and Irminger Sea, in this process are in debate. 

This study analyses pathways connecting the Labrador and Irminger Sea in detail, using simulated Lagrangian particle trajectories. To give further insight on interconnectivity and flow patterns we used two setups with different velocity fields, a high-resolution ocean model (VIKING20X) and a gridded Argo float displacement climatology. Both setups indicate two distinct pathways with interconnectivity on the order of 20% of the total amount of seeded particles between the Labrador Sea and Irminger Sea. One pathway is following the recirculation in the Labrador Sea along the Greenland shelf break; the other is along the Newfoundland shelf break turning to the north/northwest at the Orphan-Knoll region towards the central Irminger Sea. For the Argo based advective-diffusive particle trajectory integration a 2.5–3.5 year travel time scale was derived between the Labrador and the Irminger Sea, while the experiments with the temporarily varying high-resolution model output revealed significantly shorter spreading times of about 1.5–2 years. While both pathways are represented in either setup, the pathway following the Newfoundland shelf break is populated stronger in the model-based experiments. In general we found that connectivity between the two regions is weaker in the experiments based on the climatological mean velocity output of the model than in those based on the Argo derived fields, first results indicate that this is due to stronger boundary currents and a weaker recirculation in the Labrador Sea.

How to cite: Handmann, P., Visbeck, M., and Biastoch, A.: Spreading dynamics of central Labrador and Irminger Sea Waters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8602, https://doi.org/10.5194/egusphere-egu2020-8602, 2020.

EGU2020-8889 | Displays | OS1.7

The AMOC at 47° North in Observations and a High Resolution Ocean Model

Simon Wett, Monika Rhein, Arne Biastoch, Claus W. Böning, and Klaus Getzlaff

The Atlantic Meridional Overturning Circulation (AMOC) is the main driver of northward oceanic volume and heat transport in the Atlantic. Due to its definition via the streamfunction the exact calculation of the AMOC requires knowledge of the full velocity field. Since the early 2000s, observations of the AMOC are available at 47° North in the form of hydrographic sections across the Atlantic and continuous current measurements from moored instruments at specific locations. However, the spatial resolution of current measurements is coarse and shipbased hydrographic sections are mostly done only once a year. Also the observational timeseries still remain too short to come to conclusions about decadal trends in the AMOC variability. Thus, today our knowledge about the role of the AMOC in the global climate system is mainly based on model simulations. Comparing these model simulations against observations remains an important task to accurately predict the future of the AMOC and adapt to changes.

We present first results of a model observations comparison in the subpolar North Atlantic between observations at 47° North and the high resolution ocean model VIKING20X. The model has a 1/20° nest in the Atlantic embedded in a global 1/4° model. It covers the years from 1980 to 2018 and thus overlaps with the whole observational period. This comparison will help assessing different methods of estimating the AMOC strength from observations.

How to cite: Wett, S., Rhein, M., Biastoch, A., Böning, C. W., and Getzlaff, K.: The AMOC at 47° North in Observations and a High Resolution Ocean Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8889, https://doi.org/10.5194/egusphere-egu2020-8889, 2020.

EGU2020-3503 | Displays | OS1.7

Towards quantification of the subpolar North Atlantic circulation

Hannah Nowitzki, Monika Rhein, Achim Roessler, Christian Mertens, and Dagmar Kieke

It is well known that ocean circulation impacts climate and weather patterns. One of the key regions influencing Europe is the subpolar North Atlantic, Here warm and saline water from the subtropics is imported with the North Atlantic Current (NAC), meeting cold and fresh water masses intruding from the north. To quantify the strength of the NAC, Uni Bremen and BSH Hamburg started in 2006 to continuously deploy instruments to quantify the volume transport. In a first step, the crossing of the NAC from the western into the eastern basin at the western flank oft the Midatalantic Ridge was covered, followed by boundary current moorings and PIES at a nominally zonal section at 47N in the western basin. In the last years, PIES and moorings have also been deployed in the eastern basin. Here we report about the recent results, focusing on the eastern basin.

How to cite: Nowitzki, H., Rhein, M., Roessler, A., Mertens, C., and Kieke, D.: Towards quantification of the subpolar North Atlantic circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3503, https://doi.org/10.5194/egusphere-egu2020-3503, 2020.

EGU2020-9130 | Displays | OS1.7

The penetration of Labrador Slope Water to Cape Hatteras and its role in Gulf Stream Dynamics

Adrian New, David Smeed, Adam Blaker, and Jenny Mecking

Labrador Slope Water is known to exist in the Slope Sea off the US eastern shelf as a relatively fresh and cool water mass deriving from the Labrador Current further north, and is present between the upper layer US shelf-derived water masses and the deeper Deep Western Boundary current waters, typically near 400-600m. This LSLW  is investigated in the EN4 database and shown to penetrate as far south as Cape Hatteras (74-75°W), having previously only been described as far west as the Gulf of Maine (66°W). We then examine, using both EN4 and Line W observations, the changes of this water mass between 2005-2008, when the strength of Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID array at 26°N, was high, and 2009-2015, when the AMOC was low. We show that in the AMOC high period, there was a larger volume of the LSSW present on the northern side of the Gulf Stream system which resulted in an increased meridional slope of the isopycnals near these depths, commensurate with increased geostrophic transport, and also in a more southerly position, of the Gulf Stream after separation at Cape Hatteras. The LSLW could therefore play an important role in decadal timescale variations in the North Atlantic climate system through its impact on the Gulf Stream and AMOC.

How to cite: New, A., Smeed, D., Blaker, A., and Mecking, J.: The penetration of Labrador Slope Water to Cape Hatteras and its role in Gulf Stream Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9130, https://doi.org/10.5194/egusphere-egu2020-9130, 2020.

EGU2020-9328 | Displays | OS1.7

Response of the North Atlantic Meridional Overturning Circulation to the Greenland Ice Sheet Freshwater input in the CESM 2.1 model

Carolina Ernani da Silva, Miren Vizcaino, and Caroline Katsman

Coupled climate models predict a weakening of the Atlantic Meridional Overturning (AMOC) circulation in the future. However, it is not clear what is the cause of the AMOC weakening. Studies have suggested that the freshwater (FW) is an important factor in the AMOC reduction. There are different sources of FW that may play a role, such as, river discharge, sea ice melt, and precipitation. Currently, due to global warming, the Greenland Ice Sheet (GrIS) melt rate is rising, which increases the amount of freshwater (ice discharge) into the ocean. Thus, it is possible that this input of freshwater would affect the ocean circulation on a regional and global scale. Hence, the GrIS freshwater cannot be neglected. The goal of this study is to understand the impact of the freshwater from GrIS on the North AMOC (NAMOC) strength in the future. We used the Community Earth System Model (CESM) version 2.1, which contains a fully coupled and an active ice sheet, to simulate an idealized greenhouse gas scenario (1% CO2). The CO2 concentration is 1140 ppm at the end of the simulation. The results show that GrIS delivers, on average, about 0.062 Sv/yr of FW to the Subpolar North Atlantic Ocean. The bulk of the total freshwater input comes from the southeastern and southwestern parts of the ice sheet:  the regions where some fast-flowing marine-terminating glaciers are located (e.g. Helheim and Kangerlussuaq). The NAMOC index (maximum barotropic stream function from above 28°N and from 500 m to 5500 m depth) was calculated. It displays a fast weakening, approximately 16.7 Sv (0.11 Sv/yr), during the first 150 yrs. After that, the NAMOC reaches a stable state where the index is around 5.7 Sv (year 350). When the NAMOC index was compared to the FW from GrIS time series, we observed that change in AMOC occurs before the FW starts to increase (from year 200). Our results thus suggest that the FW input from GrIS does not cause significant changes in the AMOC strength. It is necessary to further investigate other possible causes for the strong NAMOC decline in this model.

How to cite: Ernani da Silva, C., Vizcaino, M., and Katsman, C.: Response of the North Atlantic Meridional Overturning Circulation to the Greenland Ice Sheet Freshwater input in the CESM 2.1 model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9328, https://doi.org/10.5194/egusphere-egu2020-9328, 2020.

EGU2020-9395 | Displays | OS1.7

Unravelling the North Alantic ‘nutrient stream’ variability

Lidia I. Carracedo, Elaine McDonagh, Richard Sanders, Edward Mawji, Sinhué Torres-Valdés, Molly Baringer, Herlé Mercier, and Virginie Thierry

The Florida Current (FC), upstream extension of the Gulf Stream, is a very intense current (~32 Sv) confined to a 800-m depth narrow passage off the east coast of Florida. Associated with an intense poleward (subsurface maximum) transport of nutrients, this current has earned the name of North Atlantic ‘nutrient stream’. Since the biological sequestration of carbon (namely the Biological Carbon pump, BCP) is limited by the presence of nutrients in the upper ocean (euphotic zone), the FC can be seen as precursor of the nutrient induction process downstream feeding the subtropical gyre productivity. However, the relevance of this current is not only limited to its interplay with the subtropical gyre. The FC also comprises the bulk of the warm upper limb of the meridional overturning circulation reaching subpolar latitudes. Therefore, disentangling the range of intra-annual variability of the nutrient transport by the FC is crucial to better understand its linkage to and influence on the BCP magnitude and efficiency at higher latitudes. Based on a high-quality nutrient and velocity dataset from repeated hydrography between May 2015 and Oct 2018, we present an analysis of the nutrient transport by the FC in its 3 main water masses (the surface water, upper thermocline water and lower thermocline water). Our results show that the transport of inorganic nutrients is dominated by the upper and lower thermocline waters, whose transport-weighted nutrient concentration reaches a maximum in winter. Conversely, the transport of organic nutrients is dominated by the surface and upper thermocline waters, peaking in spring and autumn. Inorganic transport-weighted nutrient concentrations strongly correlate with volume transport. The correlation is positive in the surface water and the lower thermocline water (of South Atlantic origin), whereas it correlates negatively in the lower thermocline water (North Atlantic recirculated water). This indicates a southern remote source for the inorganic nutrient supply, which is mainly driven by advection. Organic nutrients, however, do not show a clear correlation with volume transport. Only the upper thermocline water shows a certain positive correlation, confirming the North Atlantic subtropical gyre as a main source of organic nutrients, ultimately driven not only -nor mainly- by circulation, but also by biological activity upstream.

How to cite: Carracedo, L. I., McDonagh, E., Sanders, R., Mawji, E., Torres-Valdés, S., Baringer, M., Mercier, H., and Thierry, V.: Unravelling the North Alantic ‘nutrient stream’ variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9395, https://doi.org/10.5194/egusphere-egu2020-9395, 2020.

EGU2020-9721 | Displays | OS1.7

A novel masking technique to investigate atmosphere-ocean interaction over Western Boundary Currents

Fumi Hayashi, Arnaud Czaja, and Benoit Vanniere

Western Boundary Currents (WBC), such as the Gulf Stream, leave a strong imprint on the ocean-atmosphere boundary in the form of strong gradients and high variability of Sea Surface Temperature (SST). Recent studies have shown that midlatitude oceanic fronts have an influence throughout the depth of the troposphere by means of synoptic systems such as weather fronts. An understanding of how the midlatitude ocean influences the synoptic system is crucial for better climate projection, however, this has been challenging. For example, in model simulations the sensitivity of the atmosphere to SST anomalies are dependent on its resolution, with low resolution models unable to capture the air-sea interactions occurring over warm sectors of midlatitude cyclones, possibly leading to underestimations of the oceanic influence on the atmosphere. A novel modelling technique is developed in which an interactive “mask” is used to systematically isolate and study the air-sea interaction over different synoptic regimes (warm and cold sector). Here, simulations using an idealised aqua-planet atmospheric general circulation model (AGCM) are used to study the atmospheric response to a tightening of SST gradient (comparable to that of the Gulf Stream) over the cold sector (“cold path”) and the warm sector (“warm path”) separately. Same experiments will also be performed on models with higher resolution to investigate the difference in atmospheric response between the high and low resolution models and what physical processes are responsible for such change in response.

How to cite: Hayashi, F., Czaja, A., and Vanniere, B.: A novel masking technique to investigate atmosphere-ocean interaction over Western Boundary Currents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9721, https://doi.org/10.5194/egusphere-egu2020-9721, 2020.

EGU2020-11095 | Displays | OS1.7

Deep convection variability in the Labrador versus Irminger Sea over the last decades

Siren Rühs, Arne Biastoch, Claus W. Böning, Michael Dowd, Klaus Getzlaff, Paul G. Myers, and Eric Oliver

Deep convection in the subpolar North Atlantic has been suggested to be a key process impacting the strength and variability of the Atlantic Meridional Overturning Circulation as well as the ocean’s uptake and deep storage of heat and anthropogenic CO2. However, the spatial pattern and strength of deep convection are subject to variability on interannual-to-decadal timescales and despite intense research in the field the nature of this variability is not fully understood. In this work, we employ a hindcast simulation with the eddy-rich (1/20°) ocean/sea-ice model configuration VIKING20X to analyze the variability of deep convection in the subpolar North Atlantic over the last decades (1980-2018). A special focus is set on mixed layer depth (MLD) pattern and deep water formation characteristics in the Labrador versus Irminger Sea. We show that, in agreement with observations, the VIKING20X hindcast captures strong convection events with particularly deep MLDs in the winters of the early 1980s, late 1980s and early 1990s, as well as in recent years. Yet, there are large differences in the spatial pattern of the deep convection events, as well as in the volume and thermohaline properties of the newly formed deep water. Most notably, in recent years deep convection intensity, and in particular its spatial extent, increased in the Irminger Sea and decreased in the Labrador Sea compared to the late 1980s and early 1990s. We finally discuss potential drivers of the simulated changes, thereby contrasting the relative importance of wintertime atmosphere-ocean buoyancy fluxes and oceanic preconditioning.

How to cite: Rühs, S., Biastoch, A., Böning, C. W., Dowd, M., Getzlaff, K., Myers, P. G., and Oliver, E.: Deep convection variability in the Labrador versus Irminger Sea over the last decades, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11095, https://doi.org/10.5194/egusphere-egu2020-11095, 2020.

EGU2020-13803 | Displays | OS1.7

The modern Northeast Atlantic Radiocarbon Content viewed along a basin transect from 29°- 61°N

Norbert Frank, Markus Miltner, Steffen Therre, Marleen Lausecker, Nadine Tisnerat-Laborde, Paolo Montagna, and Ronny Friedrich

The Northeast Atlantic is crucial regarding the northward heat and carbon export into the Arctic Ocean. At the surface and at mid-depth (0 -1000 m), however, most of the water re-circulates through two basin scale gyres, with warm and salty waters in the sub-tropical gyre (STG) and significantly fresher and colder waters in the sub-polar gyre (SPG). In addition, the Azores Front (AF) separates the northeastern branch from the southeastern branch of the STG, which is today positioned around 34.5°N in the east Atlantic. Underneath both gyres newly formed deep waters from the Arctic Ocean and Labrador Sea are spread into the Atlantic basin. Here we investigate, whether these water masses and recirculation patterns reveal distinct histories of carbon uptake and advection in the eastern North Atlantic. Therefore, water samples spanning the entire water column have been collected at 6 stations along a north-south transect at 25°W spanning from 42° to 61°N in 2012 (N/O Thalassa ICE-CTD cruise). In 2018 (RV Meteor M151 ATHENA cruise) samples further south were collected spanning until 29.5°N thus including the present day AF.

The radiocarbon content of Dissolved Inorganic Carbon (DIC) from 8 profiles (N>60, 30° to 61°N, 50-3000m depth) were measured at the AMS facility at CEZA facility in Mannheim following CO2 extraction from seawater at Heidelberg University. Δ14C values range from +50 ‰ in the upper layers of the subtropical Atlantic to -100 ‰ in 3000 m depth also in the subtropical Atlantic. Three main feature appear in the radiocarbon distribution. The surface shows a moderate difference between SPG and STG Δ14C values of <15‰ with a decreasing trend towards the North, hence indicating equilibration with the atmosphere. Underneath, between 100-1000 m depth SPG (46° – 61°N) Δ14C values of nearly 0‰ are found identical to the modern Northern Hemisphere atmosphere. In contrast, the STG (30°-43°N) reveals up to 50‰ enriched water reflecting limited carbon uptake from the atmosphere. Thus this layer will act as a source of radiocarbon to the polar seas and atmosphere in the near future. Below, between 1000 and 2000 m water masses north of the AF reveal a nearly homogeneous Δ14C value of -10‰ with a moderate decreasing trend with depth. South of the AF Δ14C values show a strong decrease with depth from 0 to -75‰, hence water masses remain still little affected by the advection of bomb radiocarbon (thus anthropogenic carbon). Thus, as expected the AF and the mid-depth gyre play a crucial role in distributing carbon throughout the east Atlantic.

How to cite: Frank, N., Miltner, M., Therre, S., Lausecker, M., Tisnerat-Laborde, N., Montagna, P., and Friedrich, R.: The modern Northeast Atlantic Radiocarbon Content viewed along a basin transect from 29°- 61°N, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13803, https://doi.org/10.5194/egusphere-egu2020-13803, 2020.

EGU2020-16250 | Displays | OS1.7

Examining the respective roles of greenhouse-gas and aerosol forcing for modes of multi-decadal variability

Andrea Dittus, Ed Hawkins, Laura Wilcox, Dan Hodson, Jon Robson, and Rowan Sutton

The respective roles of aerosol and greenhouse-gas forcing in modulating the phasing and amplitude of large-scale modes of multi-decadal variability remain poorly understood, despite the attention that has been devoted to trying to separate the influence of forcing from internal variability in modes such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation, for instance. However, understanding what drives multidecadal variability in these basins is imperative for improving near-term climate projections.

Here, we show how aerosol and greenhouse-gas forcing interact with internal climate variability to generate indices of multi-decadal variability in the Atlantic, using a large ensemble of historical simulations with HadGEM3-GC3.1 for the period 1850-2014, where anthropogenic aerosol emissions are scaled to sample a wide range in historical aerosol forcing. These results are complemented by early results from new stabilised warming simulations with the same climate model and analysis of future projections from models partaking in the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6).

How to cite: Dittus, A., Hawkins, E., Wilcox, L., Hodson, D., Robson, J., and Sutton, R.: Examining the respective roles of greenhouse-gas and aerosol forcing for modes of multi-decadal variability , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16250, https://doi.org/10.5194/egusphere-egu2020-16250, 2020.

EGU2020-16753 | Displays | OS1.7

AMOC response to changing resolution in the Finite-volumE Sea ice–Ocean Model

Dmitry Sidorenko, Sergey Danilov, Nikolay Koldunov, and Patrick Scholz

The Atlantic meridional overturning circulation (AMOC) is the most common diagnostics of numerical simulations. Generally it is computed as a streamfunction of zonally averaged flow along the constant depth. More rarely it is computed as zonally averaged along constant isopycnals. The latter computation, however, allows one to better distinguish between water masses and physical processes contributing to the meridional transport. We analyze the AMOC in global simulations based on the Finite-volumE Sea ice–Ocean Model (FESOM 2.0) using eddy permitting to eddy resolving configurations in the North Atlantic. We (1) split the AMOC computed in density space into the constitutes induced by surface buoyancy fluxes and cross isopycnal transformations, (2) identify the water masses which contribute to the formation of the North Atlantic Deep Water and (3) study the AMOC response to the permitting or resolving eddies in the North Atlantic ocean.

How to cite: Sidorenko, D., Danilov, S., Koldunov, N., and Scholz, P.: AMOC response to changing resolution in the Finite-volumE Sea ice–Ocean Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16753, https://doi.org/10.5194/egusphere-egu2020-16753, 2020.

The Atlantic Ocean is influenced by large-scale physical variability like changes in the Subpolar Gyre (SPG), the Atlantic Multidecadal Variability (AMV), the Atlantic Meridional Mode (AMM) or changes in the South Atlantic Anticyclone (SAA). Associated changes in temperature and salinity may severely impact open-ocean and deep-sea ecosystems. We study the variability of potential temperature and salinity profiles associated with large-scale physical variations focusing on 12 marine regions across both the North and South Atlantic Ocean (subpolar Mid-Atlantic Ridge off Iceland; Rockall Trough to Porcupine Abyssal Plain; central Mid-Atlantic Ridge; northwest Atlantic; Sargasso Sea; eastern tropical North Atlantic; central equatorial Atlantic; ecosystems from Angola to the Congo Lobe; the Benguela Current region; ecosystems off Brazil; the Vitória-Trindade Seamount Chain off Brazil; Malvinas Upwelling Current off Argentina). These regions were selected within the framework of the EU Horizon 2020 iAtlantic project. They are in proximity to major ocean circulation pathways as well as ocean monitoring arrays and are important for international conservation, Blue Growth and Blue Economy attempts.

Our methodology builds on recent work (Johnson et al., accepted, Frontiers in Marine Science) that shows that climate indices are associated with statistically-significant and spatially-coherent changes in bottom conditions across the northern North Atlantic. We use the same composite approach to investigate the relationship between indices of physical variability and potential temperature and salinity but extend the analysis to include additional indices (e.g. AMM, SAA) and to cover the entire Atlantic basin. Additionally, we use profile data instead of a gridded data product and investigate the full water column by density class, rather than focusing on bottom conditions. This enables physical mechanisms of any observed signals across the Atlantic Ocean as a whole to be explored.

How to cite: Burmeister, K., Inall, M., and Johnson, C.: Hydrographic changes across the Atlantic Ocean on interannual to decadal time scales – an EN4 profile data analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17986, https://doi.org/10.5194/egusphere-egu2020-17986, 2020.

EGU2020-19096 | Displays | OS1.7

Impact of a realistic Greenland ice sheet melting on the North Atlantic over the period 1920-2014

Didier Swingedouw, Marion Devilliers, Juliette Mignot, Julie Deshayes, Gilles Garric, and Mohamed Ayache

Greenland experienced intensive melting over the last century, especially in the 1920s and over the last decades. The supplementary input into the ocean is influencing the freshwater budget of the North Atlantic. Simultaneously, some signs of a recent weakening of the Atlantic meridional overturning circulation (AMOC) have been reported. In order to better understand the possible impact of the melting on the North Atlantic circulation, salinity and temperature trends, we construct an observation-based estimate of the freshwater fluxes from 1840 to 2014 associated to the runoff fluxes from Greenland ice sheet and surrounding glaciers and ice caps. Input from iceberg melting is also included and spatially distributed over the North Atlantic following an observed climatology. We force historical simulations of the IPSL-CM6A-LR coupled climate model with this reconstruction from 1920 to 2014. The 10-member ensemble mean displays freshened and cooled waters around Greenland, which spread in the subpolar gyre, and then towards the subtropical gyre and the Nordic Seas. Over the whole period, the convection is reduced in the Labrador and Nordic Seas, while it is slightly enhanced in the Irminger Sea, and the AMOC is reduced by 0.32±0.35 Sv at 26°N. This highlights that the AMOC decrease due to Greenland melting remains modest in these simulations and can only explain a very moderate amount of the 3±1 Sv weakening suggested in a recent study. The multi-decadal trend of the North Atlantic surface temperature obtained with the additional freshwater forcing is more in line with observations than in standard historical simulations. We also show a clear improvement of the representation of the 1995 abrupt warming in the subpolar gyre in the melting ensemble, which may thus be partly forced by Greenland ice sheet melting. Mechanisms at play imply changes in the variability of the AMOC in the melting ensemble as compared to the historical one. Such an impact on forced decadal variability has crucial consequences for decadal prediction systems that may gain skill by including observed Greenland ice sheet melting.

How to cite: Swingedouw, D., Devilliers, M., Mignot, J., Deshayes, J., Garric, G., and Ayache, M.: Impact of a realistic Greenland ice sheet melting on the North Atlantic over the period 1920-2014, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19096, https://doi.org/10.5194/egusphere-egu2020-19096, 2020.

EGU2020-22348 | Displays | OS1.7

Labrador Sea waters export routes in an idealised model and a global high-resolution ocean model

Caroline Katsman, Sotiria Georgiou, Juan-Manuel Sayol, Stefanie Ypma, Nils Brüggemann, Henk Dijkstra, and Julie Pietrzak

The water masses exiting the Labrador Sea, and in particular the dense water mass formed by convection (i.e. Labrador Sea Water, LSW), are important components of the Atlantic Meridional Overturning Circulation (AMOC). Several studies have suggested that the eddy activity within the Labrador Sea is of high importance for the properties of the LSW and the export routes. In this study, the pathways and the associated timescales of the water masses exiting the Labrador Sea are investigated by using a Lagrangian particle tracking tool. This method is applied to two different model simulations: to an eddy- permitting idealized model able to reproduce the essential features of the Labrador Sea, and to a high-resolution global ocean model simulation under a repeated annual climatological forcing.

In both model configurations, the Lagrangian trajectories reveal that the water masses that exit the Labrador Sea have followed either a fast route within the boundary current or a slow route that involves extensive boundary current-interior exchanges. Regions characterized by enhanced eddy activity play a significant role in determining the properties and the timescales of the water masses exiting the marginal sea, as the interior-boundary current exchange is governed by eddy activity.

Analysis of the properties of the water masses along the different pathways shows that the water masses that pass through the interior experience stronger densification than those that follow the boundary current.

This study highlights the importance of the exchanges between the boundary current and the convection area in the interior in setting the properties of the water masses that leave the Labrador Sea and the associated timescales.

How to cite: Katsman, C., Georgiou, S., Sayol, J.-M., Ypma, S., Brüggemann, N., Dijkstra, H., and Pietrzak, J.: Labrador Sea waters export routes in an idealised model and a global high-resolution ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22348, https://doi.org/10.5194/egusphere-egu2020-22348, 2020.

EGU2020-22545 | Displays | OS1.7

Decomposition of the Atlantic Multidecadal Variability in a historical climate simulation

Ioana Colfescu and Edwin Schneider

The Atlantic Multidecadal Variability (AMV) modulates various climate features worldwide with enormous societal and economic implications, including variations in hurricane activity in the Atlantic, sea-level changes, West African and Indian monsoon rainfall, European climate, and hemispheric‐ scale surface temperature. Leading hypotheses regarding the nature and origin of AMV focus primarily on its links with oceanic and coupled ocean-atmosphere internal variability, and on its response to external forcing. The role of another possible process, that of atmospheric noise forcing of the ocean, has received less attention. This is addressed here by means of historical coupled simulations and diagnostic experiments, which isolate the influences of external and atmospheric noise forcings. Our findings show that external forcing is an important driver of the simulated AMV. They also demonstrate that weather noise is key in driving the simulated internal AMV in the southern part of the (0o-60oN) AMV region, and that weather noise forcing is responsible for up to 10%-20% of the multidecadal internal SST variability in some isolated areas of the sub-polar gyre region. Ocean dynamics independent from the weather noise forcing is found to be the dominant cause of multidecadal SST in the northern part of the AMV region.

 

How to cite: Colfescu, I. and Schneider, E.: Decomposition of the Atlantic Multidecadal Variability in a historical climate simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22545, https://doi.org/10.5194/egusphere-egu2020-22545, 2020.

OS1.9 – Tropical & subtropical climate variability: ocean processes, air-sea interactions, climate modes, teleconnections and impacts

A fascinating character of the Arctic summer atmospheric circulation is the anomalous anticyclone circulation centered over the Arctic Ocean. Previous studies have related the underlying mechanisms to the atmospheric internal variability, the earlier spring Eurasian snowmelt, and the tropical Pacific forcing. Here we show that the Arctic summer anomalous anticyclone circulation is strongly associated with positive sea surface temperature anomalies (SSTAs) at midlatitudes of the extratropical North Pacific which are surrounded by significant negative SSTAs, resembling the negative phase of the Pacific Decadal Oscillation (PDO) but without significant signals in the tropics. The numerical experiments from the Whole Atmosphere Community Climate Model, prescribed with negative PDO-like SSTAs from May to August with the influence of El Niño-Southern Oscillation being reduced in advance, have simulated the observed positive air temperature and geopotential height anomalies over the Arctic and the circumpolar easterly anomaly at high latitudes in summer. The observational and simulated results strongly suggest that the extratropical SSTAs can influence the Arctic atmospheric temperature and circulation in summer.

How to cite: He, S. and Furevik, T.: Contributions from extratropical North Pacific to Arctic summer atmospheric temperature and circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5390, https://doi.org/10.5194/egusphere-egu2020-5390, 2020.

EGU2020-20544 | Displays | OS1.9

The Various Aspects of the Large- scale Atmospheric Circulation Response to the Northern Hemispheric Ocean Western Boundary Currents

Nour-Eddine Omrani, Fumiaki Ogawa, Hisashi Nakamura, Noel Keenlyside, Sandro Lubis, and Katja Matthes

Semi-idealized Atmospheric General Circulation-Model (AGCM) experiments are used, in order to study the different aspects of the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation that are maintained by the North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs). Here we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport in agreement with the Bjerknes compensation. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. It is shown that the impact of OWBCs on northern hemisphere circulation has significant implication for stratosphere/troposphere dynamical coupling, time-scales on the NAM, frequency of Sudden stratospheric warming and potential formation of polar stratospheric clouds.

 

Reference:

Omrani et al., 2019: Key Role of the ocean Western Boundary currents in shaping the Northern Hemisphere climate, Scientific Reports, https://doi.org/10.1038/s41598-019-39392-y

 

How to cite: Omrani, N.-E., Ogawa, F., Nakamura, H., Keenlyside, N., Lubis, S., and Matthes, K.: The Various Aspects of the Large- scale Atmospheric Circulation Response to the Northern Hemispheric Ocean Western Boundary Currents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20544, https://doi.org/10.5194/egusphere-egu2020-20544, 2020.

EGU2020-12639 | Displays | OS1.9

ENSO Frequency Asymmetry and the Pacific Decadal Oscillation in Observations and 19 CMIP5 Models

Fei Zheng, Renping Lin, and Xiao Dong

Using observational data and the pre-industrial simulations of 19 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), the El Niño (EN) and La Niña (LN) events in positive and negative Pacific Decadal Oscillation (PDO) phases are examined. In the observational data, with EN (LN) events the positive (negative) SST anomaly in the equatorial eastern Pacific is much stronger in positive (negative) PDO phases than in negative (positive) phases. Meanwhile, the models cannot reasonably reproduce this difference. Besides, the modulation of ENSO frequency asymmetry by the PDO is explored. Results show that, in the observational data, EN is 300% more (58% less) frequent than LN in positive (negative) PDO phases, which is significant at the 99% confidence level using the Monte Carlo test. Most of the CMIP5 models exhibit results that are consistent with the observational data.

How to cite: Zheng, F., Lin, R., and Dong, X.: ENSO Frequency Asymmetry and the Pacific Decadal Oscillation in Observations and 19 CMIP5 Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12639, https://doi.org/10.5194/egusphere-egu2020-12639, 2020.

EGU2020-7603 | Displays | OS1.9

Broad consistency between observed and simulated trends in sea surface temperature patterns

Dirk Olonscheck, Maria Rugenstein, and Jochem Marotzke

A realistic representation of sea surface temperature (SST) patterns in climate models is important for constraining local climate change and estimates of climate sensitivity (Gregory et al, 2016; Zhou et al, 2016; Armour, 2017; Marvel et al, 2018; Andrews et al, 2018). However, it is debated whether global climate models are capable of simulating the observed local SST patterns (Zhou et al, 2016; Coats et al, 2017; Marvel et al, 2018; Kostov et al, 2018; Seager et al, 2019). Using seven single-model initial condition ensembles with 30-100 ensemble members and the two multi-model ensembles CMIP5 and CMIP6, we here show that observed and simulated regional trends in SST patterns are consistent when accounting for internal variability. Some individual ensemble members simulate SST trend patterns that resemble the observed patterns in large areas across different basis. We find that observed and simulated SST trends are also consistent in critical regions such as the Southern Ocean, the North Atlantic, and the equatorial Pacific east-to-west SST gradient. Observed regional trends that lie at the outer edge of the models' internal-variability range allow two non-exclusive interpretations: a) observed trends are unusual realizations of the Earth's possible behavior and/or b) the models are systematically biased but large local variability leads to some good matches with the observations. Furthermore, we find that the large internal variability influences the existing range of SST trends more strongly than differences in the model formulation or in the observational data set.

 

References:

Andrews, T., Gregory, J. M., Paynter, D., Silvers, L. G., Zhou, C., Mauritsen, T., Webb, M. J., Armour, K. C., Forster, P. M., & Titchner, H. (2018). Accounting for Changing Temperature Patterns Increases Historical Estimates of Climate Sensitivity. Geophysical Research Letters, 45 (16), 8490–8499.

Armour, K. C. (2017). Energy budget constraints on climate sensitivity in light of inconstant climate feedbacks. Nature Climate Change, 7, 331–335.

Coats, S., & Karnauskas, K. B. (2017). Are Simulated and Observed Twentieth Century Tropical Pacific Sea Surface Temperature Trends Significant Relative to Internal Variability? Geophysical Research Letters, 44 (19), 9928–9937.

Gregory, J. M., & Andrews, T. (2016). Variation in climate sensitivity and feedback parameters during the historical period. Geophysical Research Letters, 43 (8), 3911–3920.

Kostov, Y., Ferreira, D., Armour, K. C., & Marshall, J. (2018). Contributions of Greenhouse Gas Forcing and the Southern Annular Mode to Historical Southern Ocean Surface Temperature Trends. Geophysical Research Letters, 45 (2), 1086–1097.

Marvel, K., Pincus, R., Schmidt, G. A., & Miller, R. L. (2018). Internal Variability and Disequilibrium Confound Estimates of Climate Sensitivity From Observations. Geophysical Research Letters, 45 (3), 1595–1601.

Seager, R., Cane, M., Henderson, N., Lee, D.-E., Abernathey, R., & Zhang, H. (2019). Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases. Nature Climate Change, 9 (7), 517–522.

Zhou, C., Zelinka, M. D., & Klein, S. A. (2016). Impact of decadal cloud variations on the Earth’s energy budget. Nature Geoscience, 9 (12), 871–874.

How to cite: Olonscheck, D., Rugenstein, M., and Marotzke, J.: Broad consistency between observed and simulated trends in sea surface temperature patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7603, https://doi.org/10.5194/egusphere-egu2020-7603, 2020.

EGU2020-146 | Displays | OS1.9 | Highlight

Interannual variability of Tropical Atlantic and its influence on drought and flood events in the Amazon Basin

Katherine Lisbeth Ccoica López, Ricardo Hallak, and Victor Raúl Chavez Mayta

The Tropical Pacific and Tropical Atlantic Ocean modulate the interannual precipitation over the Amazon region and the decadal and interdecadal variation as well. During El Niño Southern Oscillation (ENSO), below-average rainfall is recorded in the North and Northeast of the Basin, while deficit of precipitation is observed in the West and South. On the other hand, during La Niña years, rainfall is above of normal in the North and Northeast of Amazon Basin. However, there are also drought events, such as in 1964 and 2005, unrelated to the El Niño event, but influenced by warm conditions in the Tropical North Atlantic. In fact, the exceptional drought recorded in 2010 was influenced by a combined effect of the El Niño event during the peak of rainy season, followed by warm conditions in the Tropical North Atlantic during final of rainy season and dry season.

Therefore, the main aim of this study is exploring the Atlantic Sea Surface Temperature (SST) condition in modulating patterns that influence the development of drought and flood events in the Amazon Basin. First of all, the Atlantic Ocean is divided into Tropical North Atlantic (TNA), Tropical South Atlantic (TSA) and Subtropical South Atlantic (STSA), to analyze the behavior of each region separately. Atlantic Index, in each region, is the first principal component (PC1) time series, which comes from the empirical orthogonal function (EOF) analysis applied to Hadley Center Global Sea Ice and Sea Surface Temperature (HadISST) dataset for the 1870-2107 period. The Tropical North Atlantic, Tropical South Atlantic and Subtropical South Atlantic indices show the main years when drought and flood events reaching the Amazon Basin (droughts in 2005, 2010 and 2015, and floods in 2009 and 2012, mainly), and 5-years moving correlations indicate that these three ocean basin have been coupled and decoupled periodically each other in the last century.

The equatorial Pacific, North Atlantic and South Atlantic indices were also correlated with rainfall over the Amazon for three databases: the Tropical Rainfall Mission Measurements (TRMM), the Global Precipitation Climatology Centre (GPCC) and the HyBAm Observed Precipitation. All three databases showed the same results. An increase of the SST in Eastern Pacific influences in low precipitation over the central and west of the Amazon Basin during the rainy season (December to February), increase of the SST in Central Pacific influences for droughts over the northeast region and the TSA influences in the central Amazon. Increase of the SST in TNA and STSA influences mainly in the dry season (May to September), intensifying it. TNA is responsible for precipitation below normal over the central and west Amazon Basin, while STSA only influences in the central region of the basin. Finally, analysis of extreme events indicate that droughts and floods in the Amazon are intensified (de-intensified) if we consider warm (cold) phases of the AMO (Atlantic Multidecadal Oscillation) and the PDO (Pacific Decadal Oscillation).

How to cite: Ccoica López, K. L., Hallak, R., and Chavez Mayta, V. R.: Interannual variability of Tropical Atlantic and its influence on drought and flood events in the Amazon Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-146, https://doi.org/10.5194/egusphere-egu2020-146, 2020.

EGU2020-10222 | Displays | OS1.9

Mean-state dependence of future tropical-Atlantic sector climate projections

Wonsun Park, Mojib Latif, and Arielle Stela Imbol Nkwinkwa Njouodo

Mean state and internal variability in the tropics are crucially linked to air-sea interactions. State-of-the-art climate models exhibit long-standing problems not only in simulating tropical mean climate, such as too cold sea surface temperature (SST) over the central tropical Pacific and too warm SST over the eastern tropical Pacific and Atlantic, but also with respect to seasonal and longer variability. These biases question the credibility of future climate projections with the models, and it has not been shown to date whether or how such SST biases affect the projections. Here we focus on the tropical Atlantic (TA) and investigate how the mean state influences climate projections over the region.

We use two versions of the Kiel Climate Model (KCM) in global warming simulations, in which only atmosphere model resolution differs: one version carries ECHAM5 with a horizontal resolution of T42 (~2.8°) and 31 vertical levels, and the other ECHAM5 with a horizontal resolution of T255 (~0.47°) and 62 levels. Although only the atmospheric resolutions differ, the two KCM versions exhibit very different mean states over the tropical TA, with the higher-resolution version, among others, featuring much reduced warm SST bias over the eastern basin.

The response to increasing atmospheric carbon dioxide levels is found to be sensitive to the mean state. The model employing high atmospheric resolution and featuring a small SST bias projects an eastward-amplified SST warming over the TA, consistent with the pattern of interannual SST variability simulated under present-day conditions and in line with the observed SST trends since the mid-20th century. The model employing low-resolution and exhibiting a large SST bias projects more uniform SST change. Atmospheric changes also vastly differ among the two model versions.

Analysis of models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) support the KCM’s results: models with small SST bias project stronger warming over the eastern TA, while models with large SST bias either project uniform warming across the equator or largest warming in the west. This study suggests that reducing model bias may enhance global warming projections over the TA sector.

How to cite: Park, W., Latif, M., and Imbol Nkwinkwa Njouodo, A. S.: Mean-state dependence of future tropical-Atlantic sector climate projections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10222, https://doi.org/10.5194/egusphere-egu2020-10222, 2020.

EGU2020-7848 | Displays | OS1.9

A satellite era warming hole in the equatorial Atlantic Ocean

Hyacinth Nnamchi, Mojib Latif, Noel Keenlyside, and Wonsun Park

Although the globally averaged surface temperature of the Earth has considerably warmed since the beginning of global satellite measurements in 1979, a warming hole, with hardly any surface warming that is most pronounced in boreal summer, has been observed in the equatorial Atlantic region during this period. The warming hole occurs in an extended area of the equatorial Atlantic that includes the cold tongue, the region of locally cooler ocean surface waters that develops just south of the equator in boreal summer, partly reflecting the upwelling of deep cold waters by the action of the southeasterly trade winds. This lack of surface warming of the cold tongue denotes an 11% amplification of the mean annual cycle of the sea surface temperature during the satellite era. The warming hole is driven by an intensification of the equatorial upwelling of cold waters into the ocean surface layers and damped by the surface heat flux. In observations, the tendency for surface cooling appears to reflect intrinsic variability of the climate system and is not unusual during the instrumental period. The warming hole is associated with wind-induced ocean circulation changes to the south and north of the northward of the equator. Coupled model ensembles forced by the observed varying concentrations of atmospheric greenhouse gases and natural aerosols as well as unforced runs were analyzed. The ensembles suggest a strong role for atmospheric aerosols in the warming hole. However, although aerosols can cause a cooling of the cold tongue, intrinsic climate variability as represented in the unforced experiment can potentially cause larger cooling than has been observed during the satellite era. This study highlights the difficulty in reconciling observations and the climate models for the attribution of the warming hole.

How to cite: Nnamchi, H., Latif, M., Keenlyside, N., and Park, W.: A satellite era warming hole in the equatorial Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7848, https://doi.org/10.5194/egusphere-egu2020-7848, 2020.

EGU2020-499 | Displays | OS1.9

Seasonal forecasting of the Atlantic Niño

Chloé Prodhomme, Javier García-Serrano, Noel Keenlyside, Eleftheria Exarchou, Ingo Richter, and Aurore Voldoire

The Atlantic Niño is the leading mode of interannual variability in the Tropical Atlantic, which has impacts not only on the African monsoon but also in remote regions. In the present study, we investigate the predictability of the Atlantic Niño's mature phase (June-July) at seasonal time-scale, as well as its conditioning. We analyze a large set of state-of-the art forecasts systems from the North American Multi-Model Ensemble (NMME) and Copernicus Climate Change Service (C3S) multi-models. The prediction skill of the ATL3 index has considerably improved as compared to previous forecast quality assessments, with Anomaly Correlation Coefficient (ACC) reaching up to 0.8 for the May start date. Most of the models achieve skillful prediction of the Atlantic Niño from the May start-date, and some outperform persistence. For the start-dates of April, March and February, most of the models perform better than persistence and some achieve significant correlation skill for ATL3. While there has been improvement in forecasting capability, overall the warm SST bias and associated drift remain large in the equatorial Atlantic in most of the systems. Our results suggests that the skill in predicting the Atlantic Niño in summer is weakly related to the local SST drift during the first month of the forecast, but not to the magnitude of the SST bias during the peak. In addition, we find evidence that the skill in the equatorial Atlantic is related to the ability of the models to properly represent the large-scale atmospheric circulation pattern in the South Atlantic (i.e. St. Helena high).

How to cite: Prodhomme, C., García-Serrano, J., Keenlyside, N., Exarchou, E., Richter, I., and Voldoire, A.: Seasonal forecasting of the Atlantic Niño, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-499, https://doi.org/10.5194/egusphere-egu2020-499, 2020.

EGU2020-2172 | Displays | OS1.9

Why do Benguela Niños lead Atlantic Niños?

Serena Illig, Marie-Lou Bachèlery, and Joke Lübbecke

We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. It is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds. Yet, we observe that coastal SST anomalies off Angola tend to precede the ones in the equatorial cold tongue region by ~1 month.

We explain this counter-intuitive behavior using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we simulate warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor its seasonal variability controls the timing between events. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños.

Both warmings originate from a reduction in the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern equatorial SST. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely-forced coastal warming prematurely.

In conclusion, this study shows evidence that Atlantic and Benguela Niños are connected via an ocean teleconnection associated with equatorial and coastal wave propagations, but they are also tied by a large-scale atmospheric circulation and ocean-atmosphere interactions.

How to cite: Illig, S., Bachèlery, M.-L., and Lübbecke, J.: Why do Benguela Niños lead Atlantic Niños?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2172, https://doi.org/10.5194/egusphere-egu2020-2172, 2020.

EGU2020-11198 | Displays | OS1.9

The intra-annual variability as a potential driver for the mean deep circulation in the tropical oceans

Audrey Delpech, Claire Ménesguen, Frédéric Marin, Sophie Cravatte, and Yves Morel

The deep tropical ocean circulation is dominated by systems of vertically and meridionally alternating zonal jets, known as the Equatorial Deep Jets (EDJs) and Extra-Equatorial Jets (EEJs) respectively. The energy sources and physical mechanisms responsible for this circulation are still poorly understood. Recent studies have suggested the importance of intra-annual equatorial waves to transfer their energy to the EDJs.

In this study, we use idealized numerical simulations forced with a wave-like surface momentum flux to investigate how intra-annual variability can be relevant to the formation of the EEJs. It is shown that the amplitude of the jets, their meridional scales and their vertical and latitudinal extensions are sensitive to the period and wavelength of the forced wave. Short intra-annual waves with periods around ~70 days and wavelength ~300 km are found to reproduce the observed circulation most realistically. Focusing on the dominant barotropic mode, the underlying physical processes are detailed. A spectral analysis reveals that the energy transfer between the forced waves and the jet-structured circulation is compatible with a decay instability occurring in waves triadic interactions.

In parallel, a statistical analysis is performed on observations of the 1000m-velocities inferred from Lagrangian Argo floats drifts to document the amplitude and scales of the deep intra-annual variability in the tropical Pacific and Atlantic oceans. It gives evidence for the presence of short intra-annual waves that share common properties with the most unstable waves found for the EEJ generation.

How to cite: Delpech, A., Ménesguen, C., Marin, F., Cravatte, S., and Morel, Y.: The intra-annual variability as a potential driver for the mean deep circulation in the tropical oceans, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11198, https://doi.org/10.5194/egusphere-egu2020-11198, 2020.

EGU2020-1961 | Displays | OS1.9

The important role of mixing in scale interactions in the tropics and the coupled ocean/atmosphere system

Kelvin Richards, Andrei Natarov, and Yanli Jia

Our focus is the way various scales of motion in the tropical ocean are linked through mixing and its modification by larger scales. Enhanced mixing caused by small vertical scale features (SVSs) in the equatorial thermocline is known to impact the state of the ocean and its interaction with the atmosphere, in particular the sea surface temperature of the Pacific cold tongue and ENSO variability. The SVSs are produced by wind variability (from, for instance, the MJO) and instabilities, with an equatorial enhancement caused by a combination of factors including the characteristics of the forcing and propagation of internal waves and near-equator inertial and sub-harmonic parametric instabilities. Numerous scale interactions are at play. For instance, an eastward extension of the warm(fresh) pool in the western tropical Pacific, typical under El Niño conditions, stratifies the upper ocean. This stratification can produce a dramatic decrease in the downward propagation of wind-generated inertia-gravity waves and a decrease in the mixing in the main thermocline. The associated changes to the thermocline are advected to the east and impact the eastern cold tongue and hence the coupling with the atmosphere. Using a combination of observations and models we investigate the properties of SVS activity, its impact on mixing, and interaction with larger scales. Of particular interest is the dependency on stratification, the spatial and temporal variability of wind forcing, the impact on larger scales, and the resolution of both observations and models. The good news is that with enough resolution the relevant scales can be captured in both observations and models.

How to cite: Richards, K., Natarov, A., and Jia, Y.: The important role of mixing in scale interactions in the tropics and the coupled ocean/atmosphere system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1961, https://doi.org/10.5194/egusphere-egu2020-1961, 2020.

EGU2020-12170 | Displays | OS1.9

Internal Wave Generation in Tropical Upwelling Regions: the Angolan and Peruvian Shelves

Kevin Lamb, Peter Brandt, and Marcus Dengler

The Angolan and Peruvian shelves are located in upwelling regions along the eastern boundaries of the tropical Atlantic and Pacific Oceans. They are sites of important fisheries supported by high productivity which is driven by fluxes of nutrients from deep to near surface water along the coast. Mixing associated with internal waves is believed to play a role in this process. Recent field observations have shown the presence of an active internal wave field that includes internal solitary waves. In this talk results of high-resolution two-dimensional simulations of internal wave generation by tide-topography interactions on the Angolan and Peruvian shelves are presented. The simulations show the generation of internal wave beams at near-critical slopes and the generation of high-frequency internal solitary waves. The high-frequency IW spectrum is enhanced when small scale bathymetric ripples are included. Wave generation during winter and summer stratifications will be compared.

How to cite: Lamb, K., Brandt, P., and Dengler, M.: Internal Wave Generation in Tropical Upwelling Regions: the Angolan and Peruvian Shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12170, https://doi.org/10.5194/egusphere-egu2020-12170, 2020.

EGU2020-20036 | Displays | OS1.9 | Highlight

Characterization and Evolution of Marine Heat Waves in the Peruvian Upwelling System

Alice Pietri, François Colas, Rodrigo Mogollon, Dante Espinoza-Morriberón, Adolfo Chamorro, Jorge Tam, and Dimitri Gutiérrez

Rapidly developing extreme events such as anomalously warm water events, known as Marine Heat Waves (MHWs), have received considerable attention in the past few years due to the significant impact they have on regional ecosystems and socioeconomic activity. The Peruvian Coastal Upwelling System (PCUS), one of the most productive ecosystem in the world in terms of fisheries, is highly exposed to climate variability in particular because of its geographic location close to the equator, and the influence of the subtropical high pressure cell variability.

The PCUS is highly influenced by El Niño events, which have been intensively studied, and whose variability is related to the longest and most intense MHWs in the region. However the very visible El Niño events probably overshadowed the MHWs of shorter duration that also have an important impact on the coastal environment as they can often go with other extreme events such as nearshore hypoxia. To date, a census of MHWs of shorter duration (less than 30 days) is lacking in the region.

Here, we investigate the characteristics (spatial variability, frequency, intensity and duration) and evolution of such MHWs in the South Tropical Eastern Pacific, with a focus on the PCUS coastal area where the ecological vulnerability is higher. Several sea surface temperature satellite products are compared to test the sensitivity of the results.

The distinction between El Niño events and regular MHWs has a major impact on the statistical distribution of MHWs properties in the South Equatorial and South Tropical Eastern Pacific as well as on their evolution over the last 35 years. First results indicate that in the equatorial region and along the Peruvian coast, fewer MHWs and of shorter duration are observed north than south of 15°S. The observed trend is an increase of MHWs occurrences, duration and intensity in the South Tropical Eastern Pacific over the last 35 years, with the exception of the coastal region off Peru where the trend in occurrences and duration is the same but the average temperature anomaly associated to MHWs has decreased. It also seems that there is no apparent preferential season for the occurrence of MHWs. A study of the possible drivers is performed in an attempt to disentangle the role of the local (wind stress, heat fluxes) and remote (equatorial wave activity) forcing.

How to cite: Pietri, A., Colas, F., Mogollon, R., Espinoza-Morriberón, D., Chamorro, A., Tam, J., and Gutiérrez, D.: Characterization and Evolution of Marine Heat Waves in the Peruvian Upwelling System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20036, https://doi.org/10.5194/egusphere-egu2020-20036, 2020.

EGU2020-21190 | Displays | OS1.9

Multidecadal simulation of the tropical and subtropical South Atlantic Ocean with a high resolution ocean model forced by ERA-5 reanalysis data

Martin Schmidt, Hadi Bordbar, Fernanda Nascimento, and Claudia Frauen

High resolution regional ocean circulation models are needed to investigate regional ecosystem dynamics. However, these models may suffer from biases due to shortcomings in reanalysis datasets like NCEP or ERA-Interin, that have traditionally been used as atmospheric forcing. More realistic results can be achieved by replacing the reanalysed wind with scatterometer based winds. However, inconsistencies between different scatterometers like ASCAT and QuikSCAT introduce new uncertainty, which prevents a discussion of long-term trends in these models. The ERA-5 reanalysis offers a new consistent data set to force highly resolving regional ocean models. Based on such a simulation we analyse trends and anomalies in poleward currents in the Eastern Boundary Current off Southern Africa and Northern Benguela upwelling intensity due to changing wind stress and wind stress curl. Model results are validated with remote sensing as well as shipborne and mooring data. Further, variability of oxygen conditions in the Northern Benguela and the Angola Gyre oxygen minimum zone is discussed. 

How to cite: Schmidt, M., Bordbar, H., Nascimento, F., and Frauen, C.: Multidecadal simulation of the tropical and subtropical South Atlantic Ocean with a high resolution ocean model forced by ERA-5 reanalysis data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21190, https://doi.org/10.5194/egusphere-egu2020-21190, 2020.

EGU2020-846 | Displays | OS1.9

Seasonal Variability of Mesoscale Instabilities in the Azores Current System

João Bettencourt and Carlos Guedes Soares

The Azores Current-Front system coincides with the northern limit of the subtropical gyre in  the Eastern North Atlantic. The mean zonal jet is positioned south of the Azores archipelago  and extends from west of the mid-atlantic ridge to the Gulf of Cadiz, where it partially  turns south. North of the main jet, a sub-surface counter-current is found, flowing westwards. The associated thermal front separates the warm subtropical waters from the colder subpolar waters. The instantaneous flow in the Azores Current/Front system is characterized by the presence of meandering currents with length scales of 200 km that regularly shed anticyclonic warm water and cyclonic cold water eddies to the north and south of the mean jet axis, respectively, due to vortex stretching and the planetary beta effect. The time scale of eddy shedding is 100-200 days. On the meandering arms of the current, downwelling 
and upwelling cells are found and sharp thermal gradients are formed and a residual poleward heat transport is observed. The instability cycle that originates the mesoscale meanders and the eddies is well-known from quasi-geostrophic and primitive equation models initialized from a basic baroclinic state: a first phase of baroclinic instability feeds on available potential energy to raise eddy kinetic energy levels, that, in a second phase feed the mean kinetic energy by Reynolds stress convergence. The cycle repeats itself as long as the APE reservoir is filled at the end of each cycle.

However, seasonal variability of the zonal jet dynamics has not been addressed before and it can provide valuable insights in to the variations of the Eastern North Atlantic between the subtropical and subpolar gyres. We use a primitive equation regional ocean model of the Eastern Central North Atlantic with realistic climatological wind and thermal forcing to study the yearly cycle of meandering, eddy shedding and restoration of the mean jet in the Azores/Current system. We observe an semi-annual cycle in the jet's kinetic energy with maxima in Summer/Winter and minima in early Spring/Autumn. Potential energy conversion by baroclinic instability occurs throughout the year but is predominant in the first half of the year. The mean kinetic energy draws from the turbulent kinetic energy through Reynolds stress convergence in periods of 50 - 100 days, that are followed by short barotropic instability periods. During Winter, Reynolds stress convergence, and thus mean jet reinforcement from the mesoscale eddy field, occurs along the jet meridional extent, in the top 500 m of the water column, but from Spring to Autumn it is observed only in the southern flank of the mean jet axis.

How to cite: Bettencourt, J. and Guedes Soares, C.: Seasonal Variability of Mesoscale Instabilities in the Azores Current System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-846, https://doi.org/10.5194/egusphere-egu2020-846, 2020.

EGU2020-11183 | Displays | OS1.9

Links between climate and the upper ocean structure in the Canary current upwelling system

Tina Georg, Maria C. Neves, Paulo Relvas, and Kate Malmgren

Sea surface temperature differences between coastal and offshore waters and Ekman transport inferred from the wind velocity have been used to construct upwelling indices. Those indices have been widely used in climatological studies. In the present research we look to the upper layer structure of the ocean, down to 500 m depth, to infer relations between climate and the upwelling regimes. In particular, we explore the links between climate variability and the three-dimensional spatial structure of the upwelling activity along the Canary Current Upwelling System (CCUS) sector limited to 25-35° N, where upwelling is permanent, but intensified during the summer. The vertical structure of the CCUS is studied using vertical profiles of temperature, salinity, density and spiciness from the World Ocean Atlas (WOA). Monthly grids are retrieved for the past 30 years and vertical profiles exported at selected locations. The aim is to identify inter-annual and seasonal changes in the thermocline and the mix layer depth and link them to the upwelling characteristics. We then relate periods of strong upwelling with large-scale modes of climate variability, namely the North Atlantic Oscillation (NAO) and Eastern Atlantic pattern (EA). Time series of winter composites of NAO and EA are separated into positive and negative phases and their signatures quantified through composites of SST, salinity and density. The results provide the first assessment of inter-annual variability of the Canary upwelling current at both the surface and throughout depth and contributes towards understanding the connection between the vertical ocean structure and the large-scale climate modes. The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Georg, T., Neves, M. C., Relvas, P., and Malmgren, K.: Links between climate and the upper ocean structure in the Canary current upwelling system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11183, https://doi.org/10.5194/egusphere-egu2020-11183, 2020.

EGU2020-1433 | Displays | OS1.9

Coupled Interannual Variability of Wind and Sea Surface Temperature in the Caribbean Sea and the Gulf of Mexico

Geidy Rodríguez-Vera, Rosario Romero-Centeno, Christopher L. Castro, and Víctor Mendoza Castro

This work describes dominant patterns of coupled interannual variability of the 10-m wind and sea surface temperature in the Caribbean Sea and the Gulf of Mexico (CS&GM) during the period 1982–2016. Using a canonical correlation analysis (CCA) between the monthly mean anomalies of these fields, four coupled variability modes are identified: the dipole (March–April), transition (May–June), interocean (July–October), and meridional-wind (November–February) modes. Results show that El Niño–Southern Oscillation (ENSO) influences almost all the CS&GM coupled modes, except the transition mode, and that the North Atlantic Oscillation (NAO) in February has a strong negative correlation with the dipole and transition modes. The antisymmetric relationships found between the dipole mode and the NAO and ENSO indices confirm previous evidence about the competing remote forcings of both teleconnection patterns on the tropical North Atlantic variability. Precipitation in the CS and adjacent oceanic and land areas is sensitive to the wind–SST coupled variability modes from June to October. These modes seem to be strongly related to the interannual variability of the midsummer drought and the meridional migration of the intertropical convergence zone in the eastern Pacific. These findings may eventually lead to improving seasonal predictability in the CS&GM and surrounding land areas.

How to cite: Rodríguez-Vera, G., Romero-Centeno, R., Castro, C. L., and Mendoza Castro, V.: Coupled Interannual Variability of Wind and Sea Surface Temperature in the Caribbean Sea and the Gulf of Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1433, https://doi.org/10.5194/egusphere-egu2020-1433, 2020.

            Based on a nonlinear reduced gravity model simulation, formation cause of Subtropical Countercurrent(STCC) in the Pacific Ocean are investigated. The model reproduces well the characteristics of circulation of thermocline in the North pacific Ocean. The results suggest that the variation of the west boundary topography, especially the witdh of the luzon strait, play a key role on the formationg of STCC as well as the wind sress meridional gradient. When the witdh of the luzon strait gradually decrease, the STCC increase . the model results also reveal that the wind stress dipole curl of west ot the hawaii islands is key to the HLCC formation.

How to cite: Zhang, Z. and Xue, H.: The possible formation mechanism of the Subtropical Countercurrent in the Pacific Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6635, https://doi.org/10.5194/egusphere-egu2020-6635, 2020.

Mooring measurements at ~140°E in the western equatorial Pacific documented greatly intensified eastward subsurface currents, which largely represents the nascent Equatorial Undercurrent (EUC), to ~67 cm s-1 in boreal summer of 2016. The eastward currents occupied the entire upper 500 m, with the westward surface currents nearly diminished. Similar variations were also observed during previous El Niño events, as suggested by historical in-situ data. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140°E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along New Guinea coast. A 1.5-layer reduced-gravity ocean (RGO) model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments are performed to understand the underlying dynamics. The observed circulation anomalies are largely the delayed ocean response to the strong equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Niño (September-April). Downwelling equatorial Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly wind anomalies in the eastern Pacific. Upon reaching western Pacific, the Southern Hemisphere lobe of Rossby waves encounter the slanted New Guinea island and deflects equatorward, establishing a local sea surface height maximum near the equator and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.

How to cite: Lyu, Y.: Upper-Ocean Circulation Anomalies of the Western Equatorial Pacific Observed in 2016 Summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2333, https://doi.org/10.5194/egusphere-egu2020-2333, 2020.

EGU2020-1759 | Displays | OS1.9

Decadal variability of the Kuroshio Extension

Guidi Zhou and Xuhua Cheng

The decadal variability of the Kuroshio Extension (KE) is investigated using altimeter observations (AVISO) and the output of an ocean model (OFES). It is shown that the KE decadal variability is manifested in its strength, latitudinal position, and zonal extent, as well as the associated mesoscale eddy activity. Two differences between the two datasets are identified: (a) In OFES, the eddy activity positively correlates with the KE mode index when it leads by a few years, whereas in AVISO the two are negatively and concurrently correlated. (b) In OFES, the positive KE mode is associated with large meanders of the Kuroshio south of Japan, but in AVISO they are irrelevant. These differences indicate that the generation mechanism of KE's decadal variability is different in OFES and the real ocean. The sea surface height anomaly (SSHA) is then decomposed into major components including the wind-driven Rossby waves and residual (intrinsic) variability. The relationship between the two components are virtually the same in OFES and in AVISO, showing a negative correlation when the wind-driven part leads by a few years. Further diagnostics based on OFES reveals that the residual SSHA originates from the downstream region over the Shatsky Rise, slowly propagates westward, and is driven by eddy potential energy transfer. The OFES results partly conform to the intrinsic relaxation oscillation theory put forth by idealized model analyses, but in the latter the SSHA signal originates from the upstream Kuroshio. A new mechanism is then proposed for OFES: the decadal variability of the KE is first a result of the intrinsic relaxation oscillation probably excited by wind forcing, which regulates the strength of the KE’s inflow and thus modulates the downstream topography interaction, resulting in different downstream mesoscale eddy activity that further feeds back on the mean-flow. The mechanism for the real ocean is also reassessed.

How to cite: Zhou, G. and Cheng, X.: Decadal variability of the Kuroshio Extension, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1759, https://doi.org/10.5194/egusphere-egu2020-1759, 2020.

EGU2020-6666 | Displays | OS1.9

Enhanced latent heat flux with moisture disequilibrium during rapid intensification of the strongest supertyphoon Mangkhut in 2018

Sok Kuh Kang, Sebastian Landwehr, Eun Jin Kim, Kyeong Ok Kim, and Jae Hyoung Park

A summertime field survey in 2018 has been carried out to investigate the air-sea interaction over warm eddy in the Northwest Pacific. The strongest Supertyphoon Mangkhut in the Northwest Pacific got rapidly intensified from 80 kts to 120 kts during Sep.10, 2018, and it lasted as category 5 typhoon during Sep. 11 to 14. The maximum wind speed during intensification reached 100 kts on Sep.10. At distance 500 km off typhoon track the observations by sensors equipped on wave gliders were carried out in order to measure air-sea interaction parameters, as well as using instruments equipped on research vessel Isabu.

During the intensification of typhoon Mangkhut the enhanced bulk latent heat fluxes (LHF) were estimated over the several days. The latent heat flux was estimated from gust wind and 1 minute mean parameters. Peak LHF from gust wind reached over 1,100 W/m2, while 1 minute mean LHF reached about 900 W/m2. Data analysis reveals that the enhanced heat flux appears to exist due to an increase in moisture disequilibrium between the ocean and atmosphere. This supports the hypothesis that enhanced buoyant forcing from the ocean is likely to be an important mechanism in tropical cyclones over warm oceanic mesoscale eddy (Jaimes et al., 2016).

This work was supported by the project of “Study on air-sea interaction and process of rapidly intensifying typhoon in the Northwestern Pacific” funded by the Ministry of Ocean and Fisheries, Korea,

How to cite: Kang, S. K., Landwehr, S., Kim, E. J., Kim, K. O., and Park, J. H.: Enhanced latent heat flux with moisture disequilibrium during rapid intensification of the strongest supertyphoon Mangkhut in 2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6666, https://doi.org/10.5194/egusphere-egu2020-6666, 2020.

Using a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), South China Sea (SCS), adjacent land region (LD) and Pacific Ocean (PO) to the interannual summer precipitation variations in Southwest China (SWC) are investigated. Results show that on average, IO, SCS, LD, and PO contribute 46.8%, 25.3%, 21.8% and 2.3% of total moisture release in SWC in summer. In the above-normal precipitation summers, the moisture from IO and LD is increased by 48.2% and 28.8%, whereas that from SCS is decreased by 37.2%. In the below-normal precipitation summers, the moisture from IO and LD is decreased by 34.6% and 25.2%, while that from SCS is increased by 23.7%. In addition, the moisture anomalies from the four source regions can explain 85% of the total variances of the SWC summer precipitation.

The variations in the moisture from IO, SCS, and LD to SWC are not independent to one another and strongly influenced by the large-scale atmospheric circulation anomalies in the lower troposphere analogous to the Pacific-Japan (PJ) pattern and further studies showed that the PJ pattern was stimulated by the SST anomaly in the equatorial Atlantic. The anomalous warming in the tropical Atlantic that can modify the Walker circulation and introduce an anomalous descending over the central Pacific, thus inducing the anomalous anticyclone in the western North Pacific as the classical Matsuno-Gill response. The resultant suppressed precipitation in the western North Pacific excites the PJ pattern. The observed impacts of the tropical Atlantic SSTs on the atmospheric circulation can be well reproduced in an atmospheric general circulation model and the ability of the CMIP5 and CMIP6 models to reappear this relationship is verified, which will help the models to improve the simulation performance of summer large-scale circulation anomalies and precipitation in East Asia.

How to cite: Mengzhou, Y., Chaoxia, Y., Wenmao, L., and Yahan, Z.: Interannual Variations of Summer Precipitation in Southwest China: Anomalies in the Moisture Transport and Roles of the Tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2033, https://doi.org/10.5194/egusphere-egu2020-2033, 2020.

This study investigates the relationship between the preceding late spring Sea Surface Temperature (SST) over the tropical Atlantic and the East Asian Summer Monsoon (EASM) based on the observational data and Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations. The results show that warm (cold) tropical Atlantic SST (TASST) during May tends to be followed by a strong (weak) EASM with positive (negative) precipitation anomalies over the subtropical frontal area. Evidence is also provided that the atmospheric teleconnections propagating in both east and west directions are the key mechanisms linking the EASM with the preceding May TASST. That is, the warm TASST anomaly during late spring can persist through the subsequent summer, which, in turn, induces the Gill-type Rossby wave response in the eastern Pacific, exciting the westward relay of the Atlantic signal, as well as the eastward propagation of the Rossby wave along the jet stream. Furthermore, the westward (eastward) propagating teleconnection signal may induce the anomalous anticyclone in the lower troposphere over the Philippine Sea (anomalous tropospheric anticyclone with barotropic structure over the Okhotsk Sea). The anomalous anticyclonic circulation over the Philippine Sea (Okhotsk Sea) brings warm and humid (cold) air to higher latitudes (lower latitudes). These two different types of air mass merge over the Baiu-Meiyu–Changma region, causing the enhanced subtropical frontal rainfall. To support the observational findings, CMIP5 historical simulations are also utilized. Most state-of-the-art CMIP5 models can simulate this relationship between May TASST and the EASM.

Reference: Choi, Y., Ahn, J. Possible mechanisms for the coupling between late spring sea surface temperature anomalies over tropical Atlantic and East Asian summer monsoon. Clim Dyn 53, 6995–7009 (2019) doi:10.1007/s00382-019-04970-3

Acknowledgment: This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2018-01213.

 

How to cite: Ahn, J.-B. and Choi, Y.-W.: Relationship between sea surface temperature anomalies over tropical Atlantic in late spring and East Asian summer monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6293, https://doi.org/10.5194/egusphere-egu2020-6293, 2020.

            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.

How to cite: Zhang, L., Li, Y., and Li, J.: Impact of the Equatorial Wind Stress on the Indian Ocean Shallow Meridional Overturning Circulation During the IOD Mature Phase, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1751, https://doi.org/10.5194/egusphere-egu2020-1751, 2020.

Several studies suggest that the mean atmospheric and oceanic features in south-eastern Atlantic have experienced changes over the last few decades with, in particular, a shift in the mean position of the Saint Helena Hight Anticyclone and an increase in the mean ocean stratification. Modification of the wind forcing and the mean state at the equator and along the south-western coast of Africa will most likely impact the characteristics of the eastward propagating interannual Equatorial Kelvin Waves (EKW) and subsequent Coastal Trapped Waves (CTW) in the south-eastern Atlantic. These changes will also affect the interannual variability in the Benguela Upwelling System, especially since the remote equatorial ocean dynamics is instrumental in the development of extreme warm and cold Benguela Niño/Niña events. The objective of this study is to document the low-frequency change in the characteristics (amplitude, duration and timing) of the interannual Benguela Niño/Niña events. Using model solutions and sensitivity experiments, we investigate the mechanisms that control the low-frequency modulation of the coastal interannual variability off the coasts of Angola/Namibia. Our results reveal that the decadal modulation of the interannual variability off the Angolan coast is controlled by change in the EKW activity. In the Southern Benguela, the modulation of the interannual is dominated by the influence of the local alongshore winds. However, periods during which the equatorial forcing is intensified, EKW propagate and imprint the oceanic variability off the coast of Namibia.

 

How to cite: Bachelery, M.-L., Illig, S., and Rouault, M.: How low-frequency Equatorial Kelvin Wave activity and local coastal winds modulate the south-eastern interannual Atlantic variability?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21343, https://doi.org/10.5194/egusphere-egu2020-21343, 2020.

Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal jets that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models, most likely due to a lack of vertical resolution. However, when the vertical resolution is sufficiently high, EDJ oscillating at a period of about 4.5 years are present in idealised model configurations driven by steady wind forcing. We have used such a model for a basin-wide reconstruction of the intraseasonal eddy momentum flux convergence, which has recently been shown to be a large contributor to the EDJ maintenance (Greatbatch et al., 2018). When we apply the diagnosed momentum flux convergence that is oscillating at the EDJ period as forcing in a model without wind forcing, EDJ develop, allowing us to verify the mechanism proposed in Greatbatch et al. (2018). Additionally, we can isolate the nonlinear effect that the EDJ have on the time mean zonal flow at intermediate depths. We can show that the EDJ drive time mean zonal flow that is similar in structure to the mean flow measured by Argo floats at 1000 m depth, contributing (in our idealised setup) about one fifth of the total magnitude of the observed flow, the rest likely resulting from direct forcing by downward propagating intraseasonal waves as shown before in other studies.

 

References:

Greatbatch, R. J., M. Claus, P. Brandt, J.-D. Matthießen, F. P. Tuchen, F. Ascani, M. Dengler, J. Toole, C. Roth, J. T. Farrar, 2018: Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability. Geophysical Research Letters, 45, 1923-1929.

How to cite: Bastin, S., Claus, M., Brandt, P., and Greatbatch, R. J.: Equatorial deep jets and their influence on the equatorial mean circulation in an idealised model forced by intraseasonal momentum flux convergence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3039, https://doi.org/10.5194/egusphere-egu2020-3039, 2020.

EGU2020-10934 | Displays | OS1.9

Decadal variability of circulation and oxygen in the equatorial Atlantic

Peter Brandt, Johannes Hahn, Schmidtko Sunke, Franz Philip Tuchen, Robert Kopte, Rainer Kiko, Bernard Bourlès, and Marcus Dengler

The upper ocean circulation of the tropical Atlantic experiences long-term changes associated with different climate modes, but is at the same time expected to adjust to changes in the meridional overturning forced by climate warming. While observations of decadal variability of the surface circulation are generally based on satellite altimetry, direct observations of subsurface circulation mostly rely on very few long-term mooring sites typically covering energetic currents such as the Atlantic Equatorial Undercurrent (EUC). Here we focus on the period 2006 to 2018 that was covered by an intense field program including oxygen and circulation observations in the equatorial and tropical North Atlantic. During the observational period, a strengthening of the EUC of about 20% was detected based on data of an equatorial current meter mooring at 23°W. The EUC strengthening is related to a similar strengthening of the subtropical cells (STC). These STC changes were forced by a trade wind intensification in both hemispheres, however, more pronounced in the north and in the western basin. The STC strengthening is found to be consistent with the observed 12-year oxygen increase in the equatorial band (i.e. south of about 5°N) in the upper 400m obtained from repeat ship sections along 23°W. Such strongly enhanced oxygen levels relative to climatological mean were also observed in the upper 300-400m during a recent cruise along the whole Atlantic equator from Africa to South America. Our results are discussed with regard to the superposition of internal climate variability likely associated to a recent phase shift in the Atlantic multidecadal variability and changes due to global warming including ocean deoxygenation and enhanced thermocline stratification.

How to cite: Brandt, P., Hahn, J., Sunke, S., Tuchen, F. P., Kopte, R., Kiko, R., Bourlès, B., and Dengler, M.: Decadal variability of circulation and oxygen in the equatorial Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10934, https://doi.org/10.5194/egusphere-egu2020-10934, 2020.

EGU2020-5693 | Displays | OS1.9

Seasonal to interannual variability of observed temperatures in the equatorial Pacific

Angelo Rubino, Davide Zanchettin, Francesco de Rovere, and Michael J. McPhaden

On large (global and hemispheric) scales, sea surface temperature (SST) anomalies are considered to be good surrogates for marine air temperature (MAT) anomalies. Here we investigate how MAT and SST anomalies from instrumental measurements compare regarding a few crucial aspects of their variability including seasonality and multiannual trends. We make use of MAT and SST data acquired by moored buoys constituting the Tropical Atmosphere Ocean (TAO) array. Buoys are managed by the Pacific Marine Environmental Laboratory (PMEL), which is part of the National Oceanic and Atmospheric Administration (NOAA) agency of the United States of America. We  aim at answering the following questions: How do the monthly average anomalies of SST and MAT compare? Do observed MAT and SST data contain significantly different multiannual trends?

How to cite: Rubino, A., Zanchettin, D., de Rovere, F., and McPhaden, M. J.: Seasonal to interannual variability of observed temperatures in the equatorial Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5693, https://doi.org/10.5194/egusphere-egu2020-5693, 2020.

The Atlantic Warm Pool (AWP) is a big body of warm water with SST greater or equal to 28.5◦ C, that appears in the Gulf of Mexico, the Caribbean and the western tropical North Atlantic and it is a key element of the climate system. Previous studies have focused on climate variability within the AWP, but did not take into account the distinctive properties of AWP sub-regions. In other cases, obtained results had not been tested against selected databases. This work will try to deal systematically with these limitations. Ocean reanalysis databases have been used in order to detect AWP climate variability, mechanisms through which thermal component of ocean-atmosphere interactions operates and the effect of remote phenomena such as El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO).  Empirical Orthogonal Functions, spectral analysis, linear correlation and composites analysis techniques have been used. A large portion of AWP variability comes from Caribbean Sea and Gulf of Mexico while North tropical Atlantic contains a large internal variability. The thermal component of ocean-atmosphere interactions appears partitioned in Gulf of Mexico and Atlantic from Caribbean Sea. SST/latent heat feedback mechanism operates not globally in the AWP but stronger in the open Atlantic sub-region. ENSO+ enhances AWP development, while ENSO- is opposite to both development and decay of AWP. NAO effect is stronger in its negative phase by enhancing the AWP decay.

How to cite: Povea Perez, Y.: Atlantic Warm Pool: climate variability, thermal ocean-atmosphere interactions and remote response to ENSO and NAO., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4893, https://doi.org/10.5194/egusphere-egu2020-4893, 2020.

EGU2020-9788 | Displays | OS1.9

A multi-mode model applied to the tropical oceans

Markus Dunkel, Martin Claus, and Richard Greatbatch

A new model for the tropical ocean is described in which variations in the vertical are taken care of using vertical normal modes and the horizontal structure is taken care of by a set of linear shallow water models. The code is written in python and run in parallel and uses domain splitting in the horizontal. The model can be run in fully nonlinear mode and using generalized vertical mixing. The advantage of this approach is an almost continuous representation in the vertical and the ability to easily diagnose mode-mode interactions. The model performance is illustrated using the model of McCreary. In its original form, the McCreary model is linear and uses a very special form for the vertical mixing. Here we show preliminary results using more general vertical mixing and including nonlinearity.

How to cite: Dunkel, M., Claus, M., and Greatbatch, R.: A multi-mode model applied to the tropical oceans, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9788, https://doi.org/10.5194/egusphere-egu2020-9788, 2020.

EGU2020-22633 | Displays | OS1.9

Seasonal variability of salt in the western tropical Atlantic

Yaci Alvarez and Andre Luiz Belem

The western boundary regime of the tropical South Atlantic Ocean is the main pathway of an important meridional transfer of warm and cold water masses that balances the global temperature on Earth, known as Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system that depends on a delicate balance of temperature and salinity effects on density, and is considered one of the main elements of the terrestrial system. The objective of this work was to study the variability of the salinity in the Western Tropical Atlantic Ocean, in order to identify salt transport anomalies in the circulation of the Atlantic Meridional Overturning Circulation as a result of climate change. Based on 3 decades of hydrographic observations of the Northern Brazilian Current and of the Deep Western Boundary Current, neutral density surfaces, salinity anomalies, geostrophic transport and salt transport were calculated. In general, the results reveal a coherent decadal change in salinity in 5°S and 11°S. In the upper ocean, both water masses, the South Atlantic Central Water and the Antarctic Intermediate Water, presented an increase of the salinity. The Antarctic Intermediate Water shows small trends with a decrease in salinity values in the upper part of the layer and an increase at the border to the North Atlantic Deep Water. In the deep ocean, the North Atlantic Deep Water layers the salinity generally decreases and, as expected for a warmer ocean in the Southern Hemisphere, the Antarctic Bottom Water layer shows an increase in salinity. The geostrophic and salt transports suggest a multidecadal variability and the changes in upper layer salinity are consistent with an increased Agulhas leakage, as described in literature. In the deep ocean, water mass changes seem to be likely related to changes in weather patterns in the North Atlantic as well as in tropical circulation changes.

How to cite: Alvarez, Y. and Belem, A. L.: Seasonal variability of salt in the western tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22633, https://doi.org/10.5194/egusphere-egu2020-22633, 2020.

EGU2020-4047 | Displays | OS1.9

Atlantic Zonal Mode-Monsoon teleconnection in CMIP6 Models

Sabeerali Cherumadanakadan Thelliyil, Ravindran Ajayamohan, and Praveen Veluthedathekuzhiyil

Atlantic Zonal Mode (AZM) and Indian summer monsoon rainfall (ISMR) are known to have an inverse relationship, which means that the cold (warm) phases of AZM result in strong (weak) ISMR. The realistic simulation of AZM and its teleconnection with ISMR in coupled models is important for the better seasonal prediction of ISMR. Here, we evaluated the performance of 26 CMIP6 models in simulating the AZM-ISMR teleconnection using 40 years of historical simulations. The skill of most CMIP6 models in simulating the teleconnection between AZM and ISMR is poor. Out of the 26 models analyzed, only 10 models show the correct sign of AZM related rainfall response over central India. The underlying mechanism responsible for the models' failure in capturing AZM-teleconnection is studied using the large-scale dynamical/thermodynamical variables. By choosing a set of good and bad models we unravel the common biases responsible for the wrong teleconnection between AZM-ISMR. This study highlights the importance of correcting AZM‐ISMR teleconnection in climate models for better seasonal monsoon prediction.

How to cite: Cherumadanakadan Thelliyil, S., Ajayamohan, R., and Veluthedathekuzhiyil, P.: Atlantic Zonal Mode-Monsoon teleconnection in CMIP6 Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4047, https://doi.org/10.5194/egusphere-egu2020-4047, 2020.

General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic, as well as its linkage to the tropical Pacific. While, on average, mean state biases have improved little relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.

How to cite: Richter, I. and Tokinaga, H.: Evaluating the performance of CMIP6 models in the tropical Atlantic: mean state, variability, and remote impacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12722, https://doi.org/10.5194/egusphere-egu2020-12722, 2020.

EGU2020-22388 | Displays | OS1.9

Seasonal to interannual variations of wind forcing in the Peruvian upwelling system

Sadegh Yari and Volker Mohrholz

The Humboldt (Peruvian) Upwelling System (HUS) is the most productive among the main Eastern Boundary Upwelling Systems (EBUS), namely California, North West Africa, Benguela and itself. In spite of comparable upwelling intensity its fisheries production exceeds that of the other upwelling systems considerably (Chavez and Messie 2009). Wind is the major driving force of the coastal and curl driven upwelling, that controlls the nutrient supply from the deep water pool to the euphotic surface layer. Strength, spatial and temporal variability of the wind forcing are subjected to seasonal and interannual changes. The core of this study is describe the wind driven upwelling cells in the Peruvian coastal area in detail using long-term data which is not well understood. A better understanding of the state and dynamics of HUS seems essential for fututre regional climate predictions. ASCAT wind stress data for the period of 11 years (2008-2018) is analyzed to assess the spatio-temporal variations of the wind stress field, coastal upwelling and Ekman pumping along the Peruvian coast. The meridional component of wind stress off the peruvian coast, which is the main driver of offshore transport, has been marginally inensified over the entire priod. However, a high level of interannual variability is evident. The El-Niño years show anomalously high wind stress and associated Ekman transoprt. Our results indicate that the southern sector is more influenced by ENSO cycle than the northern sector. Additionally, a strong seasonality in the wind stress is observed. During the austral summer (December-February) the wind stress show the minimum value while the high values are observed in July-September.

How to cite: Yari, S. and Mohrholz, V.: Seasonal to interannual variations of wind forcing in the Peruvian upwelling system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22388, https://doi.org/10.5194/egusphere-egu2020-22388, 2020.

EGU2020-16996 | Displays | OS1.9

Intra-seasonal Oscillation of Abyssal Currents in the Middle East Pacific Ocean

Fangfang Kuang, Junpeng Zhang, Aijun Pan, and Dayong Zhu

In this work, the intra-seasonal oscillation of the abyssal currents in the Middle East Pacific Ocean is investigated using direct observations from ADCP instruments, which are mounted on a subsurface mooring deployed at 154oW,10oN. The observation shows that the intra-seasonal (20-100 days) oscillation part of the kinetic energy accounts for more than 40% of the low-frequency flow kinetic energy between 200~2000m, while accounts for more than 50% under 2000m; the intra-seasonal oscillation of meridional flow is more obvious than that of zonal flow. The meridional velocity in the upper layer (100-1000m) shows an oscillation at periods of 50~90 days, which is most obvious at the depth of 500m; from 200m to the bottom layer currents shows an synchronous oscillation at a period of 30 days lasting for several months, and the oscillation signal is the strongest in the deep layer (4600m); The correlation is good between the 20~40 day band passed meridional current at the bottom layer and that of the geostrophic current. The observed temperature of 4000m and 5000m also shows similar characteristics of 30 days period oscillation, which has good correlation to the sea level height. The reanalysis data shows the 30 days oscillation of the abyssal currents is propagated from west to east at a speed of about 0.29m/s while the 40~100 day oscillation is propagated at a speed of about 0.1m/s; the intensity of the intra-seasonal oscillation has obvious interannual variations, which may be related to the change of the eddy energy of the sea surface.

How to cite: Kuang, F., Zhang, J., Pan, A., and Zhu, D.: Intra-seasonal Oscillation of Abyssal Currents in the Middle East Pacific Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16996, https://doi.org/10.5194/egusphere-egu2020-16996, 2020.

EGU2020-11616 | Displays | OS1.9

Water Mixing and Circulation within the South Atlantic Basin Constrained by Seismic Reflection Images

Jingxuan Wei, Robert Reece, Will Fortin, and Tanner Acquisto

South Atlantic water masses and circulation significantly influence the dynamics and water mass structure of the Atlantic Meridional Overturning Circulation (AMOC). Previous research in the South Atlantic has mostly focused on energetic regions such as the Brazil/Malvinas Confluence Zone along the western boundary and the Agulhas retroflection to the east. However, it is also important to understand water circulation and diapycnal mixing within the South Atlantic Basin (SAB). Previous studies have observed low salinity patches of the Antarctic Intermediate Water within the western side of the SAB at 30o S, but the temporal variability of the scales, locations and structures of these low salinity patches are still uncertain. Former studies also show an increased level of mixing within the SAB above the Mid-Atlantic Ridge, but did not evaluate mixing on smaller scales such as mesoscale and sub-mesoscale.

Here we present a water mass structure analysis at 30o S from Rio Grande Rise to the Mid-Atlantic Ridge by using Seismic Oceanography (SO). SO is being applied around the world to image mesoscale water mass structures using the seismic reflection method. Reflections in the seismic images are essentially temperature gradients that are proxies for isopycnal surfaces. We paid particular attention in seismic processing to imaging of structures that characterize the boundary between water masses. We imaged the upper South Atlantic Central Water, and identified discontinuous water boundaries (about 150 km long) between the Antarctic Intermediate Water and the North Atlantic Deep Water that could correspond to the intermittent appearance of low salinity patches. We combine seismic images with previous hydrographic measurements to investigate the temporal change of these low salinity patches. We use a horizontal slope spectra to quantify mixing rate from tracked seismic horizons to evaluate mesoscale and sub-mesoscale mixing events such as internal waves and eddies. Through SO, we hope to better constrain South Atlantic circulation and contribute to the understanding of AMOC as a whole.

How to cite: Wei, J., Reece, R., Fortin, W., and Acquisto, T.: Water Mixing and Circulation within the South Atlantic Basin Constrained by Seismic Reflection Images, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11616, https://doi.org/10.5194/egusphere-egu2020-11616, 2020.

EGU2020-17853 | Displays | OS1.9

Climate change in the tropical Atlantic

Noel Keenlyside, Lander Crespo, Shunya Koseki, Lea Svendsen, and Ingo Richter

The tropical Atlantic SST have warmed by about 1 degree over the historical period, with greatest warming in the east, along the African coast and in the Gulf of Guinea. Experiments performed from the Coupled Model Intercomparison Projects (CMIP) indicate that models fail to reproduce this warming pattern, instead showing a rather uniform warming. Future projections with these models also tend to show rather uniform warming. In constrast. results from anomaly coupled models indicate that model biases impact the ability of climate models to simulate warming patterns in the tropical Atlantic. Here we investigate the role of model biases on climate change in the tropical Atlantic in the CMIP experiments. In addition, we have analyzed impacts of global warming on tropical Atlantic climate variability, and we assess the sensitive of the results are to model biases.

How to cite: Keenlyside, N., Crespo, L., Koseki, S., Svendsen, L., and Richter, I.: Climate change in the tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17853, https://doi.org/10.5194/egusphere-egu2020-17853, 2020.

Based on direct current measurements by ADCP moorings conducted during 2014-2018, seasonal-to-interannual variabilities of the Western Equatorial Pacific currents in different depth layers are analyzed. GODAS, Tropflux and NCEP reanalysis2 data are used to study the climatological factors influencing the current variabilities. The results show that both Equatorial Under Current (EUC) and Equatorial Intermediate Current (EIC) have significant seasonal-to-interannual variabilities. Both are closely related to the ENSO cycle, but through different mechanisms. Variations of the zonal velocity of Western Pacific EUC have noticeable correlations with subtropical SST, SLP and wind velocity, suggesting an influence of the Pacific meridional mode. The EIC, however, changes basically in corresponding to the Pacific zonal mode (ie. canonical ENSO mode). ENSO signals of the Eastern Equatorial Pacific might impact the Western Pacific EIC through vertical propagation of Rossby wave. This study gives an example on how atmospheric signals influence the subsurface ocean currents up to 800m depth.

How to cite: Tang, X., Wang, F., and Lyu, Y.: Variability of Subsurface and Intermediate Currents in the Western Equatorial Pacific Ocean and Their Impacting Factors, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18792, https://doi.org/10.5194/egusphere-egu2020-18792, 2020.

EGU2020-20615 | Displays | OS1.9

Representation of Northwest African upwelling in CMIP5 models

Antonio Castaño-Tierno, Belén Rodríguez-Fonseca, Elsa Mohino, and Teresa Losada

The Northwest Africa (NWA) upwelling region is located along the Senegalese and Mauritanian coast, between 10°N and 25°N and in a very narrow longitudinal band. In this region, most of the upwelled waters are due to alongshore surface winds through Ekman pumping.

The rapid increase in the upper ocean upwelling in this region along the 20th century and the contradictions found about future projections put forward the need for a better understanding of model’s ability to simulate Ekman induced upwelling processes.

In this work we assess intermodel variability to better understand the causes of different responses and spread among a set of CMIIP5 models.  

Results suggest that the seasonal cycle of NWA upwelling is qualitatively well simulated by CMIP5 models, although models tend to show strong biases for the permanent upwelling latitudes (north of 20°N) and the seasonal upwelling area (around 15°N in boreal spring). The maximum vertical temperature gradient shown by CMIP5 models is higher than that of SODA reanalysis and prevents cold waters from deeper layers to reach the surface, thus making coastal upwelling less effective in affecting sea surface temperatures.

Most of the intermodel variance is explained by the two first EOF modes of intermodel variability. The first mode shows a latitudinal structure, with a maximum in the permanent upwelling season, while. the second one is more seasonal. Both modes are very related to changes in the North-West Africa land-sea surface pressure gradient. In the case of the leading mode, incoming solar radiation differences between the North African desert and the ocean are the cause of the pressure gradients. For the second mode pressure changes in the Atlantic Ocean are driven by ITCZ shifts in response to interhemispheric differential warming.

How to cite: Castaño-Tierno, A., Rodríguez-Fonseca, B., Mohino, E., and Losada, T.: Representation of Northwest African upwelling in CMIP5 models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20615, https://doi.org/10.5194/egusphere-egu2020-20615, 2020.

EGU2020-21321 | Displays | OS1.9

Temporal and spatial Sea Surface Temperature (SST) variability off the Southwest African coast

Fernanda P. S. Nascimento, Martin Schmidt, and Volker Mohrholz

An understanding of Benguela Nino events is important for local economy, ecosystem and ocean dynamics. Aiming to see if Benguela Nino events can be seen in observations and reproduced by the models, an investigation of sea surface temperature (SST) temporal and spatial variability was done throughout the Southwest African coast.  Using SST obtained from satellite observations and from four different numerical models, a coastal strip of 1o width from 8S to 28S was calculated and averaged longitudinally. Even though models were warmer than the observations, variability seen on observations were reproduced by the models. Highly anomalous warm and cold periods that coincides with years of Benguela Niño and Niña were found both on observations and in the models, as well as SST weakening after 2000.

How to cite: Nascimento, F. P. S., Schmidt, M., and Mohrholz, V.: Temporal and spatial Sea Surface Temperature (SST) variability off the Southwest African coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21321, https://doi.org/10.5194/egusphere-egu2020-21321, 2020.

EGU2020-7291 | Displays | OS1.9

Interaction between the Boreal Spring and Summer Tropical Atlantic Interannual Variability Modes

Marta Martín-Rey, Jose Luis Pelegrí, Emilia Sánchez-Gómez, and Christophe Cassou

Traditionally, the interannual Tropical Atlantic variability (TAV) is thought to be governed by two air-sea coupled modes denoted as Meridional Mode (MM) and Equatorial Mode (EM), peaking in boreal spring and summer respectively. Several studies have proposed a possible connection between the MM and EM, but without reaching a consensus about its frequency, type and associated mechanisms. Remarkably, recent findings brought to light decadal changes in the structure, intensity and teleconnections of the EM along the observational record. In particular, new overlooked equatorial modes called ‘non-canonical EM’ and ‘Horse-Shoe mode’ have been reported, which exhibit significant sea surface temperature anomalies in the north tropical Atlantic region. This gives robustness to the connection between the boreal spring and summer interannual modes.

Here, using observational and CMIP6 model datasets, we demonstrate the existence of distinct interannual modes in the tropical Atlantic basin along the record. Furthermore, the emergence of these modes is not stationary on time and varies from some decades to the others.  In this study, using observations and coupled climate models we explore the connection between the MM and EM to generate the diverse of tropical Atlantic variability reported in previous works. Moreover, the air-sea mechanisms and ocean dynamics involved in the evolution of these modes and the role of the mean state in the connection between them is assessed.

How to cite: Martín-Rey, M., Pelegrí, J. L., Sánchez-Gómez, E., and Cassou, C.: Interaction between the Boreal Spring and Summer Tropical Atlantic Interannual Variability Modes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7291, https://doi.org/10.5194/egusphere-egu2020-7291, 2020.

The dimensional and temporal distribution of Antarctic Intermediate Water (AAIW) and North Pacific Intermediate Water (NPIW) in the Philippines Sea were explored using Argo profiles. As the salinity minimum of intermediate water from mid-high latitude of the southern and northern hemisphere of the Pacific Ocean, the properties of AAIW and NPIW merges at about 10°N with different properties in the Philippine Sea. The core of AAIW is located below 600dbar with potential density of 27≤σθ≤27.3 kg m-3 and salinity of 34.5≤S≤34.55 psu. The core of NPIW is located between 300-700dbar with potential density of 26.2≤σθ≤27 kg m-3 and salinity of 34≤S≤34.4 psu. The volume of AAIW and NPIW during January 2004 to December 2017 is negative correlated.  The time series of AAIW and NPIW is dominated by semi-annual signals. The variations of AAIW and NPIW was mainly affected by volume transport through 130°E section by North Equatorial Current (NEC) and North Equatorial Undercurrent (NEUC).

How to cite: Zang, N.: The long-term variation of the Intermediate Water in the Philippines Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21599, https://doi.org/10.5194/egusphere-egu2020-21599, 2020.

EGU2020-22282 | Displays | OS1.9

Climatological annual cycle of Benguela upwelling system using different upwelling indices

Mohammad Hadi Bordbar, Volker Mohrholz, and Martin Schmidt

Several long-established upwelling indices derived from the observed wind fields, Chlorophyll-a concentration, sea surface temperature (SST) are used to investigate the climatology annual cycle of Benguela Upwelling System (BUS). Chlorophyll-a concentration is taken as an indicator of ocean primary production. In addition, we analyze a multi-decadal simulation of a state-of-the-art eddy resolving ocean model which was forced by observed atmospheric heat and momentum fluxes. We take the vertically averaged of simulated vertical velocity in water column as a direct measure of upwelling strength.

The Ekman offshore transport tends to have two distinctive upwelling cells near the coast of Lüderitz (26.3°S) and Cape Frio (17°S) with large seasonal cycles. The former peaks between September and December. The latter features a biannual cycle with two peaks over April-June and September-December, which is concurrent with meridional migration of Angola-Benguela SST front. The offshore (30-200 km) vertical velocity, primarily induced by Ekman transport divergence, depicts a similar annual cycle, but with smaller magnitude. It becomes broader from south to north with four distinctive upwelling cells located near the coast of Cape Columbine (33°S), Orange River (28°S), Walvis Bay (23°S) and northern part of Cape Frio (16°S). The spatial and temporal variation of Ekman pumping and Chlorophyll-a, as measures of upwelling, show a clear correlation. However, such a correlation is not evident when Ekman coastal transport is taken. SST-based index depicts a very similar spatial pattern. However, the seasonal cycle does not match with other observational and simulated indices. Our finding suggests that the local SST anomalies are strongly influenced by horizontal heat advection and surface heat flux anomaly which can dominate over the anomalies associated with the upwelling; meaning that SST-index alone may not give a realistic estimate of upwelling strength over the region.

How to cite: Bordbar, M. H., Mohrholz, V., and Schmidt, M.: Climatological annual cycle of Benguela upwelling system using different upwelling indices, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22282, https://doi.org/10.5194/egusphere-egu2020-22282, 2020.

EGU2020-22335 | Displays | OS1.9

Multidecadal Modulations of ENSO influence on cyclogenesis in the Tropical Atlantic

Adama Badiane, Belén Rodríguez-Fonseca, Teresa Losada, Abdou Lahat Dieng, and Saidou Moustapha Sall

The impact of ENSO (El Niño Southern Oscillation) events on the cyclogenesis of the eastern tropical North Atlantic is highlighted, focusing on decadal variations of the interannual relationship at the Senegalese coast, which is the main cyclone development region (MDR). SST anomalies in the Equatorial Pacific associated with ENSO events affect vertical wind shear over the eastern Atlantic, by inducing strong subsidence of dry air over the eastern Atlantic which tends to inhibit deep convection and thus be unfavorable to cyclonic activity. Based on 20yr- correlations between the number of cyclones that are born in the MDR and ENSO index, we have selected two different periods of study (period1: 1950-1969; and period2: 1996-2015).The results show that period2 presents the highest scores of negative correlations between ENSO and tropical Atlantic cyclogenesis. Although there is an intensification of ENSO events during period2 compared to period1, we have found that decadal changes in climatology have a more significant effect on the MDR than the interannual changes. Additionally, the changes in the interannual signal appear to be related to the concomitant action of interannual SST anomalies over the whole tropical basins.

How to cite: Badiane, A., Rodríguez-Fonseca, B., Losada, T., Dieng, A. L., and Sall, S. M.: Multidecadal Modulations of ENSO influence on cyclogenesis in the Tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22335, https://doi.org/10.5194/egusphere-egu2020-22335, 2020.

EGU2020-10459 | Displays | OS1.9

Atlantic Ocean Influence on Middle East Summer Surface Air Temperature

Muhammad Azhar Ehsan, Dario Nicolì, Fred Kucharski, Mansour Almazroui, Michael Tippett, Alessio Bellucci, Paolo Ruggieri, and In-Sik Kang

Middle East surface air temperature (ME−SAT), during boreal summer (June to August: JJA), shows robust multidecadal variations for the period 1948−2016. Here using observational and reanalysis datasets as well as coupled atmosphere−ocean model simulations, we linked the observed summer ME−SAT variability to the multidecadal variability of sea surface temperature (SST) in the North Atlantic Ocean (AMV). This Atlantic−ME connection during summer involves ocean−atmosphere interactions through multiple ocean basins, with an influence from the Indian Ocean and the Arabian Sea. The downstream response to Atlantic SST is a weakening of the subtropical westerly jet stream that impacts summer ME−SAT variability through a wave−like pattern in the upper tropospheric levels. The Atlantic SST response is further characterized by positive geopotential height anomalies in the upper levels over the Eurasian region and dipole−like pressure distribution over the ME lower levels. For positive Atlantic SST anomalies, this pressure gradient initiates anomalous low−level southerly flow, which transports moisture from the neighboring water bodies toward the extremely hot and dry ME landmass. The increase in atmospheric moisture reduces the longwave radiation damping of the SAT anomaly, increasing further ME−SAT. A suite of Atlantic Pacemaker experiments skillfully reproduces the North Atlantic−ME teleconnection. Our findings reveal that in observations and models the Atlantic Ocean acts as a critical pacemaker for summer ME−SAT multidecadal variability and that a positive AMV can lead to enhanced summer warming over the Middle East.

How to cite: Ehsan, M. A., Nicolì, D., Kucharski, F., Almazroui, M., Tippett, M., Bellucci, A., Ruggieri, P., and Kang, I.-S.: Atlantic Ocean Influence on Middle East Summer Surface Air Temperature, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10459, https://doi.org/10.5194/egusphere-egu2020-10459, 2020.

EGU2020-4858 | Displays | OS1.9

Understanding the 2017 warm event in North Tropical Atlantic

Ana Trindade, Marta Matín-Rey, Marcos Portabella, Eleftheria Exarchou, Pablo Ortega, and Iñigo Gómara

The Atlantic Ocean has suffered tremendous warming during recent decades as a consequence of anthropogenic forcing, modulated by the natural low frequency variability. Special attention should be paid to the high temporal frequency of warm interannual events in the North Tropical Atlantic (NTA) since the early 2000s, resulting in the most intense hurricane seasons on record (Hallam et al., 2017; Lim et al., 2018; Murakami et al., 2018; Klotzbach et al., 2018; Camp et al., 2018). Moreover, NTA sea surface temperature anomalies during boreal spring have been suggested as a potential precursor to the Equatorial Mode (Foltz and McPhaden, 2010ab; Burmeister et al., 2016; Martín-Rey and Lazar, 2019; Martín-Rey et al., 2019). 

This study aims to investigate the development of the 2017 NTA spring-summer warming event, which was the strongest of the last decade, as well as the importance of an accurate ocean forcingin the simulation of this event. For such purpose, a set of four simulations using distinct ocean wind forcing products, namely from the EC-Earth model, ERA-Interim (ERAi) reanalysis and a new ERAi-corrected ocean wind product (ERAstar), have been performed and analysed.The latter consists of average geolocated scatterometer-based corrections applied to ERAi output (Trindade et al., 2019).In this sense, ERAstar includes some of the physical processes missing or misrepresented by ERA-i, and corrects for large-scale NWP parameterization and dynamical errors.

The air-sea processes underlying the onset and development of the warm 2017 NTA event and the wave activity present in the equatorial Atlantic will be explored to determine their possible connection with the equatorial sea surface temperature variability. Furthermore, the comparison between the different experiments allows us to validate the new surface wind dataset and evaluate the importance of accurate, high-resolution ocean forcing in the representation of tropical Atlantic variability.

How to cite: Trindade, A., Matín-Rey, M., Portabella, M., Exarchou, E., Ortega, P., and Gómara, I.: Understanding the 2017 warm event in North Tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4858, https://doi.org/10.5194/egusphere-egu2020-4858, 2020.

EGU2020-22337 | Displays | OS1.9

Weaker AMOC in coupled models inhibits north tropical Atlantic impact on ENSO

Belen Rodriguez-Fonseca, Irene Polo, Elsa Mohino, Teresa Losada, Marta Martín-Rey, Noel Keenlyside, and C. Roberto Mechoso

Observational studies have reported that interannual variability of sea surface temperature in two tropical Atlantic regions can act as ENSO predictors in different seasons and periods: boreal summer Atlantic Nino (AN) in negative phases of the Atlantic Multidecadal Variabil- ˜ ity (AMV); and boreal spring tropical north Atlantic (TNA) in positive AMV. The robustness of the AMV role in the interbasin connection remains an open question due to the short observational record. Using observations and pre-industrial climate model simulations, we demonstrate for the first time that latitudinal displacements of the Atlantic ITCZ act as a switch for the type of inter-basin teleconnection. During periods in which the Atlantic ITCZ is further equatorward (northward) AN (TNA) impacts ENSO. This ITCZ location can be 1 affected by several factors, including the inter-hemispheric SST gradients associated with AMV.Coupled models success in capturing the AN-ENSO connection. Nevertheless, they have difficulties in reproducing the TNA-ENSO connection because they overestimate rainfall in the southern tropical Atlantic. The TNA-ENSO connection occurs sporadically during periods when the ITCZ is shifted further northward in association with strong heat transports by the AMOC. Weaker AMOC periods in coupled models don't present the TNA-ENSO connection. State-of-the-art models still need to improve for correctly representing tropical Atlantic impact on ENSO.

How to cite: Rodriguez-Fonseca, B., Polo, I., Mohino, E., Losada, T., Martín-Rey, M., Keenlyside, N., and Mechoso, C. R.: Weaker AMOC in coupled models inhibits north tropical Atlantic impact on ENSO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22337, https://doi.org/10.5194/egusphere-egu2020-22337, 2020.

OS1.10 – Understanding the Indian Ocean’s past, present, and future

EGU2020-1264 | Displays | OS1.10

Low Dinitrogen Fixation Rates in the Bay of Bengal during Summer Monsoon

Arvind Singh, Himanshu Saxena, Deepika Sahoo, Mohammad Atif Khan, Sanjeev Kumar, and Athiyarath K Sudheer

Nitrogen is a staple element for every living organism in addition to carbon, since all the major cellular components (e.g., DNA and RNA), proteins, and energy carrier molecules (e.g., ATP) are stemmed from these elements. Biological dinitrogen (N2) fixation exerts an important control on oceanic primary production by providing bioavailable form of nitrogen (such as NH4+) to photosynthetic microorganisms. We hypothesized that the oligotrophic nature of the Bay of Bengal might create a suitable niche for N2 fixing microorganisms.

In the Bay of Bengal, fresh water influx driven stratification prevent the vertical influx of nutrients to the sunlit layers. Most of the riverine nutrients are used within estuarine and coastal regions, and thus these have negligible contribution on open ocean biological productivity. Atmospheric deposition contribution to the nutrients supply is equally low (< 3%) in the Bay. Thus, the recently observed high new production rates in the Bay of Bengal suggests the higher probability of N2 fixation in this basin than the Arabian Sea. In addition, nitrogen isotopic composition of sedimentary organic matter (low δ15N values) in the Bay of Bengal can also be alluded to the presence of diazotrophy in the Bay. Hence, we further strengthened our hypothesis that N2 fixers play a crucial role for the primary production in the Bay.

We commenced the first N2 fixation study in the sunlit layer of the Bay of Bengal using 15N2 gas tracer incubation experiments on a cruise expedition during summer monsoon 2018. N2 fixation rates varied from 4 to 124 μmol N m-2 d-1 – these rates were very low compared to that observed in the Bay’s western counterpart in the Indian Ocean, i.e., the Arabian Sea. The contribution of N2 fixation to primary production was small (< 1%). Noteworthily, the upper bound of observed N2 fixation rates in our study was still higher than that measured in other oceanic regimes such as Eastern Tropical South Pacific, Tropical Northwest Atlantic, and Equatorial and Southern Indian Ocean. Strong monsoonal winds, turbidity due to copious riverine discharge and cloud cover over the Bay of Bengal might have inhibited N2 fixation. Therefore, a more detailed study covering all the seasons is needed to understand the role of N2 fixation rates on primary productivity in the Bay of Bengal.

How to cite: Singh, A., Saxena, H., Sahoo, D., Khan, M. A., Kumar, S., and Sudheer, A. K.: Low Dinitrogen Fixation Rates in the Bay of Bengal during Summer Monsoon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1264, https://doi.org/10.5194/egusphere-egu2020-1264, 2020.

EGU2020-8642 | Displays | OS1.10

Last 12 ky record of various organic geochemical proxies in the Eastern Arabian Sea

Ayusmati Manaskanya, Aninda Mazumdar, Aditya Peketi, Svetlana Fernandes, and Rheane D. A. Silva

Here we present high-resolution biogeochemical study using nitrogen/carbon isotope ratio measurement and molecular proxies from a sediment core (length = 2.9 m) collected from the center (588 mbsl; Lat: 16049.88’N and Long: 710 58.55’ E) of the oxygen minimum zone off west coast of India. The core archives the depositional record covering 1.114 to 12.025 ky BP. The concentrations of total organic carbon (TOC) and total nitrogen (TN) range from 0.7 to 4.9 wt% and 0.068 to 0.5 wt % respectively. TOC and TN show parallel trends and the TOC/TN ratio varies within a narrow range of 8 to 11.5. The δ13C values range from -20.5‰ to -21.9‰ (V-PDB). The TOC/TN and δ13C suggest typical marine organic matter source. This observation is also further supported by the n-alkane distribution pattern where the dominance of nC21 to nC24 and the absence of odd alkane dominance over even suggest predominantly marine organic source. The δ15N profile shows a steady increase from 5.7‰ at 203 cmbsf (5.5 ky BP) to 7.5‰ at 2 cmbsf (~1ky BP) suggesting gradual increase in denitrification possibly liked to reduced ventilation in the Arabian Sea, whereas, between 5.5 to 12 ky BP, the δ15N values show marked fluctuations (5.2 to 7.1‰) possibly indicating variable level of oxygenation which in turn controlled the extent of denitrification. Possible influence of diagenesis (microbial degradation of organic matter) on the δ15N values also need to be investigated for a better understanding of the water column processes.

How to cite: Manaskanya, A., Mazumdar, A., Peketi, A., Fernandes, S., and Silva, R. D. A.: Last 12 ky record of various organic geochemical proxies in the Eastern Arabian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8642, https://doi.org/10.5194/egusphere-egu2020-8642, 2020.

EGU2020-13738 | Displays | OS1.10

Projected response of Arabian sea Oxygen minimum zone to climate change: Insights from a set of downscaled experiments

Parvathi Vallivattathillam, Zouhair Lachkar, Marina Levy, and Shafer Smith

The land-locked northern boundary and seasonal high productivity in the Arabian sea (AS) leads to the formation and the maintenance of one of the most intense and thickest open ocean oxygen minimum zones (OMZ) there. Earlier studies based on both observation and model sensitivity experiments have reported that this perennial OMZ is highly sensitive to the strength of the monsoonal circulation and surface heating. Model simulations from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in the Indian monsoonal circulation and the atmospheric heat fluxes under climate change. However, the future projection of AS OMZ under climate change remains largely uncertain and ill-understood. This is mainly due to a poor representation of the AS OMZ in the CMIP5 simulations and an important spread in their future oxygen projections for the region. Here we explore how downscaling CMIP5 global simulations with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model can help improving the representation of the AS OMZ and reduce the spread in CMIP5 projections. To this end, we performed a climatological “reference” simulation, i.e., the control simulation, where ROMS is forced with observed atmospheric and lateral boundary conditions, and a set of corresponding downscaled sensitivity experiment where ROMS is forced with atmospheric and lateral boundary conditions derived from global CMIP5 simulations. For the downscaling experiment, we chose two best performing models from the CMIP5 database based on their skill in simulating the present day (historical) climatology. The control simulation has been extensively validated against the observations for its skill in simulating the physical and biogeochemical variables. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model resolution from 1/3deg to 1/12deg and expanding the model domain from a small AS-limited domain to one encompassing the full Indian Ocean. We show that the downscaled experiments improve the representation of different classes of oxygen (Oxic - O2 > 60mmol/l; Hypoxic - 60mmol/l >= O2 > 4mmol/l; and the Suboxic  - 4 mmol/l > O2 > 0 mmol/l) within the 0-1500m depth range. In particular, the downscaled experiments simulate a much smaller fraction of suboxic waters relative to hypoxic and oxic fractions, in agreement with observations.

 

How to cite: Vallivattathillam, P., Lachkar, Z., Levy, M., and Smith, S.: Projected response of Arabian sea Oxygen minimum zone to climate change: Insights from a set of downscaled experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13738, https://doi.org/10.5194/egusphere-egu2020-13738, 2020.

EGU2020-13104 | Displays | OS1.10

Impact of Climate Change on the Agulhas Bank, South Africa ― a 100 year projection with consequences for the squid fishery

Sarah Asdar, Michael J. Roberts, Zoe Jacobs, and Ekaterina Popova

The South and East coast of South Africa is strongly influenced by the warm, fast-flowing Agulhas Current. The Agulhas Bank, a shallow shelf on the southern tip of Southern Africa, is a crucial area for productivity which support fisheries of high economic importance for South Africa.  In this context of climate change, perturbations of this diverse, complex and highly variable marine environment could affect the productivity and lead to dramatic social and economic consequences for the region. To predict potential changes over the eastern and central Bank, we employ a high-resolution global coupled ocean-biogeochemistry model, NEMO-MEDUSA, simulated to year 2099. We find that even though the Agulhas Bank is warming over the next century, primary production does not experience a significant decrease. Additionally, we show that the Agulhas Current might shift its position, with intensification surface current velocity on the Bank hence reducing water retention over the Bank. This change in local circulation over the Bank could have a serious impact on the ecosystem of the region.

How to cite: Asdar, S., Roberts, M. J., Jacobs, Z., and Popova, E.: Impact of Climate Change on the Agulhas Bank, South Africa ― a 100 year projection with consequences for the squid fishery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13104, https://doi.org/10.5194/egusphere-egu2020-13104, 2020.

This study estimates variability in meridional velocity and transport of the subtropical circulation in the south Indian Ocean using in-situ hydrographic observations, satellite altimetry and two reanalysis products for the period from 2006 to 2017. Previous studies used the zonal difference of satellite sea surface height (SSH) between the western and eastern parts of the basin as an index to variability in basinwide meridional geostrophic transport. This study estimates meridional geostrophic velocity in the upper 1800 m from in-situ observations and compares results with SSH variability. Results show that zonal SSH difference represents a surface trapped variability in meridional velocity, the amplitude of which is large in the upper 250 m and decreases to zero at about 1000 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1000 m depth. It is likely that wind variability both in the south Indian Ocean and tropical Pacific Ocean is responsible for this surface trapped variability, as is suggested by past studies. Results of this study also show meridional velocity variability at subsurface, which peaks in magnitude at about 400 to 800 m depth and is not correlated with zonal SSH difference. Waves radiated from the eastern boundary are possibly responsible for the generation of this subsurface flow, but detailed forcing mechanisms are not known in this study. This subsurface flow can contribute to interannual variability in mode water transport and warrants a further study.

How to cite: Nagura, M.: Interannual to decadal variability in subtropical circulation transport in the south Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11992, https://doi.org/10.5194/egusphere-egu2020-11992, 2020.

EGU2020-2707 | Displays | OS1.10

Interactions of the Indian Ocean climate with other tropical oceans

Matthieu Lengaigne and the Coauthors

Ocean-atmosphere interactions in the tropics have a profound influence on the climate system. El Niño–Southern Oscillation (ENSO), which is spawned in the tropical Pacific, is the most prominent and well-known year-to-year variation on Earth. Its reach is global, and its impacts on society and the environment are legion. Because ENSO is so strong, it can excite other modes of climate variability in the Indian Ocean by altering the general circulation of the atmosphere. However, ocean-atmosphere interactions internal to the Indian Ocean are capable of generating distinct modes of climate variability as well. Whether the Indian Ocean can feedback onto Atlantic and Pacific climate has been an on-going matter of debate. We are now beginning to realize that the tropics, as a whole, are a tightly inter-connected system, with strong feedbacks from the Indian and Atlantic Oceans onto the Pacific. These two-way interactions affect the character of ENSO and Pacific decadal variability and shed new light on the recent hiatus in global warming.

Here we review advances in our understanding of pantropical interbasins climate interactions with the Indian Ocean and their implications for both climate prediction and future climate projections. ENSO events force changes in the Indian Ocean than can feed back onto the Pacific. Along with reduced summer monsoon rainfall over the Indian subcontinent, a developing El Niño can trigger a positive Indian Ocean Dipole (IOD) in fall and an Indian Ocean Basinwide (IOB) warming in winter and spring. Both IOD and IOB can feed back onto ENSO. For example, a positive IOD can favor the onset of El Niño, and an El Niño–forced IOB can accelerate the demise of an El Niño and its transition to La Niña. These tropical interbasin linkages however vary on decadal time scales. Warming during a positive phase of Atlantic Multidecadal Variability over the past two decades has strengthened the Atlantic forcing of the Indo-Pacific, leading to an unprecedented intensification of the Pacific trade winds, cooling of the tropical Pacific, and warming of the Indian Ocean. These interactions forced from the tropical Atlantic were largely responsible for the recent hiatus in global surface warming.

Climate modeling studies to address these issues are unfortunately compromised by pronounced systematic errors in the tropics that severely suppress interactions with the Indian and Pacific Oceans. As a result, there could be considerable uncertainty in future projections of Indo-Pacific climate variability and the background conditions in which it is embedded. Projections based on the current generation of climate models suggest that Indo-Pacific mean-state changes will involve slower warming in the eastern than in the western Indian Ocean. Given the presumed strength of the Atlantic influence on the pantropics, projections of future climate change could be substantially different if systematic model errors in the Atlantic were corrected. There is hence tremendous potential for improving seasonal to decadal climate predictions and for improving projections of future climate change in the tropics though advances in our understanding of the dynamics that govern interbasin linkages.

How to cite: Lengaigne, M. and the Coauthors: Interactions of the Indian Ocean climate with other tropical oceans, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2707, https://doi.org/10.5194/egusphere-egu2020-2707, 2020.

EGU2020-2486 | Displays | OS1.10

IndOOS-2: A Roadmap to Better Observations of the Rapidly Warming Indian Ocean

Michael J. McPhaden, Lisa M. Beal, Jerome Vialard, Matthew K. Roxy, and Authors of the CLIVAR-GOOS IndOOS-2 Report

Over the last decade the Indian Ocean has absorbed 60% of the global oceanic heat uptake and the fate of this heat and its impact on future change is unknown. Projections foresee accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. Almost two-thirds of humanity live around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture. Coastal population growth is conflating with climate change to further increase exposure and vulnerability of these populations. The Indian Ocean observing system (IndOOS), established in 2006, is a multi-national network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. However, gaps in the IndOOS have so far limited forecasting efforts, left large discrepancies in the basin energy budget, and kept us in the dark about ecosystem stressors. A three-year, international review of the IndOOS by more than 60 scientific experts provides a roadmap to an improved observing network — IndOOS-2 — that can better meet future scientific and societal challenges. Core findings include the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) better resolved upper-ocean processes to yield improved sub-seasonal to seasonal forecasts; 3) expansion into the western Arabian Sea; and 4) expansion into key coastal regions and the deep ocean to better constrain heat and freshwater changes. IndOOS-2 will require new resources and partnerships, creating opportunities for Indian Ocean rim countries to enhance their monitoring and forecasting capacity as part of a growing Global Ocean Observing System community.

How to cite: McPhaden, M. J., Beal, L. M., Vialard, J., Roxy, M. K., and CLIVAR-GOOS IndOOS-2 Report, A. O. T.: IndOOS-2: A Roadmap to Better Observations of the Rapidly Warming Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2486, https://doi.org/10.5194/egusphere-egu2020-2486, 2020.

EGU2020-1582 | Displays | OS1.10

Could an ocean climate atlas generated by a model compete with an observational one?

Georgy I. Shapiro, Jose M. Gonzalez-Ondina, Xavier Francis, Hyee S. Lim, and Ali Almehrezi

Modern numerical ocean models have matured over the last decades and are able to provide accurate fore- and hind-cast of the ocean state. The most accurate data could be obtained from the reanalysis where the model run in a hindcast mode with assimilation of available observational data. An obvious benefit of model simulation is that it provides the spatial density and temporal resolution which cannot be achieved by in-situ observations or satellite derived measurements. It is not unusual that even a relatively small area of the ocean model can have in access of 100,000 nodes in the horizontal, each containing vertical profiles of temperature, salinity, velocity and other ocean parameters with a temporal resolution theoretically as high as a few minutes. Remotely sensed (satellite) observations of sea surface temperature can compete with the models in terms of spatial resolution, however they only produce data at the sea surface not the vertical profiles. On the other hand, in-situ observations have a benefit of being much more precise than model simulations. For instance a widely used CTD profiler SBE 911plus has accuracy of about 0.001 °C, which is not achievable by models.

In the creation of a climatic atlas the higher accuracy of individual profiles provided by in-situ measurements may become less beneficial. Assuming the normal distribution of data at each location, the standard error of the mean (SEM) is calculated as SE=S/SQRT(N), where S is the standard deviation of individual data points around the mean, and N is the number of data points. The climatic data are obtained by averaging a large number of individual data points, and here the benefit of having more data points may become a greater advantage than the accuracy of a single observation.  

In this study we have created an ocean climate atlas for the northern part of the Indian Ocean including the Red Sea and the Arabian Gulf using model generated data. The data were taken from Copernicus Marine Environment Monitoring Service (CMEMS) reanalysis product GLOBAL_REANALYSIS_PHY_001_030 with 1/12° horizontal resolution and 50 vertical levels for the period 1998 to 2017. The model component is the NEMO platform driven at the surface by ECMWF ERA-Interim reanalysis. The model assimilates along track altimeter data, satellite Sea Surface Temperature, as well as in-situ temperature and salinity vertical profiles where available. The monthly data from CMEMS were then averaged over 20 years to produce an atlas at the surface, 10, 20, 30, 75, 100, 125, 150, 200, 250, 300, 400, and 500 m depths.  The standard error of the mean has been calculated for each point and each depth level on the native grid (1/12 degree).

The atlas based on model simulations was compared with the latest version of the World Ocean Atlas (WOA)  2018 published by the NCEI.  WOA has objectively analysed climatological mean fields on a ¼  degree grid. The differences between the mean values and SEMs from observational and simulated atlases are analysed, and the potential causes of mismatch are discussed.

How to cite: Shapiro, G. I., Gonzalez-Ondina, J. M., Francis, X., Lim, H. S., and Almehrezi, A.: Could an ocean climate atlas generated by a model compete with an observational one?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1582, https://doi.org/10.5194/egusphere-egu2020-1582, 2020.

EGU2020-513 | Displays | OS1.10

A regional study of Bay of Bengal processes using radiation boundary condition in Modular Ocean Model
not presented

Siddhesh Tirodkar, Manasa Ranjan Behera, and Sridhar Balasubramanian

The ocean exchanges heat and mass with the atmosphere in form of shortwave and longwave radiations, precipitation, and evaporation. The regional scale ocean processes governed by this exchange play a vital role in modulating the local dynamics of the Indian Ocean. For instance, the meso-scale eddies and waves control the ocean vertical temperature structure, mixed layer depth, and the thermocline. The Indian Ocean Observing System (IndOOS) recommends the need of proper understanding of heat budget in the Indian Ocean to resolve the mesoscale and submesoscale processes, which trigger large scale ocean circulation, cyclonic eddies, plumes etc. In a regional domain, the stability of ocean also depends on the local parameters, namely, wind pattern, precipitation, runoff and exchange of heat and mass fluxes near the domain boundary. The main objective of this study is to understand the effect of atmospheric wind and solar radiation on the ocean surface and sub-surface characteristics using Modular Ocean Model (MOM5).

A regional domain in the Bay of Bengal (BoB) is selected, which has unique features, such as, large amount of freshwater flux, seasonal wind reversal and high amount of solar radiation due the geographic location. The dynamics in BoB is important for understanding the Indian summer and winter monsoon seasons and associated weather patterns. A regional ocean modelling approach is adopted using MOM5 with horizontal grid resolution (0.250) while maintaining the vertical grid-size as 1m near the surface region which increases with depth. For the regional domain, radiation open boundary condition (OBC) is implemented on three lateral boundaries of domain, based on the technique proposed by Orlanski (1976). The OBC at the lateral boundaries help in smooth exchange of current and tracers. K-profile parameterization (KPP) vertical mixing scheme is used that accounts for effects of shear, wave breaking, and double diffusion. The model is started from a state of rest and simulated for a period of 10 years using 6-hourly solar radiation (Japanese 25-year reanalysis (JRA-25)) and daily averaged wind stress (SODA reanalysis) dataset. After five years of model spin-up, the last five years of simulated output is considered to ensure consistency of model results. Heat budget calculation shows good agreement with WHOI OA Air-Sea Fluxes (OAFlux). Smooth exchange of mass and fluxes is observed near boundary, which confirms successful implementation of OBC. Implementation of KPP scheme enhances mixing in the upper ocean layers with more realistic thermocline formation and turbulent kinetic energy (TKE). The model is able to mimic the seasonal variability in the ocean currents enforced due to winds. The Sea Surface Temperature (SST) is in good agreement with SODA reanalysis data.

A plume like mesoscale feature in the SST plot is captured in the present study (that is also observed in microwave SST), but found to be missing in earlier BoB study with sponge boundary conditions. Finer scale resolution (0.1250) study is in progress, which is expected to show secondary mesoscale structures and their evolution. The results from this study would help in better understanding of the influence regional-scale processes on local ocean dynamics.

How to cite: Tirodkar, S., Behera, M. R., and Balasubramanian, S.: A regional study of Bay of Bengal processes using radiation boundary condition in Modular Ocean Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-513, https://doi.org/10.5194/egusphere-egu2020-513, 2020.

EGU2020-7886 | Displays | OS1.10

The influence of water temperature-phytoplankton feedback in a Regional Earth System Model upon the hydrography and biogeochemistry of the northern Indian Ocean

Stanislav D. Martyanov, Dmitry V. Sein, Vladimir A. Ryabchenko, Anton Y. Dvornikov, and Pankaj Kumar

Within the Coordinated Regional Climate Downscaling Experiment (CORDEX) framework, a high-resolution Regional Earth System Model (RESM) was used to understand the effects of various parameterizations of the attenuation of short-wave radiation (SWR) penetrating into the ocean. The RESM comprises of the Max Planck Institute Ocean Model and the Hamburg Ocean Carbon Cycle model (MPIOM/HAMOCC) coupled via the OASIS coupler to the Regional atmosphere Model (REMO), and the Hydrological Discharge model (HD). Two runs of the RESM for the historical period 1950-2017 were performed. In the first run, the model utilized a simple light attenuation parameterization based on the Jerlov water types when the attenuation coefficient varies spatially depending on the water type but does not vary in time. In the second run, the feedback between the ocean and atmosphere through the marine ecosystem was implemented by using the parameterization of light attenuation coefficient as the function of not only water attenuation itself but also phytoplankton concentration, which was implemented in both the physical and biogeochemical model blocks. The obtained model results correspond well to the observed climatic characteristics. In the calculation with phytoplankton-dependent light attenuation parameterization, the average SST was lower than in the case of Jerlov-based parameterization. The greatest difference in SST (more than 1 °C) occurs in the spring and summer periods during the phytoplankton bloom. The SST differences in autumn and winter are less pronounced and do not exceed 0.2 °C and 0.6 °C, respectively. Also, during the period of intensive heating (spring and summer) the SWR in the ocean upper layers calculated by the feedback-based model run is more strongly absorbed in these layers and, as a result, a significant cooling of subsurface layers (25-200 m) occur (up to 1-1.5 ° С). The phytoplankton primary production and its dispersion in the feedback-based model run turned out to be higher, especially during the periods of winter and summer blooms, and the surface concentration of dissolved nitrates was lower than in the reference run (Jerlov-based parameterization) almost the whole year.

The work was supported by the Russian Science Foundation (Project 19-47-02015) and by the grant DST/INT/RUS/RSF/P-33/G of the Department of Science and Technology, Govt. of India.

How to cite: Martyanov, S. D., Sein, D. V., Ryabchenko, V. A., Dvornikov, A. Y., and Kumar, P.: The influence of water temperature-phytoplankton feedback in a Regional Earth System Model upon the hydrography and biogeochemistry of the northern Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7886, https://doi.org/10.5194/egusphere-egu2020-7886, 2020.

EGU2020-19919 | Displays | OS1.10

Importance of wind variations and intersecting waveguides near Sri Lanka for the intraseasonal sea level variability along the west coast of India

Iyyappan Suresh, Jerome Vialard, Matthieu Lengaigne, Takeshi Izumo, and Muraleedharan Pillathu Moolayil

Remote wind forcing plays a strong role in the Northern Indian Ocean, where oceanic anomalies can travel long distances within the coastal waveguide. Previous studies for instance emphasized that remote equatorial forcing is the main driver of the sea level and currents intraseasonal variability along the west coast of India (WCI). Until now, the main pathway for this connection between the equatorial and coastal waveguides was thought to occur in the eastern equatorial Indian Ocean, through coastal Kelvin waves that propagate around the Bay of Bengal rim and then around Sri Lanka to the WCI. Using a linear, continuously stratified ocean model, the present study demonstrates that two other mechanisms in fact dominate. First, the equatorial waveguide also intersects the coastal waveguide at the southern tip of India and Sri Lanka, creating a direct connection between the equator and WCI. Rossby waves reflected from the eastern equatorial Indian Ocean boundary indeed have a sufficiently wide meridional scale to induce a pressure signal at the Sri Lankan coast, which eventually propagates to the WCI as a coastal Kelvin wave. Second, local wind variations in the vicinity of Sri Lanka generate strong intraseasonal signals, which also propagate to the WCI along the same path. Sensitivity experiments indicate that these two new mechanisms (direct equatorial connection and local wind variations near Sri Lanka) dominate the WCI intraseasonal sea level variability, with the “classical” pathway around the Bay of Bengal only coming next. Other contributions (Bay of Bengal forcing, local WCI forcing) are much weaker.

We further show that the direct connection between the equatorial waveguide and WCI is negligible at seasonal timescale, but not at interannual timescales where it contributes to the occurrence of anoxic events. By providing an improved understanding of the mechanisms that control the WCI thermocline and oxycline variability, our results could have socio-economic implications for regional fisheries and ecosystems.

How to cite: Suresh, I., Vialard, J., Lengaigne, M., Izumo, T., and Pillathu Moolayil, M.: Importance of wind variations and intersecting waveguides near Sri Lanka for the intraseasonal sea level variability along the west coast of India , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19919, https://doi.org/10.5194/egusphere-egu2020-19919, 2020.

EGU2020-6513 | Displays | OS1.10

The characteristics of the mid-depth striations in the North Indian Ocean

Yan Du, Yifan Xia, Bo Qiu, Xuhua Cheng, Tianyu Wang, and Qiang Xie

Argo trajectory data is used to estimate the velocities of mid-depth (1000db) currents in the North Indian Ocean (NIO). Based on these estimated velocities rather than an assumed level of “no motion”, the structure of upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles. The derived flow field reveals that eastward zonal velocities have a striation-like structure in the Arabian Sea, while barely observed in the Bay of Bengal. The striation-like structure is most prominent in the layer from 500db, with a meridional scale of about 300km. Both the meridional scale and the distribution of these mid-depth striations are unique as compared to the other ocean basins. The nonlinear 1 1/2 -layer reduced gravity model and the baroclinic Rossby wave triad interaction theory capture the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared to the Pacific Ocean, the narrower meridional scale in the NIO is because of the smaller basin scale in the equatorial zone rather than semiannual wind stress forcing period or slope of the eastern boundary. Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea.

How to cite: Du, Y., Xia, Y., Qiu, B., Cheng, X., Wang, T., and Xie, Q.: The characteristics of the mid-depth striations in the North Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6513, https://doi.org/10.5194/egusphere-egu2020-6513, 2020.

EGU2020-21178 | Displays | OS1.10

Variability of Sea Surface Salinity in the Southeastern Arabian Sea: Driving mechanisms and influence on the Arabian Sea mini Warm Pool

Akhil Valiya Parambil, Matthieu Lengaigne, Jerome Vialard, Krishnamohan Krishnapillai Sukumarapillai, and Keerthi Madhavan Girijakumari

With sea surface temperatures (SST) exceeding 30˚C in May, the southeastern Arabian Sea (SEAS) hosts one of the warmest open ocean region globally, which appears to play an important role in the summer monsoon onset. Freshwater input from the Bay of Bengal precede the SEAS warm pool build-up by a few months, and are believed to influence its temperature through its impact on oceanic stability and vertical mixing of heat. SSS interannual variations in the SEAS region have not been extensively described before, and their potential feedback on the warm pool build-up and the monsoon are still debated. In the present study, we describe the SEAS SSS seasonal and interannual variability, its driving mechanisms and potential impact on the monsoon. To that end, we analyse experiments performed with a regional 25-km ocean model, both forced and coupled to a regional atmospheric model. The forced and coupled simulations both reproduce the main oceanic features in the SEAS region, including the salinity seasonal cycle and interannual variability. Winter salinity stratification inhibits the vertical mixing of heat, thereby warming the mixed layer by ~0.5°C.month-1. This salinity-induced warming is however compensated by a salinity-induced cooling by air-sea fluxes. Salinity stratification indeed yields a thinner mixed layer which is more efficiently cooled by negative surface heat fluxes at this season. Overall, salinity has thus a negligible impact on the SST seasonal cycle. SEAS SSS interannual variations are largely remotely driven by the Indian Ocean Dipole (IOD), an indigenous interannual climate mode in the equatorial Indian Ocean. The IOD remotely impacts coastal currents along the Indian coastline, and hence modulates freshwater transport from the Bay of Bengal into the SEAS. This yields positive SSS anomalies in the SEAS during the boreal winter that follows positive IOD events. Those SSS anomalies however do not appear to significantly alter the interannual surface layer heat budget. Coupled model sensitivity experiments, in which the influence of haline stratification on vertical mixing is neglected, further confirm that the SEAS winter freshening does not significantly influence the SEAS warm-pool build-up nor the monsoon onset

How to cite: Valiya Parambil, A., Lengaigne, M., Vialard, J., Krishnapillai Sukumarapillai, K., and Madhavan Girijakumari, K.: Variability of Sea Surface Salinity in the Southeastern Arabian Sea: Driving mechanisms and influence on the Arabian Sea mini Warm Pool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21178, https://doi.org/10.5194/egusphere-egu2020-21178, 2020.

EGU2020-11181 | Displays | OS1.10

Drivers of oceanic exchange through the Indonesian Throughflow since the late 1800s – a synthesis of coral δ18O and high-resolution ocean models

Sujata Murty, Caroline Ummenhofer, Markus Scheinert, Erik Behrens, Arne Biastoch, and Claus Böning

The Indonesian Throughflow (ITF) serves as an important oceanic teleconnection for Indo-Pacific climate, altering heat and buoyancy transport from the Pacific to the Indian Ocean. Equatorial Pacific wind forcing transmitted through the ITF impacts interannual to interdecadal Indian Ocean thermocline depth and heat content, with implications for preconditioning Indian Ocean Dipole events. Yet the modulation of Indian Ocean thermal properties at seasonal timescales is still poorly understood. Here we synthesize coral δ18O records, instrumental indices (El Niño Southern Oscillation (ENSO), Asian Monsoon), and simulated ocean variability (sea surface salinity (SSS) and temperature (SST), heat content, mixed layer depth) from state-of-the-art NEMO ocean model hindcasts to explore drivers of seasonal to multi-decadal variability. All coral sites are located within main ITF pathways and are influenced by monsoon-driven, buoyant South China Sea (SCS) surface waters during boreal winter that obstruct surface ITF flow and reduce heat transport to the Indian Ocean. Makassar and Lombok Strait coral δ18O co-varies with simulated SSS, subsurface heat content anomalies (50-350m) and mixed layer depth at the coral sites and in the eastern Indian Ocean. At decadal timescales, simulated boreal winter ocean variability at the coral sites additionally indicates a potential intensification of the SCS buoyancy plug from the mid- to late-20th century. Notably, the variability in these coral and model responses reveals sensitivity to phase changes in the Interdecadal Pacific Oscillation and the East Asian Winter Monsoon. These results collectively suggest that the paleoproxy records are capturing important features of regional hydrography and Indo-Pacific exchange, including responses to regional monsoon variability. Such proxy-model comparison is critical for understanding the drivers of variability related to changes in ITF oceanic teleconnections over the 19th and 20th centuries.

How to cite: Murty, S., Ummenhofer, C., Scheinert, M., Behrens, E., Biastoch, A., and Böning, C.: Drivers of oceanic exchange through the Indonesian Throughflow since the late 1800s – a synthesis of coral δ18O and high-resolution ocean models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11181, https://doi.org/10.5194/egusphere-egu2020-11181, 2020.

EGU2020-12313 | Displays | OS1.10

Interannual variability of the Eastern Indian Ocean with focus on the Ningaloo Niño and negative Indian Ocean Dipole event in 2010/2011

Svenja Ryan, Caroline Ummenhofer, Glen Gawarkiewicz, Patrick Wagner, Markus Scheinert, Arne Biastoch, and Claus Böning

The dominant mode of sea surface temperature (SST) variability in the southeast Indian Ocean off the coast of Western Australia is called Ningaloo Niño/Niña. An unprecedented Ningaloo Niño, or marine heatwave, occurred during the austral summer of 2010/2011 with mean SSTs at 3°C above the long-term mean and had drastic impacts on the ecosystem. This event was attributed to a combination of an anomalous strong Leeuwin Current and high local air-sea heat fluxes. A number of local and remote forcing mechanisms have been investigated in recent years, however, little is known about the depth-structure of these ocean extremes and their general connections to large-scale ocean interannual to decadal variability. Using a suite of simulations with a high-resolution global Ocean General Circulation Model from 1958-2016, we investigate eastern Indian Ocean variability with focus on Ningaloo Niño and corresponding cold Ningaloo Niña events. In particular, we are interested in the impacts of large-scale ocean and climate variability, such as the Indonesian Throughflow, El Niño - Southern Oscillation and the Indian Ocean Dipole (IOD), on the study region. Spatial composites reveal large-scale surface and subsurface anomalies that extend from the western Pacific across the Indonesian Archipelago into the tropical eastern Indian Ocean. In particular, strong anomalies in temperature, salinity and mixed layer depth are found to the west of Sumatra and Java, a region that is generally strongly impacted by the IOD. We therefore investigate the connection with Ningaloo Niño/Niña events, at surface and subsurface, with a focus on 2010/2011 where a strong negative IOD event occurred prior to the unprecedented Ningaloo Niño. Furthermore, we find that major heatwaves in 2000 and 2011 are associated with pronounced fresh anomalies. Sensitivity experiments allow us to assess the relative role of buoyancy and wind-forcing as drivers of the observed patterns. Our work can provide valuable contributions for advancing the understanding of Ningaloo Niño/Niña drivers from surface to depth and regional to large scales.

How to cite: Ryan, S., Ummenhofer, C., Gawarkiewicz, G., Wagner, P., Scheinert, M., Biastoch, A., and Böning, C.: Interannual variability of the Eastern Indian Ocean with focus on the Ningaloo Niño and negative Indian Ocean Dipole event in 2010/2011, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12313, https://doi.org/10.5194/egusphere-egu2020-12313, 2020.

EGU2020-2381 | Displays | OS1.10

Interannual monsoon wind variability over the South tropical Indian ocean drives East African small pelagic fisheries

Fatma Jebri, Zoe Jacobs, Dionysios Raitsos, Meric Srokosz, Stuart Painter, Stephen Kelly, Mike Roberts, Lucy Scott, Sarah Taylor, Matthew Palmer, Hellen Kizenga, Yohana Shaghude, Juliane Wihsgott, and Ekaterina Popova

Small pelagic fisheries play a critical role in food security and economic stability for East African coastal communities ― a region of least developed countries. Using satellite and field observations together with modelling, we show the links between the small pelagic fisheries along the East African coast and the changes in Western Indian Ocean currents due to the interannual variability of the monsoonal wind field. The annual variations in phytoplankton biomass and fisheries yield are strongly associated. During the Northeast monsoon, the enhanced phytoplankton biomass is triggered by local wind-driven upwelling. During the Southeast monsoon, however, the enhanced phytoplankton biomass is due to two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of waters with higher nutrient concentrations. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. Interannually, an extreme increase (decrease) in chlorophyll concentrations is induced by strengthened (weakened) surface currents, which occur during anomalously “strong” (“weak”) Southeast monsoon years. For years where the effects of El Niño / La Niña are weak, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. Such changes have important implications for the predictability of fisheries yield, its response to climate change, policy and resource management.  

How to cite: Jebri, F., Jacobs, Z., Raitsos, D., Srokosz, M., Painter, S., Kelly, S., Roberts, M., Scott, L., Taylor, S., Palmer, M., Kizenga, H., Shaghude, Y., Wihsgott, J., and Popova, E.: Interannual monsoon wind variability over the South tropical Indian ocean drives East African small pelagic fisheries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2381, https://doi.org/10.5194/egusphere-egu2020-2381, 2020.

EGU2020-2678 | Displays | OS1.10

Millennial scale monsoon variability in the northeastern Arabian Sea: A multiproxy approach

Andreas Lückge, Jeroen Groeneveld, Martina Hollstein, Mahyar Mohtadi, Enno Schefuß, and Stephan Steinke

The Dansgaard-Oeschger oscillations and Heinrich events described in Greenland ice core records are also expressed in the climate of the tropical realm as for instance documented in Arabian Sea sediments. However, little is known about these fluctuations beyond the reach of the Greenland ice cores. Here, we present high-resolution organic- and inorganic geochemical, sedimentological as well as micropaleontological data from two cores retrieved off the coast of Pakistan, extending the monsoon record to the past 200,000 years in millennial scale resolution.

The stable oxygen isotope (δ18O) record of the planktic foraminifera G. ruber shows a strong correspondence to Greenland ice core δ18O, whereas the deepwater δ18O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Strong shifts in benthic δ18O during stadials are interpreted to show frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. Alkenone-derived SSTs vary between 23 and 28°C. Highest temperatures were encountered during interglacial MIS 5. Millennial scale SST changes of 2°C magnitude are modulated by long-term SST fluctuations. Interstadials (of glacial phases) and the cold phases of interglacials are characterized by sediments enriched in organic carbon (TOC) whereas sediments with low TOC contents appear during stadials. Abrupt shifts (50-60 year duration) at climate transitions, such as interstadial inceptions, correlate with changes in productivity-related and anoxia-indicating proxies. Interstadial inorganic data consistently show that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. In contrast, stadials are characterized by an increased contribution of aeolian dust probably from the Arabian Peninsula. Heinrich events are especially dry and dusty, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.

These results strengthen the evidence that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, and had a massive impact on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.

How to cite: Lückge, A., Groeneveld, J., Hollstein, M., Mohtadi, M., Schefuß, E., and Steinke, S.: Millennial scale monsoon variability in the northeastern Arabian Sea: A multiproxy approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2678, https://doi.org/10.5194/egusphere-egu2020-2678, 2020.

EGU2020-9188 | Displays | OS1.10

Frontal variability and its impact on chlorophyll in the Arabian Sea

Yuntao Wang, Wentao Ma, Feng Zhou, and Chai Fei

Sixteen years satellite observations are used to investigate the frontogenesis, frontal variability and its impact on chlorophyll in the Arabian Sea. Large frontal probability (FP) and high chlorophyll mainly happens near the coast, e.g., near Somalia and Oman, and its value generally decreases with offshore distance. An Empirical Orthogonal Function (EOF) is used to disentangle the spatial and temporal variability of front and chlorophyll. Prominent seasonal cycle of frontal activities is identified, peaking in summer when southwest wind prevails. The seasonality for chlorophyll is same with wind and front near Somalia, which largely impacted by monsoon. During summer, the southwest monsoon drives offshore Ekman transport and induces coastal upwelling. It transports subsurface cold water and nutrients to the surface layer, which generates fronts and enhances chlorophyll, respectively. The frontal activities can be used as an indicator to determine the chlorophyll level that high chlorophyll happens when frontal probability gets higher than 2%. At anomalous field, stronger wind can induce higher frontal activities and chlorophyll. The impact of wind on frontogenesis can extend 1,000km offshore and a simplified linear regression is applied to quantify their relationship. The variability of wind leads chlorophyll by lags increasing with distance, indicating a horizontal offshore transport of coastal water. In winter, the northeast wind is not upwelling favorable, thus the frontal activities and chlorophyll are greatly reduced off Somalia. During this period, large chlorophyll is found in the north off Oman because of mixing, thus its relationship with front is less pronounced. In the upwelling regions, fronts act as an intermedia process that connecting the wind forcing and responses of ecosystem. The frontal activities in Arabian Sea is fundamentally important to improve our understanding of monsoon related ocean dynamics.

How to cite: Wang, Y., Ma, W., Zhou, F., and Fei, C.: Frontal variability and its impact on chlorophyll in the Arabian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9188, https://doi.org/10.5194/egusphere-egu2020-9188, 2020.

EGU2020-12390 | Displays | OS1.10

Seasonality of Submesoscale processes in the Bay of Bengal

Lanman Li and Xuhua Cheng

Mesoscale eddies that known as a dominant reservoir of kinetic energy has been studied extensively for its dynamics and variation.In order to maintain energy budget equilibrium,the energy stored in mesoscale eddies is dissipated by small scale processes around centimeters.Submesoscale processes that lie between mesoscale and microscale motions effectively extract energy from mesoscale motions and transfer to smaller scales.The Bay of Bengal(the BOB) receives large fresh water from precipitation and river runoff resulting in strong salinity fronts that conducive to the generation of submesoscale processes.Using the Regional Ocean Modeling System(ROMS) data with two horizontal resolutions:a high-resolution(~1.6km) that is partially resolve submesoscale,and a low-resolution(~7km) that not resolves submesoscale,we focus on the seasonality of submesoscale processes in the Bay of Bengal.To ensure that only the submesoscale motions is considered,we choose 40km as the length to separate submesoscale from the flow field.Results show that submesocale processes is ubiquitous in the BOB,mainly trapped in the mixed layer.As resolution increasing,submesoscale motions become much stronger.Seasonality of submesoscale in the BOB is apparent and is different from the Gulf stream region  which is strongest in winter and weakest in summer.Submesoscale features in this region mostly present in fall,which the most important mechanisms is frontogenesis due to strong horizontal buoyancy flux associated with large strain.Submesoscale motions is also vigorous in winter.The proposed mechanism is that the depth of mixed layer is deep enough which contributes to the occurrence of mixed layer instability.During the whole year,mesoscale strain field is weakest in summer,which makes submesoscale weakest.

How to cite: Li, L. and Cheng, X.: Seasonality of Submesoscale processes in the Bay of Bengal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12390, https://doi.org/10.5194/egusphere-egu2020-12390, 2020.

EGU2020-3793 | Displays | OS1.10

Capturing the 2018 Monsoon Onset in the Bay of Bengal from in-situ ship and mooring network observations

Amit Tandon, Emily Shroyer, Ramasamy Venkatesan, Andrew Lucas, J. Thomas Farrar, and Michael McPhaden

Air-Sea interaction in the Bay of Bengal has a strong coupling with the Monsoon rains over the South Asian region. The wet and dry spells, or active-break cycles of the Asian summer monsoon are governed by different modes of intra-seasonal variability with implied northward and westward propagation. Multiple hypotheses exist as to how air-sea interaction and the ocean mixed layer influence the propagation of Monsoon Intra-seasonal Oscillations (MISO), but the multi-scale nature of atmosphere-ocean coupling is not well understood. Multi-country collaborative initiatives MISOBOB (Oceanic Control of Monsoon Intra-seasonal Oscillations in the Tropical Indian Ocean and the Bay of Bengal-USA), RIO-MISO (Role of the Indian Ocean on Monsoon Intra-Seasonal Oscillations-USA), and OMM (Ocean Mixing and Monsoons-India) have led to a combination of ocean observations, atmospheric observations, and associated modeling to study this phenomenon.

 

We present observations analyzed using the OMNI (Ocean Moored Buoy Network for Northern Indian Ocean) buoy network of India and RAMA 15N mooring along with MISOBOB field program in June 2018, which captured the onset of the 2018 Monsoon from a heavily instrumented ship that simultaneously made measurements in the atmospheric and oceanic boundary layers. The shortwave and net heat fluxes show dramatic changes during the active phase with the in-situ net heat flux reversing sign. The Monsoon onset cooled all of the Central and North Bay of Bengal by 1.5 K, leading to large heat losses in the Bay, as the oceanic surface mixed layer deepened from 20m to about 40m. This talk will also explore the role of sub-surface salinity stratification in modulating cooling of the upper ocean at multiple locations across the Bay, providing a basin-wide view. Observations suggest that the air-sea interaction and ocean stratification in the Bay likely has strong feedback on the organized convection in the atmosphere.

How to cite: Tandon, A., Shroyer, E., Venkatesan, R., Lucas, A., Farrar, J. T., and McPhaden, M.: Capturing the 2018 Monsoon Onset in the Bay of Bengal from in-situ ship and mooring network observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3793, https://doi.org/10.5194/egusphere-egu2020-3793, 2020.

EGU2020-19411 | Displays | OS1.10

Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations

Takeshi Izumo, Maratt Satheesan Swathi, Matthieu Lengaigne, Jérôme Vialard, and Dr Ramesh Kumar

A strong Low-Level Jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations  only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmosphericdeep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific / North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.

 

Related reference: Swathi M.S, Takeshi Izumo, Matthieu Lengaigne, Jérôme Vialard and M.R. Ramesh Kumar:Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, accepted in Climate Dynamics.

How to cite: Izumo, T., Swathi, M. S., Lengaigne, M., Vialard, J., and Ramesh Kumar, D.: Remote influences on the Indian monsoon Low-Level Jet intraseasonal variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19411, https://doi.org/10.5194/egusphere-egu2020-19411, 2020.

EGU2020-12760 | Displays | OS1.10

Dramatic reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014-2019

Denis Volkov, Michael Rudko, and Sang-Ki Lee

The interannual-to-decadal variability of heat content and sea level in the South Indian Ocean (SIO) is strongly influenced by its connection with the Pacific and large-scale climatic forcing in the Indo-Pacific region primarily associated with El Niño-Southern Oscillation (ENSO). Besides the advection by the Indonesian Throughflow, signals generated in the Pacific can enter the SIO as coastally trapped Kelvin waves and propagate along the coast of Western Australia. In the southeast tropical and subtropical Indian Ocean, these signals along the eastern boundary can radiate westward as Rossby waves and eventually impact sea level and heat content in the SIO interior and near the western boundary. Local wind forcing, through Ekman pumping over the open ocean and coastal upwelling, is also able to generate Rossby waves and/or modify those emanated from the eastern boundary.

As measured by Argo floats and satellite altimetry, a decade-long increase of the upper-ocean heat content and sea level in the SIO in 2004-2013 ended with a remarkable drop returning to the initial values in 2004. This basin-wide heat release was associated with one of the strongest on record El Niño events in 2014-2016. Surprisingly, the basin-averaged heat content and sea level quickly recovered during the weak La Niña event in 2017-2019. Here we present an analysis of the evolution and mechanisms of 2014-2016 cooling and subsequent warming in the SIO subtropical gyre. We show that the 2014-2016 El Niño did contribute to the reduced heat content in the eastern SIO, while the local wind forcing (via increased Ekman upwelling) largely contributed to the heat reduction in the western SIO. We find no evidence to support that the 2017-2018 warming was forced by the weak La Niña, because the upper-ocean heat content in eastern SIO was still below normal during 2016-2018. The recovery largely occurred in the western SIO due to local wind forcing (via increased Ekman downwelling) primarily associated with changes in the strength of the southeasterly trade winds.

Because sea level is a good proxy for the oceanic heat content in the SIO, we extend our analysis back to 1993 using satellite altimetry records. Using a simple model of wind-forced Rossby waves, we estimate the relative contributions of sea level signals propagating from the eastern boundary, the origin of which is strongly linked to ENSO, and the local wind forcing in the SIO interior to the observed sea level variability. The local wind forcing appears to dominate the sea level (and, hence, the upper-ocean heat content) variability in the western SIO, especially in 2013-2019, while the ENSO-related signals are dominant in the eastern SIO. The local wind forcing over the SIO interior effectively suppressed the cooling associated with the most recent 2014-2016 El Niño event. In contrast, the cooling associated with the strongest on record 1997-1998 El Niño was amplified by the local wind forcing in the basin’s interior.

How to cite: Volkov, D., Rudko, M., and Lee, S.-K.: Dramatic reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014-2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12760, https://doi.org/10.5194/egusphere-egu2020-12760, 2020.

EGU2020-21824 | Displays | OS1.10

Latitudinal distributions of 234Th in the upper western Indian Ocean

Suk Hyun Kim, Intae Kim, and Hyunmi Lee

We conducted an onboard measurement of dissolved- and particulate 234Th in seawater of upper Indian Ocean. The study region covers the meridional section of upper (<500 m depth) Indian Ocean (3⁰N to 15⁰S at 67⁰E in July 2017, and 5⁰S to 13⁰S at 60⁰E and 5⁰S to 24⁰S 67⁰E in April 2018). Dissolved and particulate (>1.2 μm) 234Th ranged 0.8 – 2.7 dpm L-1 and 0.05 – 0.7 dpm L-1, respectively. In July 2017, the large deficiency of dissolved 234Th were consistently observed at ~50m depth where the subsurface chlorophyll maximum (SCM) present, along the entire section (5⁰S to 13⁰S). After then, the 234Th/238U were almost ~1 in ≥100 m depths. In contrast, in April 2018, the significant deficits of dissolved 234Th were observed in entire upper water columns, 0 – 200m depths. This difference in distribution patterns between two years appears to be related to the annual-/seasonal- variations of SCM patterns. In 2018, SCM were shown in 70 – 80 m depths near equator (5⁰S degree), and gradually deepens in lower latitude (SCM presents in 130 m depths in 24⁰S). Interestingly, the unusually lowest dissolved 234Th (and very low particulate 234Th also,) were observed in 5⁰S 60⁰E, near the Seychelles–Chagos thermocline ridge (SCTR) region. There are two hypotheses to explain this extremely lower concentrations of 234Th. The one is that the large input of lithogenic particles from SCTR, seems to be due to largest 234Th removal in the water column of extremely shallow area (<300 m of bottom depth). The other is that unusually strong eastward currents (>1 m/s of zonal velocity, based on ADCP observations) can laterally transport the 234Th. In this presentation, we will also present the preliminary results of vertical export fluxes of some particulate trace elements (Al, Fe, Mn, Cu, Zn, Ni, Pb, and etc.) in the upper Indian Ocean estimated by using this 234Th tracer.

How to cite: Kim, S. H., Kim, I., and Lee, H.: Latitudinal distributions of 234Th in the upper western Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21824, https://doi.org/10.5194/egusphere-egu2020-21824, 2020.

EGU2020-20849 | Displays | OS1.10

Meridional Distribution of Surface CO2 along 67°E of the Indian Ocean

Dong-Jin Kang, Sang-Hwa Choi, Daeyeon Kim, and Gyeong-Mok Lee

Surface seawater carbon dioxide was observed from 3 °S to 27 °S along 67 °E of the Indian Ocean in April 2018 and 2019. Partial pressure of CO2(pCO2) in the surface seawater and the atmosphere were observed every two minutes using an underway CO2 measurement system (General Oceanics Model 8050) installed on R/V Isabu. Surface water temperature and salinity were measured as well. The pCO2 was measured using Li-7000 NDIR. Standard gases were measured every 8 hours in five classes with concentrations of 0 µatm, 202 µatm, 350 µatm, 447 µatm, and 359.87 µatm. The fCO2 of atmosphere remained nearly constant at 387 ± 2 µatm, but the surface seawater fCO2 peaked at about 3 °S and tended to decrease toward the north and south. The distribution of fCO2 in surface seawater according to latitude tends to be very similar to that of sea surface temperature. In order to investigate the factors that control the distribution of fCO2 in surface seawater, we analyzed the sea surface temperature, sea surface salinity, and other factors. The effects of salinity are insignificant, and the surface fCO2 distribution is mainly controlled by sea surface temperature and other factors that can be represented mainly by biological activity and mixing.

How to cite: Kang, D.-J., Choi, S.-H., Kim, D., and Lee, G.-M.: Meridional Distribution of Surface CO2 along 67°E of the Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20849, https://doi.org/10.5194/egusphere-egu2020-20849, 2020.

EGU2020-21350 | Displays | OS1.10

Chlorophyll and phytoplankton spatial distribution in the Arabian/Persian Gulf and the Sea of Oman

Igor Polikarpov, Maria Saburova, and Faiza Al-Yamani

Spatial distribution of the chlorophyll-a and phytoplankton community composition related to different water masses were studied during regional cruise in February-March 2006 across the Arabian/Persian Gulf and the Sea of Oman, the marginal seas of the Western Indian Ocean.

Chlorophyll-a concentrations were measured using in vitro method with fluorescence detection and also were assessed as in vivo fluorescence measured by submersible fluorometer. Nearly four hundred species of phytoplankton were enumerated and identified using microscopy in the samples collected at the same stations.

High phytoplankton abundance was associated with diatom-dominated phytoplankton blooms in the central and northwestern part of the Gulf, in the Strait of Hormuz and in the Sea of Oman. The average concentration of in vitro measured surface chlorophyll-a in the studied area was 2.5 mg/m3, with the maximum over 9 mg/m3. The relationships between the concentrations of satellite remotely sensed chlorophyll and in vitro measured chlorophyll-a were found to be mostly in good agreement. The highest concentrations of the surface chlorophyll (> 4 mg/m3) were observed in the areas where diatom-dominated blooms were identified. It was revealed a significant relationship between the phytoplankton composition and water masses indexed by salinity.

The main significance of this study is in the first data set of in vitro measured precise chlorophyll-a concentrations that were obtained along with phytoplankton abundance and taxonomic diversity from the entire region of the Arabian/Persian Gulf and the Sea of Oman. This data set can be used for remote sensing measurements validation and as a baseline for future studies of the biological productivity changes in the Western Indian Ocean.

How to cite: Polikarpov, I., Saburova, M., and Al-Yamani, F.: Chlorophyll and phytoplankton spatial distribution in the Arabian/Persian Gulf and the Sea of Oman, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21350, https://doi.org/10.5194/egusphere-egu2020-21350, 2020.

EGU2020-19117 | Displays | OS1.10

No N2 fixation in the Bay of Bengal?

Carolin Löscher, Wiebke Mohr, Hermann Bange, and Donald Canfield

The Bay of Bengal (BoB) has long stood as a biogeochemical enigma with subsurface waters containing extremely low, but persistent, concentrations of oxygen (O2) in the nanomolar range which - for some, yet unconstrained reason- are prevented from becoming anoxic. One reason for this may be
the low productivity of the BoB waters due to nutrient limitation, and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key to understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N2) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB’s nitrogen input, suggesting a potential role of N2 fixation in those waters.
Here, we present a survey of N2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N2 fixers comparable to other OMZ regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10 m and 560 m water depth). While similar communities of N2 fixers were shown to actively fix N2 in other OMZs, N2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ.
Consistent with this, no N2 fixation signal was visible in δ15N signatures. We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB. To explore how the onset of N2 fixation by cyanobacteria compared to nonphototrophic N2 fixers would impact on OMZ O2 concentrations, a simple model exercise was carried out. We observed that both, photic zone-based and OMZ-based N2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which would exhaust remaining O2 traces in the BoB.       

How to cite: Löscher, C., Mohr, W., Bange, H., and Canfield, D.: No N2 fixation in the Bay of Bengal?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19117, https://doi.org/10.5194/egusphere-egu2020-19117, 2020.

Subantarctic Mode Water (SAMW) is formed by deep mixing in winter in the Subantarctic Zone and transported into the adjacent subtropical gyres after subduction, which plays a vital role in heat, freshwater, carbon and nutrient budgets in the global oceans. The changes in SAMW properties and its impact on spiciness variation in the southern Indian Ocean have been investigated using the gridded Argo dataset in 2004-2018. Annual mean potential temperature and salinity of the SAMW have undergone significant variations during 2004-2018, with an increase (a decrease) trend for potential temperature (salinity). An analysis of decomposition shows that the heaving process contributes to warming and salinification while spiciness causes cooling and freshening, both of which modulate the SAMW properties. A strong deepening of the isopycnal surfaces caused by positive wind stress curl anomalies over the subtropical southern Indian Ocean leads to warming/salinification heaving contribution to the changes in SAMW. The cooling/freshening contribution from spiciness process is due to a southward shift of sea surface potential density favoring colder and fresher water into the interior ocean, which is driven by an increase in wintertime sea surface temperature and salinity in the SAMW formation region. The colder and fresher water carried with the SAMW spreads along isopycnal surfaces via the Indian Ocean subtropical gyre, which results in cooling and freshening spiciness trends over the all basin of the subtropical southern Indian Ocean.

 

How to cite: Zhang, Y., Du, Y., and Feng, M.: Changes in Subantartic Mode Water Properties and its Impact on Spiciness Variation in the Southern Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21228, https://doi.org/10.5194/egusphere-egu2020-21228, 2020.

EGU2020-11975 | Displays | OS1.10

Projected future changes of meridional heat transport and heat balance of the Indian Ocean

Jie Ma, Ming Feng, Jian Lan, and Dunxin Hu

An ocean downscaling model product, forced under the RCP8.5 future climate change scenario, has been used to understand the ocean heat balance of the Indian Ocean in a warming climate. Towards the end of the 21th century, the model simulates a significant reduction of Indonesian Throughflow (ITF) transport, which reduces the Pacific to Indian Ocean heat transport by 0.20 PW; whereas across S in the southern Indian Ocean (SIO), the southward heat transport is reduced by 0.28 PW, mainly contributed from the weakening western boundary current, the Agulhas Current (0.21 PW). The projected weakening of the Agulhas Current is to compensate for the reduction of the ITF transport, with additional contribution from the spin-down of the SIO subtropical gyre. Thus, being amplified by the ocean circulation changes in the SIO, the projected Indian Ocean warming trend is much faster than the direct air-sea heat flux input.

How to cite: Ma, J., Feng, M., Lan, J., and Hu, D.: Projected future changes of meridional heat transport and heat balance of the Indian Ocean , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11975, https://doi.org/10.5194/egusphere-egu2020-11975, 2020.

EGU2020-3153 | Displays | OS1.10

The Agulhas Bank Circulation

Ricardo Matano

The southern tip of Africa is the gateway between the Indian and Atlantic oceans, one of the most widely recognized chokepoints of the meridional overturning circulation (MOC). The oceanic circulation in this region connects a western boundary current, the Agulhas Current, to an eastern boundary current, the Benguela Current, a connection not replicated elsewhere and quite important, not only because of its peculiarity, but also because of its role in the MOC. During the last few decades numerous international research programs have collected large amounts of oceanographic data of this region. All these efforts, however, have been largely focused on the deep-ocean, leaving the coastal region practically unattended. In this presentation we will use the results of a suite of process-oriented numerical experiments to discuss the circulation along the Agulhas Bank (AB)—the shelf region sandwiched between the eastern and western margins of the African continent; in particular to illustrate its connections and interactions with the deep-ocean region. As we shall show these shelf/deep-ocean interactions, are not only important to the shelf but also to the Indian/Atlantic interoceanic exchange.

How to cite: Matano, R.: The Agulhas Bank Circulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3153, https://doi.org/10.5194/egusphere-egu2020-3153, 2020.

EGU2020-3889 | Displays | OS1.10

New Micropalaeontological Evidence for an Early Pleistocene Existence of the Agulhas Leakage

Thore Friesenhagen and Michael Knappertsbusch

The Agulhas Leakage is considered to influence the Atlantic Meridional Overturning Circulation and the climate system via transport of saltier and warmer water masses from the Indian Ocean into the Atlantic Ocean (Laxenaire et al. 2018). Therefore, reconstructing the time of the possible onset of the Agulhas Leakage will allow an improvement of global palaeo-climate models. Since the Agulhas Leakage is known to disperse the Indian Ocean biota into the Atlantic Ocean, Caley et al. (2012) proposed that the Agulhas Leakage exists since about 1.3Ma.

Here, we provide new evidence for an early Pleistocene existence of the Agulhas Leakage by comparing the test size evolution of G. menardii between the tropical eastern Atlantic Ocean ODP Site 667 and the Indian Ocean IODP Site 1476, which is located in the Mozambique Channel.

At Site 667 and parallel to a climate cooling trend, we observe a test size decrease from a maximal axial length (max δY) of 875µm during the Mid-Pliocene Warmth Period (3.2Ma) to a maximal axial length of 520µm by the end of Pliocene (ca. 2.6Ma). This trend is followed by a relatively rapid test size increase until ca. 2.1Ma, during which the size more than doubles (max δY = ca. 1200µm). This pattern in the test size evolution of G. menardii was also observed in the western tropical Atlantic Ocean (Knappertsbusch 2016).

In the Mozambique Channel, we do not observe a decrease of the test size at the end of the Pliocene. The values stay almost stable throughout the Pliocene (max δY = ca. 900µm) until 2.3Ma. Between 2.3 and 2Ma, the maximal test size increases to a value very similar to that observed in the eastern tropical Atlantic (max δY = ca. 1250µm).

It has been observed that relatively large G. menardii specimens occurred in the Mozambique Channel, while the Atlantic only harboured relatively small specimens during the late Pliocene and earliest Pleistocene, and that both localities show a similar test size at ca. 2Ma. This suggests the possibility of a dispersal of the Indian Ocean giant G. menardii into the Atlantic between 2.3 and 2Ma, probably via a strengthening Agulhas Leakage.

 

Caley, T., Jiraudeau, J., Malaizé, B., Rossignol, L. & Pierre, C. (2012), ‘Agulhas leakage as a key process in the modes of Quaternary climate changes’, PNAS 109(18), 6835–6839.

Knappertsbusch, M. W. (2016), ‘Evolutionary prospection in the Neogene planktic foraminifer Globorotalia menardii and related forms from ODP Hole 925B (Ceara Rise, western tropical Atlantic): evidence for gradual evolution superimposed by long distance dispersal?’, Swiss Journal of Palaeontology.

Laxenaire, R., Speich, S., Blanke, B., Chaigneau, A., Pegliasco, C. & Stegner, A. (2018), ‘Anticyclonic Eddies Connecting the Western Boundaries of Indian and Atlantic Oceans’, Journal of Geophysical Research: Oceans 123, 7651–7677.

How to cite: Friesenhagen, T. and Knappertsbusch, M.: New Micropalaeontological Evidence for an Early Pleistocene Existence of the Agulhas Leakage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3889, https://doi.org/10.5194/egusphere-egu2020-3889, 2020.

OS1.11 – Changes in the Arctic Ocean, sea ice and subarctic seas systems: Observations, Models and Perspectives

EGU2020-6981 | Displays | OS1.11

The Atlantic Water boundary current North of Svalbard in 2018-2019: background properties, dynamics and turbulence.

Zoé Koenig, Eivind Kolås, Kjersti Kalhagen, and Ilker Fer

North of Svalbard is a key region for the Arctic Ocean heat and salt budget as it is the gateway for one of the main branches of Atlantic Water in the Arctic. As the Atlantic Water layer advances into the Arctic Ocean, its core deepens from about 250 m depth around the Yermak Plateau to 350 m in the Laptev Sea, and gets colder and less saline due to mixing with surrounding waters. The complex topography in the region facilitates vertical and horizontal exchanges between the water masses and, together with strong shear and tidal forcing driving increased mixing rates, impacts the heat and salt content of the Atlantic Water layer that will circulate the Arctic Ocean.

In summer 2018, 6 moorings organized in 2 arrays were deployed across the Atlantic Water Boundary current for a year, within the framework of the Nansen Legacy project. In parallel, turbulence structure in the Atlantic Water boundary current was measured north of Svalbard in two different periods (July and September), using a Vertical Microstructure Profiler (Rockland Scientific) in both cruises and a Microrider (Rockland Scientific) mounted on a Slocum glider in September.

Using mooring observations, we investigated the background properties of the Atlantic Water boundary current (transport, vertical structure, seasonal variations) and the possible sources of the low-frequency variations (period of more than 2 weeks).

Using observations during the cruise periods, we investigated changes in the mixed layer through the summer and the sources of vertical mixing in the water column. In the mixed layer, depth-integrated turbulent dissipation rate is about 10-4 W m-2. Variations in the turbulent heat, salinity and buoyancy fluxes are strong, and hypothesized to be affected by the evolution of the surface meltwater layer through summer. When integrated over the Atlantic Water layer, the turbulent dissipation rate is about 3.10-3 W m-2. Whilst the wind work exerted in the mixed layer accounts for most of the variability in the mixed layer, tidal forcing plays an important role in setting the dissipation rates deeper in the water column.

How to cite: Koenig, Z., Kolås, E., Kalhagen, K., and Fer, I.: The Atlantic Water boundary current North of Svalbard in 2018-2019: background properties, dynamics and turbulence. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6981, https://doi.org/10.5194/egusphere-egu2020-6981, 2020.

EGU2020-20578 | Displays | OS1.11

Ocean role in the winter sea ice openings north of Svalbard

Christophe Herbaut, Marie-Noelle Houssais, and Anne-Cecile Blaizot

The winter trend in the sea ice coverage in the Atlantic sector of the Arctic Ocean has been linked to the Atlantic Water heat transport, providing significant skill to decadal prediction (Yeager et al, 2015, Årthun et al., 2019). The Atlantic Water meets the sea ice north of Svalbard where it has the potential to melt significant amounts of ice and contribute to the formation of a cool, fresh surface layer (Rudels et al., 2004). In this study we investigate the origin of the intra-seasonal variability of winter sea ice melt north of Svalbard and evaluate its contribution to recurrent sea ice openings in this region. 

Based on outputs of a simulation with a high resolution regional ice-ocean model over the period 1995-2017, a number of large, short-term ice melt events could be identified in winter which can contribute up to 40% of the total winter ice melt. Most of these events show enhanced signature along the Atlantic Water path. However different types of events have been established depending on the scenario responsible for enhanced sea ice melt. Enhanced melt can happen concomitantly to large ice edge convergence over preexisting warm surface waters, a scenario which predominates during close-up of large ice openings. Large melt rates can also be driven by entrainment of warm water into the mixed layer in response to strong winds or to enhanced advection of warm water during episodes of increased transport in the boundary current. The latter process is however less efficient than entrainment. We conclude that increased southerly winds, which can sustain altogether ice edge retreat and efficient ice melt through entrainment and advection of heat into the region, create optimal conditions for major ice openings such as those observed north of Svalbard.

How to cite: Herbaut, C., Houssais, M.-N., and Blaizot, A.-C.: Ocean role in the winter sea ice openings north of Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20578, https://doi.org/10.5194/egusphere-egu2020-20578, 2020.

EGU2020-6076 | Displays | OS1.11 | Highlight

Causality and Evolution of Summer Polynyas off the Coast of Northern Greenland

Younjoo Lee, Wieslaw Maslowski, Robert Osinski, Jaclyn Clement Kinney, Anthony Craig, John Cassano, Bart Nijssen, and Mark Seefeldt

The summer polynya along the northern coast of Greenland has been observed only six months later after the winter polynya in 2018, which has prompted concerns about the stability of some of the thickest sea-ice in the Arctic region. This study combines retrospective remotely sensed sea-ice measurements with results from the Regional Arctic System Model (RASM) to examine the causes, effect, and evolution of open-water areas/polynyas in the region.

RASM is a limited-domain, fully-coupled climate model, consisting of the atmosphere (Weather Research and Forecasting, WRF3.7), ocean (Los Alamos National Laboratory Parallel Ocean Program, POP2), sea-ice (Community Sea Ice Model, CICE5), land hydrology (Variable Infiltration Capacity, VIC4) and streamflow routing (RVIC) components. The ocean and sea-ice models are configured with the horizontal resolution of 1/12-degree with 45 vertical levels and 5 sea-ice thickness categories, respectively. The atmosphere and land hydrology components are set up on a 50-km grid with 40-vertical levels and 3-soil layers, respectively. The Climate Forecast System Reanalysis (CFSR) and version 2 (CFSv2) output are used as boundary conditions for dynamic downscaling.

Analysis of the sea-ice conditions off the coast of northern Greenland revealed that RASM, in agreement with satellite measurements, has simulated five summer polynya events, i.e. in August of 1984, 1985, 2002, 2004 and 2018, over the 39-year period (1980-2018). All these events were primarily dynamically forced, with the thermodynamic forcing playing the secondary, yet still important role. While the thermodynamically driven sea-ice melting exhibited a relatively little year-to-year variability, between 87 km3 and 115 km3, its relative contribution to the total sea-ice loss increased by 2.5 times, from 16% in 1984 to 40% in 2018. This implies that with continuing thinning of sea-ice, increasingly less mechanical forcing may be required to generate and maintain a polynya or open water north of Greenland in summers to come.  

How to cite: Lee, Y., Maslowski, W., Osinski, R., Clement Kinney, J., Craig, A., Cassano, J., Nijssen, B., and Seefeldt, M.: Causality and Evolution of Summer Polynyas off the Coast of Northern Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6076, https://doi.org/10.5194/egusphere-egu2020-6076, 2020.

EGU2020-462 | Displays | OS1.11

The impact of wintertime sea-ice retreat on convection in the Nordic Seas

Yue Wu, David Stevens, Ian Renfrew, and Xiaoming Zhai

The Nordic Seas have a significant impact on global climate due to their role in providing dense overflows to the North Atlantic Ocean. However, the dramatic loss of sea ice in recent decades is creating a new atmosphere-ice-ocean environment where large swathes of the ocean that were previously ice-covered are now exposed to the atmosphere. Despite the largest sea-ice loss occurring in summer and autumn, the sea-ice loss in winter and spring is arguably more important for the climate system. Atmosphere-ocean coupling is the most intense in the extended winter, when convective mixing leads to water-mass modification processes, impacting the densest waters of the Atlantic Meridional Overturning Circulation. Here we focus on the marginal-ice-zone of the Nordic Seas where the air-sea temperature difference is large, promoting high heat flux events during periods of off-ice winds. We use both transient and control simulations of the coupled climate model HiGEM, which allows us to isolate the climate change response from the sea-ice retreat response. We find that wintertime sea-ice retreat leads to remarkable changes in ocean surface heat exchanges and wind energy input. As the sea ice edge retreats towards the Greenland coastline, there is a band of exposed ocean which was previously covered by ice. This exposure allows enhanced mechanical mixing by the wind and a greater loss of buoyancy from the ocean leading to deeper vertical mixing in the upper ocean. Sensible and latent heat fluxes from the ocean to the atmosphere provide the greatest loss of buoyancy. However, climate warming inhibits this process as the atmosphere warms more rapidly than the ocean which reduces the sea-air temperature difference. Further away from the retreating ice edge, toward the centre of the Greenland Sea, the upper ocean warms, resulting in a more stratified water column. As a consequence, the depth of convective mixing reduces over the deep ocean and increases over shallower regions close to the coast. This leads to changes in the formation and properties of some of the water masses that enter the North Atlantic and thus may modify the ocean circulation in the subpolar seas in response to sea-ice decline. 

How to cite: Wu, Y., Stevens, D., Renfrew, I., and Zhai, X.: The impact of wintertime sea-ice retreat on convection in the Nordic Seas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-462, https://doi.org/10.5194/egusphere-egu2020-462, 2020.

In the recent few years the topic of accelerated sea ice loss, and related changes in the vertical structure of water masses in the East-Atlantic sector of the Arctic Ocean, including the Barents Sea and the western part of the Nansen Basin, has been in the foci of multiple studies. This region even earned the name the “Arctic warming hotspot”, due to the extreme retreat of sea ice and clear signs of change in the vertical hydrographic structure from the Arctic type to the sub-Arctic one. A gradual increase in temperature and salinity in this area has been observed since the mid-2000s. This trend is hypothetically associated with a general decrease in the volume of sea ice in the Arctic Ocean, which leads to a decrease of ice import in the Barents Sea, salinization, weakening of density stratification, intensification of vertical mixing and an increase of heat and salt fluxes from the deep to the upper mixed layer. The result of such changes is a further reduction of sea ice, i.e. implementation of positive feedback, which is conventionally refereed as the “atlantification. Due to the fact that the Barents Sea is a relatively shallow basin, the process of atlantification might develop here much faster than in the deep Nansen Basin. Thus, theoretically, the hydrographic regime in the northern part of the Barents Sea may rapidly transform to a “Nordic Seas – wise”, a characteristic feature of which is the year-round absence of the ice cover with debatable consequences for the climate and ecosystem of the region and adjacent land areas. Due to the obvious reasons, historical observations in the Barents Sea mostly cover the summer season. Here we present a rare oceanographic data, collected during the late winter - early spring in 2019. Measurements were occupied at four sequential oceanographic surveys from the boundary between the Norwegian Sea and the Barents Sea – the so called Barents Sea opening to the boundary between the Barents Sea and the Kara Sea. Completed hydrological sections allowed us to estimate the contribution of the winter processes in the Atlantic Water transformation at the end of the winter season. Characteristic feature of the observed transformation is the homogenization of the near-to-bottom part of the water column with remaining stratification in the upper part. A probable explanation of such changes is the dominance of shelf convection and cascading of dense water over the open sea convection. In this case, complete homogenization of the water column does not occur, since convection in the open sea is impeded by salinity and density stratification, which is maintained by melting of the imported sea ice in the relatively warm water. The study was supported by RFBR grant # 18-05-60083.

How to cite: Ivanov, V., Frolov, I., and Filchuk, K.: Transformation of Atlantic Water in the Barents Sea in winter: overview of “Transarctika-2019” cruise results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11531, https://doi.org/10.5194/egusphere-egu2020-11531, 2020.

EGU2020-7544 | Displays | OS1.11

Analysis of tidal sea-ice movement using a drifting ice beacon array in the Barents Sea

Amey Vasulkar, Lars Kaleschke, Martin Verlaan, and Cornelis Slobbe

In an experiment to validate an ice forecast and route optimization system, an array of 15 ice drift beacons/buoys were deployed between Edgeøya and Kong Karls Land in the east of Svalbard to measure the sea ice movement. These beacons recorded data at a sampling frequency of 15 minutes in the duration from March 2014 to May 2014 with different start and end dates based on their life. The particularly short time step captures the small scale effect of tides on the drifting ice. In this region of the Barents Sea, the frequency of the inertial motion is very close to the M2 tidal frequency. Hence, it is not possible to extract the tidal motion from the time series data of the buoys by using a Fourier analysis. It is also likely that these effects will interact. Instead, we develop a physics-based free drift ice model that can simulate the drift at all tidal and other frequencies.

The model is forced by winds obtained from the ERA5 Reanalysis dataset of ECMWF and ocean currents obtained from the Global Ocean Analysis product of CMEMS. Due to the effect of tides, the model is also forced by the tides obtained from the Global Tide and Surge Model (GTSM v3.0) which is built upon Delft3D-FM unstructured mesh code. This free drift model is validated against 8 of the 15 beacon trajectories. The model along with the observed data can be then be used to obtain insights on the relationship between the sea ice velocities and the tides. This will be particularly useful to obtain the effect of ice drift on tides in tidal models.

The model uncertainty is mainly due to oceanic and atmospheric drag coefficients, Cdw and Cda, respectively, and the sea ice thickness, hi. This study also focuses on optimizing the ratio of drag coefficients (Cdw/Cda) for the different beacon trajectories while varying the ice thickness between 0.1 m - 1.5 m and the ice-air drag coefficient between (0.5-2.5)x10-3. This ratio facilitates the evaluation of the frictional drag between the ice-water interface and thus, helps in determining the effect of ice on tides in tidal models.

How to cite: Vasulkar, A., Kaleschke, L., Verlaan, M., and Slobbe, C.: Analysis of tidal sea-ice movement using a drifting ice beacon array in the Barents Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7544, https://doi.org/10.5194/egusphere-egu2020-7544, 2020.

EGU2020-8697 | Displays | OS1.11

The long-lived anthropogenic radionuclides I-129 and U-236 as tracers of water mass provenance, circulation timescales and mixing in the Arctic Ocean and Fram Strait

Anne-Marie Wefing, Núria Casacuberta, Marcus Christl, John N. Smith, Paul A. Dodd, Elena Chamizo, Mercedes López-Lora, and Hans-Arno Synal

Anthropogenic chemical tracers are powerful tools to study ocean circulation timescales, water mass provenance and mixing regimes. In the Arctic Ocean, the releases of artificial radionuclides from European nuclear reprocessing plants (RPs) act as valuable transient tracers as they label the inflowing Atlantic Waters with a distinct anthropogenic signal. In recent years, the combination of the two long-lived radionuclides 129I and 236U has emerged as a new tracer pair and several studies have shown their potential to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait.

The circulation times of Atlantic-origin waters in the Arctic Ocean that were inferred using this tracer pair (in combination with the naturally occurring 238U) agree to those obtained by means of other transient tracers. Moreover, the combination of 129I and 236U promises to be a useful marker of water mass mixing regimes both in the surface waters and the subsurface Atlantic layer. In particular, the interface between Atlantic and Pacific Waters in the polar surface layer of the Arctic Ocean can be easily identified as these two water masses are labelled by very different 129I/236U and 236U/238U atom ratios.

Here we present a compilation of 129I and 236U in a quasi-synoptic pan-arctic section including the Fram Strait and we show how this data can be used to gain information about circulation patterns. We discuss timescales and transport characteristics of Atlantic Water flow, the position and variability of the front between Atlantic and Pacific Waters and the temporal variability of Pacific Waters in the Fram Strait.

How to cite: Wefing, A.-M., Casacuberta, N., Christl, M., Smith, J. N., Dodd, P. A., Chamizo, E., López-Lora, M., and Synal, H.-A.: The long-lived anthropogenic radionuclides I-129 and U-236 as tracers of water mass provenance, circulation timescales and mixing in the Arctic Ocean and Fram Strait, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8697, https://doi.org/10.5194/egusphere-egu2020-8697, 2020.

EGU2020-8569 | Displays | OS1.11

On the inter-connectivity of volume transports through Arctic Straits

Agatha De Boer, Estanislao Gavilan Pascual-Ahuir, David Stevens, Léon Chafik, David Hutchinson, Qiong Zhang, Louise Sime, and Andrew Willmott

Arctic heat and freshwater budgets are highly sensitive to volume transports through Arctic-Subarctic straits. Here we investigate how the volume transports through these straits adjust to each other to maintain a mass balance in the Arctic on annual timescales. To this end, we use three models; two coupled global climate models, one with a third-degree horizontal ocean resolution (HiGEM1.1) and one with a twelfth-degree horizontal ocean resolution (HadGEM3), and one ocean-only model with an idealized polar basin (tenth-degree horizontal resolution). The two global climate models indicate that there is a strong anti-correlation between the Bering Strait throughflow and the transport through the Nordic Seas, a second strong anti-correlation between the transport through the Canadian Artic Archipelago (CAA) and the Nordic Seas transport, and a third strong anti-correlation between the Fram Strait and the Barents Sea throughflows. We find that part of the strait correlations is due to the strait transports being coincidentally driven by large-scale atmospheric forcing patterns such as the Arctic Oscillation. However, there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year. Idealized experiments with an ocean model (NEMO3.6) that investigate such causal strait relations suggest that perturbations in the Bering Strait are compensated preferentially in the Fram Strait due to the narrowness of the western Arctic shelf and the deeper depth of the Fram Strait.

How to cite: De Boer, A., Gavilan Pascual-Ahuir, E., Stevens, D., Chafik, L., Hutchinson, D., Zhang, Q., Sime, L., and Willmott, A.: On the inter-connectivity of volume transports through Arctic Straits, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8569, https://doi.org/10.5194/egusphere-egu2020-8569, 2020.

EGU2020-6748 | Displays | OS1.11 | Highlight

Variability of Lagrangian pathways and coherent structures in the Arctic and its effect on the predictability of MOSAiC drift and material transport

Chris Wilson, Stefanie Rynders, Myriel Vredenborg, Stephen Kelly, and Yevgeny Aksenov

Lagrangian particle tracking and associated diagnostics may be used to examine advective pathways of material and to identify coherent structures in the flow.  Lagrangian coherent structures are material transport barriers and act to separate different flow regimes.

The drift of the International Multidisciplinary Observatory for the Study of Arctic Climate (MOSAiC) expedition onboard R/V Polarstern began in October 2019 and will continue for the full year.   Our study has the goals to (i) characterise advective pathways and (ii) examine potential predictability of the MOSAiC drift.  Eddies, jets and boundary currents feature large spatiotemporally varying velocity gradients.  Since operational ocean forecasts have a limited time horizon (~weeks), we focused on hindcast to examine typical sea ice/ocean circulation scenarios for 2005-15.  We applied off-line ARIANE particle tracking in an eddying 1/12 deg. global NEMO sea ice-ocean model to estimate the most likely drift pathways. 

Over 10,000 trajectories were initialised in October each year, started at the best estimated MOSAiC location, advected for one year and analysed for key coherent drift structures.  The advection and deformation of the initial particle cluster provided information about MOSAiC drift predictability, but also elucidated transport processes of the biogeochemical tracers, such as nutrients and carbon, and spread of pollution and microplastics. We analysed observations from a newly curated dataset of the Arctic to examine various watermass properties, their origin, fate and connectivity.

The MOSAiC surface drift trajectories depend on release time and location, but to leading-order, they are governed by the interannual variability of the wind and of the underlying ocean circulation.  Mesoscale flow deformation is linked to a spreading of the cluster of particles and is associated with reduced potential predictability of separation of particles within the cluster (~ 450 km after 12 months).  Gyre-scale flow affects the ensemble drift path over long times and influences whether particular coherent structures are encountered by the particles, their location and strength (in terms of velocity magnitude and gradient).  Saddle-type structures play a major role in bifurcation of particle trajectories.  In the examples studied, saddles north of Nares Strait, near Northern Greenland and Northern Iceland, topologically associated with streamline connectivity between gyres, coastal boundary currents and inflow/outflow at the Arctic gateways, were significant.  On seasonal-interannual scales, the position and strength of the Beaufort Gyre, as well as an anomalous cyclonic gyre in the eastern basin, affected both the ensemble drift path and the coherent flow structures.

The variability of ensemble drift path, cluster deformation and coherent flow structures across the full Arctic basin were often very different from the climatological advective behaviour of Trans-Polar Drift.  For estimation of advective pathways and sea ice drift it is important to consider the varying flow from gyre-scale to mesoscale, where velocity gradients are large, and to identify robust Lagrangian measures for steady features.

The study is supported from NE/R012865/1 (APEAR), part of the Changing Arctic Ocean programme, jointly funded by the UKRI Natural Environment Research Council (NERC) and the German Federal Ministry of Education and Research (BMBF).

 

How to cite: Wilson, C., Rynders, S., Vredenborg, M., Kelly, S., and Aksenov, Y.: Variability of Lagrangian pathways and coherent structures in the Arctic and its effect on the predictability of MOSAiC drift and material transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6748, https://doi.org/10.5194/egusphere-egu2020-6748, 2020.

EGU2020-21849 | Displays | OS1.11

Understanding eddy field in the Arctic Ocean from high-resolution satellite observations

Igor Kozlov, Anastasia Artamonova, Larisa Petrenko, Evgeny Plotnikov, Georgy Manucharyan, and Arseny Kubryakov

The Arctic Ocean is a host to major ocean circulation systems, many of which generate eddies that can transport water masses and corresponding tracers over long distances from their formation sites. However, comprehensive observations of critical eddy characteristics are currently not available and are limited to spatially and temporally sparse in situ observations.

Here we use multi-mission high‐resolution spaceborne synthetic aperture radar (SAR) measurements to detect eddies over open ocean and marginal ice zones (MIZ) of Fram Strait and Beaufort Gyre regions. We provide the first estimate of eddy properties, including their locations, size, vorticity sign and monthly distribution during summer period (from June to October). The results of historical Envisat ASAR observations for 2007 and 2011 are then compared to Sentinel-1 and ALOS-2 PALSAR-2 measurements acquired in 2016 and 2018, to infer the possible changes in the intensity and locations of eddy generation over the last decade.

The most prominent feature of the obtained results is that cyclonic eddies strongly dominate over anticyclones. Eddies range in size between 0.5 and 100 km and are frequently found over the shelf and near continental slopes but also present in the deep basin. For MIZ eddies, the number of eddies clearly depends on sea ice concentration with more eddies detected at the ice edge and over low ice concentration regions. The obtained results clearly show that eddies are ubiquitous in the Arctic Ocean even in the presence of sea ice and emphasize the need for improved ocean observations and modeling at eddy scales.

A special focus is also given to infer eddy dynamics over the Arctic marginal ice zones. The use of sequential Sentinel-1 SAR images enables to retrieve high-resolution velocity field over MIZ on a daily basis and observe eddy-driven MIZ dynamics down to submesoscales. The obtained eddy orbital velocities are in agreement with historical observations and may reach up to 0.5-0.7 m/s. We believe that this information is critical for better understanding of the key dynamical processes governing the MIZ state, as well as for improving and validation of sea ice and coupled ice-ocean models.

The analysis of eddies in this work was supported by RFBR grant 18‐35‐20078. Processing and analysis of Sentinel‐1 and ALOS‐2 Palsar‐2 data were done within RSF grant 18‐77‐00082. Software development for data analysis in this work was made under the Ministry of Science and Higher Education of the Russian Federation contract 0555‐2019‐0001.

How to cite: Kozlov, I., Artamonova, A., Petrenko, L., Plotnikov, E., Manucharyan, G., and Kubryakov, A.: Understanding eddy field in the Arctic Ocean from high-resolution satellite observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21849, https://doi.org/10.5194/egusphere-egu2020-21849, 2020.

EGU2020-13159 | Displays | OS1.11

Cascading and the pathways of the key biogeochemical tracers in the Canadian Basin: from models and observations.

Maria Luneva, Yevgeny Aksenov, Vladimir Ivanov, Stephen Kelly, and Fedor Tuzov

We explore dense water cascading (DWC; a type of bottom-trapped gravity current) on multi-decadal time scales using a pan-Arctic regional ocean-ice model. DWC is particularly important in the Arctic Ocean as the main mechanism of ventilation of interior waters when open ocean convection is blocked by strong density stratification. We identify the locations where the most intense DWC events occur and evaluate the associated cross-shelf mass, heat and salt fluxes. 

 

A detailed analysis of specific cascading sites around the Beaufort Gyre and adjacent regions is performed. We find that autumn upwelling of warm and saltier Atlantic waters on the shelf and subsequent cooling and mixing of uplifted waters trigger the cascading on the West Chukchi Sea shelf break. We also perform Lagragian particle tacking of low salinity Pacific waters originating at the surface in the Bering Strait; these waters are shown to be modified by brine rejection and cooling, and through subsequent mixing become dense enough to reach depths of 160-200m and below. We examine the role of cascading and shelf upwelling on the shelf waters transformation, pathways and spread of the biological important tracers (O18, Si., DIC snd DIN).

How to cite: Luneva, M., Aksenov, Y., Ivanov, V., Kelly, S., and Tuzov, F.: Cascading and the pathways of the key biogeochemical tracers in the Canadian Basin: from models and observations., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13159, https://doi.org/10.5194/egusphere-egu2020-13159, 2020.

EGU2020-11874 | Displays | OS1.11

Summer N₂O dynamics in the western Arctic Ocean : Distributions, Processes, and Fluxes

Seong-Su Kim, Sung-Ho Kang, Eun Jin Yang, and Il-Nam Kim

We collect seawater samples from 32 stations for N2O analysis between August 6 and August 25, during 2017 ARA08B cruise in western Arctic Ocean (WAO), covering from Southern Chukchi Sea (SC) to Northern Chukchi Sea (NC). At surface depth (~50 m), N2O concentrations were 10.9‒19.4 nmol L-1, and distinct pattern was observed between SC and NC. N2O concentrations were increased from surface to bottom (~50 m) at SC, corresponding to positive relationship of ∆N2O (N2Omeasured - N2Oequilibrium) with DIN (NO- + NO2-) and negative relationship between ∆N2O and N*. It suggests that nitrification and denitrification are the main processes to produce N2O at SC. On the other hand, N2O concentration at NC increased from the south to north, and remained vertically constant. It may be the result of physical processes such as dilution by sea ice melting water, and high solubility that affected by low temperature and low salinity. The highest N2O concentrations were observed at intermediate depth (50‒200 m), ranging 13.4‒21.9 nmol L-1. It would be determined by high solubility and active biogeochemical processes synthetically. Concentrations of N2O were rapidly diminished to 400 m, ranging 10.2‒14.1 nmol L-1, and did not be remarkably altered under 400 m, ranging 11.3‒13.7 nmol L-1. It might be affected by advection of Atlantic Water (AW) and existence of Arctic Bottom Water (ABW), and influence of biogeochemical processes was negligible at deep and bottom depth (below 200 m). N2O flux was calculated to determine that the WAO is sources or sinks region for atmospheric N2O. Positive N2O flux was observed at SC, and it indicate that N2O gas is released to atmosphere at SC. Negative value of N2O flux at NC suggest that atmospheric N2O is absorbed into NC. Furthermore, positive relationship of N2O flux with environmental parameters (temperature, salinity, and ∆N2O) also observed in WAO. These results provide comprehensive information of the spatial N2O distribution and main processes which decide N2O distribution in WAO, and also suggest that air-sea N2O flux could be affected by changing environments of the Arctic Ocean.

How to cite: Kim, S.-S., Kang, S.-H., Yang, E. J., and Kim, I.-N.: Summer N₂O dynamics in the western Arctic Ocean : Distributions, Processes, and Fluxes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11874, https://doi.org/10.5194/egusphere-egu2020-11874, 2020.

EGU2020-2418 | Displays | OS1.11

Spatial and temporal distribution of polycyclic aromatic hydrocarbons in sediments from the Canadian Arctic Archipelago

Anne Corminboeuf, Jean-Carlos Montero-Serrano, and Richard St-Louis

In Arctic, because of the ongoing diminution of the sea ice coverage, it is speculated that anthropogenic activities such as cargo transportation and oil explorations/exploitations could increase in the Canadian Arctic Archipelago (CAA). However, the vast majority of the channels within the CAA, as well as the adjoining continental shelf and slopes, are characterized by a substantial knowledge gap regarding the regional-scale sediment composition and associated contaminants. Overall, knowing that sediments are a sink for pollutants, a wider spatial coverage of sedimentary records across the marine CAA is essential to provide fundamental baseline information on the physical and geochemical sediment properties in this Arctic region. In this context, a total of 118 surface sediment samples were collected over a large area covering the Canadian Beaufort Sea to the Baffin Bay in order to characterize the modern spatial distribution patterns and the temporal trends of polycyclic aromatic hydrocarbons (PAHs) within the CAA. Sampling was performed in 2016, 2017, 2018 and 2019 aboard the CCGS Amundsen as part of the ArticNet program. Extractions were performed using one-step accelerated solvent extraction and clean-up, followed by gas chromatography coupled to a mass spectrometer analysis. To characterize the temporal concentrations of PAHs, the top 10 cm of 8 push-cores distributed across the archipelago were sub-sampled and analyzed just as the surface samples. Sedimentation rates for each core were obtained by 210Pb dating and allow to reconstruct the PAHs inputs from the last century. Results of the surface sediment samples indicate that the sum concentrations of 23 PAHs ranged from 6 ng/g (dry weight basis) in the North Baffin Bay to 437 ng/g in the Canadian Beaufort Shelf, with a mean value of 67 ng/g. PAHs source characterization was investigated through diagnostic ratio: fluoranthene over the sum of fluoranthene and pyrene. This ratio tends to point a profile with mainly petrogenic sources (i.e., igneous rock-derived, petroleum or crude oil spill) for the majority of the CAA. Some samples in the Beaufort Sea have a mixed profile with petrogenic sources and pyrogenic sources from incomplete combustion of fossil fuel that could indicate an anthropogenic input. Along Ellesmere Island, ratios are mainly pyrogenic from biomass combustion. Results of the push-cores suggest that the inputs of PAHs to the sediment in CAA were relatively stable during the last years. Taken as a whole, our study will provide a baseline of PAHs levels in surface sediment within the CAA before an increase in maritime transport in this area and a comparison of the modern concentrations versus the last century tendencies.

How to cite: Corminboeuf, A., Montero-Serrano, J.-C., and St-Louis, R.: Spatial and temporal distribution of polycyclic aromatic hydrocarbons in sediments from the Canadian Arctic Archipelago, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2418, https://doi.org/10.5194/egusphere-egu2020-2418, 2020.

EGU2020-20073 | Displays | OS1.11

Ice Season forecast under ClimateChange: Tipping element approach

Elena Surovyatkina and Roman Medvedev

The Sea of Okhotsk is a marginal sea of the western Pacific Ocean. It is one of the world's richest in biological resources and famous for the fishing industry. In winter, navigation on the Sea is difficult, if not impossible, due to the harsh conditions of the North and the presence of sea ice. On average, the ice-free period lasts from June to November. However, the start and end dates of the ice season vary from year to year within a month. Such variability is impossible to capture by meteorological methods, which have a limit of predictability for 10 days. The absence of a long-term forecast of the navigational period in the Sea of Okhotsk affects the safety of navigation and the reliability of transit transport.

Most of the studies of the distribution of ice floes focus on such factors as the location, time of year, water currents, and sea temperatures. In our study, we use the distribution of temperature in the atmosphere and wind direction (NCEP/NCAR re-analyses data set) because most of the area of the Sea of Okhotsk is located in monsoon climate zone. We propose a new approach to forecast predicting the upcoming ice advance/ retreat date by developing our Tipping element approach [1] elaborated for prediction of the Indian Summer Monsoon, which proved to be successful for prediction upcoming monsoon four years in a row (2016-2019).

The physical mechanism underlying forecast is the following. There is an atmospheric feature that appears at the beginning of the transition to the ice season. We show, for the first time, the evidence in observational data that a transition from open water season to ice season begins when the near-surface air temperature crosses a critical threshold. It appears in the form of spatially organized critical transitions in the atmosphere over the see. This event happening 2-3 months before the ice season is a starting point forecasting date of ice advance. We perform forecast in critical areas - tipping elements of the spatial structure of ice formation, which we identified via data analysis.

The retrospective test (over the period 2001-2017) confirms that the methodology allows forecasting the ice advance/retreat date more than one month in advance, with a success rate in 88% of the years within the error of +/- 4 days. Forecasts of the upcoming season 2018-2019 show successful results as well.

Climate change affects the ice season in the Sea of Okhotsk in the following aspects: there has been a declining trend in sea ice cover in recent years due to delays in the ice advance date. Season shift because it takes for the atmosphere longer time cooling down in autumn. The novel approach allows for accounting climate change effects.

ES acknowledges financial support of the EPICC project (18_II_149_Global_A_Risikovorhersage) funded by BMU, RM acknowledges the Russian Foundation for Basic Research (RFBR) (No. 20-07-01071)

[1] Stolbova, V., E. Surovyatkina, B. Bookhagen, and J. Kurths (2016): Tipping elements of the Indian monsoon: Prediction of onset and withdrawal. GRL 43, 1–9 [doi:10.1002/2016GL068392]

How to cite: Surovyatkina, E. and Medvedev, R.: Ice Season forecast under ClimateChange: Tipping element approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20073, https://doi.org/10.5194/egusphere-egu2020-20073, 2020.

EGU2020-4076 | Displays | OS1.11

Atlantic influence on content of freshwater in upper layer of the Arctic Ocean

Genrikh Alekseev, Andrey Pnyushkov, Alexander Smirnov, Anastasia Vyazilova, and Natalia Glok

The interdecadal changes in layer of the Atlantic water (AW) and the fresh water content (FWC)  in the  Arctic Basin  (AB) are traced for the 1960s - 2010s  in order to assess the influence of the influx from the Atlantic on the FWC changes. The results showed that the upper boundary of the AB layer, identified on zero isotherm, everywhere rose in the 1990s - 2010s by several tens of meters relative to its position before the start of the warming in the 1970s. The lower boundary of the layer, also determined by the depth of the zero isotherm, fell. Such displacements of the layer boundaries indicate an increase in the volume of the AW in the AB. A reduction in the volume of the upper freshened layer it is necessary to maintain balance. Our calculations confirmed that in the 1990s, the FWC in the layer 0–100 m decreased to 2 m or more in the Eurasian part of the Arctic Basin west of 180 °E and increased to east of 180 °E closer to the shores of Alaska and the Canadian archipelago,. This trend intensified in the 2000s and in the 2010s. A comparison of the distributions of the FWC and the position of the upper boundary of the AB layer over different decades by the method of spatial correlation confirmed a close relationship between both distributions. The response on changes of water temperature in the tropical region of the Atlantic is traced in the Barents Sea and in the Arctic basin.  That indicates the influence of low latitude SST on changes in AW layer and serves as an indicator of tropical effect on the Arctic processes. The study is supported by the RFBR grant 18-05-60107.

How to cite: Alekseev, G., Pnyushkov, A., Smirnov, A., Vyazilova, A., and Glok, N.: Atlantic influence on content of freshwater in upper layer of the Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4076, https://doi.org/10.5194/egusphere-egu2020-4076, 2020.

EGU2020-15246 | Displays | OS1.11

Assessing the Atlantic Water Pathways To The Arctic In a High-Resolution NEMO Model

Christopher Roach, Christophe Herbaut, and Marie-Noëlle Houssais

Most Atlantic Water (AW) enters the Arctic Ocean through the West Spitzbergen Current, passing north of Svalbard, either moving along the upper slope or passing over and around the Yermak Plateau. Recent model studies (Koenig et al. 2017 and Crews et al. 2019) have improved our understanding of these pathways but were limited to periods of 1-5 years. This is insufficient for examining the contributions of AW inflow to climate-scale problems such as the ‘Atlantification’ of the Arctic.

In this study we use 23 years (1995-2018) of high resolution (~1/24°) velocity fields from a NEMO 3.6 model (DOI: 10.5281/zenodo.2682406) allowing us to examine the geographic distributions and strengths of AW inflow pathways using a Lagrangian particle tracking approach. Virtual particles were released on a section at 30° E and tracked backwards in time using the PARCELS 2.0 particle tracking system (Delandmeter and van Sebille 2019).

For the present analysis, we focus on trajectories of particles which are contained in AW layer at the release line (SA>34.9 and CT>2°C) and could be tracked backwards to the Nowegian Sea (here taken as south of 75° N). A control line was selected across the Yermak Plateau to allow us to separate particles passing through the Svalbard and Yermak branches. Using these particle trajectories, we created a time-series of transport of AW reaching the southern rim of the western Nansen Basin. The transport was found to vary between 0.5 Sv and 3.75 Sv, comparable to previous studies (e.g. Beszczynska-Möller et al. 2012), and to be dominated, on average, by the Yermak Branch.

How to cite: Roach, C., Herbaut, C., and Houssais, M.-N.: Assessing the Atlantic Water Pathways To The Arctic In a High-Resolution NEMO Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15246, https://doi.org/10.5194/egusphere-egu2020-15246, 2020.

EGU2020-22642 | Displays | OS1.11

Atlantic water north of Svalbard 1899-2018

Marika Marnela, Frank Nilsen, Ragnheid Skogseth, and Kjersti Kalhagen

As part of the Nansen Legacy project, waters north of Svalbard are studied. The warm and saline Atlantic water, brought northward by the West Spitsbergen Current cools and freshens as it flows eastward along the slope north of Svalbard, bringing heat and salt into the Arctic Ocean. Hydrographic CTD data are available from various cruises and databases, the main source here being the UNIS Hydrographic Database. Changes in the Atlantic water properties and its horizontal and vertical location on the slope and shelf are mapped from decadal averages of historical data from 1899 to 2018. The mean width of the boundary current following the slope eastward is estimated for five cross-shelf/slope sections from the decadal averages. An Atlantification is present from 1996-2005 to 2006-2018 with warmer and more saline water covering a larger area across the slope and reaching further east.

How to cite: Marnela, M., Nilsen, F., Skogseth, R., and Kalhagen, K.: Atlantic water north of Svalbard 1899-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22642, https://doi.org/10.5194/egusphere-egu2020-22642, 2020.

EGU2020-15215 | Displays | OS1.11

High-frequency dynamics near the shelf break southwest of Svalbard

Pierre-Marie Poulain, Elisa Cozzani, Giuliana Pennucci, Craig Lewis, Cyril Lathuiliere, Lucie Bordoix, and Luca Centurioni

The high-frequency dynamics (including tidal and inertial currents, internal and coastal-trapped waves) on the shelf break/slope southwest of Svalbard were explored in September-October 2019 using a variety of mobile and fixed sensors operated as part of the NARVAL19 Sea Trial. Ocean currents, temperature and salinity were measured in the water column with 6 moorings, 3 gliders and a wirewalker profiler. In addition near-surface (15 m) currents were measured with 24 satellite-tracked drifters.

The collected data show some variability, mostly near the surface, associated with the lateral displacements or meandering of the Polar Front separating cool and low salinity waters on the shelf and warmer/saltier waters of Atlantic origin. The most striking signal, however, is at depth (in and below the thermocline) in the form of internal waves at semidiurnal tidal frequency.

Preliminary results of spectral and harmonic analyses of the data collected by all the platforms are presented and discussed.

How to cite: Poulain, P.-M., Cozzani, E., Pennucci, G., Lewis, C., Lathuiliere, C., Bordoix, L., and Centurioni, L.: High-frequency dynamics near the shelf break southwest of Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15215, https://doi.org/10.5194/egusphere-egu2020-15215, 2020.

EGU2020-8815 | Displays | OS1.11

Deep and intermediate water properties based on Argo floats data in the eastern Nordic Seas

Malgorzata Merchel and Waldemar Walczowski

The Nordic Seas play an important role in global climate change. Compared with other areas, this region has the largest ocean surface and air positive temperature anomalies in the world. It is particularly important for the water masses formation and modification and for interactions between the ocean and atmosphere. This region is also the main route for freshwater and heat exchange between the North Atlantic and the Arctic Ocean.

Because the ship-borne measurements are performed usually during the spring to the autumn season, there is no data to analyze seasonal changes in the intermediate and deep water. The Argo floats, operating throughout the whole year, allow observation of seasonal changes that occur in particular regions. This is especially important in the Nordic Seas, where conditions of the oceanographic observations are very difficult even during the summer.

In this study we analyze hydrographic data collected by the Argo floats in the eastern part of the Nordic Seas region in 2008-2017. Based on the data, both the temporal and spatial variability of the basic physical parameters of the intermediate and deep water were analyzed. It allowed determining how the properties of these waters changed both seasonally and spatially.

The study was funded by the Ministry of Science and Higher Education, Poland under grant agreement No. DIR/WK/2016/12 for the research infrastructure EURO-ARGO ERIC and the National Science Centre, Poland within the DWINS Project (2016/21/N/ST10/02920).

How to cite: Merchel, M. and Walczowski, W.: Deep and intermediate water properties based on Argo floats data in the eastern Nordic Seas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8815, https://doi.org/10.5194/egusphere-egu2020-8815, 2020.

EGU2020-11821 | Displays | OS1.11 | Highlight

Variability and decadal trends in the Isfjorden (Svalbard) ocean climate and circulation - an indicator for climate change in the European Arctic

Ragnheid Skogseth, Lea L. A. Olivier, Frank Nilsen, Marius O. Jonassen, and Eva Falck

Isfjorden, a broad Arctic fjord in western Spitsbergen, has shown significant changes in hydrography and inflow of Atlantic Water (AW) the last decades that only recently have been observed in the Arctic Ocean north of Svalbard. Variability and trends in this fjord’s climate and circulation are therefore analysed from observational and reanalysis data during 1987 to 2017. Isfjorden experienced a shift in summer ocean structure in 2006, from AW generally in the bottom layer to AW (with increasing thickness) higher up in the water column. This shift, and a concomitant shift to less fast ice in Isfjorden are linked to positive trends in the mean sea surface temperature (SST) and volume weighted mean temperature (VT) in winter (SSTw/VTw: 0.7 ± 0.1/0.9 ± 0.3 °C 10yr-1) and summer (SSTS/VTS: 0.7 ± 0.1/0.6 ± 0.1 °C 10yr-1). The local mean air temperature shows similar trends in winter (1.9 ± 0.4 °C 10yr-1) and summer (0.7 ± 0.1 °C 10yr-1). Positive trends in volume weighted mean salinity in winter (0.21 ± 0.06 10yr-1) and summer (0.07 ± 0.05 10yr-1) suggest increased AW advection as a main reason for Isfjorden’s climate change. Local mean air temperature correlates significantly with sea ice cover, SST, and VT, revealing the fjord’s impact on the local terrestrial climate. In line with the shift in summer ocean structure, Isfjorden has changed from an Arctic type fjord dominated by Winter Deep and Winter Intermediate thermal and haline convection, to a fjord dominated by deep thermal convection of Atlantic type water (Winter Open). AW indexes for the mouth and Isfjorden proper show that AW influence has been common in winter over the last decade. Alternating occurrence of Arctic and Atlantic type water at the mouth mirrors the geostrophic control imposed by the Spitsbergen Polar Current (carrying Arctic Water) relative to the strength of the Spitsbergen Trough Current (carrying AW). During high AW impact events, Atlantic type water propagates into the fjord according to the cyclonic circulation along isobaths determined by the winter convection. This study demonstrates that Isfjorden and its ocean climate can be used as an indicator for climate change in the Arctic Ocean. The used methods may constitute a set of helpful tools for future studies also outside the Svalbard Archipelago.

How to cite: Skogseth, R., Olivier, L. L. A., Nilsen, F., Jonassen, M. O., and Falck, E.: Variability and decadal trends in the Isfjorden (Svalbard) ocean climate and circulation - an indicator for climate change in the European Arctic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11821, https://doi.org/10.5194/egusphere-egu2020-11821, 2020.

EGU2020-8142 | Displays | OS1.11

Impact of Atlantic water variability on sea ice changes in the Fram Strait and north of Svalbard

Agata Grynczel, Agnieszka Beszczynska-Moeller, and Waldemar Walczowski

Recent satellite passive microwave observations indicate significant negative Arctic sea ice extent trends in all months and substantial reduction of winter sea ice in the Atlantic sector. Warm and salty oceanic water masses from the North Atlantic flow towards the Arctic Ocean along the eastern Fram Strait, carried by the West Spitsbergen Current (WSC). Fram Strait, as well as the region north of Svalbard, play a key role in controlling the amount of oceanic heat supplied to the Arctic Ocean and are the place of dynamic interaction between the ocean and sea ice. The north of Svalbard area is one of the regions where the substantial changes in sea ice concentrations are observed both in summer and in winter. One of the possible reasons can be sought in the observed warming of Atlantic water, carried through Fram Strait into the Arctic Ocean. The main goal of this work is to analyse and explain the sea ice variability along main pathways of the Atlantic origin water (AW) in the context of observed warming of Atlantic water layer. Shrinking sea ice cover in the southern part of Nansen Basin (north of Svalbard) and shifting the ice edge in Fram Strait are driven by the interplay between increased advection of oceanic heat in the Atlantic origin water and changes in the local atmospheric conditions that result in the increased ocean-air-sea ice exchange in winter seasons. The basis for this hypothesis is warming of winter mean surface air temperature observed north of Svalbard and withdrawal of the sea ice cover towards the northeast, along with the pathways of water inflow in the Atlantic sector of the Arctic Ocean. Hydrographic data from vertical CTD profiles were collected during annual summer expeditions of the research vessel "Oceania", conducted in Fram Strait and the southern part of the Nansen Basin over the past two decades. The measurement strategy of the original research program AREX, which consists of the performance of cross-sections perpendicular to the presumed direction of the West Spitsbergen Current, allowed to observe changes in the properties and transport of the Atlantic Water carried to the Arctic Ocean. The analysis of past and present changes in the sea ice cover in relation to Atlantic water variability and atmospheric forcing employs hydrographic data from the repeated CTD sections, systematically collected since 1996 during annual summer Arctic long-term monitoring program AREX, satellite products of sea ice concentration and drift, and selected reanalysis data sets.

How to cite: Grynczel, A., Beszczynska-Moeller, A., and Walczowski, W.: Impact of Atlantic water variability on sea ice changes in the Fram Strait and north of Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8142, https://doi.org/10.5194/egusphere-egu2020-8142, 2020.

EGU2020-18793 | Displays | OS1.11

Structure and transport of Atlantic water north of Svalbard from observations in summer and fall 2018

Eivind Hugaas Kolås, Zoe Koenig, Ilker Fer, Frank Nilsen, and Marika Marnela

The transport of warm Atlantic waters north of Svalbard is one of the major heat and salt sources to the Arctic Ocean. The circulation pathway and the associated heat transport influence the variability in the Arctic sea ice extent and the onset of freezing. We present observations obtained from research cruises and autonomous underwater glider missions in summer and fall 2018 to describe the hydrographic structure, volume transport rates and circulation patterns of the warm boundary current between 12E and 24E north of Svalbard.

A composite section is constructed along a representative, average bathymetry across the shelf break, using all available observations in order to obtain the hydrographic structure and the absolute geostrophic transport of the boundary current. The Atlantic water volume transport reaches a maximum of 3.0 ± 0.2 Sv in October, with an intraseasonal variability of 1 Sv. During summer and late fall, we observed Atlantic water flowing eastward (a counter current), in the outer part of the section away from the shelf break, in the Sofia Deep. The intensity of the Atlantic water counter current and the Atlantic water boundary current are very sensitive to the wind stress curl: we observed a near doubling of the volume transport in less than a week.

The composite section also reveals a bottom-intensified current flowing parallel to the boundary current, between the 1500 m and 2000 m isobaths. A composite of all historical data collected in the region, constructed identical to our observations, support the presence of the bottom intensified current.

How to cite: Hugaas Kolås, E., Koenig, Z., Fer, I., Nilsen, F., and Marnela, M.: Structure and transport of Atlantic water north of Svalbard from observations in summer and fall 2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18793, https://doi.org/10.5194/egusphere-egu2020-18793, 2020.

EGU2020-19977 | Displays | OS1.11

Drivers of interannual sea ice variability on the Arctic continental margin north of Svalbard

Øyvind Lundesgaard, Arild Sundfjord, and Angelika H. H. Renner

Sea ice concentration along the Arctic continental margin north of Svalbard is in decline, but superimposed on this trend is considerable interannual variability. Many factors impact sea ice in this region, including atmospheric cooling and heating, winds, sea ice advection, and oceanic heat transport associated with the inflow of Atlantic Water, and regional sea ice cover remains difficult to predict. We present observations of upper ocean temperature between 2012 and 2017 from an ocean mooring located on the continental shelf break north of the Barents Sea, together with concurrent time series of atmospheric variables and sea ice concentration, drift, and thickness, derived from satellite and reanalysis data. While the inflow of Atlantic Water undoubtedly plays a key role in maintaining the area north of Svalbard ice-free through much of the year, variations in upper ocean temperature do not explain major interannual sea ice anomalies during the study period. Instead, we find that the magnitude of sea ice advection from the north and east was a major driver of interannual sea ice variability during our study.

How to cite: Lundesgaard, Ø., Sundfjord, A., and Renner, A. H. H.: Drivers of interannual sea ice variability on the Arctic continental margin north of Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19977, https://doi.org/10.5194/egusphere-egu2020-19977, 2020.

In this study we develop a novel sea ice classification scheme based on remote sensing Synthetic-aperture Radar (SAR) data, and use it to classify sea ice types over the spatial and temporal range of the Norwegian Young sea ICE cruise (N-ICE2015). Ice type classification will be conducted on wide-swath SAR datasets including RADARSAT-2 and Sentinel-1 data. We use a classification scheme that takes into account different rates of decrease in backscatter intensity with incidence angle variation for different classes. In addition, it examines texture features of different sea ice types, and also variations of surface texture with changing incidence angles, and incorporates this relationship into the classification process. Sea ice classifications using high-resolution SAR images collected over the same period and also field data retrieved from the N-ICE2015 expedition will be used for ground truthing. Earlier N-ICE2015 studies with high resolution SAR and deformation suggest high lead and pressure ridge formation. We will use our lower-resolution results to explore potential increase in the fraction of deformed and lead ice from January to June 2015 in the region north of Svalbard.

How to cite: Guo, W., Itkin, P., and Philipp Lohse, J.: Sea ice classification using wide-swath SAR data considering incidence angle depenency of backscatter intensity and surface texture, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11045, https://doi.org/10.5194/egusphere-egu2020-11045, 2020.

EGU2020-5592 | Displays | OS1.11

Semidiurnal current dynamics in the Arctic Ocean's eastern Eurasian Basin

Till Baumann, Igor Polyakov, Laurie Padman, Seth Danielson, Ilker Fer, Susan Howard, Jenny Hutchings, Markus Janout, An Nguyen, and Andrey Pnyushkov

In the Arctic Ocean, semidiurnal-band processes including tides and wind-forced inertial oscillations are significant drivers of ice motion, ocean currents and shear contributing to mixing. Two years (2013-2015) of current measurements from seven moorings deployed along 125°E from the Laptev Sea shelf (~50 m) down the continental slope into the deep Eurasian Basin (~3900 m) are analyzed and compared with models of baroclinic tides and inertial motion to identify the primary components of semidiurnal-band current (SBC) energy in this region. The strongest SBCs, exceeding 30 cm/s, are observed during summer in the upper ~30 m throughout the mooring array. The largest upper-ocean SBC signal consists of wind-forced oscillations during the ice-free summer. Strong barotropic tidal currents are only observed on the shallow shelf.  Baroclinic tidal currents, generated along the upper continental slope, can be significant. Their radiation away from source regions is governed by critical latitude effects: the S2 baroclinic tide (period = 12.000 h) can radiate northwards into deep water but the M2 (~12.421 h) baroclinic tide is trapped to the continental slope. Baroclinic upper-ocean tidal currents are sensitive to varying stratification, mean flows and sea ice cover.  This time-dependence of baroclinic tides complicates our ability to separate wind-forced inertial oscillations from tidal currents. Since the shear from both sources contributes to upper-ocean mixing that affects the seasonal cycle of the surface mixed layer properties, a better understanding of both, inertial motion and baroclinic tides is needed for projections of mixing and ice-ocean interactions in future Arctic climate states.

How to cite: Baumann, T., Polyakov, I., Padman, L., Danielson, S., Fer, I., Howard, S., Hutchings, J., Janout, M., Nguyen, A., and Pnyushkov, A.: Semidiurnal current dynamics in the Arctic Ocean's eastern Eurasian Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5592, https://doi.org/10.5194/egusphere-egu2020-5592, 2020.

EGU2020-8445 | Displays | OS1.11

Eddy statistics validation of an ORCA12 ocean and sea ice model for the Arctic with satellite data

Stefanie Rynders, Yevgeny Akesenov, and Igor Kozlov

As sea ice and ocean models are moving to higher resolution it becomes possible to permit eddy formation even in the Arctic Ocean. Eddies can affect the three dimensional ocean state through causing mixing and even ventilation of subsurface ocean layers if they are deep enough. To ensure models have the potential to simulate the density structure correctly it is therefore necessary to start doing model validation of not only the large scale ocean state, but also of the eddy field. Eddy statistics for the Arctic are available from satellite for the Western Arctic Ocean and the Fram Strait, in particular on number, size and cyclonicity of eddies for open ocean versus ice covered sites. These are compared to a NEMO-LIM 1/12 degree sea ice and ocean simulation (resolution 2-5km), upon which the model based statistics are expanded to the whole Arctic. In the model it is also possible to examine the depth structure of eddies, allowing to generate size vs. depth statistics. This, together with climatological mixed layer depth, provides a first estimate to get satellite-based information on mixing from eddies in the Arctic. We also map the maximum depth of eddies, to examine ventilation and identify sites with especially deep eddies, for instance at the boundary current. Acknowledgements: Grant NE/R000654/1 “Towards a Marginal Sea Ice Cover” funded by the UK Natural Research Council (NERC) and the UK-Russia Arctic bursaries program funded by the United Kingdom’s Department for Business, Energy and Industrial Strategy. The study is also supported from the project “The Advective Pathways of nutrients and key Ecological substances in the Arctic (APEAR)” (grant NE/R012865/1) funded by the Joint UK NERC/German Federal Ministry of Education and Research Changing Arctic Ocean Programme. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 821926 (IMMERSE). IK acknowledges the support from RFBR grant No 18-35-20078.

How to cite: Rynders, S., Akesenov, Y., and Kozlov, I.: Eddy statistics validation of an ORCA12 ocean and sea ice model for the Arctic with satellite data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8445, https://doi.org/10.5194/egusphere-egu2020-8445, 2020.

EGU2020-1013 | Displays | OS1.11

Arctic Ocean near-surface circulation: altimetry and in situ observations along the Transpolar Drift.

Francesca Doglioni, Robert Ricker, Benjamin Rabe, and Torsten Kanzow

Recent decades have seen a strong intensification of major circulation systems in the Arctic Ocean, namely the Beaufort Gyre and Transpolar Drift. Observing and studying seasonal, interannual and decadal variability of large-scale Arctic Ocean surface circulation is a key element to understand changes in climate-relevant export of both sea ice and fresh surface water from the Arctic. However, lack of in-situ ocean surface velocity observations have prevented further investigation until recently.

In the past decade, charts of the Arctic geostrophic surface flow field have been derived from new satellite altimetry missions over the ice-covered oceans, such as CryoSat-2, which was launched in 2010. The altimetric measurements allow the detection of leads and therefore to retrieve sea surface height (SSH) across the ice-covered Arctic Ocean. Aiming to characterize the seasonal to interannual variability of geostrophic surface currents in the Transpolar Drift, we use SSH observations from the Cryosat-2 mission between 2011 and 2018.

Here we present an evaluation of optimally interpolated altimetric SSH anomalies against in situ ocean observations of both bottom pressure and dynamic Height in Fram Strait and north of Arctic Cape, in the years between 2016 and 2018. Following the assessment of the quality of altimetry-based SSH, we discuss the timescales of SSH variability in seasonally ice-covered regions. Moreover, from the comparison with ocean bottom pressure and dynamic height we will attribute the relative importance of mass and steric contributions to the variability of SSH along the two transects. From first preliminary results in a test year (2011), SSH at a meridional transect in central Fram Strait between 78°N and 80°N shows a seasonal cycle with minimum in the months of March and April, enhanced at the most southern mooring. 

How to cite: Doglioni, F., Ricker, R., Rabe, B., and Kanzow, T.: Arctic Ocean near-surface circulation: altimetry and in situ observations along the Transpolar Drift., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1013, https://doi.org/10.5194/egusphere-egu2020-1013, 2020.

EGU2020-21510 | Displays | OS1.11

Variations of surface waters characteristics in the Eastern Arctic Ocean in 2010-2019 at different scales: in situ and satellite study

Anastasiia Tarasenko, Alexandre Supply, Jacqueline Boutin, Nikita Kusse-Tiuz, Mikhail Makhotin, Vladimir Ivanov, and Gilles Reverdin

The last 10 years of Arctic Ocean observations showed the dramatic changes and new records of the sea ice minimum. The largest variations were observed in the Eastern Arctic: the Kara, the Laptev, the East-Siberian seas. This region is a key area of the important freshwater input from the great Siberian rivers (Ob’, Yenisei, Lena). This remote area remains one of the less studied in the Arctic Ocean, although several regular expeditions (such as NABOS or Transdrift) together with special expeditions following the Northern Route, such as Tara-2013 expedition, or recent Transarktika-2019 expedition help to monitor the changes of surface waters in recent years.

The use of new satellite-derived datasets, (e.g., SST blended product from Danish Meteorological Institute or REMSS, SSS SMOS from LOCEAN University of Sorbonne) fill the gaps and help to better understand the complex dynamics of surface waters in the Eastern Arctic ocean.

In this work, we discuss the surface waters variations using in situ and satellite data at different scales.  Synoptic scales are studied with continuous and point in situ measurements (thermosalinographs and CTD data). The recent scientific results of Transarktika-2019 expedition are presented. In the summer season of 2019 (July-October) Transarktika expedition did oceanographic measurements following the Northern Route twice, from Vladivistok to Murmansk and back to Vladivostok. The seasonal variations are analyzed over the period of 10 years, comparing with climatological data. The difference between the climatological values of SST or SSS can reach 5 or more units in some areas of the Eastern Arctic. The results of interannual variations analysis using satellite data, suggest the salinification (“Atlantification”) of the southern areas and freshening of the northern parts of the Eastern Arctic.

The development of SSS SMOS Arctic product was supported by the French CNES-TOSCA SMOS-OCEAN project. Anastasiia Tarasenko, Nikita Kusse-Tiuz, Mikhail Makhotin and Vladimir Ivanov acknowledge financial support from the Ministry of Science and Higher Education of the Russian Federation, project RFMEFI61619X0108

How to cite: Tarasenko, A., Supply, A., Boutin, J., Kusse-Tiuz, N., Makhotin, M., Ivanov, V., and Reverdin, G.: Variations of surface waters characteristics in the Eastern Arctic Ocean in 2010-2019 at different scales: in situ and satellite study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21510, https://doi.org/10.5194/egusphere-egu2020-21510, 2020.

EGU2020-15898 | Displays | OS1.11

Pacific Water Pathways through the Arctic Ocean

Paul A. Dodd, Tore Hattermann, Michael Karcher, Frank Kauker, and Colin Stedmon

The volume, characteristics and sources of freshwater circulating in the Arctic Ocean vary in time and are expected to change under a declining sea ice cover, influencing the physical environment and Arctic ecosystem. Relatively fresh (S = 32) Pacific Water, which enters the Arctic Ocean via the Bering Strait makes up a significant part of the liquid freshwater exiting the Arctic Ocean through Fram Strait. If transported to the Nordic Seas and North Atlantic via the East- and West Greenland Currents freshwater from the Pacific could have an effect on convection and dense water formation in those regions.

More than 30 repeated sections of nutrient measurements have been collected across Fram Strait between 1980 and 2019. The fraction of Pacific Water along these repeated sections can be estimated from the ratio of nitrate to phosphate. The time-series of repeated Fram Strait sections indicates that the fraction of Pacific Water passing out of the Arctic Ocean has changed significantly over the last 30 years. Pacific water fractions remained high from 1980 to 1998, but in 1999 Pacific water almost disappeared from Fram Strait, reappearing from 2011 to 2012, when there was a peak in freshwater export though Fram Strait.

Several hypotheses suggest how variations in the large-scale atmospheric circulation over the Arctic Ocean may influence the transport and pathways of Pacific Water. We show how anomalies in reanalysis wind fields are associated with the reappearance of Pacific Water in Fram Strait in recent years. Repeated sections across Fram Strait are compared with sea ice back-trajectories in the Polar Pathfinder 4 product and a simulated Pacific Water tracer in the NAOSIM numerical model to investigate likely Pacific water pathways through the Arctic Ocean and upstream drivers of changes observed in Fram Strait.

How to cite: Dodd, P. A., Hattermann, T., Karcher, M., Kauker, F., and Stedmon, C.: Pacific Water Pathways through the Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15898, https://doi.org/10.5194/egusphere-egu2020-15898, 2020.

EGU2020-10530 | Displays | OS1.11

Extension of Pacific summer waters in the Arctic Ocean based on field and model data

Mikhail Makhotin and Leonid A. Timokhov

During the last few decades we observe fast climatic changes. These changes are well expressed in the Polar Region that is more sensitive to all environmental shifts. The temperature and salinity anomalies were observed not only on the surface but also in the Pacific origin halocline layer. Due to hydrostatic imbalance and atmospheric circulation, water from the North Pacific flows through the Bering Strait, transits the upper levels of the Arctic Ocean, and penetrates to the North Atlantic. Pacific Summer Water (PSW) (flows through the Bering Strait in summer) is a main freshwater source in the Canadian Basin and influences thermohaline structure of the whole Arctic Ocean. Based on the field oceanographic data obtained during 1991-2014 we found the high interannual variability of PSW extent and maximum temperatures of PSW core. Since 1991 the area of PSW distribution decreased and the boundary of PSW extension shifted from the Makarov Basin towards the Canada Basin. At the end of the 2000s the PSW boundary extension returned to approximately early 1990s conditions. Rapid changes in the Arctic wind forcing regime occurred in 2007 led to anomaly extension of the PSW boundary towards the Lomonosov Ridge in 2008. For study the distribution of PSW under the conditions of a lack of field data, we used the modeling data of the GLORYS12V1 product of Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu/). The model component is the NEMO platform driven at the surface by ECMWF ERA-Interim reanalysis. Based on the calculated data we revealed the maximum temperatures and extension of PSW core in the Arctic Ocean from 1993 to 2018. As a result of comparing of field and calculated data, we found a good correspondence the values of maximum temperatures and the position of the PSW core in the centre of the Canada Basin. For example, in 2011 observed (1.0 °С) and calculated (0.9 °С) maximum temperature of PSW were very close. Based on model data we calculated the heat content of PSW which reached maximum values in recent years. The research was supported by the Ministry of Science and Higher Education of the Russian Federation (project RFMEFI61619X0108).

How to cite: Makhotin, M. and Timokhov, L. A.: Extension of Pacific summer waters in the Arctic Ocean based on field and model data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10530, https://doi.org/10.5194/egusphere-egu2020-10530, 2020.

In the summer Arctic, bump-like vertical temperature profiles of the upper layer in the Canada Basin suggest a near-surface temperature maximum (NSTM) beneath the mixed layer. This paper concentrates on describing the decadal variance of these NSTMs. Essentially, the temporal evolution of the summer NSTM revealed three decadal phases. The first period is before 2003, when the summer NSTM could rarely be observed except around the marginal of the Canada Basin. The second period is between 2003 and 2015, when the summer NSTM nearly occurred over the whole basin as accelerated decline of summer sea ice. The third period is from 2016 to 2017, when the summer NSTM almost disappeared due to prevailing warm surface water. Furthermore, for the background behind the decadal variance of summer NSTM, linear trends of the September minimum sea ice extent and surface water heat content in the Canada Basin from 2003 to 2017 were –2.75±1.08×104km2yr–1 and 2.29±1.36MJ m–2yr–1, respectively. According to a previous theory, if we assume that the trend of the summer surface water heat content was only contributed by NSTM, it would cause a decrease in sea ice thickness of approximately 13 cm. The analysis partially explains the reason for sea ice decline in recent years.

How to cite: Lin, L. and He, H.: Decadal variance of summer near-surface temperature maximum in Canada Basin of Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6853, https://doi.org/10.5194/egusphere-egu2020-6853, 2020.

Knowledge of sea ice state (distribution, characteristics and movement) is interesting both from a practical point of view and for fundamental science. The western part of the Barents Sea is a region of increasing activity – oil and gas exploration may growth in addition to traditional fishing and transport. So theinformation is requested by industry and safety authorities.

Three last years (2017-19) the Arctic Technology Department of the University Centre in Svalbard (UNIS) performed expeditions on MS Polarsyssel in April in the sea ice-marginal zone of the Western Barents Sea, as a part of teaching and research program. In (Marchenko 2018), sea ice maps were compared with observed conditions. The distinguishing feature of ice in this region is the existence of relatively small ice floes (15-30 m wide) up to 5 m in thickness, containing consolidated ice ridges. In (Marchenko 2019) we described several such floes investigated by drilling, laser scanning and ice mechanical tests, on a testing station in the place with very shallow water (20 m) where ice concentrated. In this article, we summarise three years results with more attention for level ice floes and ice floe composition, continuing to feature ice condition in comparison with sea ice maps and satellite images.

These investigations provided a realistic characterization of sea ice in the region and are a valuable addition to the long-term studies of sea ice in the region performed by various institutions.

How to cite: Marchenko, N.: Three years (2017-19) of field observation of the marginal ice the Western Barents Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4043, https://doi.org/10.5194/egusphere-egu2020-4043, 2020.

EGU2020-6089 | Displays | OS1.11 | Highlight

Wave and coastal sea ice interaction along the Arctic coast

Lucia Hosekova, Mika Malila, Jim Thomson, Nirnimesh Kumar, Erick W. Rogers, Lettie Roach, and Emily Eidam

Rapid decline in seasonal sea ice has been linked to increased surface wave activity and shoreline erosion in the coastal Arctic. This trend poses a risk to communities vulnerable to flooding and storm surges. Here we focus on quantifying the relationship between coastal erosion, increasing wave activity and the role of sea ice in protecting the coast. 

In November 2019, we observed a three day wave event in the Chukchi Sea along the coastal barrier system near Icy Cape, Alaska. The wave event was sampled using multiple drifting SWIFT (Surface Wave Instrument Float with Tracking) buoys, a cross-shore mooring array, and ship-based CTD casts.  This provided datasets for different ice types in both Eulerian and Lagrangian reference frames. Pancake and frazil sea ice near the coast attenuated the incident wave field, such that the significant wave height reduced from 3 to 1.5 m over less than 5 kilometers. The wave data combined with in-situ ice observations and satellite imagery are used to calculate spectral attenuation of wave energy segregated by ice type. Furthermore, observed temperature, mean circulation and surface heat fluxes are used to address the evolution of sea ice throughout the event. 

 

Supported by the National Science Foundation and the Office of Naval Research. 

How to cite: Hosekova, L., Malila, M., Thomson, J., Kumar, N., Rogers, E. W., Roach, L., and Eidam, E.: Wave and coastal sea ice interaction along the Arctic coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6089, https://doi.org/10.5194/egusphere-egu2020-6089, 2020.

The Arctic Ocean is undergoing significant changes, with rapid sea ice decline, unprecedented freshwater accumulation and pronounced regional sea level rise. In this work we analyzed the sea level variation in the Arctic Ocean based on a global simulation with 4.5 km resolution in the Arctic Ocean using the multi-resolution Finite Element Sea ice-Ocean Model (FESOM). The simulation reasonably reproduces both the main spatial features of the sea surface height (SSH) and its temporal evolution in the Arctic Ocean in comparison to tide gauge and satellite data. Using the model results we investigated the low-frequency variability of the Arctic SSH. The decadal variability is the dominant mode of the annual-mean SSH evolution in the Arctic Ocean, which can be mainly attributed to the variability of the halosteric height. The atmospheric circulation associated with the Arctic Oscillation drives the accumulation and release of freshwater in the Arctic deep basin, thus leading to the decadal variability of the SSH. The associated redistribution of water mass changes the ocean mass over the continental shelf, so the change in SSH is opposite between the shelf seas and the deep basin. By using a dedicated sensitivity simulation in which the recent sea ice decline is eliminated, we find that the sea ice decline contributed considerably to the observed sea level rise in the Amerasian Basin in the recent decades. Although the sea ice decline did not change the mean SSH averaged over the Arctic Ocean, it significantly changed the spatial pattern of the SSH trend. Our finding indicates that both the wind regime and on-going sea ice decline should be considered to better understand and predict the changes in regional sea level in the Arctic Ocean.

How to cite: Chen, M., Xiao, K., Wang, Q., Wang, X., and Zhang, W.: Low-frequency Sea Level Variability and Impact of Recent Sea Ice Decline on the Sea Level Trend in the Arctic Ocean from a High-Resolution Simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8580, https://doi.org/10.5194/egusphere-egu2020-8580, 2020.

EGU2020-10052 | Displays | OS1.11

Correlation between Arctic river discharge and sea ice formation in Laptev Sea using sea surface salinity from SMOS satellite

Carolina Gabarro, Justino Martinez, Veronica Gonzalez-Gambau, Cristina González-Haro, Estrella Olmedo, Antonio Turiel, Laurent Bertino, Jiping Xie, Roshin Raj, Rafael Catany, Manuel Arias, Roberto Sabia, and Diego Fernandez

During the last 3 decades, the Arctic rivers have increased their discharge around 10%, mainly due to the increase of the global atmospheric temperature. The increase of the river discharge carries higher loads of dissolved organic matter (DOM) and suspended matter (SM) entering to the Arctic Ocean. This results in increased absorption of solar energy in the mixed layer, which can potentially contribute to the general sea ice retreat. Observation based studies (e.g. Bauch et al., 2013) showed correlation between river water discharge and local sea ice melting on the Laptev sea shelf due to the change on the ocean heat. Previous studies are based with a limited number of observations, both in space and in time.

Thanks to the ESA SMOS (Soil Moisture and Ocean Salinity) and NASA SMAP (Soil Moisture Active Passive) missions we have daily the sea surface salinity (SSS) maps from the Arctic, which permit to observe the salinity variations due to the river discharges. The Arctic sea surface salinity products obtained from SMOS measurements have been improved considerable by the Barcelona Expert Center (BEC) team thanks to the project Arctic+Salinity, funded by ESA. The new version of the product (v3) covers the years from 2011 up to 2018, have a spatial resolution of 25km and are daily maps with 9 day averages. The Arctic+ SSS maps provide a better description of the salinity gradients and a better effective spatial resolution than the previous versions of the Arctic product, so the salinity fronts are better resolved. The quality assessment of the Arctic+SSS product is challenging because, in this region, there are scarce number of in-situ measurements.

The high effective spatial resolution of the Arctic+ SSS maps will permit to study for the first time scientific physical processes that occurs in the Arctic. We will explore if a correlation between the Lena and Ob rivers discharge with the sea ice melting and freeze up is observed with satellite data, as already stated with in-situ measurements by Bauch et al. 2013. Salinity and sea ice thickness maps from SMOS and sea ice concentration from OSISAF will be used in this study.

 

Bauch, D.,Hölemann, J. , Nikulina, A. , Wegner, C., Janout, M., Timokhov, L. and Kassens, H. (2013): Correlation of river water and local sea-ice melting on the Laptev Sea shelf (Siberian Arctic) , Journal of Geophysical Research C: Oceans, 118 (1), pp. 550-561 . doi: 10.1002/jgrc.20076

How to cite: Gabarro, C., Martinez, J., Gonzalez-Gambau, V., González-Haro, C., Olmedo, E., Turiel, A., Bertino, L., Xie, J., Raj, R., Catany, R., Arias, M., Sabia, R., and Fernandez, D.: Correlation between Arctic river discharge and sea ice formation in Laptev Sea using sea surface salinity from SMOS satellite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10052, https://doi.org/10.5194/egusphere-egu2020-10052, 2020.

EGU2020-12269 | Displays | OS1.11

Multiyear variability of atmospheric processes and ice cover in the Laptev Sea since 1942 to 2019

Anna Timofeeva, Vladimir Ivanov, Alexander Yulin, and Stepan Khotchenkov

The Laptev Sea is influenced by synoptic regions of the Atlantic-Eurasian sector of the Northern Hemisphere. Types of large-scale processes are consider according to the G. J. Vangengeim typization: West (W) circulation form, with dominating zonal transport of air masses, East (E) and meridional (C) circulation forms with opposite phases of geographic orientation in the troposphere of the anticyclones ridges axes, blocking the Western transfer of air masses and developing the meridional circulation at high and middle latitudes. The Laptev Sea ice extent at the end of the melting season has a strong interannual variability, the oscillations amplitude reaches 86%.

The paper considers analysis of long-term trends of the large-scale atmosphere processes realignment and multiyear variability of the air temperature and ice cover anomalies in the Laptev Sea. According to multiyear course of integral anomalies values four steady periods of homogeneous  tendency of climatic processes revealed and described for data series from 1942 to 2019 (air reconnaissance and satellite data).

The types of ice conditions development (severe, medium, mild) at the end of the melting season were determined for the entire series of observations. More than half of cases during 78 years are distinguished as medium type of ice conditions. The repeatability of severe and mild types is almost the same numerically but varies in time according to revealed periods.

During 1942-1947 years in the Laptev Sea the “warming” period occurred (same for the whole polar region), known as the warming of the Arctic of 30th. At this period positive temperature anomalies and negative anomalies of sea ice extent (mean -2%) were dominated. During subsequent period 1948-1989 years the positive temperature trend has changed to the steady negative. The most dramatic temperature drops were observed in the 60-70th. Positive ice anomalies increased (mean 9%), in August Laptev Sea remained mostly covered by ice. Of the 42 years 28 refer to the medium type of ice conditions, 11 to the severe. During the period 1990-2004 years frequent interannual rearrangements of the atmosphere circulation and multidirectional fluctuations of temperature and ice cover anomalies were observed. On average, the temperature and ice cover during the period are close to the long-term norm. After 2005 temperature regime in the polar climate system has changed. This period is the warmest for the whole observations series in the Laptev Sea. Ice extent at the end of the melting season steady decreases and shows dramatic growth of negative anomalies values and occur of extremely low anomaly for the entire observation period (up to -54-55%). The average negative ice anomaly for the period is -26.4 %. Of the 15 years 9 refer to the mild type of ice conditions.

How to cite: Timofeeva, A., Ivanov, V., Yulin, A., and Khotchenkov, S.: Multiyear variability of atmospheric processes and ice cover in the Laptev Sea since 1942 to 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12269, https://doi.org/10.5194/egusphere-egu2020-12269, 2020.

EGU2020-20077 | Displays | OS1.11 | Highlight

Air-Sea Fluxes following the Unusually Early Ice Retreats in the Arctic

Dongxiao Zhang, Chidong Zhang, Jessica Cross, Calvin Mordy, Edward Cokelet, Chelle Gentemann, Andy Chiodi, Phyllis Stabeno, Richard Jenkins, Christian Meinig, Noah Lawrence-Slava, Heather Tabisola, and Muyin Wang

The Arctic has been rapidly changing over the last decade, with more frequent unusually early ice retreats in late spring and summer. Vast Arctic areas that were usually covered by sea ice are now exposed to the atmosphere because of earlier ice retreat and later arrival. Assessment of consequential changes in the energy cycle of the Arctic and their potential feedback to the variability of Arctic sea ice and marine ecosystems critically depends on the accuracy of surface flux estimates. In the Pacific sector of the Arctic, earlier ice retreat generally follows the warm Pacific water inflow into the Arctic through the Bering and Chukchi Seas. Due to ice coverage and irregularity of seasonal ice retreats, air-sea flux measurements following the ice retreats has been difficult to plan and execute. A recent technology development is the Unmanned Surface Vehicles (USVs): The long-range USV saildrones are powered by green energy with wind for propulsion and solar energy for instrumentation and vehicle control. NOAA/PMEL and University of Washington scientists have made surface measurements of the ocean and atmosphere in the Pacific Arctic using saildrones for the past several years. In 2019, for the 1st time a fleet of six saildrones capable of measuring both turbulent and radiative heat fluxes, wind stress, air-sea CO2 flux and upper ocean currents was deployed to follow the ice retreat from May to October, with five of the USVs into the Chukchi and Beaufort Seas while one staying in the Bering Sea. These in situ measurements provide rare opportunities of estimating air-sea energy fluxes during a period of rapid reduction in Arctic sea ice in different scenarios: open water after ice melt, free-floating ice bands, and marginal ice zones. In this study, Arctic air-sea heat and momentum fluxes measured by the saildrones are compared to gridded flux products based on satellite data and numerical models to investigate the circumstances under which they agree and differ, and the main sources of their discrepancies. The results will quantify the uncertainty margins in the gridded flux products and provide insights needed to improve their accuracy. We will also discuss the feasibility of using USVs in sustained Arctic observing system to collect benchmark datasets of the changing surface energy fluxes due to rapid sea ice reduction and provide real time data for improved weather and ocean forecasts.  

How to cite: Zhang, D., Zhang, C., Cross, J., Mordy, C., Cokelet, E., Gentemann, C., Chiodi, A., Stabeno, P., Jenkins, R., Meinig, C., Lawrence-Slava, N., Tabisola, H., and Wang, M.: Air-Sea Fluxes following the Unusually Early Ice Retreats in the Arctic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20077, https://doi.org/10.5194/egusphere-egu2020-20077, 2020.

EGU2020-21070 | Displays | OS1.11

Numerical study on ocean response to storm in Arctic Ocean

Hailun He and Long Lin

EGU2020-8178 | Displays | OS1.11

Arctic connections between sea ice, ocean dynamics and biogeochemistry in the UK Earth System Model (UK ESM1): present climate and future scenarios

Yevgeny Aksenov, Andrew Yool, Julien Palmieri, Katya Popova, Stephen Kelly, Stefanie Rynders, David Schroeder, and Bablu Sinha

We present analysis of Arctic sea ice and ocean dynamics in the ensemble of the UK Earth System Model (UK ESM1) simulations completed under the Coupled Model Intercomparison Project Phase 6 (CMIP6) protocol. The focus of the investigation is on the future changes in the Arctic sea ice and oceanic connections and on the impact of the nutrient advection on the Arctic marine biogeochemistry and ecosystems. Changes in the balance of the oceanic inflows from the North Atlantic and North Pacific Oceans are found to have a first order effect on the watermasses and nutrients balances in the central Arctic Ocean. The simulations show that the total primary production in the Arctic Ocean is increased by 100% in the 2090s as compared to the present climate. This is caused by higher nutrients availability in the Atlantic inflowing waters and prolonged ice- free season. The faster connections through the Arctic and milder oceanic environment allows species to survive through the winter and from the second half of the century the Arctic Ocean could become a key oceanic gateway connecting the global oceans. The study is supported from the project APEAR (NE/R012865/1) NERC-BMBF and from the NERC ACSIS Programme (NE/N018044/1).

How to cite: Aksenov, Y., Yool, A., Palmieri, J., Popova, K., Kelly, S., Rynders, S., Schroeder, D., and Sinha, B.: Arctic connections between sea ice, ocean dynamics and biogeochemistry in the UK Earth System Model (UK ESM1): present climate and future scenarios, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8178, https://doi.org/10.5194/egusphere-egu2020-8178, 2020.

EGU2020-17906 | Displays | OS1.11

Further development on sea-ice HBI biomarker proxies.

Maria Luisa Sánchez-Montes, Nikolai Pedentchouk, Thomas Mock, Simon Belt, and Lukas Smik

Sea ice is a crucial component of the Earth’s climate system, which helps regulate global ocean and atmosphere’s temperature. The alarming decline in sea-ice extent and thickness under modern climate conditions has created the urgency to understand the long-term sea-ice variability and mechanisms of change. In recent years, the highly branched isoprenoid (HBI) lipid biomarker IP25 has emerged as a powerful proxy measure of past sea ice in the Arctic, and its analysis in a variety of marine sediments has provided the foundation for a large number of palaeo sea ice reconstructions spanning thousands to millions of years before present. To date, IP25 and related HBI-based studies have focussed largely on reconstructions of sea-ice extent and seasonal dynamics. Here we aim to further develop such sea ice proxies by measuring the changes in distribution and isotopic composition of HBIs in HBI-producing diatoms grown under different controlled laboratory conditions. We present preliminary results from the diatom Haslea ostrearia and outline the next steps of our research in the coming year.

How to cite: Sánchez-Montes, M. L., Pedentchouk, N., Mock, T., Belt, S., and Smik, L.: Further development on sea-ice HBI biomarker proxies., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17906, https://doi.org/10.5194/egusphere-egu2020-17906, 2020.

OS1.12 – The Southern Ocean in a changing climate: open-ocean physical and biogeochemical processes

EGU2020-164 | Displays | OS1.12 | Highlight

Causes and consequences of Southern Ocean change: the IPCC SROCC assessment

Michael Meredith, Martin Sommerkorn, Sandra Cassotta, Chris Derksen, Alexey Ekaykin, Anne Hollowed, Gary Kofinas, Andrew Mackintosh, Jess Melbourne-Thomas, Monica Muelbert, Geir Ottersen, Hamish Pritchard, Ted Schuur, Andrew Meijers, Andrew Hogg, Robert Hallberg, Alessandro Tagliabue, Shengping He, and Victoria Peck

Climate change in the polar regions exerts a profound influence both locally and over all of our planet.  Physical and ecosystem changes influence societies and economies, via factors that include food provision, transport and access to non-renewable resources.  Sea level, global climate and potentially mid-latitude weather are influenced by the changing polar regions, through coupled feedback processes, sea ice changes and the melting of snow and land-based ice sheets and glaciers.

Reflecting this importance, the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) features a chapter highlighting past, ongoing and future change in the polar regions, the impacts of these changes, and the possible options for response.  The role of the polar oceans, both in determining the changes and impacts in the polar regions and in structuring the global influence, is an important component of this chapter.

With emphasis on the Southern Ocean and through comparison with the Arctic, this talk will outline key findings from the polar regions chapter of SROCC. It will synthesise the latest information on the rates, patterns and causes of changes in sea ice, ocean circulation and properties. It will assess cryospheric driving of ocean change from ice sheets, ice shelves and glaciers, and the role of the oceans in determining the past and future evolutions of polar land-based ice. The implications of these changes for climate, ecosystems, sea level and the global system will be outlined.

How to cite: Meredith, M., Sommerkorn, M., Cassotta, S., Derksen, C., Ekaykin, A., Hollowed, A., Kofinas, G., Mackintosh, A., Melbourne-Thomas, J., Muelbert, M., Ottersen, G., Pritchard, H., Schuur, T., Meijers, A., Hogg, A., Hallberg, R., Tagliabue, A., He, S., and Peck, V.: Causes and consequences of Southern Ocean change: the IPCC SROCC assessment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-164, https://doi.org/10.5194/egusphere-egu2020-164, 2020.

EGU2020-20837 | Displays | OS1.12

Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection

Thomas Rackow, Sergey Danilov, Helge F. Goessling, Hartmut H. Hellmer, Dmitry V. Sein, Tido Semmler, and Thomas Jung

Despite ongoing global warming and strong sea ice decline in the Arctic, the sea ice extent around the Antarctic continent has not declined during the satellite era since 1979. This is in stark contrast to existing climate models that tend to show a strong negative sea ice trend for the same period; hence the confidence in projected Antarctic sea-ice changes is considered to be low. In the years since 2016, there has been significantly lower Antarctic sea ice extent, which some consider a sign of imminent change; however, others have argued that sea ice extent is expected to regress to the weak decadal trend in the near future.

In this presentation, we show results from climate change projections with a new climate model that allows the simulation of mesoscale eddies in dynamically active ocean regions in a computationally efficient way. We find that the high-resolution configuration (HR) favours periods of stable Antarctic sea ice extent in September as observed over the satellite era. Sea ice is not projected to decline well into the 21st century in the HR simulations, which is similar to the delaying effect of, e.g., added glacial melt water in recent studies. The HR ocean configurations simulate an ocean heat transport that responds differently to global warming and is more efficient at moderating the anthropogenic warming of the Southern Ocean. As a consequence, decrease of Antarctic sea ice extent is significantly delayed, in contrast to what existing coarser-resolution climate models predict.

Other explanations why current models simulate a non-observed decline of Antarctic sea-ice have been put forward, including the choice of included sea ice physics and underestimated simulated trends in westerly winds. Our results provide an alternative mechanism that might be strong enough to explain the gap between modeled and observed trends alone.

How to cite: Rackow, T., Danilov, S., Goessling, H. F., Hellmer, H. H., Sein, D. V., Semmler, T., and Jung, T.: Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20837, https://doi.org/10.5194/egusphere-egu2020-20837, 2020.

EGU2020-12247 | Displays | OS1.12

Evaluation of the numerical wave model (WaveWatch III) for wave simulation in the Antarctic marginal ice zone

Marzieh H. Derkani, Katrin Hessner, Stefan Zieger, Filippo Nelli, Alberto Alberello, and Alessandro Toffoli

The Southern Ocean is the birthplace of the fiercest waves on the Earth, which play a fundamental role in global climate by regulating momentum, heat and gas exchanges between the atmosphere and ocean. At high latitudes, waves interact with Antarctic sea ice, another crucial player of the Earth's climate system, modulating its expansion in the winter and its retreat in summer and hence affecting the global albedo. Despite the impact of waves on climate, global wave models are considerably biased in the Southern Hemisphere, due to the scarcity of observations in these remote waters. This is exacerbated in the marginal ice zone, the region of ice-covered water between the compact ice or land and the open ocean, where surface waves, upper ocean and atmosphere interact with sea ice but the dominant physics are still largely unknown. To improve our understanding of physical processes in Southern Ocean and model capabilities, the Antarctic Circumnavigation Expedition (ACE) sailed these waters from December 2016 to March 2017 to acquire wave data (among other climate variables) both in the open ocean and Antarctic marginal ice zone. Observations were gathered using a radar-based wave and surface current monitoring system (WaMoS-II) built on board of the research icebreaker Akademik Tryoshnikov. Here, we discuss how these observations underpin the set up, calibration and validation of the WaveWatch III wave model over a domain covering the entire Southern Hemisphere, therefore spanning from tropical waters to the edge of sea ice (open waters only). The calibrated model will then be used to carry out a thorough assessment of different sea ice modules, to evaluate accuracy of predictions in the marginal ice zone. Test cases of waves-in-ice recorded during the Antarctic Circumnavigation Expeditions will be discussed in details.

How to cite: H. Derkani, M., Hessner, K., Zieger, S., Nelli, F., Alberello, A., and Toffoli, A.: Evaluation of the numerical wave model (WaveWatch III) for wave simulation in the Antarctic marginal ice zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12247, https://doi.org/10.5194/egusphere-egu2020-12247, 2020.

Wintertime surface ocean heat loss is the key driver of Subantarctic Mode Water (SAMW) formation. However, until now there have been very few direct observations of fluxes, particularly during winter. Here, we present results from the first concurrent (2015-17 with gaps), air-sea flux mooring deployments in two key SAMW formation regions: the Southern Ocean Flux Site (SOFS) in the Southeast Indian sector and the Ocean Observatories Initiative (OOI) mooring in the Southeast Pacific sector. Gridded Argo and ERA5 reanalysis provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is found to be on average 1.5 times larger at the Southeast Indian than Southeast Pacific sites with stronger extreme heat flux events in the Southeast Indian leading to larger cumulative winter heat loss. For the first time, we show that turbulent heat loss events in the Southeast Indian sector occur in two atmospheric regimes (a direct cold air pathway from the south and an indirect pathway circulating dry Antarctic air via the north). In contrast, heat loss events in the Southeast Pacific sector occur in a single atmospheric regime (cold air from the south). On interannual timescales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. Using ERA5, we show that this is part of a larger zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific SAMW formation regions, associated with anomalous meridional atmospheric circulation. These recent results will be placed in the context of multidecadal variability in the SAMW formation region dominant heat flux patterns over the past 40 years over all 3 sectors of the Southern Ocean (Pacific, Indian and Atlantic).

How to cite: Josey, S., Tamsitt, V., Cerovecki, I., Gille, S., and Schulz, E.: New insights into concurrent air-sea heat flux forcing of Subantarctic Mode Water formation from mooring observations in the Southeast Indian and Southeast Pacific sectors of the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7580, https://doi.org/10.5194/egusphere-egu2020-7580, 2020.

EGU2020-19451 | Displays | OS1.12

Nutrient budgets in Southern Ocean mode waters controlled by nitrification

Pearse Buchanan, Robyn Tuerena, Alessandro Tagliabue, Claire Mahaffey, and Raja Ganeshram

Southern Ocean mode and intermediate waters supply nitrate-rich but silicate-poor waters to the lower latitudes, impeding diatom growth throughout the extra-polar ocean and weakening the ocean’s ability to absorb carbon dioxide from the atmosphere. This silicate deficiency is widely attributed to high silicate to nitrate uptake by iron-limited diatoms. Here, we show that nitrification, by rapidly regenerating nitrate in shallow waters, drives the silicate deficiency. Measurements of nitrate dual isotopes and complementary modelling independently suggest that 15-35% of the nitrate within mode waters is generated by nitrification. Our results reveal that without nitrification, the silicate deficiency would disappear, which would allow the diatomaceous niche to expand. Nitrification therefore provides a key buffering service that mitigates against change in the silicate deficit and subsequently restricts diatom dominance to the polar ocean. This insight highlights the critical importance for understanding Southern Ocean processes, such that the large-scale effects of ongoing environmental change may be realised.

How to cite: Buchanan, P., Tuerena, R., Tagliabue, A., Mahaffey, C., and Ganeshram, R.: Nutrient budgets in Southern Ocean mode waters controlled by nitrification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19451, https://doi.org/10.5194/egusphere-egu2020-19451, 2020.

EGU2020-18205 | Displays | OS1.12

Seasonal carbon dynamics in the Southern Ocean based on a neural network mapping of ship measurements

Lydia Keppler, Peter Landschützer, Nicolas Gruber, Siv Lauvset, and Irene Stemmler

We present a monthly climatology of dissolved inorganic carbon (DIC) in the upper 2000 m of the Southern Ocean north of 65°S, based on a 2-step neural network method that establishes and applies statistical relationships between global fields of physical and biogeochemical properties and direct DIC measurements from the GLODAPv2.2019 database from 2004 trough 2017. We test our method using synthetic data from a global hindcast simulation of the HAMOCC ocean biogeochemistry model, and independent observational datasets. At the month and location of biogeochemical floats from the SOCCOM array, our estimate is on average ~10 μmol kg-1 lower than the calculated DIC based on the float measurements. This difference can be partially explained by the difference in time period (SOCCOM floats used: 2014 through 2017; our estimate: 2004 through 2017). We find that the surface seasonal cycle of DIC has a mean amplitude of ~20 μmol kg-1 in the Southern Ocean, and the months of the highest surface DIC concentrations tend to be in austral spring when vertical mixing dominates the seasonal maximum. We also find that the nodal depth of DIC, the depth where the phase of the seasonal cycle of DIC shifts due to photosynthesis near the surface and remineralisation below, partially extends to several hundred meters. Using the nodal depth allows us for the first time to estimate the basin-wide seasonal net community production (NCP) based on direct DIC measurements. We find a mean NCP of ~2 mol C m-2, which is considerably lower than in the temperate northern hemisphere.

How to cite: Keppler, L., Landschützer, P., Gruber, N., Lauvset, S., and Stemmler, I.: Seasonal carbon dynamics in the Southern Ocean based on a neural network mapping of ship measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18205, https://doi.org/10.5194/egusphere-egu2020-18205, 2020.

EGU2020-10433 | Displays | OS1.12

Variations of the Carbonate Counter Pump in the Southern Ocean during the Mid-Brunhes event and their contribution to the global biospheric productivity

Margaux Brandon, Stéphanie Duchamp-Alphonse, Amaëlle Landais, Elisabeth Michel, Gulay Isguder, and Thomas Extier

During the last 800,000 years, atmospheric CO2 concentrations have varied with an amplitude of more than 100 ppm, with the fastest increases registered during deglaciations. The mechanisms behind the increases of CO2 are still discussed since several parameters are involved. Biological productivity on land and in the ocean played a major role in the variations of atmospheric CO2. Particularly, productivity variations in the Southern Ocean along deglaciations are key because changes in the efficiency of the Soft Tissue Pump (STP) and the Carbonate Counter Pump (CCP) in the Subantarctic Zone significantly impact the exchanges between ocean and atmospheric reservoirs. As calcifying organisms, coccolithophores and planktonic foraminifera represent the major producers of CaCO3 and are therefore good tools to reconstruct past variations of CCP.

Among the last 9 deglaciations, Termination V registers the strongest global productivity (20% higher) compared to the other 8 interglacial periods. Associated with the Mid-Brunhes event, it is followed by the warm MIS 11, the longest interglacial (~ 30 ka). MIS 11 also registers a strong carbonate production in the ocean, most probably favoured by the low eccentricity during this period. Studying the variations of the CCP during this specific period of time is therefore important to better understand its relation with biospheric productivity changes and its impact on atmospheric CO2.

Here we present micropaleontological (coccoliths and foraminifera) and geochemical (CaCO3) data from marine core MD04-2718, located in the Indian sector of the Southern Ocean (48°53 S; 65°57 E) throughout Termination V and MIS 11, that we compared with other productivity data from the Southern Ocean as well as reconstruction of global biospheric productivity data (Δ17O of O2).

 Results show that coccolith and foraminifera abundances and masses increase during Termination V and MIS 11. The good correlation between variations of CaCO3 in the sediment and calcite mass from coccoliths and foraminifera shells proves that exported CaCO3 is essentially of planktonic origin and reveals that CCP significantly increases over this period.

We suggest that the strengthening of CCP through the increase in production and export of calcite associated to coccolith and foraminifera in the Southern Ocean may have contributed to increase the atmospheric CO2 during Termination V and MIS 11, while the strong biological productivity registered during this period would have permitted to maintain the CO2 level relatively low

How to cite: Brandon, M., Duchamp-Alphonse, S., Landais, A., Michel, E., Isguder, G., and Extier, T.: Variations of the Carbonate Counter Pump in the Southern Ocean during the Mid-Brunhes event and their contribution to the global biospheric productivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10433, https://doi.org/10.5194/egusphere-egu2020-10433, 2020.

EGU2020-18595 | Displays | OS1.12

A new method for characterising the Antarctic Circumpolar Currents using Argo float temperature and salinity profiles

Luke Roberts, Rhiannon Jones, Matthew Donnelly, and Katharine Hendry

Argo is an array of automated profiling floats, which have allowed the rapid development of high-resolution and high-quality oceanographic data acquisition. The international program has been in operation since the 1990s providing continuous hydrographic data globally. There are now over a million individual float profiles, contributing to our understanding of global ocean physical properties, such as circulation processes at both a local and regional scale. With these innovations come the challenges of data processing, and compilation of user-friendly data products. For example, the Southern Ocean is a critical region that modulates our climate, via heat exchange, carbon storage, biogeochemistry, and primary productivity. An improved quantified understanding of Southern Ocean currents, informed by Argo, must be implemented in policy-relevant high-resolution climate models to advance our understanding of future change.

 

In May 2019, a new collaboration was formed between the Southern Ocean Argo Resource Centre (British Oceanographic Data Centre) and the University of Bristol. The aims were two-fold: to produce a method for characterising Southern Ocean frontal zones using Argo floats, and to train early career researchers in the University sector in data processing and management. We have created a publicly available code that characterises physical features of the Antarctic Circumpolar Current using Argo float profiles, using minimal software, and without the need to access high-performance computers. The code categorises each profile based on the temperature and salinity ‘fingerprints’ of zones between each Southern Ocean front. This allows the user to produce output surface plots from user-specified time-slices and geographic areas, and so compare frontal movement in time and space.

How to cite: Roberts, L., Jones, R., Donnelly, M., and Hendry, K.: A new method for characterising the Antarctic Circumpolar Currents using Argo float temperature and salinity profiles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18595, https://doi.org/10.5194/egusphere-egu2020-18595, 2020.

EGU2020-1957 | Displays | OS1.12

Jets of the Antarctic Circumpolar Current in the Drake Passage Based on Hydrographic Section Data

Roman Tarakanov and Alexander Gritsenko

We have analyzed the fine structure of Antarctic Circumpolar Current jets in the Drake Passage based on CTD and SADCP measurements over two hydrographic sections in January 2010 and October–November 2011. Eleven jets with a local horizontal velocity maximum were revealed in 2010, and nine jets were in 2011. These individual jets were various combinations of 12 jets of the Antarctic Circumpolar Current, which we revealed earlier in the region south of Africa on the basis of the section data in December 2009. Daily satellite altimetry data available at http://www.aviso.altimetry.fr were also used to interpret the synoptic patterns of currents over the sections. These results allow us to suggest that the multi-jet structure with a number of jets exceeding nine reported by Sokolov&Rintoul, 2009 is common for the entire circumpolar circle and even for regions with significant contraction of the ACC, such as the Drake Passage. However, the question about the number of jets and its temporal and spatial permanency remains open. Investigation was supported by Russian Foundation of Basic Research grant No 18-05-00283.

How to cite: Tarakanov, R. and Gritsenko, A.: Jets of the Antarctic Circumpolar Current in the Drake Passage Based on Hydrographic Section Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1957, https://doi.org/10.5194/egusphere-egu2020-1957, 2020.

EGU2020-13460 | Displays | OS1.12

Characterization of the ocean mesoscale eddies in the Antarctic Circumpolar Current from in situ, model and remotely sensed data

Yuri Cotroneo, Lavinia Patara, Milena Menna, Pierpaolo Falco, Jan Klaus Rieck, Giulio Notarstefano, Giannetta Fusco, Giorgio Budillon, and Pierre-marie Poulain

Mesoscale variability and associated eddy fluxes play crucial roles in the ocean dynamics, transport of water mass properties and ecology of the upper ocean. In the Southern Hemisphere, where the nearly zonal flow of the Antarctic Circumpolar Current (ACC) acts as a barrier to the direct poleward transport toward the Antarctica, the eddy flux across the ACC is the main mechanism that guarantees the heat budget and distributes physical and biogeochemical properties between subtropical and polar regions. We focused on a high dynamical region located between the South-West Indian Ridge and the South Pacific Ridge. In this area, the interaction between the ACC and the major bathymetric features produces relatively large values of eddy kinetic energy and eddy heat fluxes as well as a relevant forcing for the ACC path.

The aim of this study is to evaluate the actual efficiency of mesoscale eddies to exchange heat and other properties across the different ACC fronts and to describe the vertical properties of the eddies, their tracks and evolution. To this end, we used in-situ and satellite data in conjunction with a hindcast simulation from 1958 to 2018 performed with a 1/10° ocean biogeochemistry model.

Eddies are identified and tracked in both the model output and altimetry data while their thermohaline properties and vertical extension are described using model outputs and in situ data, which include available repeated XBT sections (i.e. New Zealand – Antarctica and Hobart – Antarctica) and Argo float profiles located inside these structures.

Thanks to the joint analysis of model and observational data, we are able to 1) assess the ability of the 1/10° ocean model of simulating the eddy field properties, and to 2) better interpret the spatial and temporal variability of the observed eddy characteristics in the larger and longer framework of the ocean simulation.

How to cite: Cotroneo, Y., Patara, L., Menna, M., Falco, P., Rieck, J. K., Notarstefano, G., Fusco, G., Budillon, G., and Poulain, P.: Characterization of the ocean mesoscale eddies in the Antarctic Circumpolar Current from in situ, model and remotely sensed data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13460, https://doi.org/10.5194/egusphere-egu2020-13460, 2020.

EGU2020-5944 | Displays | OS1.12

A new convective model for the Maud Rise Polynya

Daan Boot, Rene van Westen, and Henk Dijkstra

The Maud Rise Polynya, a large hole in the Antarctic sea-ice, was first observed in the 1970s and reappeared again in 2017. The general paradigm is that the polynya formed due to deep convection caused by static instability of the water column. There is, however, no consensus on the processes responsible for the initialisation of deep convection. Both atmospheric and oceanic processes have been suggested by observational and model studies. Deep convection is viewed as an irregular event caused by densification of the surface layer. Heat accumulation in the subsurface layer is also considered to be vital for the formation of the polynya. This study investigates the initiation of deep convection using a simple 1D convective model introduced by Martinson et al. (1981) which is further extended with a dynamical subsurface layer. This extended version of the model allows us to study the contribution of both surface- and subsurface forcing on the initiation of deep convection. Two model set-ups with different subsurface characteristics have been used: (1) with a constant subsurface layer; (2) with periodic subsurface accumulation of heat and/or salt. Model set-up 1 results in either one or no polynya events. This does not agree with observations, since multiple polynya events have been observed. Model set-up 2 results in in periodically returning polynyas with the same period as the subsurface accumulation. Therefore, model set-up 2 is able to again multiple events as observed. Adding noise to the simulations does not change the conclusions for both model set-ups. The results suggest that subsurface forcing is a dominant process in Maud Rise Polynya formation. Our results indicate that densification of the surface layer plays a much smaller role than previously assumed by various literature. Based on these results and previous studies, we suggest that subsurface processes govern both the initial formation and reccurrence of the Maud Rise Polynya.

How to cite: Boot, D., van Westen, R., and Dijkstra, H.: A new convective model for the Maud Rise Polynya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5944, https://doi.org/10.5194/egusphere-egu2020-5944, 2020.

The variability of the Atlantic Meridional Overturning Circulation (AMOC) is known to have considerable impacts on the global climate system. In particular, the South Atlantic is thought to control the stability of the AMOC. That’s why significant resources have been invested in measuring the AMOC in the South Atlantic since 2009 mainly through mooring deployments accompanied by yearly cruise campaigns in the eastern and western boundaries. In January 2017, the RV Maria S. Merian conducted the first hydrographic (GO-SHIP) transect along the SAMOC-SAMBA line located at 34.5°S in the South Atlantic. In this study, we present analyses of the data obtained during the cruise. Volume and heat transports were calculated using the slow varying geostrophic density field and Ekman transport from daily satellite wind stress data. The general structure of these transports is in close agreement with other estimations at near latitudes. Two defined overturning cells are distinguished with differences in the abyssal circulation between the west and east. While the west shows Antarctic Bottom Waters flowing northward (ɣ > 28.27 kg.m-3) and Lower Circumpolar Deep Waters (LCDW, 28.10 kg.m-3 <  ɣ < 28.27 kg.m-3) above flowing southward, only LCDW are found in the east, flowing northward and southward from 6°E, respectively. The section presented a positive mass imbalance. By assuming the latter to be the barotropic transport component we equally distributed it along the vertical in order to satisfy a net-zero transport. The overturning maximum is located at 1250 meters deep and is 18 Sv with net northward heat transport. Moreover, by using the Laxenaire et al., (2018) eddy detection method, we identified the various mesoscale eddies crossed during the cruise. This analysis provided evidence of the presence of 13 cyclonic and 12 anticyclonic eddies. Among the anticyclonic eddies, three were identified as Agulhas rings. One of these eddies was located close to the Brazilian slope and was more than 5 years old since its first detection. During the cruise, a particularly intense cyclonic eddy was crossed near the African coast. The upper layer velocity within the eddies exceeded 1 m.s-1, with an asymmetric stronger northward flow. Although mesoscale eddies dominate the upper 1000 meters circulation, no significant differences were found in the overturning circulation when replacing the upper 1200 meters layer of 34.5°S observed section with a climatology generated with the ARGO profiles located outside eddies.

How to cite: Manta, G., Speich, S., and Karstensen, J.: Overturning Circulation and mesoscale eddies in the first GO-SHIP section at 34.5ºS across the South Atlantic during January 2017, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5883, https://doi.org/10.5194/egusphere-egu2020-5883, 2020.

The dominant Subantarctic Mode Water (SAMW) formation regions are located in the Indian, and in the Pacific sector of the Southern Ocean. Strong wintertime (Jul-Sep) surface air pressure anomalies with variance maxima at approximately 100°E and 150°W drive a zonal dipole structure in the SAMW formation and thickness, in both the Indian and Pacific sector of the Southern Ocean. This has been documented within gridded Argo data for years 2005-2019. A much weaker surface air pressure anomaly variance maxima is located in the Atlantic Ocean centered at approximately 25°W.

Anomalously strong positive pressure anomalies result in deepening of the wintertime mixed layers and an increase in the SAMW formation in the eastern part of the Pacific and Indian sector; these effects are due to cold southerly winds, strengthened zonal winds and increased surface ocean heat loss. 
Anomalously strong negative pressure anomalies result in shoaling of the wintertime mixed layers and a decrease in SAMW formation in these regions, while at the same time deepening the wintertime mixed layers and increasing SAMW formation in the western Indian Ocean and in the central Pacific.

In years with strong El Nino, the interannual variability of the strength of two surface air pressure anomalies does not co-vary in phase with each other. Strong isopycnal heave in SAMW density range emanates from locations where winter surface air pressure anomalies and mixed layers are most strongly coupled.  

How to cite: Cerovecki, I. and Meijers, A.: Strong atmospheric surface pressure anomalies drive a see-saw in Subantarctic Mode Water formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12128, https://doi.org/10.5194/egusphere-egu2020-12128, 2020.

EGU2020-4670 | Displays | OS1.12

Variability of the Subantarctic Mode Water volume in the South Indian Ocean during 2004-2018

Yu Hong, Yan Du, Tangdong Qu, and Wenju Cai

Analysis of the Argo data reveals that the Subantarctic Mode Water (SAMW) in the South Indian Ocean, characterized by a vertical potential vorticity minimum, decreases by 10% in volume from 2004 to 2015. Most of this volume decrease occurs in the density range 26.8-26.9 kg m-3 which forms southwest of Australia, while a slight volume increase occurs in 26.6-26.8 kg m-3. Further analysis of the data indicates that a reduction of subtropical high and westerly winds in the South Indian Ocean weakens (intensifies) the E-P, heat loss, Ekman pumping and shoals (deepens) the mixed layer southwest of Australia (west of 90°E), which leads the decrease in 26.8-26.9 kg m-3 (increase in 26.6-26.8 kg m-3) by 3 years (see the figure below). This result suggests that the subtropical wind system variation plays an important role in the volume variation of SAMW in the South Indian Ocean in the Argo period.

How to cite: Hong, Y., Du, Y., Qu, T., and Cai, W.: Variability of the Subantarctic Mode Water volume in the South Indian Ocean during 2004-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4670, https://doi.org/10.5194/egusphere-egu2020-4670, 2020.

EGU2020-8015 | Displays | OS1.12

Reshuffling of Nutrients in the Southern Ocean

Ivy Frenger, Ivana Cerovecki, and Matthew Mazloff

Deep waters upwell in the Southern Ocean, replete with nutrients. Some of these nutrients enter lighter mode and intermediate waters (MIW), fueling upper ocean productivity in the otherwise nutrient depleted (sub)tropical waters. However some of the upwelled nutrients are retained in the Southern Ocean or leak into denser bottom waters (AABW), making them unavailable for upper ocean productivity. Despite its fundamental importance for the global ocean productivity, this “reshuffling” of nutrients between Southern Ocean water masses, and its driving forces and temporal variability, have not been quantified to date.

We analyze the globally major limiting macronutrient, nitrate (NO3), using the results of a data-assimilating coupled ocean-sea-ice and biogeochemistry model, the Biogeochemical Southern Ocean State Estimate (B-SOSE), for the years 2008 – 2017. Using a water mass framework, applied to five day averaged SOSE output south of 30oS, we quantify the processes controlling NO3 inventories and fluxes. The water mass framework enables us to assess the relative importance of physical processes (such as surface buoyancy fluxes and diapycnal mixing) and biogeochemical processes (such as productivity and remineralization) in driving the transfer of NO3 from upwelling deep waters (CDW) to MIW and AABW, and its interannual variability.

Our results show that two thirds of the NO3 supplied to MIW occurs through lightening, or transforming, of CDW waters during the course of the overturning circulation. The other third of the NO3 supplied to MIW occurs through upward mixing of NO3 from NO3-enriched CDW. This means that physical processes determine the mean MIW NO3 content. Biology does not have a net effect on MIW NO3: while biological uptake draws down the MIW concentration of  NO3 near the surface, remineralization of organic matter compensates for this MIW loss below the surface. Also, we find that the productivity in the subtropical waters south of 30oS is fed through both, the canonical upward mixing of NO3 through the thermocline, and through the near surface supply from MIW. Thus, again, water mass transformation is playing a large role in nutrient distributions. 

In ongoing work, we assess the drivers of variability of the reshuffling of NO3 between water masses and their potential sensitivity to climate change.

How to cite: Frenger, I., Cerovecki, I., and Mazloff, M.: Reshuffling of Nutrients in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8015, https://doi.org/10.5194/egusphere-egu2020-8015, 2020.

EGU2020-3957 | Displays | OS1.12

Nitrate isotopic constraints on routes of nutrient supply to global ocean pycnocline

Francois Fripiat, Aflredo Martinez-Garcia, Dario Marconi, Sarah E. Fawcett, Daniel M. Sigman, and Gerald H. Haug

The circulation of the ocean plays a fundamental role in restoring the surface nutrients necessary to maintain global ocean biological production. However, our quantitative understanding of the physical mechanisms that return deep-ocean water and nutrients to the upper ocean is currently limited. The nitrate isotopes are investigated here as a new data constraint on the percentage of gross water transport into the global pycnocline that derives from the Southern Ocean as opposed to the deep ocean (which we term the “pycnocline recipe”). Based on a comparison between large-scale observations of nitrate isotopes and the output of a box model, we estimate that the pycnocline recipe is 75 ± 10%; this result implies that ~ 64% of the nutrients supplied to the low latitude pycnocline pass through the Southern Ocean. Our simulations also highlight the shortcomings of a purely advective view of the ocean’s transport of water and nutrients, confirming that mixing with both the deep ocean and the Southern Ocean ventilating area are key to the exchange of water and nutrients between the pycnocline and higher-density deep and polar surface waters. Our calculations support a pure advective-diffusive balance in the deep ocean. In contrast, in the Southern Ocean, our findings provide independent evidence for the importance of air-sea fluxes of momentum and buoyancy in driving the circulation.

How to cite: Fripiat, F., Martinez-Garcia, A., Marconi, D., Fawcett, S. E., Sigman, D. M., and Haug, G. H.: Nitrate isotopic constraints on routes of nutrient supply to global ocean pycnocline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3957, https://doi.org/10.5194/egusphere-egu2020-3957, 2020.

EGU2020-3950 | Displays | OS1.12

Limitation by iron and manganese on phytoplankton communities in the Drake Passage.


Jenna Balaguer, Florian Koch, and Scarlett Trimborn

The Southern Ocean (SO) accounts for over 40% of anthropogenically derived CO2 uptake. It is the world’s largest High-Nutrient Low-Chlorophyll (HNLC) region and the scarcity of trace metals such as iron (Fe) drives phytoplankton composition and biomass build up. Besides Fe, manganese (Mn) is the second most abundant trace metal since it is present in the thylakoids. As dissolved manganese (dMn) concentrations in the Atlantic sector of the SO are very low (0.04 nM), phytoplankton growth may not only be limited by Fe but also by Mn availability, a theory previously described by Martin et al. (1990). However, mechanistic studies investigating the effects of multiple trace metals limiting or co-limiting on growth and photosynthesis are lacking. This study focuses on the identification of the Fe-Mn co-limitation of natural phytoplankton assemblages to elucidate the impact of different Fe and Mn additions on species composition. To this end, two shipboard Fe-Mn addition bottle incubation experiments were conducted during the ‘RV Polarstern’ expedition PS97 in the Western and Eastern Drake Passage (DP) in 2016. This study highlights the importance of Mn in the otherwise Fe-limited Drake Passage. From microscopy samples, the addition of Fe and Mn together triggered the highest abundance of the genus Fragilariopsis sp. in the Western DP. In the Eastern DP, the nanophytoplankton fraction, detected by flow cytometry, reached the highest abundance only when both trace elements were provided, confirmed by highest chlorophyll-a build up. Moreover, the distinct response of Mn depletion relative to the Fe depletion support the findings that Fe and Mn do not substitute to each other. This experimental study highlights that both trace elements act as drivers of the ecology across the Drake Passage.

How to cite: Balaguer, J., Koch, F., and Trimborn, S.: Limitation by iron and manganese on phytoplankton communities in the Drake Passage.
, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3950, https://doi.org/10.5194/egusphere-egu2020-3950, 2020.

EGU2020-5855 | Displays | OS1.12

Iron and manganese colimitation in the Southern Ocean examined with a proteomic allocation model

J. Scott P. McCain, Eric P. Achterberg, Alessandro Tagliabue, and Erin M. Bertrand

Iron (Fe) limits primary productivity in a large part of the ocean; but recent geochemical profiles and bottle incubation assays indicate that manganese (Mn) can also limit primary productivity in the Southern Ocean. Fe and Mn can interact to influence primary productivity, but the extent to which these elements colimit phytoplankton in the Southern Ocean is uncertain. In addition, current models are insufficient to assess colimitation as they assume a single, most scarce, resource. In order to examine Fe and Mn colimitation in phytoplankton, we developed a modeling framework to predict proteomic profiles under varying Fe, Mn, and light conditions. In our model, proteins are optimally allocated to various coarse-grained cellular pools, governed by environmental conditions. We predict that Fe controls cellular Mn quotas, largely because of paired stoichiometry within photosynthetic machinery. Our model suggests that the diffusion-limitation paradigm of Fe should be revisited, as diffusive flux of Fe does not appear to be limiting. We then use our model to explore various cellular mechanisms leading to phytoplankton Fe limitation. Lastly, using Fe and Mn biogeochemical model output, we predict regions in the Southern Ocean where Fe/Mn colimitation is most likely to occur.

How to cite: McCain, J. S. P., Achterberg, E. P., Tagliabue, A., and Bertrand, E. M.: Iron and manganese colimitation in the Southern Ocean examined with a proteomic allocation model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5855, https://doi.org/10.5194/egusphere-egu2020-5855, 2020.

The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies,  which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\% strengthening of the Southern Hemispheric westerlies leads to a $\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\circ$S, the amplitude is three times greater in the poleward strengthening case, which is  not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell,  which leads to an increase in natural and total carbon north of 35$^\circ$S in the upper 2000 m.

How to cite: Spence, P., Menviel, L., and Waugh, D.: Natural carbon release over-rides anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12282, https://doi.org/10.5194/egusphere-egu2020-12282, 2020.

EGU2020-10767 | Displays | OS1.12

The role of changing Southern Ocean circulation for the uptake and storage of CFC-12, anthropogenic CO2 and oxygen

Lavinia Patara, Jan Klaus Rieck, Toste Tanhua, Iris Kriest, and Claus Boening

Recent studies highlighted an increase in the Southern Ocean ventilation since several decades and, at the same time, pronounced decadal fluctuations in its carbon sink. The role of changing ventilation for the anthropogenic CO2 uptake (and thus for the overall Southern Ocean carbon sink) and for oxygen concentrations of mode and intermediate waters (with possible impacts on low-oxygen regions downstream) are still poorly understood. The aim of this study is to assess the role of changing Southern Ocean ventilation for the uptake and storage of atmospheric gases such as CFC-12, CO2 and oxygen. A related question is whether CFC-12 can be used as a proxy of anthropogenic CO2 in the Southern Ocean, since CO2 equilibrates significantly more slowly in seawater than CFC-12 and, while the solubility of CFC-12 increases with decreasing temperature and salinity, the solubility of anthropogenic CO2 decreases. We developed a suite of global configurations based on the NEMO-LIM2 ocean sea ice model including the passive tracer CFC-12 and the biogeochemical model MOPS. The suite includes ORCA05 (1/2° resolution), ORCA025 (1/4° resolution) and ORION10 (featuring a 1/10° nest between 68°S and 30°S). Hindcast and sensitivity experiments performed with ORCA025 under the JRA-55-do atmospheric forcing are used to unravel the role of changing wind and buoyancy forcing on the gas uptake and storage. First results highlight that anthropogenic CO2 is taken up in lighter density classes than CFC-12, meaning that increased ventilation of lighter mode waters would be particularly effective in taking up anthropogenic CO2. This effect is more pronounced in the higher-resolution model ORION10, indicating that mesoscale eddies inject anthropogenic CO2 in lighter waters than lower-resolution models.

How to cite: Patara, L., Rieck, J. K., Tanhua, T., Kriest, I., and Boening, C.: The role of changing Southern Ocean circulation for the uptake and storage of CFC-12, anthropogenic CO2 and oxygen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10767, https://doi.org/10.5194/egusphere-egu2020-10767, 2020.

EGU2020-1755 | Displays | OS1.12

Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks over the last eight glacial cycles

Karl Stein, Axel Timmermann, Eun Young Kwon, and Tobias Friedrich

The Southern Ocean (SO) played a prominent role in the exchange of carbon between ocean and atmosphere on glacial timescales through its regulation of deep ocean ventilation. Previous studies indicated that SO sea ice could dynamically link several processes of carbon sequestration, but these studies relied on models with simplified ocean and sea ice dynamics or snapshot simulations with general circulation models. Here we use a transient run of the LOVECLIM intermediate complexity climate model, covering the past eight glacial cycles, to investigate the orbital-scale dynamics of deep ocean ventilation changes due to SO sea ice. Cold climates increase sea ice cover, sea-ice export, and Antarctic Bottom Water formation, which are accompanied by increased SO upwelling, stronger poleward export of Circumpolar Deep Water, and a reduction of the atmospheric exposure time of surface waters by a factor of ten. Moreover, increased brine formation around Antarctica enhances deep ocean stratification, which could act to decrease vertical mixing by a factor of four compared to the current climate. The impact of the two mechanisms on carbon sequestration was then tested within a steady-state carbon cycle. The two mechanisms combined can reduce atmospheric carbon by 40 ppm, of which approximately 30 ppm is due to ocean stratification. Moreover, ocean stratification from increased SO sea ice production acts early within glacial cycles to amplify the carbon cycle response.

How to cite: Stein, K., Timmermann, A., Kwon, E. Y., and Friedrich, T.: Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks over the last eight glacial cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1755, https://doi.org/10.5194/egusphere-egu2020-1755, 2020.

EGU2020-22442 | Displays | OS1.12

Long time-series of export fluxes in the western Ross Sea (Antarctica)

Patrizia Giordano, Federico Giglio, Mariangela Ravaioli, Marco Capello, Laura Cutroneo, Robert B. Dunbar, David A. Mucciarone, Walker O. Smith, Clara Manno, and Leonardo Langone

The export of particulate organic carbon (POC) from the sea surface is an essential part of the biological pump. Export fluxes are the result of what is produced in surface water and how much is consumed during particle sinking in the water column. In the Ross Sea, fluxes of POC and total mass are well correlated implying that particle fluxes are dominated by biogenic debris.

Here, we report new and reference data of vertical particle fluxes to below the productive layer obtained on decadal time scales (1990-2017) by automatic sediment traps tethered to moorings in the western Ross Sea (Antarctica). Compilation of all data available in the Ross Sea (23 sites, >1000 samples) shows that annual POC fluxes to below 200 m average 4.4±3.3 g C m-2  y-1. Particle fluxes are relatively low when primary production is high (spring-summer) followed by enhanced sedimentation in late summer-fall. The high degree of decoupling between production and sedimentation is unusual compared to records of Antarctic Peninsula and may represent low grazing rates. Furthermore, data exhibit a large interannual variability and a decreasing trend over time, with a clear shift after 2000. Do the reduced export fluxes depend on lower biological production, enhanced OM consumption, or other processes (e.g., lateral transfer of biogenic particles outside the study area)?

Satellite observations allow us to reconstruct the seasonal and interannual change of chlorophyll biomass, and sea ice extent and duration. Water temperature recorded at mid-depth is used to monitor the different intrusion over time of CDW, the main driver of temporal variability of Fe supply for the Ross Sea. Time series of particle fluxes, chlorophyll, sea ice cover and mid-depth temperature will be compared in order to test if the recent reduction of downward particle fluxes depend on primary production changes.

How to cite: Giordano, P., Giglio, F., Ravaioli, M., Capello, M., Cutroneo, L., Dunbar, R. B., Mucciarone, D. A., Smith, W. O., Manno, C., and Langone, L.: Long time-series of export fluxes in the western Ross Sea (Antarctica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22442, https://doi.org/10.5194/egusphere-egu2020-22442, 2020.

EGU2020-19394 | Displays | OS1.12

Cocccolithophore contribution to carbonate export to the deep sea in the Australian-New Zealand sector of the subantarctic Southern Ocean

Andres Rigual-Hernandez, Thomas W. Trull, Scott D. Nodder, José A. Flores, Helen Bostock, Fátima Abrantes, Ruth S. Eriksen, Francisco J. Sierro, Diana M. Davies, Anne-Marie Ballegeer, Miguel A. Fuertes, Alba González Lanchas, and Lisa C. Northcote

Coccolithophores are ubiquitous marine unicellular algae belonging to the Class Prymnesiophyceae, division Haptophyta. They are distinct from other phytoplankton groups in their capacity to produce minute calcite platelets (termed coccoliths) with which they cover their cells. During the Cretaceous and throughout the Cenozoic era, pelagic sedimentation of carbonate was largely controlled by coccolithophores as evidenced by their major contribution to deep-sea oozes and chalks. In the modern Southern Ocean, coccolithophores represent an important component of the phytoplankton communities and carbon cycle. However, their contribution to total Particulate Inorganic Carbon (PIC) for large regions of the Southern Ocean remains undocumented.

Here we report the Particulate Inorganic Carbon (PIC) and coccolithophore fluxes collected over a year by sediment traps placed at two sites of the subantarctic Southern Ocean. We present coccolith mass estimates of the most abundant coccolithophore species and quantitatively partition annual PIC fluxes amongst heterotrophic calcifying plankton and coccolithophores. Our results reveal that coccolithophores account for approximately half of the annual PIC export in the subantarctic Southern Ocean. Moreover, in contrast to satellite estimations, that mainly reflect coccospheres and detached coccoliths of Emiliania huxleyi, less abundant but larger species make the largest contribution to CaCO3. Lastly, comparison of our data with previous studies suggest that projected environmental change in the Southern Ocean may result in a decline of coccolithophore PIC production and export.

 

How to cite: Rigual-Hernandez, A., Trull, T. W., Nodder, S. D., Flores, J. A., Bostock, H., Abrantes, F., Eriksen, R. S., Sierro, F. J., Davies, D. M., Ballegeer, A.-M., Fuertes, M. A., González Lanchas, A., and Northcote, L. C.: Cocccolithophore contribution to carbonate export to the deep sea in the Australian-New Zealand sector of the subantarctic Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19394, https://doi.org/10.5194/egusphere-egu2020-19394, 2020.

EGU2020-7505 | Displays | OS1.12

Ocean productivity and bottom water oxygenation across the onset of the Cenozoic cooling trend

Sophie M. Alexander, Philip F. Sexton, Pallavi Anand, and Steven M. Bohaty

The onset of the Cenozoic global cooling trend represents a major transition in Earth’s climate from the extremely warm early Eocene ‘greenhouse’ towards much colder unipolar glaciation by the end of the Eocene. Data across this interval of profound climatic change have long suggested contemporaneous changes to marine biological productivity and the carbon cycle. Modern observations, conceptual models from the last glacial cycle and models documenting change at Cenozoic timescales, all predict heterogeneous biological productivity responses between high and low latitudes. We test for this heterogeneity between latitudes across the onset of the Cenozoic global cooling trend. Here we utilise bulk sediment elemental compositions, and concentration of benthic foraminifera and ichthyoliths to reconstruct changes in palaeoproductivity, export and preservation of CaCO3 across the early-to-middle Eocene interval (~42 to 50 Ma) at multiple deep-sea sites in the Atlantic basin. We also present evidence of changes in bottom-water oxygenation across the Southern Atlantic sites. We find opposing trends in Biogenic barium (BioBa) versus benthic foraminifera accumulation rates (BFAR) and Ichythyolith Accumulation Rates (IAR) for many of our sites across the high southern latitudes. These trends could be explained by increased organic carbon flux to the sediment, which would have increased the BFAR and IAR while at the same time potentially causing oxygen depletion and reductive barite dissolution in the sediments driving our observed synchronous decreases in BioBa and Mn/Al. This trend of synchronous decreases in BioBa and Mn/Al is evident within the Atlantic Sector of the Sub-Antarctic Zone. Within the Antarctic zone, an opposing trend of decreasing BioBa coincides with increasing Mn/Al and BFAR, suggesting a differing mode of production, export and fate of marine CaCO3. Ongoing work seeks to determine the nature of biological productivity responses across the high and low latitude oceans during the Eocene ‘greenhouse’ to ‘icehouse’ transition.

How to cite: Alexander, S. M., Sexton, P. F., Anand, P., and Bohaty, S. M.: Ocean productivity and bottom water oxygenation across the onset of the Cenozoic cooling trend, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7505, https://doi.org/10.5194/egusphere-egu2020-7505, 2020.

OS1.13 – Under cover: ice-ocean interactions from the boundary layer to the Southern Ocean

EGU2020-21107 | Displays | OS1.13

Productivity and carbon export potential in the Weddell Sea, with a focus on the waters near Larsen C Ice Shelf

Raquel Flynn, Jessica Burger, Shantelle Smith, Kurt Spence, Thomas Bornman, and Sarah Fawcett

Net primary production (NPP) is indicative of the energy available to an ecosystem, which is central to ecological functioning and biological carbon cycling. The Southern Ocean’s Weddell Sea (WS) represents a point of origin where water masses form and exchange with the atmosphere, thereby setting the physical and chemical conditions of much of the global ocean. The WS is particularly understudied near Larsen C Ice Shelf (LCIS) where harsh sea-ice conditions persist year-round. We measured size-fractionated rates of NPP, nitrogen (N; as nitrate, ammonium, and urea) uptake, and nitrification, and characterized the phytoplankton community at 19 stations in summer 2018/2019, mainly near LCIS, with a few stations in the open Weddell Gyre (WG) and at Fimbul Ice Shelf (FIS). Throughout the study region, NPP and N uptake were dominated by nanophytoplankton (3-20 μm), with microphytoplankton (>20 μm) becoming more abundant later in the season, particularly at FIS. Here, we observed high phytoplankton biomass and diversity, and the community was dominated by diatoms known to enhance carbon export (e.g., Thalassiosira spp.). At LCIS, by contrast, the community comprised mainly Phaeocystis Antarctica. In the open WG, a population of small and weakly-silicified diatoms of the genus Corethron dominated the phytoplankton community. Here, euphotic zone-integrated uptake rates were similar to those at LCIS even though the depth-specific rates were lower. Mixed-layer nitrification was below detection at all stations such that nitrate uptake can be used as a proxy for carbon export potential sensu the new production paradigm – this was highest near FIS in late summer. Our observations can be explained by melting sea ice near the ice shelves that supplies iron and enhances water column stratification, thus alleviating iron and/or light limitation of phytoplankton and allowing them to consume the abundant surface macronutrients. That the sea ice melted completely at FIS but not LCIS may explain why late-summer productivity and carbon export potential were highest near FIS, more than double the rates measured in early summer and near LCIS. The early-to-late summer progression near the ice shelves contrasts that of the open Southern Ocean where iron is depleted by late summer, driving a shift towards smaller phytoplankton that facilitate less carbon export.

How to cite: Flynn, R., Burger, J., Smith, S., Spence, K., Bornman, T., and Fawcett, S.: Productivity and carbon export potential in the Weddell Sea, with a focus on the waters near Larsen C Ice Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21107, https://doi.org/10.5194/egusphere-egu2020-21107, 2020.

Sea ice growth into seawater proceeds by congelation of almost pure ice crystals, with most sea salts being rejected into a solute-enriched boundary layer. This solute enrichment does locally change the freezing point and thereby alter crystal growth due to a process termed "Constitutional Supercooling" (heat diffusing much faster than solute). It also sets up solute gradients that drive convection in a boundary layer of thickness D/V, where D is the coefficient of solute diffusion and V the ice growth velocity. The crystal pattern arising from such interaction of solute and temperature gradients, and respective solute and heat fluxes at the freezing interface, has been observed for other systems. It may be described by the theory of "Morphological Instability" formulated half a century ago by Mullins and Sekerka (1964). In principal this theory predicts the cell or dendrite spacing at a freezing interface of a saline solution. However, application of the theory to the case of sea ice growing from seawater has been incomplete so far due to three aspects: (i) detailed observations of the sea ice - ocean interface are difficult to obtain, as this high-porosity regime is rather fragile and often lost during sampling, and/or altered during cooling when the sample is removed from the water. However, from thin sections further up in the ice the interface is known to be lamellar, consisting of vertical oriented plates with a distance of 0.3 to 1 mm found for natural growth conditions; (ii) observations of the solutal boundary layer are even sparser and limited to a few laboratory studies, where this layer was heavily convecting; (iii) from a theoretical point it turns out that the classical Mullins-Sekerka theory needs to be modified due to such a convecting boundary layer. In the present talk I review the existing observations, and present novel 3-d observations of the microstructure near the interface of growing sea-ice. I propose an application of morphological stability theory to predict the plate spacing of sea ice and the salt fluxes through a convectively unstable solutal boundary layer. The predictions are consistent with observed plate spacings over a wide range of ice growth velocities, ranging from fast (100 cm/day) laboratory ice growth to slow (0.01 cm/day) accretion at the bottom of marine ice shelves. These predictions are not only of importance to predict ice properties near the interface, yet indicate the potential to trace sea ice and ice shelf growth rates through microstructure observations. Regarding the solutal boundary layer, the theory is found to be consistent with published observations of salt fluxes from growing sea ice.

W. W. Mullins and R. F. Sekerka, J. Appl. Phys., 1964, 35, 444.

How to cite: Maus, S.: Microstructure and solutal boundary layer at the sea ice - ocean interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6039, https://doi.org/10.5194/egusphere-egu2020-6039, 2020.

EGU2020-463 | Displays | OS1.13

Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean

Ria Oelerich, Karen J. Heywood, Gillian M. Damerell, and Andrew F. Thompson

The Bellingshausen Sea, located between the West Antarctic Peninsula and the Amundsen Sea, is poorly observed, compared with its neighbours. The Antarctic Slope Front (ASF), that rings the continental slope of Antarctica, supports a westward current (the Antarctic Slope Current). The structure and variability of this current affect exchange processes close to Antarctica such as the transport of warm Circumpolar Deep Water onto the Antarctic continental shelf. This water mass is responsible for the transport of heat across the shelf and therefore the basal melting of ice shelves. Due to the lack of observations, it is still unclear if the ASF even exists in the Bellingshausen Sea or if there are other processes moderating the transport of warm water onto the shelf.

We present ship-based and glider-based CTD data collected in 2007 and 2019, which in total provide 7 cross-slope sections in the Bellingshausen Sea. Geostrophic velocities are referenced to lowered ADCP data, shipboard ADCP data and the Dive Average Current. Cumulative transports show remarkable differences between the years 2007 and 2019. The sections of 2007 provide cumulative transports of up to 3.5 Sv eastward. In contrast, the sections in 2019 have cumulative transports up to 2 Sv westward. The sections from 2007 and 2019 are in very similar locations, indicating a temporal change rather than a spatial change.

We compare the cross-slope sections from the observations with sections from the NEMO 1/12 ° model output. A time series of cumulative transports from the model, covering the years from 2000 to 2010, allows us to identify seasonality and interannual variability in this current system.

How to cite: Oelerich, R., Heywood, K. J., Damerell, G. M., and Thompson, A. F.: Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-463, https://doi.org/10.5194/egusphere-egu2020-463, 2020.

EGU2020-20332 | Displays | OS1.13

Radiative heating of ice-covered waterbodies of varying morphologies

Hugo N. Ulloa, Kraig B. Winters, Alfred Wüest, and Damien Bouffard

Millions of small, high-altitude lakes freeze their surface waters each winter season. Observations, however, reveal that their water temperature is increasing and the ice-on period is declining. Altering the thermal regime of ice-covered lakes have multiple impacts, including the loss of ecosystem services to increments in greenhouse gas emissions. These pervasive changes are affected by the heating rate of under-ice waters, which in turn regulates the water-to-ice heat flux, and therefore the rate of ice melting. In such aquatic systems, solar radiation warms the water beneath a diffusive boundary layer, thereby increasing its density and providing energy for convection in a diurnally-active mixing layer. Shallow regions are differentially heated to warmer temperatures, driving downslope buoyancy-driven currents that transport warm water to the interior basin. We characterize the energetics of these processes, focusing on the rate at which solar radiation supplies energy that is available to drive fluid motion. Using numerical simulations, we show that advective fluxes due to differential heating contribute to the evolution of the mixed-layer in waterbodies with significant shallow areas. We use a heat balance to assess the relative importance of differential heating to the one-dimensional effects of radiative heating and diffusive cooling at the ice-water interface in waterbodies of varying morphologies.

How to cite: Ulloa, H. N., Winters, K. B., Wüest, A., and Bouffard, D.: Radiative heating of ice-covered waterbodies of varying morphologies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20332, https://doi.org/10.5194/egusphere-egu2020-20332, 2020.

EGU2020-17820 | Displays | OS1.13

Impact of the Mertz Glacier Tongue calving on the emergence of polynyas in the d'Urville Trough, East Antarctica

Lucile Ricard, Marie-Noelle Houssais, Christophe Herbaut, Alexander Fraser, Rob Massom, and Anne-Cecile Blaizot

 


Passive microwave remote sensing observations and atmospheric data are used to characterize the impact of the Mertz Glacier Tongue (MGT) calving in February 2010 on the sea ice conditions in the D’Urville Trough, East Antarctic shelf (139°E-141°E). The main objective is to determine if conditions for dense shelf water production in this area were possibly influenced by the calving. In particular, we look for the existence of winter polynyas capable of sustaining significant sea ice production, a prerequisite for the formation of dense, saline waters. We show that polynyas in the D'Urville area are part of a complex icescape made of fast ice and drifting pack ice. The seasonal evolution of this icescape has been profoundly modified with the calving of the MGT and opening of new polynyas. Pre-calving and post-calving sea ice concentrations are analyzed to identify major patterns of variability. Examination of the fast ice distribution and atmospheric forcing helps to develop a scenario for the formation of low sea ice concentration regions and their relation to the sea ice fluxes, supporting the conclusion that the role of the Adelie Bank as a barrier to the drift ice may have strengthened after the calving.

How to cite: Ricard, L., Houssais, M.-N., Herbaut, C., Fraser, A., Massom, R., and Blaizot, A.-C.: Impact of the Mertz Glacier Tongue calving on the emergence of polynyas in the d'Urville Trough, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17820, https://doi.org/10.5194/egusphere-egu2020-17820, 2020.

EGU2020-13781 | Displays | OS1.13

From red to white: the time-varying nature of ocean heat flux to Arctic sea ice

Srikanth Toppaladoddi and Andrew Wells

Arctic sea ice is one of the most sensitive components of the Earth’s climate system. The underlying ocean plays an important role in the evolution of the ice cover through its heat flux at the ice-ocean interface which moderates ice growth and melt. Despite its importance, the spatio-temporal variations of this heat flux are not well understood. In this work, we combine direct numerical simulations of turbulent convection over fractal surfaces and analysis of time-series data from the Surface Heat Budget of the Arctic Ocean (SHEBA) program using Multifractal Detrended Fluctuation Analysis (MFDFA) to understand the nature of fluctuations in this heat flux. We identify key physical processes associated with the observed Hurst exponents calculated by the MFDFA, and how these evolve over time. We also discuss ongoing work on constructing simple stochastic models of the ocean heat flux to the ice, and potential use as a parameterisation.

How to cite: Toppaladoddi, S. and Wells, A.: From red to white: the time-varying nature of ocean heat flux to Arctic sea ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13781, https://doi.org/10.5194/egusphere-egu2020-13781, 2020.

EGU2020-22464 | Displays | OS1.13

The role of tides in ocean--ice-shelf interactions in the southwestern Weddell Sea

Ute Hausmann, Jean-Baptiste Sallée, Nicolas Jourdain, Pierre Mathiot, Clement Rousset, Gurvan Madec, Julie Deshayes, and Tore Hattermann

A novel regional ocean-sea-ice model configuration is designed to investigate the mechanisms of ocean–ice-shelf-melt interactions in the Weddell Sea. It features explicit resolution of the cavities of eastern Weddell, Larsen and Filchner-Ronne ice-shelves (FRIS, at 1.5-2.5 km horizontal resolution), as well as of the adjacent continental shelves (~2.5 km) and deep open-ocean gyre (at 2.5-4 km), in presence of interannually-varying atmospheric and ocean boundary forcing as well as explicit ocean tides. Simulated circulation, water mass and ice-shelf melt properties compare overall well with available open-ocean and cavity observations, and simulated Weddell ice-shelf melting reveals large variability on tidal, seasonal and year-to-year timescales. The presence of ocean tides, investigated explicitly, is revealed to result in a systematic time-average enhancement of both the production of ice-shelf meltwater as well as its refreezing on ascending branches of especially the FRIS cavity circulation. This tide-driven enhancement of the melt-induced FRIS cavity circulation acts to increase net ice-shelf melting (by 50%, ~50 Gt/yr) and the meltwater export by the FRIS outflow, and modulates their seasonal and lower frequency variability. The tidal impact on ice-shelf melting is consistent with being primarily driven mechanically through enhanced kinetic energy of the time-varying flow in contact with the ice drafts. The dynamically-driven tide-induced melting is thereby to almost 90% compensated by cooling through meltwater produced, but not quickly exported from regions of melting in the Weddell cold-cavity regime. Ocean boundary layer thermal adjustment underneath ice drafts, minimizing departures from the in-situ freezing point, thus substantially dampens the impact of tides on Weddell ocean–ice-shelf interactions. Simulations furthermore suggest attendant changes on the open-ocean continental shelves, characterized by overall freshening and modest year-round sea-ice thickening, as well as a marked freshening of newly-formed bottom waters in the southwestern Weddell Sea.

How to cite: Hausmann, U., Sallée, J.-B., Jourdain, N., Mathiot, P., Rousset, C., Madec, G., Deshayes, J., and Hattermann, T.: The role of tides in ocean--ice-shelf interactions in the southwestern Weddell Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22464, https://doi.org/10.5194/egusphere-egu2020-22464, 2020.

EGU2020-112 | Displays | OS1.13

Water Mass Characteristics and Distribution Adjacent to Larsen C Ice Shelf, Antarctica

Katherine Hutchinson, Julie Deshayes, Jean-Baptiste Sallee, Julian Dowdeswell, Casimir de Lavergne, Isabelle Ansorge, Hermann Luyt, Tahlia Henry, and Sarah Fawcett

The physical oceanographic environment, water mass mixing and transformation in the area adjacent to Larsen C Ice Shelf (LCIS) are investigated using hydrographic data collected during the Weddell Sea Expedition 2019. The results shed light on the ocean conditions adjacent to a thinning LCIS, on a continental shelf that is a source region for the globally important water mass, Weddell Sea Deep Water (WSDW). Modified Weddell Deep Water (MWDW), a comparatively warmer water mass of circumpolar origin, is identified on the continental shelf and is observed to mix with local shelf waters, such as Ice Shelf Water (ISW), which is a precursor of WSDW. Oxygen measurements enable the use of a linear mixing model to quantify contributions from source waters revealing high levels of mixing in the area, with much spatial and temporal variability. Heat content anomalies indicate an introduction of heat, presumed to be associated with MWDW, into the area via Jason Trough. Furthermore, candidate parent sources for ISW are identified in the region, indicating the potential for the circulation of continental shelf waters into the ice shelf cavity. This highlights the possibility that offshore climate signals are conveyed under LCIS. ISW is observed within Jason Trough, likely exiting the sub-ice shelf cavity en route to the Slope Current. This onshore-offshore flux of water masses links the region of the Weddell Sea adjacent to northern LCIS to global ocean circulation and Bottom Water characteristics via its contribution to ISW and hence WSDW properties. 

What remains to be clarified is whether MWDW found in Jason Trough has a direct impact on basal melting and thus thinning of LCIS. More observations are required to investigate this, in particular direct observations of ocean circulation in Jason Trough and underneath LCIS. Modelling experiments could also shed light on this, and so preliminary results based on NEMO global simulations explicitly representing the circulation in under-ice shelf seas, will be presented. 

How to cite: Hutchinson, K., Deshayes, J., Sallee, J.-B., Dowdeswell, J., de Lavergne, C., Ansorge, I., Luyt, H., Henry, T., and Fawcett, S.: Water Mass Characteristics and Distribution Adjacent to Larsen C Ice Shelf, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-112, https://doi.org/10.5194/egusphere-egu2020-112, 2020.

EGU2020-10388 | Displays | OS1.13

Assessing z-level modelling of the ice shelf - ocean boundary layer

Ryan Patmore, Paul Holland, and Catherine Vreugdenhil

Ice shelf dynamics play a key role in the climate. Melt-rates along the ice shelf-ocean interface are an important aspect in determining the character of global sea level rise. A representation of ice shelf melt is currently implemented in various z-level General Circulation Models (GCMs) by employing parameterisations of the small scale boundary layer dynamics. However, these parameterisations are strongly dependent on the near boundary flow and at the spatial scales for which GCMs are intended the boundary layer is not well resolved. We investigate the ability of a GCM in representing these small scale boundary effects. This is done using MITgcm in an idealised setting with a sloping ice-ocean interface.

How to cite: Patmore, R., Holland, P., and Vreugdenhil, C.: Assessing z-level modelling of the ice shelf - ocean boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10388, https://doi.org/10.5194/egusphere-egu2020-10388, 2020.

EGU2020-2319 | Displays | OS1.13

On the modified Circumpolar Deep Water upwelling over the Four Ladies Bank in Prydz Bay, East Antarctica

Chengyan Liu, Zhaomin Wang, Chen Cheng, Xi Liang, Yang Wu, Xiang Li, Yu Liu, and Xiaojun Yuan

We report on mooring observations of tidal currents in Prydz Bay, East Antarctica. Tides in Prydz Bay are mixed diurnal-semidiurnal and much weaker than that in the Ross Sea and the Weddell Sea, with the spatial and temporal averaged value of 2.58 cm s-1 for all the current meter observations over the continental shelf. The major axes of the tidal ellipses are generally aligned south-north, probably steered by the topography. The tidal phases are modulated by both the baroclinic and barotropic tidal components. The averaged tidal kinetic energy can account for a fraction of ~13% with respect to the total kinetic energy at the Amery Ice Shelf calving front during the observing period. The long-term average tidal heat flux across the Amery Ice Shelf calving front is negligible, but the ratio of the tidal heat flux standard deviation to the residual heat flux standard deviation can be up to 41%. We also report on borehole observations of tide-like pulsing of potential temperature and salinity, indicating the indispensable tidal influences in the ice-ocean boundary layer. These mooring and borehole data support that the tidal processes should be highlighted in the investigations of the interaction between the Amery Ice Shelf and ocean.

How to cite: Liu, C., Wang, Z., Cheng, C., Liang, X., Wu, Y., Li, X., Liu, Y., and Yuan, X.: On the modified Circumpolar Deep Water upwelling over the Four Ladies Bank in Prydz Bay, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2319, https://doi.org/10.5194/egusphere-egu2020-2319, 2020.

EGU2020-50 | Displays | OS1.13

Turbulence Observations in the Grounding Zone Region of Thwaites Glacier

Peter Davis, Keith Nicholls, and David Holland

Antarctic ice shelves restrain the flow of grounded ice into the ocean, and are thus an important control on Antarctica’s contribution to global sea level rise. West Antarctica represents the largest source of uncertainty in future sea level projections, and Thwaites Glacier has the potential to influence sea level more than any other outlet glacier in this region. The future behaviour of Thwaites Glacier is particular sensitive to basal melting in the grounding zone region. Basal melting is controlled by the turbulent transfer of heat through the ice shelf-ocean boundary layer. The physics of this boundary layer is poorly understood, however, and its inadequate representation in numerical models is hampering our ability to predict the future evolution of the Antarctic ice sheet. Using a hot-water drilled access hole, a turbulence instrument cluster was deployed in the grounding zone region of Thwaites Glacier in January 2020. By observing the momentum and scalar fluxes, these observations provide a unique opportunity to explore the important turbulent processes responsible for modulating the basal melt rate in this region. Ultimately, this observational effort will allow us to better constrain our parameterisations of the grounding zone region in large-scale numerical models, facilitating more accurate simulations of the Antarctic ice sheet in the changing climate.

How to cite: Davis, P., Nicholls, K., and Holland, D.: Turbulence Observations in the Grounding Zone Region of Thwaites Glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-50, https://doi.org/10.5194/egusphere-egu2020-50, 2020.

EGU2020-2984 | Displays | OS1.13

Subsea Water Isotope Sensors: A novel tool for continuous and in-situ analysis

Roberto Grilli and Camillle Blouzon

The isotopic composition of seawater represents an important fingerprint of water masses, containing information about conditions during their formation and evolution. Following the spatial and temporal variability of either δD or δ18O of water in the ocean will provide a direct link to the freshwater cycle, allowing to discriminate between different water masses, such as the one coming from glacier and sea ice melting, freshwater rivers and precipitations.

Information from physical parameters (e.g. temperature and salinity) are not always enough for identifying the undergoing processes, and current knowledge of water isotopes or noble gases in the ocean remains very poor due to scarcity of measurements obtained from discrete sampling followed by laboratory analysis.

Here we present a novel in-situ Membrane Inlet Laser Spectroscopy (MILS) sensor which is currently under development. The sensor will provide simultaneous and continuous measurements of water isotopes (both δD and δ18O, expected precision of ~0.05‰) and will be adapted for deployment from vessels, through boreholes into the ice shelves, and to be integrated in autonomous underwater vehicles (AUVs). The instrument will run on batteries, with an autonomy of ~12h.

From 2021, the MILS sensor will be ready for field deployments, particular in the Southern Ocean, where high resolution water isotope data inside ice shelf cavities could be coupled with modelling approaches for better understanding the processes at work in ocean - ice shelf interactions, and better constraint the ice melting processes in Antarctica, which remains today a major challenge.

How to cite: Grilli, R. and Blouzon, C.: Subsea Water Isotope Sensors: A novel tool for continuous and in-situ analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2984, https://doi.org/10.5194/egusphere-egu2020-2984, 2020.

EGU2020-20463 | Displays | OS1.13

Modeling ocean eddies and their effects on ice shelf basal melt rate in the Ross Sea

Stefanie Mack, Mike Dinniman, John Klinck, Dennis McGillicuddy, Jr., and Laurie Padman
Ice shelf basal melt rates around Antarctica are affected by the advection of warm Circumpolar Deep Water (CDW) onto the continental shelf and under the ice shelf. In ocean models, resolving mesoscale eddies is necessary to capture eddy fluxes of CDW and estimate basal melt rates of ice shelves. Where and when (not if) eddies are resolved in an ocean model depends on the baroclinic Rossby radius and thus on stratification and latitude. The Ross Sea presents some interesting scientific questions in two regards: first, it is weakly stratified in winter conditions, lowering the radius of deformation; and second, the Ross Ice Shelf melts mainly from dense shelf water at the grounding line and from light surface water at the ice shelf front, rather than CDW. An investigation using a ROMS (Regional Ocean Modelling System) model of the Ross Sea reveals that portions of the domain (48% in well-mixed winter conditions, and 33% in stratified summer conditions) do not resolve mesoscale eddies even at a horizontal grid spacing of 1.5 km. We find that smaller grid spacing (1.5 km versus 5 km) leads to increased eddy generation in the model, and eddies that cross the ice shelf front in both directions. However, there is no significant change in basal melt between low and high resolution simulations. While even higher resolution is needed to fully represent eddies in the Ross Sea, the processes that control basal melt of the Ross Ice Shelf may not be strongly affected by these eddies.

How to cite: Mack, S., Dinniman, M., Klinck, J., McGillicuddy, Jr., D., and Padman, L.: Modeling ocean eddies and their effects on ice shelf basal melt rate in the Ross Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20463, https://doi.org/10.5194/egusphere-egu2020-20463, 2020.

EGU2020-10848 | Displays | OS1.13

Toward a new ice-shelf melt rate parameterization with large-eddy simulations

Carolyn Branecky Begeman, Xylar Asay-Davis, and Luke Van Roekel

Predictions of ice shelf melting depend on dynamical insights into ocean boundary layers below ice shelves. Fundamental questions regarding the nature of stratified turbulence below the sloped and ablating ice shelf base remain. Laboratory experiments, direct numerical simulations, and observations have yielded important insights, but have yet to produce a robust relationship between ice shelf melt rates and shear- and buoyancy-driven mixing. This relationship is the target of our Large-Eddy Simulations (LES) of the ice-shelf ocean boundary layer. Several new developments were applied to the LES code PALM to produce dynamic melting as well as tides. In this presentation, we demonstrate these new model capabilities. We contrast profiles of vertical turbulent fluxes of heat, salt and momentum across different simulated ice shelf settings: cold, shear-dominated settings vs. warm, buoyancy-dominated settings. We also discuss our recent work toward a new ice-shelf melt parameterization for use in large-scale ocean models on the basis of these simulations. A new melt parameterization is a critical component of ongoing ice-ocean coupling efforts, both to place melt rate predictions on a more physical footing and to achieve convergence with vertical ocean model resolution, on which current parameterizations fail.

How to cite: Begeman, C. B., Asay-Davis, X., and Van Roekel, L.: Toward a new ice-shelf melt rate parameterization with large-eddy simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10848, https://doi.org/10.5194/egusphere-egu2020-10848, 2020.

EGU2020-8075 | Displays | OS1.13

Investigating the climate variability in the Totten area using NEMO-LIM regional model.

Guillian Van Achter, Charles Pelletier, and Thierry Fichefet

The Totten ice shelf drains over 570 000 km² of East Antarctica. Most of the ice sheet that drains through the Totten ice-shelf is from Aurora Subglacial Basin and is marine based making the region potentially vulnerable to rapid ice sheet colapse.
Understanding how the changes in ocean circulation and properties are causing increased basal melt of Antarctic ice shelves is crucial for predicting future sea level rise.
In the context of the The PARAMOUR project (decadal predictability and variability of polar climate: the role of atmosphere-ocean-cryosphere multiscale interaction), we use a high resolution NEMO-LIM 3.6 regional model to investigate the variability and the predictability of the coupled climate system over the Totten area in East Antarctica.
In this poster, we will present our on-going work about the impact of landfast ice over the variability of the system. Landfast ice is sea ice that is fastened to the coastline, to the sea floor along shoals or to grouded icebergs. Current sea ice models are unable to represent very crudely the formation, maintenance and decay of coastal landfast ice. We applyed several parameterization for modeling landfast ice over the Totten ice shelf area.

How to cite: Van Achter, G., Pelletier, C., and Fichefet, T.: Investigating the climate variability in the Totten area using NEMO-LIM regional model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8075, https://doi.org/10.5194/egusphere-egu2020-8075, 2020.

EGU2020-19054 | Displays | OS1.13

Ice melting in a turbulent stratified shear flow

Louis-Alexandre Couston, Eric Hester, Benjamin Favier, Adrian Jenkins, and Paul Holland

In this talk I will present preliminary results of direct numerical simulations of ice melting in a turbulent stratified shear flow. The model solves the evolution of the turbulent fluid phase and of the diffusive solid ice phase, due to melting and freezing, in a fully coupled way. This is done by combining a Direct Numerical Simulation (DNS) code with a novel formulation of the equations for the solid and liquid phases of water based on the phase-field method. DNS enables turbulent motions to be simulated without approximation, i.e. solving Navier Stokes equations, while the phase-field method allows the ice-ocean interface to be rough and evolve in response to melting. I will present results on the turbulent boundary layer and on the self-generated basal topography at the ice-water interface. The ultimate goal of this work is to propose a new DNS-based parameterization of the melting process at rough ice-ocean boundaries that takes into account the effects of temperature and salt stratification, and flow velocities.

How to cite: Couston, L.-A., Hester, E., Favier, B., Jenkins, A., and Holland, P.: Ice melting in a turbulent stratified shear flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19054, https://doi.org/10.5194/egusphere-egu2020-19054, 2020.

EGU2020-9934 | Displays | OS1.13

The Seasonality of submesoscale variability in the Antarctic Seasonal Ice Zone

Isabelle Giddy, Sarah Nicholson, Marcel Du Plessis, Andy Thompson, and Sebastiaan Swart

The ocean surface boundary layer in the Southern Ocean plays a critical role in heat and carbon exchange with the atmosphere. Submesoscale flows have been found to be important in setting mixed layer variability in the Antarctic Circumpolar Current (ACC). However, sparsity in observations, particularly south of the ACC in the Antarctic Seasonal Ice Zone (SIZ) where the horizontal density structure of the mixed layer is influenced by sea ice melt/formation and mesoscale stirring, brings into question the ability of climate models to correctly resolve mixed layer variability. We present novel fine-scale observations of the activity of submesoscale variability in the ice-free Antarctic SIZ using three deployments of underwater gliders over an annual cycle. Salinity-dominated density fronts of O(1)km associated with strong horizontal buoyancy gradients are observed during all deployments. There is evidence that stratifying ageostrophic eddies, energised by salinity driven submesoscale fronts are active across seasons, with intermittent equivalent heat fluxes of the same order to, or greater than local atmospheric forcing. This study highlights the need to consider future changes of Antarctic sea-ice in respect to feedback mechanisms associated with salinity (sea-ice) driven submesoscale flows.

How to cite: Giddy, I., Nicholson, S., Du Plessis, M., Thompson, A., and Swart, S.: The Seasonality of submesoscale variability in the Antarctic Seasonal Ice Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9934, https://doi.org/10.5194/egusphere-egu2020-9934, 2020.

EGU2020-12502 | Displays | OS1.13

The sensitivity of sea ice growth to the choice of ice shelf-ocean coupling algorithms.

Stefan Jendersie and Alena Malyarenko

EGU2020-12570 | Displays | OS1.13

Effects of model resolution on ice shelf-ocean boundary layer
not presented

Alena Malyarenko, Stefan Jendersie, Mike Williams, Natalie Robinson, and Pat Langhorne

Boundary layer mixing at the ice-ocean thermodynamic interface is represented by turbulent transfer coefficients, ΓT and ΓS. Commonly used expressions for these are based on observations at the sea ice-ocean and ice shelf-ocean boundaries, and result in values ranging over 5 orders of magnitude (10-7< ΓT< 10-2). To demonstrate the potential effect of the choice of turbulent transfer parameterisation we applied all of the available transfer coefficient values (12) to an idealised ice shelf-ocean cavity model experiment using the ISOMIP domain with ROMS. The mean ablation rate in warm cavity scenarios varies between 2.1 and 4.7 m/year, and in cold cavity scenarios between 0.03 and 0.17 m/year.

 ΓT and ΓS not only directly determine the ablation rate, but have effects on fresh water distribution in the ocean boundary layer. High Γ values develop deep mixed layers, while low Γ values stratify the top ocean grid cells. Thus the ocean boundary layer structure directly depends on vertical resolution in the ocean model and how well the mixing scheme can handle the stratification effects. The experiment results we are presenting here include comprehensively tested and quantified effects of tidal forcing, mixing schemes, vertical flux distribution and ocean model resolution on the ablation rates and the ocean boundary layer structure.

How to cite: Malyarenko, A., Jendersie, S., Williams, M., Robinson, N., and Langhorne, P.: Effects of model resolution on ice shelf-ocean boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12570, https://doi.org/10.5194/egusphere-egu2020-12570, 2020.

EGU2020-19677 | Displays | OS1.13

Investigating the dynamics of an Antarctic coastal polynya using a regional climate model

Pierre-Vincent Huot, Thierry Fichefet, Christoph Kittel, Nicolas Jourdain, and Xavier Fettweis

Coastal polynyas of the Southern Ocean, such as the Mertz Glacier Polynya, are paramount features of the polar climate. They allow for exchanges of heat, momentum and moisture between the atmosphere and ocean where sea ice usually prevents such interactions. Polynyas are believed to have a profound impact on polar and global climate, thanks to their sustained sea ice production and the associated formation of Dense Shelf Waters. Less is known, however, about the impact of polynyas on the atmosphere. Changes in air properties and winds induced by heat and moisture flux could for instance affect precipitation regime over the ice sheet or sea ice. As the formation and evolution of coastal polynyas are tied to the state of the atmosphere, such changes can also induce important feedbacks to polynyas dynamics. Such processes have almost never been studied, whether on the field or with the help of coupled models. Here, we propose to describe the behavior of a coastal polynya and its relationship with the ocean and atmosphere. To do so, we developed a regional coupled model of the ocean, sea ice and atmosphere (including interactive basal melt of ice shelves) and applied it to the Adélie Land area, in East Antarctica. The dynamics of the Mertz Glacier Polynya is described, together with its impact on the atmosphere, sea ice growth, dense water production and ice shelf melt. To assess the importance of potential feedbacks, we compare the dynamics of the polynya from the coupled model to a forced ocean-sea ice model. We then use the regional coupled model to investigate the implications of the Mertz ice tongue calving in early 2010 which led to a drastic decrease of the Mertz Glacier Polynya extent. This experiment aims at investigating the sensitivity of the atmosphere to the activity of the polynya and to evaluate the impact of the calving on regional climate. This work improves the understanding of the Mertz Glacier Polynya dynamics, and of the impact of coastal polynyas on polar climate. It also constitutes an additional step in the modelling of the polar regions in Earth System Models.

How to cite: Huot, P.-V., Fichefet, T., Kittel, C., Jourdain, N., and Fettweis, X.: Investigating the dynamics of an Antarctic coastal polynya using a regional climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19677, https://doi.org/10.5194/egusphere-egu2020-19677, 2020.

EGU2020-9112 | Displays | OS1.13

Onset of Double-Diffusive Convection in the Ice Shelf/Ocean Boundary Layer

Leo Middleton, Catherine Vreugdenhil, Paul Holland, and John Taylor

The interaction between Ice-Shelves and the Ocean is an important component of the response of ice sheets to future warming oceans. Observational data in the ocean boundary layer beneath ice shelves is limited and the turbulent flow in the boundary layer is not well characterised. Our work uses small scale (9m depth) direct numerical simulations (DNS) of the Ice-Shelf-Ocean Boundary Layer, inspired by field observations made beneath the George VI Ice Shelf. Here, warm water has been observed directly beneath the ice shelf, and yet the observed melt rates are modest. To study this scenario, we simulate a forced turbulent flow underlying an ice shelf where the ice base is represented by a dynamic melting boundary condition. As the ice melts, a pool of relatively cold, fresh water develops below the ice base. Thermal diffusion causes the underlying water to cool and can drive turbulent convection. At the same time, the salinity gradient in the halocline is stabilising, but develops over a longer time scale. As a result, two flow regimes exist: one with active turbulent convection driven by double-diffusion of heat and salt, and the other with stratified turbulence leading to mixing of the halocline. By varying control parameters, we identify the transition between the flow regimes in terms of the temperature contrast (thermal driving) and the level of turbulence in the far field. We consider the behaviour of the diapycnal buoyancy flux near the ice base, and it provides insight into the drivers of both the double-diffusive convection and its modification by ambient turbulence. Finally, we discuss how the double-diffusive process we have described applies to real-world ice-shelf ocean boundary layers, and how it may be quantified within observations.

How to cite: Middleton, L., Vreugdenhil, C., Holland, P., and Taylor, J.: Onset of Double-Diffusive Convection in the Ice Shelf/Ocean Boundary Layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9112, https://doi.org/10.5194/egusphere-egu2020-9112, 2020.

EGU2020-22008 | Displays | OS1.13

Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

F. Alexander Haumann, Nicolas Gruber, and Matthias Münnich

Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or increased surface freshwater fluxes from either the atmosphere or glacial meltwater. However, these mechanisms so far failed to fully explain the surface trends and the concurrently observed warming of the subsurface (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind-driven northward transport of sea ice, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward transport of sea ice, the open-ocean surface between the Subantarctic Front and the sea-ice edge is cooled by strengthening the salinity dominated oceanic stratification. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea-ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice-shelf melting. Moreover, it could account for about 8±2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea-ice changes thereby may have contributed to the slowdown of global surface warming over this period. The important role of sea-ice in driving changes in the high-latitude Southern Ocean are robust across all considered sensitivity cases, although their magnitude is sensitive to the forcing and the role of salinity in controlling the vertical stratification in the mean state. It remains yet unclear whether these sea-ice induced changes are associated with natural variability or a response to anthropogenic forcing.

How to cite: Haumann, F. A., Gruber, N., and Münnich, M.: Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22008, https://doi.org/10.5194/egusphere-egu2020-22008, 2020.

OS2.1 – Open session on coastal and shelf seas

During the last decades, many estuarine systems in Europe (e.g. the Elbe, Ems, Loire) have shown increases in tidal range and in turbidity, which are linked to local human activity (i.e., deepening). Compared to these European estuaries, the Yangtze Estuary is much larger in scales, experiences much stronger river discharge, and it is subject to a strong seasonal variation in freshwater and sediment supply from the drainage area. Moreover, the Yangtze estuary is a complex network with several branches, connecting channels. The changes in the flow and sediment dynamics in the estuary may result from both local and nonlocal human activities. Despite the intense research efforts over the past two decades, it is still unclear which impact (local or nonlocal) is responsible for the changing flow and sediment characteristics in the estuary. Deep investigation of tidal characteristic quantities such as extreme tidal level, tidal range, amplitude of tidal constituents, tidal characteristic coefficient and suspended sediment concentration is performed in a systematic manner. It is accomplished using the extreme value analysis, the wavelet analysis and harmonic analysis of water level at 11 hydrography stations along the tidal river channel (Datong-Nanjing reach) and estuarine section (downstream the Xuliujing) during 2008-2016. Similar data analysis is also performed for the last four decades of 20th century and results are compared with the analysis of the recent measurements. The driving forces of the significant changes in tidal characteristic quantities and suspended sediment concentration are discussed. Results show that the tidal dynamics in the Yangtze estuary has been enhanced. Its seasonal variation is attributed to the adjustment of runoff distribution, which is mainly caused by the operation of Three Gorges Dam. In short-term, local changes of flow/sediment dynamics, terrain changes play a major role. In the long term (on the 40-year time scale), the effect of sea level rise on the increasing M2 constituent is obvious. This has mainly resulted from the enhancing anti-clockwise rotation of the synchronous tidal phase.

 

How to cite: Cheng, H., Chen, W., Teng, L., and Yuan, X.: Variation behavior of tidal dynamics in the Yangtze Estuary: implying the amplification of hydrodynamics and sediment dynamics by the human intervention , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9331, https://doi.org/10.5194/egusphere-egu2020-9331, 2020.

Many estuaries are characterized by one or more locations where the concentration of fine sediment attains a maximum. The locations and intensities of these estuarine turbidity maxima (ETM) are sensitive to river discharge, tides, depth and sediment properties. In this contribution, results are presented of a width-averaged process-based model that describes tides, residual currents and sediment transport in an estuarine channel. The aim is to quantify the sensitivity of location and intensity of ETM to 1) flocculation and hindered settling of fine sediment and 2) sediment-induced damping of turbulence. The model is applied to the North Passage of the Yangtze Estuary, which is a prototype estuary that undergoes strong variations in environmental conditions. The sediment settling velocity is allowed to vary along the channel due to the effects of flocculation and hindered settling, by parametrizing settling velocity as the function of the subtidal near-bed sediment concentration according to results obtained from laboratory experiments. Sediment-induced turbulence damping is taken into account by parametrizing eddy viscosity and eddy diffusivity coefficients as functions of bulk Richardson number.

In the flocculation (low concentration) regime, where the settling velocity increases with sediment concentration, the rapid settling of flocs induces larger landward sediment transport due to upstream flow in the lower layer of density-driven flow, leading to a landward shift and intensification of the ETM (with respect to the case of a constant settling velocity). In the hindered settling (high concentration) regime, the settling velocity decreases with bottom concentration. This induces a decrease in upstream sediment transport due to density-driven flow and an increase in seaward sediment transport due to river flow, leading to seaward migration and attenuation of the ETM. In both regimes, sediment-induced damping of turbulence results in stronger upstream flow in the bottom layer of density-driven flow and more vertically stratified sediment distribution, which significantly intensifies the landward sediment transport due to density driven flow, and hence causes a landward shift and intensification of the ETM.

How to cite: Jiang, C., de Swart, H. E., Zhou, J., and Li, J.: On the role of flocculation, hindered settling and sediment-induced damping of turbulence in trapping sediment in estuaries, with focus on the North Passage, Yangtze Estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12437, https://doi.org/10.5194/egusphere-egu2020-12437, 2020.

EGU2020-10119 | Displays | OS2.1

Secondary circulation in shallow ocean straits: Observations and numerical modeling of the Danish Straits

Verena Haid, Emil Stanev, Johannes Pein, Joanna Staneva, and Wei Chen

We study the secondary circulation in the Danish Straits using the unstructured-grid hydrodynamic model SCHISM covering the North Sea and Baltic Sea. The resolution in the straits is up to ~100 m. Since the large-scale atmospheric variability controls the transport in these straits, we focus on the processes with subtidal time scales. We compare the in- and outflows in the straits to flood and ebb flows in estuaries and analyze similarities and differences. Very prominently, the outflow and inflow phases of the Danish Straits feature substantial differences to the tidal straining in estuaries. With a resolution of ~100 m, new transport and mixing pathways, previously unresolved, appear fundamental to the strait dynamics. The variability of the strait bathymetry leads to a strongly differing appearance of secondary circulation. Helical cells, often with a horizontal extension of ~1 km, develop in the deep parts of the channels. A comparison between the high-resolution simulation and a simulation with a coarser grid of ~500 m in the straits suggests that the coarser resolution overestimates the stratification and misrepresents the transport balance. Axial velocities and transport through the Sound are underestimated by ~12%. These differences are explained by the underdeveloped secondary circulation in the coarse-resolution simulation and the associated changes in mixing along the straits. In conclusion, the use of ultrafine resolution grids is essential to adequately resolve secondary flow patterns and two-layer exchange.

How to cite: Haid, V., Stanev, E., Pein, J., Staneva, J., and Chen, W.: Secondary circulation in shallow ocean straits: Observations and numerical modeling of the Danish Straits, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10119, https://doi.org/10.5194/egusphere-egu2020-10119, 2020.

EGU2020-4860 | Displays | OS2.1

Physical drivers of oxygen depletion in the Central and Eastern Baltic Sea

Taavi Liblik, Stella-Theresa Stoicescu, Jaan Laanemets, Oliver Samlas, Kai Salm, Irina Suhhova, Madis-Jaak Lilover, and Urmas Lips

Eutrophication and consequent increase in biomass production and sedimentation of organic material cause oxygen depletion of the deep layers and an increase in hypoxic bottom areas in the Baltic Sea.

The Baltic Sea – a semi-enclosed brackish sea – has restricted water exchange with the North Sea. High fresh water runoff and sporadic inflows of saline water through the Danish Straits maintain stratification. Seasonal thermocline and quasi-permanent halocline, their vertical location, shape and strength are sensitive to atmospheric forcing and influence the oxygen depletion in the near-bottom layer. Physical processes altering deoxygenation in the three sub-basins of the Baltic Sea (Baltic Proper, Gulf of Finland and Gulf of Riga) are under scope of the present overview. Permanent halocline is present in the deep Baltic Proper, while in the Gulf of Finland, it occasionally vanishes during winter. Complete mixing occurs in each winter in the shallow Gulf of Riga separated from the Baltic Proper by the sill. We show that the bathymetry, combined with physical drivers, causes distinct spatial and temporal patterns of oxygen depletion in the basins. The results presented here are a summary of in-situ measurement campaigns conducted by the research vessel, underwater glider, autonomous vertical profiler and bottom moorings in 2011–2020.

Large barotropic inflows from the North Sea temporarily ventilate the deep layer of the Central Baltic Proper, but rather intensify hypoxia in the Northern Baltic Proper and the Gulf of Finland. Wind-driven estuarine circulation alterations shape the hypoxic area and volume in the Gulf of Finland considerably. Seaward winds support estuarine circulation and the advection of hypoxic saltier water of the Northern Baltic Proper into the gulf deep layer. The landward wind can reverse estuarine circulation, the collapse of stratification and mixing of the whole water column in winter (when the seasonal thermocline is absent), thus, temporarily improving oxygen conditions in the deep layer of the gulf. Intrusion of cold saltier water of the Baltic Proper over the sill into the Gulf of Riga deep layer strengthens water column stratification and supports hypoxia formation in summer. Such a water exchange regime is related to the northerly wind forced upwelling along the eastern coast of the Baltic Proper. The role of submesoscale processes on vertical mixing and deep layer ventilation is still unclear, and the data of high-resolution in situ measurements in the Baltic Sea is limited yet. Preliminary results from the dedicated underwater glider surveys conducted at the coastal slope of Eastern Baltic Proper in 2019-2020 will be presented.

How to cite: Liblik, T., Stoicescu, S.-T., Laanemets, J., Samlas, O., Salm, K., Suhhova, I., Lilover, M.-J., and Lips, U.: Physical drivers of oxygen depletion in the Central and Eastern Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4860, https://doi.org/10.5194/egusphere-egu2020-4860, 2020.

The aim of the study was to test the possibility of improving prediction skills of Baltic SST values throughout the year using past NAO index value. This could be possible due to the strong effect of NAO on Central and North European winter (DJFM) temperatures and the large thermal capacity of a sea, even as shallow as the Baltic.

 

First, the correlations of monthly NAO index and SST fields have been calculated. Significant (p<0.05) correlations were noted only in the central and southern Baltic in January, February and March, unlike in the air temperature - not shown – when significant correlations last from December to March. This may suggest one month delay in NAO affecting SST. To test this delay directly correlations with one month delay were performed showing significant values for five months of the year (from December to April SST temperatures).

 

Next, the main test of the study was performed by calculating the lagged correlations of average winter (DJFM) NAO values with SST fields. The results show that winter NAO significantly affects SST first in the central and southern basins (April to June) and later (July-September) also in the Bothnian Bay in the north. In some regions (mainly close to the Gulf of Riga) the influence of winter NAO is significant even in October, seven months after the end of the DJFM period when NAO influences the air temperatures. This shows that the effect must be caused by changes of heat content of the sea water. Also the fact that the effect of winter NAO spreads northwards during the summer can be explained with advection due to sea currents.

 

The results show that winter NAO values can be used to improve the skill of seasonal predictions of Baltic SST values in spring and summer. They also imply that if the observed significant correlation of the winter NAO values with global temperature will continue in the next decades, the prevailing positive winter NAO values should result in the spring and summer Baltic SST warming at a faster rate than the adjacent land.

How to cite: Piskozub, J.: Winter NAO significantly influences Baltic SST values throughout spring and summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19185, https://doi.org/10.5194/egusphere-egu2020-19185, 2020.

EGU2020-21411 | Displays | OS2.1

Early spring SST distribution in the Baltic Sea: in search of the coldest water

Tatiana Bukanova and Irina Chubarenko

We examine three hypotheses of formation of waters of the Cold Intermediate Layer (CIL) in the Baltic Sea: the coldest baltic waters are formed (1) at the beginning of spring warming in the Arkona and Bornholm basins, (2) in the centers of mesoscale vortexes (similar to those in the Black Sea), and (3) in the convergence zones of alongshore fronts while cooling over shelves (as in the Mediterranean Sea).

In search of the coldest surface water we analyzed the dynamics of sea surface temperature (SST) in the Baltic Proper for February-April 2003-2019 from satellite imagery of infrared sensors (MODIS-Terra/Aqua and VIIRS-Suomi-NPP), and microwave sensors (AMSR-E-Aqua, AMSR-2-GCOM-W1, and WindSat-Coriolis).

Long-term mean SST maps (for February, March, April 2003-2019) show patterns that indicate rather quick, abrupt re-structuring of thermohaline fields in late March - early April, especially evident in the Arkona and Bornholm basins. This supports the idea that seasonal transfer from two-layered winter-time vertical water stratification to the summer-time three-layered stratification is driven in the Baltic Sea not by the direct heat fluxes through the surface, but rather by the large-scale north-south water exchange.

Coastal fronts may persist for a few weeks, however their location is changeable. Stable frontal zones and vortexes are not observed under long-term SST averaging. However the sequential warming of waters from south to north direction due to geographical reasons is clearly seen with long-term averaging.

The features of spring differential warming development above shallows and along shore can be observed only from daily SST maps (not from annually averaged maps).

Investigations are supported by the Russian Foundation for Basic Research, grant No. 19-05-00717 (in part of data analysis) and the State Assignment No 0149-2019-0013 (in part of satellite data collecting and processing).

How to cite: Bukanova, T. and Chubarenko, I.: Early spring SST distribution in the Baltic Sea: in search of the coldest water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21411, https://doi.org/10.5194/egusphere-egu2020-21411, 2020.

The shallow continental shelf is increasingly used to site infrastructure for marine energy conversion and aquaculture. In this shallow typically energetic environment, tides and waves cause significant sediment fluxes, which interact with and are modified by emplaced infrastructure. This contribution presents observational field data to quantify non-equilibrium turbulent stresses caused by an obstruction in a tidal flow and its impact on suspended sediment transport.

Observations of the turbulent properties of the benthic boundary layer (BBL) in an energetic nearshore environment were made over a 4-month period in Cemaes Bay, Anglesey, UK. The area experiences a high energy semi-diurnal tidal regime with a maximum range of 7.5 m. Tidal current velocities were a maximum of 1.1 m s−1 during springs tides and the strength of the tides ensures that the water column was vertically well mixed. An instrumented lander deployed in 13 m depth on a region of flat sand-sheet sampled the turbulent flows in the BBL using a pulse coherent Nortek Aquadopp and a Vector ADV. An Acoustic Backscatter System was mounted coincidently to sample suspended sediment concentrations.

Vertical profiles of mean flow show that during the flood tide an obstruction upstream of the sampling region modified the BBL causing the breakdown of the constant stress layer and a reduction in velocity shear compared to the opposing ebb tide currents. The turbulent dissipation rate computed using the inertial dissipation and structure functions methods illustrate an order of magnitude increase in dissipation and identify a strongly non-equilibrium relationship between turbulent dissipation and production during flood tides, which varies with elevation above the seabed. The non-equilibrium turbulence effects the suspension and transport of seabed sediments by modifying the vertical profile of sediment diffusivity. These effects are quantified and impacts discussed.

How to cite: Austin, M., Lincoln, B., and Walker-Springett, G.: Non-equilibrium turbulent stresses and sediment transport in the benthic boundary layer of a shallow shelf environment influenced by flow obstruction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11021, https://doi.org/10.5194/egusphere-egu2020-11021, 2020.

EGU2020-19147 | Displays | OS2.1

The impact of baroclinity on tidal ranges in the North Sea

Wenguo Li, Bernhard Mayer, and Thomas Pohlmann

Tidal range is one of significant contributors of coastal inundation. Therefore, it is very important to investigate the dynamics of tidal range variations over different time scales. The baroclinity has the potential to modulate surface tides through ocean stratification on seasonal scale. In order to better understand the impact of ocean stratification on tidal ranges in the North Sea, the numerical simulations were carried out in baroclinic and barotropic modes covering the period from 1948 to 2014, using the regional 3D hydrodynamic prognostic Hamburg Shelf Ocean Model (HAMSOM). In the barotropic mode, the river forcing was also included, which only increases the local sea level without any influence on the density. The tidal range difference between baroclinic and barotropic modes in winter (less stratification) and summer (strong stratification) are compared at 22 tide-gauge stations, where the simulated sea surface elevations agree well with observations from 1950 to 2014. The statistical analysis generally shows that the difference at 19 stations (86% of total stations) in summer is much larger than that in winter during more than 32 years (50% of the analysis period). This suggests that the stratification decouples the surface and bottom layers weakening the damping effects of bottom friction, which is visible even at the coastal tide-gauge stations, where the ocean water is well-mixed. Obviously, the signal induced by stratification is propagated by the tidal Kelvin wave through the North Sea. Additionally, the spatial distribution of tidal range differences indicate that the amphidromic points in the North Sea moved westward in the baroclinic mode. Regarding the seasonal mean sea level at the stations, the results show that the coastal sea level could be increased by baroclinity itself, since the river runoff freshens the coastal water in the baroclinic mode, and thus the local sea level increases due to steric effect. Consequently, the increased sea level could further weaken the damping effect. However, this is a relatively minor impact on the tidal range.

How to cite: Li, W., Mayer, B., and Pohlmann, T.: The impact of baroclinity on tidal ranges in the North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19147, https://doi.org/10.5194/egusphere-egu2020-19147, 2020.

EGU2020-10206 | Displays | OS2.1

Existence and Stability of Morphodynamic Equilibria in Double Inlet Systems

Xiao Deng, Thomas Boelens, Tom De Mulder, and Henk Schuttelaars

Tidal inlet systems, ubiquitous along sandy coasts, are very valuable areas in terms of ecology
(breeding and feeding areas), economy (gas–mining and dredging) and recreation, and important
for coastal safety. To properly manage these systems, good insight into their morphodynamic
behaviour is essential.
In this presentation, we focus on morphodynamic equilibria of so-called double inlet systems,
i.e., systems in which the tidal basin is connected to the open sea by two tidal inlets. In our model,
the water motion is described by the cross-sectionally averaged shallow water equations, and forced
by prescribed tidal elevations at both seaward sides. The sediment transport is modeled by an
advection–diffusion equation with source and sink terms, while the bed evolution is governed by the
convergences and divergences of sediment transports. The sediment transport consists of various
contributions, a diffusive contribution, a transport term related to the variations in topography
and an advective contribution (ter Brake and Schuttelaars, 2010).
To directly identify morphodynamic equilibria, we employ continuation methods and bifurcation
techniques. By systematically varing the amplitude φM2 at one of the inlets, while keeping all other
parameters fixed, a region in the φM2 parameter space is found where the bed level reaches the
water surface, resulting in two single inlet systems. Outside this region, morphodynamic equilibria
exist. These equilibria are characterized by their minimum water depth and location. There are
branches of stable equilibria, while there are also branches of unstable equilibria, coinciding with
the stable equilibria at so-called limit points. Varying both the amplitude and phase of the M2 tide
at one of the inlets while keeping the other parameters fixed, results in limit points in AM2 − φM2
space that form an ellipse.
In our presentation, we will systematically discuss the number and stability of morphodynamic
equilibria and compare our results to observations in the Marsdiep-Vlie system, a double inlet
system in the Nothern Dutch Wadden Sea.
References
ter Brake, M. C. and Schuttelaars, H. M. (2010). Modeling equilibrium bed profiles of short tidal embayment. on
the effect of the vertical distribution of suspended sediment and the influence of the boundary conditions. Ocean
Dynamics, 60:183–204.

How to cite: Deng, X., Boelens, T., De Mulder, T., and Schuttelaars, H.: Existence and Stability of Morphodynamic Equilibria in Double Inlet Systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10206, https://doi.org/10.5194/egusphere-egu2020-10206, 2020.

Assessing the impacts of both natural (e.g., tidal forcing from the ocean) and human-induced changes (e.g., dredging for navigation, land reclamation) on estuarine morphology is particularly important for the protection and management of the estuarine environment. In this study, a novel analytical approach is proposed for the assessment of estuarine morphological evolution in terms of tidally averaged depth on the basis of the observed water levels along the estuary. The key lies in deriving a relationship between wave celerity and tidal damping or amplification. For given observed water levels at two gauging stations, it is possible to have a first estimation of both wave celerity (distance divided by tidal travelling time) and tidal damping or amplification rate (tidal range difference divided by distance), which can then be used to predict the morphological changes via an inverse analytical model for tidal hydrodynamics. The proposed method is applied to the Lingdingyang Bay of the Pearl River Estuary, located on the southern coast of China, to analyse the historical development of the tidal hydrodynamics and morphological evolution. The analytical results show surprisingly good correspondence with observed water depth and volume in this system. The merit of the proposed method is that it provides a simple approach for understanding the decadal evolution of the estuarine morphology through the use of observed water levels, which are usually available and can be easily measured.

How to cite: Huayang, C. and Liu, F.: A novel approach for the assessment of morphological evolution based on observed water levels in tide-dominated estuaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1461, https://doi.org/10.5194/egusphere-egu2020-1461, 2020.

EGU2020-4717 | Displays | OS2.1

2D multi-layer hydrodynamic and sediment transport modelling in a tidal estuary

Kai-Yi Bai and Jiing-Yun You

This study developed a multi-layer hydrodynamic and sediment transport model for simulating tides and the estuarine flows. The flow circulation in an estuary shows complicated mixing and stratification patterns due to the combined effects from currents and tides. This kind of issues becomes more important in Taiwan in line with the more and more frequent sediment flushing operation which led to high sediment concentration flow at the estuary. In some applications,  three-dimensional (3D) models solving full Navier-Stokes equations were used. However, the extremely high computational cost, especially for the large-scale environmental problems, is always a serious concern. In the past years, continuous efforts have been devoted to the development of efficient quasi-three-dimensional models under hydrostatic and Boussinesq assumptions. Following the same state-of-the-art modelling strategy, this study develops a multi-layer shallow-water and sediment transport model with finite volume method. In this model, a terrain following coordinate with high local resolution is used to vertically divide the computational domain into multiple layers to better addressing bottom topography and velocity profile. Our model is rigorously validated against several benchmark cases including winddriven circulation, subcritical flow over a hump, tidal wave propagation, and sediment transport. The grid convergence test and accuracy both are in good agreement with analytical solutions. Subsequently, the model is applied to investigate the estuary dynamics and sediment transport under different conditions, e.g., flow discharges, bottom slopes, wind shears and tidal variations. Overall, the results show a relationship between flow conditions and sediment transport. Later, some scenarios for various upstream inflow and sediment concentration will be examined to assess the reservoir operation rules. 

Keywords: shallow water, sediment transport, multi-layer, hydrostatic, Boussinesq Assumption, a finite volume characteristics (FVC) method 
 


 
 

 

How to cite: Bai, K.-Y. and You, J.-Y.: 2D multi-layer hydrodynamic and sediment transport modelling in a tidal estuary , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4717, https://doi.org/10.5194/egusphere-egu2020-4717, 2020.

EGU2020-8781 | Displays | OS2.1

Investigation of spatial and temporal salinity distribution in river deltas through idealized numerical modelling

Constantinos Matsoukis, Laurent Amoudry, Lucy Bricheno, and Nicoletta Leonardi

The world's river deltas are increasingly vulnerable due to pressures from human activities and environmental change. In deltaic regions, the distribution of salinity controls the resourcing of freshwater for agriculture, aquaculture and human consumption; it also regulates the functioning of critical natural habitats. Despite numerous insightful studies, there are still significant uncertainties on the spatio-temporal patterns of salinity across deltaic systems. In particular, there is a need for a better understanding of the salinity distribution across deltas’ channels and for simple predictive relationship linking salinity to deltas’ characteristics and environmental conditions. We address this gap through idealized three-dimensional modelling of typical delta configurations (river, tide dominated etc.) and by investigating the relationship between salinity, river discharge and channels’ bifurcation order. Model results are then compared with data from real delta cases. Results demonstrate the existence of simple one-dimensional and analytical relationships describing the salinity field in a delta. Salinity and river discharge are exponentially and negatively correlated. There is a correlation between salinity and channels bifurcation order and salinity increases linearly with decreasing stream order. These useful parametrizations of salinity distribution following deltas’ features and geometry might be applied to real case scenarios to support the investigation of deltas vulnerability to environmental change and the management of deltaic ecosystems.

 

Keywords: salinity, salt intrusion, river deltas, numerical modelling, idealized river delta models

How to cite: Matsoukis, C., Amoudry, L., Bricheno, L., and Leonardi, N.: Investigation of spatial and temporal salinity distribution in river deltas through idealized numerical modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8781, https://doi.org/10.5194/egusphere-egu2020-8781, 2020.

EGU2020-11194 | Displays | OS2.1

Tidal processes and their spatial and temporal variability in the mid-field Guadalquivir ROFI

Maria Angeles Serrano, Manuel Díez-Minguito, Miguel Ortega-Sánchez, and Miguel Ángel Losada

In the Region Of Freshwater Influence (ROFI), located between sea exposure and estuary, characteristic physical processes of both estuarine and shelf seas overlap and impact on shelf circulation, sediment transport and ecosystem (Simpson, 1997). Although freshwater discharge typically exhibits the highest variability (Horner-Devine et al., 2015), this work focus on the tidal variability within the ROFI, which is often overlooked. This work addresses the spatial and temporal variability of tidal elevations and currents at the mid-field Guadalquivir ROFI (SW Spain), which is semidiurnal in character. Observations from five current-meter profiles, which were moored pointing upwards from June 2008 to December 2009 are analyzed. These instruments were placed along an arc, from south to north, and closing the estuary mouth.  

 

The analysis of the observations indicates that tides in the Guadalquivir ROFI have a close-to standing wave behavior. This is induced by the reflection at the continental margin of the northward-propagating tidal Kelvin wave. Regarding the M2 and M4 tidal constituents, which are relevant for residual sediment transport, their relative phase difference shows that, although the inner estuary is flood-dominant, in the mid-field ROFI zone ebb currents are slightly stronger than flood currents. Fortnightly variations are observed in the ratio of the M4 and M2 tidal amplitudes. Remarkably, the minimumvalues occur during the transition periods from neap to spring tides, whereas the maximum values are observed during neap tides. These results might suggest that there is still some influence of the tidal jet in this region.

 

Regarding the vertical structure of tidal currents, the M2 inclination varies with depth, being maximum near the bottom at all moorings. Tidal currents inclination also varies with time in the southern part of the ROFI. Moreover, in the southern part of the ROFI, current ellipses in the upper layer of the water column rotate clockwise whereas near the bottom currents rotate anti-clockwise, as revealed by the sign of the eccentricity. However, the eccentricity of tidal ellipses is uniform throughout the water column in the northern part of the ROFI. This along-coast variability of the vertical structure of the tidal ellipses suggests that the buoyant outflow circulates preferentially southwards, most likely driven by the prevailing winds.

How to cite: Serrano, M. A., Díez-Minguito, M., Ortega-Sánchez, M., and Losada, M. Á.: Tidal processes and their spatial and temporal variability in the mid-field Guadalquivir ROFI, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11194, https://doi.org/10.5194/egusphere-egu2020-11194, 2020.

EGU2020-14614 | Displays | OS2.1

The influence of wind on the evolution of freshwater fronts in the Rhine ROFI

Lennart Keyzer, Sabine Rijnsburger, Firmijn Zijl, Martin Verlaan, Mirjam Snellen, Cornelis Slobbe, Raul Flores Audibert, Alexander Horner-Devine, Alejandro Souza, and Julie Pietrzak

The Rhine River discharges freshwater into the North Sea, forming one of the largest Regions of Freshwater Influence (ROFI) in Europe. Every tidal cycle, a freshwater lens is released. These fronts were captured by the STRAINS (STRAtification Impacts Near-shore Sediment) field campaign of 2014. The data consists of current velocity, temperature and salinity at a fixed location 10 km northeast of the river mouth. Here, we explore the effect of the wind on the evolution of the freshwater lenses using a high-resolution 3D model, which is validated against the field data. We find a stratified river plume that consists of multiple freshwater fronts. On every ebb tide, a new freshwater lens is formed, which is subsequently advected by the tidal flow. Remaining lenses from previous tidal cycles are still present when the next one is formed. The properties and evolution of the lenses strongly depends on the wind magnitude and direction. Under upwelling winds, they evolve separately and the downstream plume is detached from the coast. The thickest lenses are found under downwelling winds, when their propagation speed is maximum and the downstream river plume is pushed against the coast. During storm conditions, when the wind speed exceeds 15 m/s, the river plume becomes well-mixed and no separate lenses are found. The model shows a detailed picture of the formation and evolution of the freshwater lenses in the Rhine ROFI and the vertical structure of the water column. We find a multiple front system, where lenses interact under the influence of tidal flow and prevailing winds; diverging flows causes the lenses to separate, while they seem to merge under converging flows.

How to cite: Keyzer, L., Rijnsburger, S., Zijl, F., Verlaan, M., Snellen, M., Slobbe, C., Audibert, R. F., Horner-Devine, A., Souza, A., and Pietrzak, J.: The influence of wind on the evolution of freshwater fronts in the Rhine ROFI, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14614, https://doi.org/10.5194/egusphere-egu2020-14614, 2020.

EGU2020-12979 | Displays | OS2.1

Tidal plume fronts, internal waves and sediment resuspension in a near field river plume

Julie D. Pietrzak, Sabine Rijnsburger, Raul Flores, Zeinab Safar, Alex Horner-Devine, Alex Souza, Kevin Lamb, Nicole Jones, and Claire Chassagne

Internal waves are known to be an important source of mixing in the coastal ocean. Measurements from the Columbia River Plume were some of the first to demonstrate the generation of large amplitude internal waves released by a newly formed tidal plume front. Here we explore internal waves generated by multiple tidal plume fronts and their trapping in the mid-field plume of the Rhine river plume. The internal waves are released into a shallow frictional system, and their role on mixing, near shore sediment resuspension is examined. We use data collected off the Dutch coast near the Sand Engine, during the STRAINS field campaigns at a location 10 km north of the river mouth. An ADCP measured current velocity with a frequency of 1 Hz and a resolution of 0.25 m. Temperature, salinity, velocity, sediment concentration measurements, as well as turbulent stresses were measured at the 12 m site at 0.25, 0.5 and 0.75 m above the bed. The field-data and radar images show tidal plume fronts propagating towards the Dutch coast and the generation of high frequency internal waves ahead of the fronts. As the fronts propagate onshore they increase turbulence and mixing and can also increase sediment resuspension. Using an idealised non-hydrostatic model we show that the fronts can generate high frequency internal waves as they propagate towards the coast, and that these waves can break inshore. We introduce a frontal sediment pumping mechanism, and show how this is a new mechanism for sediment resuspension and offshore transport.

How to cite: Pietrzak, J. D., Rijnsburger, S., Flores, R., Safar, Z., Horner-Devine, A., Souza, A., Lamb, K., Jones, N., and Chassagne, C.: Tidal plume fronts, internal waves and sediment resuspension in a near field river plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12979, https://doi.org/10.5194/egusphere-egu2020-12979, 2020.

We use optical imagery and video of ocean surface acquired from aerial drones (quadcopters) to study small river plumes formed in the northeastern part of the Black Sea. Quadcopters can continuously observe small river plumes with high spatial resolution from relatively low altitude. It provides unprecedented ability to study spatial structure of small river plumes, detect and measure their temporal variability, register various dynamical features of these plumes. In this work we describe and analyze strongly inhomogeneous structure of small river plumes manifested by complex and dynamically active internal frontal zones; undulate form of sharp front between small river plume and ambient sea and energetic lateral mixing across this front caused by frontal baroclinic instability; internal waves generated by river discharge near a river estuary and propagating within inner part of a plume; internal waves generated by vortex circulation of a river plume and propagating within outer part of a plume. The issues reported in this study remained mainly unaddressed before due to low spatial and/or temporal resolution of in situ measurements and satellite imagery used in previous related studies. We show that usage of aerial drones, first, strongly enhance in situ and satellite observations of structure and variability of small plumes, second, provides ability to perform accurate, continuous, and high-resolution measurements of their spatial characteristics and current velocity fields and, finally, significantly improves organization of operational field measurements. As a result, aerial drones are effective tools to obtain new qualitative insights and quantitative assessments of structure, variability, and dynamics of small river plumes.

How to cite: Osadchiev, A. and Barymova, A.: Spatial structure, temporal variability, and dynamical features of small river plumes observed by aerial drones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13161, https://doi.org/10.5194/egusphere-egu2020-13161, 2020.

This study is focused on delivery and transport of floating marine litter, which is carried by river discharge to coastal sea. This floating matter initially is contained in river plumes and its transport is governed by river plume dynamics. Despite the great importance of understanding the fate of floating marine litter (including plastic litter) in the sea, many aspects of its transport and accumulation remain unstudied. In this study we consider a large flood which happened in the northeastern part of the Black Sea in October 2018. A high resolution circulation model with a non-uniform horizontal grid (the grid bin length is decreased up to 200 m in a local area of interest) is applied to simulate transport of floating matter brought into the sea by overflowing rivers. The floating matter transport is modelled by horizontal advection of Lagrangian particles seeded in the mouths of main rivers of the study region in proportion to the actual river runoff. The particles that originated from different river mouths merge together on a horizontal velocity convergence line. These areas of accumulated marine litter remain stable during several days and are transported off the river mouths by a quasi-geostrophic alongshore current. However, some of the particles are trapped in the surf zone and form irregular contamination of the shoreline depending on local circulation features controlled by bottom topography and local wind forcing.  

How to cite: Korshenko, E., Zhurbas, V., Osadchiev, A., and Belyakova, P.: The fate of river-borne floating marine litter in the coastal sea: a case study of flooding discharge from numerous small rivers in the northeastern part of the Black Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18132, https://doi.org/10.5194/egusphere-egu2020-18132, 2020.

Understanding the evolution of estuarine hydrodynamics is essential for sustainable water resources management, since they directly link to estuarine environment by regulating the materials transportation (e.g. nutrients, sediments, organisms and pollutants). In this study, an enhanced harmonic analysis model for nonstationary tide (S_TIDE model) was used to extract the amplitudes and phases of two predominant tidal constituents (M2 and K1) in a daily scale in two tidal gauging stations (i.e., Chiwan, Sishengwei) in the Lingdingyang Bay of the Pearl River Delta from 1965 to 2016, with the purpose of exploring the spring-neap change in tidal hydrodynamics (e.g., tidal wave celerity and tidal damping/amplification rate). To understand the stepwise evolution of tidal hydrodynamics, we have divided the whole study period into three distinct periods: the pre-human (e.g., from 1965-1997), transitional (e.g., from 1998-2007) and post-human periods (e.g., from 2008-2016), based on the dynamics of wave celerity. It was shown that the long-term spring-neap change in tidal hydrodynamics was mainly driven by the highly-modified geometry (including deepening and narrowing) in the Lingdingyang Bay. To quantify the effects of estuarine morphological alterations in terms of deepening and narrowing on tidal hydrodynamics, an analytical hydrodynamics model was adopted to assess the spring-neap variations at different periods. The proposed method for evaluating the tidal dynamics owing to morphological changes is particularly useful for providing a theoretical guideline for protecting the estuarian environment in the Lingdingyang Bay and other estuaries that are subject to strong human interventions.

How to cite: Zhang, P. and Cai, H.: SPRING-NEAP CHANGE IN TIDAL HYDRODYNAMICS IN THE LINGDINGYANG BAY OF THE PEARL RIVER DELTA, China , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17473, https://doi.org/10.5194/egusphere-egu2020-17473, 2020.

Based on the field efforts in 2016 during a dry season (30 Nov-6 Dec) in the Pearl River Estuary (PRE),south China, this study aimed to investigate the tidal changes of phytoplankton variability (in terms of chlorophyll a) and their responses to multiple environmental factors.Time series analysis,principal component analysis (PCA),Pearson correlation analysis, and Delft3D model were carried out. A significant difference was found in the tidal variations of dissolved nutrients, covering both a spring tide and neap tide . Moderate differences in salinity and suspended sediment played different roles in the nitrogen and phosphate. The negative correlations of salinity and nitrogen ecologically implied a stronger diluting-mixing effect than that of phosphate, which has a large impact on the water quality. The adsorption of phosphorus by sediment particles was stronger than that of nitrogen. Nitrogen was mainly contributed by river discharge. DIN was constrained by tide-river dynamics and their mutual increase-decline trend, and a new source was supplemented along the transport from river to sea. The weak correlation between PO4 and salinity suggested a different source contribution of the terrestrial emission from coastal cities; the contribution of river discharge was less compared with nitrogen. Over site, P-limitation was detected and was more frequently resulted in eutrophication and even bloom events. Characterizing the relationships among chlorophyll a, nutrients, and hydrological factors enables us to develop effective ecosystem management strategies, and to design studies more focused on ecological health and function.

How to cite: Zeng, D., Niu, L., and Yang, Q.: Distribution Characteristics and Environmental Impacts of Nutrients in the Dry Season of the Pearl River Estuary in 2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12108, https://doi.org/10.5194/egusphere-egu2020-12108, 2020.

The water age in coastal waters can be estimated by a simplified formula defined by activity ratio of radium isotopes when neglecting the effects of four factors including recirculated seawater (RSGD), open sea water end-member, sediments and rivers. Although this formula has been widely used, sometimes it is applied without checking the assumptions of neglecting the effects of the above-mentioned four factors. Here an attempt is made to give a generalized formula for estimating water age explicitly incorporating all the above-mentioned effects. The formula is then applied in Daya Bay, China by comprehensively using all the radium quartet (223,224,226,228Ra) data to assess the water age and submarine groundwater discharge (SGD). Data analyses indicate that the factors such as RSGD, open sea water end-member, sediments and rivers should be included in the general radium model when there are various radium sources and their contributions are unknown. It is found that in Daya Bay, neglecting the effects of RSGD underestimates the water age by 25.5-45.7% and neglecting the effects of open sea water end-member overestimates the water age by 120-130%. The SGD-derived fluxes of nutrients and trace elements are significantly higher than those from local rivers. SGD can support approximately 63-70% of the total primary production. Overall, this study emphasizes again the importance of the general radium model and enhances accuracy in estimating water age and SGD. Our results also reveal that SGD significantly influences coastal primary production in Daya Bay and other similar aquatic ecosystems.

How to cite: Zhang, Y., Wang, X., and Li, H.: Improvement of evaluation of water age and submarine groundwater discharge: a case study in Daya Bay, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12872, https://doi.org/10.5194/egusphere-egu2020-12872, 2020.

EGU2020-21061 | Displays | OS2.1

The impact of channel deepening and sand mining on estuarine tidal dynamics

Nanyang Chu, Peng Yao, Suying Ou, Shuai Hu, Jie Huang, and Qingshu Yang

Lingding Bay (LDB) is a part of the Pearl River Estuary in south China. It is surrounded by large growing cities and has been subjected to heavy waterways traffic over the past 5 decades. The tide propagation pattern has been greatly modified ever since. It has widespread morphological and ecological impacts on the LDB system. However, a systematic study on the response of tide propagation pattern to channel deepening and sand mining is currently lacking to provide future management guidelines for the Lingding Bay. Based on a state-of-the-art modeling tool (Delft3D Flexible Mesh), we explore the tidal propagation pattern (tidal amplitude, tidal phase, residual current, and tidal-energy flux) of the LDB over the last century (1901-2016). Three stages of bathymetry and tidal dynamics variation are divided by our investigation. Stage I (1901-1964) is a natural evolution process, the LDB is manifested as gradually filling by fluvial sediment, the tidal amplitude decline, and tidal dynamics decrease. Stage II (1964-1989) is characterized by a slow increase in water depth and tidal dynamics, which affected by channel dredging activities. While stage III (1989-2016) is influenced by channel deepening and sand mining, shown an abrupt increase of water depth in a short time and the rapid enhancement of tidal dynamics.

The investigation indicating that the channel deepening and sand mining activities amplified the tidal dynamics distribution difference between the channel and shoal. The increased tidal dynamics in the channels may increase saline intrusion and coastal flooding risks. Spatially, these two activities may also lead to contrasting morphodynamic patterns between the inner and outer LDB. The morphology state of erosion in the inner LDB and deposition in the outer LDB reported by other studies are consistent with the hydrodynamic variation in our study. Most likely, channel deepening and sand mining in inner LDB cause the sea bed to appear to lose,lead to larger SSC levels in the vicinity. This resulting in the inner LDB formed a new sediment source, under the stronger runoff in inner LDB, the depocenter is moved southward, and outer LDB developed to a sedimentary area. Our findings conclude that channel deepening and sand mining can greatly change the tidal dynamics distribution in an estuary, thereafter affect the sediment transport pattern. We suggest that coastal engineering planning should pay more attention to sand mining activities. The insights obtained from this study are of value to the future management of the LDB and other estuaries that are also under similar stress.

How to cite: Chu, N., Yao, P., Ou, S., Hu, S., Huang, J., and Yang, Q.: The impact of channel deepening and sand mining on estuarine tidal dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21061, https://doi.org/10.5194/egusphere-egu2020-21061, 2020.

EGU2020-20697 | Displays | OS2.1

Internal hydraulics of surface buoyant jets with high aspect ratio

Adam Jiankang Yang and Gregory Lawrence

Surface buoyant jets are commonly found in natural and engineered environments. Typical examples are rivers entering into the ocean, and wastewater discharges into water courses. The surface buoyant jet structure depends on the source properties, geometry and mixing processes. Predicting the mixing and spreading is the key challenge. Recent studies based on layered models have investigated the entrainment rate and spreading rate. However, frictional effects are also important in determining the thickness of the buoyant jet and its lateral spreading.  We will address the effects of entrainment, spreading and friction.

We investigate the surface buoyant jet over a sloping bottom through internal hydraulic theory and field measurements of a river flow into the ocean. In the nearshore zone, the river flow is attached on sea bottom due to the Coanda effect. With a decrease of momentum and thickening, the buoyant jet starts to lift off. At the detachment point, the buoyant jet is critical and the isopycnals are perpendicular to the bottom. We focus on large aspect ratios (river width to the depth) and predict layer thickness, entrainment, lateral spreading and interfacial friction. Comparisons are made with field measurements in Koombana Bay, Western Australia.

How to cite: Yang, A. J. and Lawrence, G.: Internal hydraulics of surface buoyant jets with high aspect ratio, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20697, https://doi.org/10.5194/egusphere-egu2020-20697, 2020.

EGU2020-2494 | Displays | OS2.1

Quantile function based, optical characterisation of the Nelson River plume dispersion in Hudson Bay (Canada)

Atreya Basu, Anirban Mukhopadhyay, and Jens Ehn

This study hypothesizes, that sediment-based optical estimation of plume extent underestimates the dispersion limit of river discharge in marine waters. Optically active Colour Dissolved Organic Matter (CDOM) has been used as a tracer along with suspended sediments to validate the hypothesis, for the Nelson River (NR) plume in southwest Hudson Bay (HB). Remote sensing reflectance (Rrs) band ratio (Rrs 678nm/488nm) was tuned in to retrieve CDOM absorbance at 412nm: aCDOM (412nm) from Moderate Resolution Imaging Spectroradiometer (MODIS) images. Similarly, Rrs (678nm) was used for TSS concentration retrieval. Plume dispersion characteristics were compared for spring and neap tidal periods during a high (2005) and a normal (2006) discharge year. Quantile function (QF) provides variable values (aCDOM, TSS concentration) for a given cumulative probability. It was assumed that 0.90 QF (10% of the data distribution) is a representative of the river plume function, while 0.05 QF (95% of the data distribution) represents the ocean end-member values. These threshold values were calculated for each of the cumulative areas of coastal waters of southwest HB with NR mouth as the origin, limited till 500 km radial distance. Thresholds averaged over 0 km to 50km was used as the reference for estimation of river plume dilution. A conditional approach of the plume extent limit was set to the point at which the QF dilution (0.90-0.50) equals to the QF dilution (0.90-0.05). This dilution conditionality was satisfied for CDOM but was never achieved for TSS, indicating the additional source of sediment influx. Plume discharge volume was proportional to the aCDOM (412nm) plume extent threshold for all tidal periods. This threshold value was observed at ~250km during neap ebb tide (NET) for both years. While a shorter extent (150 km) was observed during the 2005 spring ebb tide (SET) and ~300km for the same tidal period during 2006. The point of minimum variance within aCDOM (412nm) dilution (0.50 to 0.90 QF) was used as a representative of the freshwater-marine boundary. An extent of 400km (SET) and 350km (NET) for 2005 was recorded, with a 500km extent observed for both ebb tides in 2006.  A lower aCDOM (412nm) dispersion threshold was observed for the SET (0.39 m-1) than the NET (0.59 m-1) for 2005 and vice-versa for 2006. A weak, negative correlation of aCDOM (412nm) minimum variance threshold between 2005 and 2006 indicated a weaker influence of discharge beyond the maximum variance threshold. This dispersion is expected to be controlled by a cyclonic eddy in southwest HB. Modeled dilution slope of aCDOM (412nm), <200km showed a significant influence of discharge with higher slope values obtained for 2005 for both spring and neap ebb tide conditions. The minimum variance of TSS concentration dilution is limited within 200-250 km, while its maximum variance is limited within 100km. Thus, TSS underestimates the river plume dispersion extent, which is better represented by CDOM of terrestrial provenance.

How to cite: Basu, A., Mukhopadhyay, A., and Ehn, J.: Quantile function based, optical characterisation of the Nelson River plume dispersion in Hudson Bay (Canada), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2494, https://doi.org/10.5194/egusphere-egu2020-2494, 2020.

The Yangtze estuary is characterized by its extremely high suspended sediment concentration (SSC) and the extensive turbidity maximum zone (TMZ). The estuary is physically forced by an upstream river discharge seasonally varying in a wide range of 6000 – 92000 m3/s and semidiurnal-diurnal mixed tides with the tidal range up to 5 m. The influences of the seasonal and interannual variations in the upstream river discharge and the tidal asymmetry on the location of the Yangtze TMZ are numerically investigated with a two-dimensional depth-averaged model. Sensitivities of SSC and hence the location of TMZ to the bottom shear stress, bed erodibility, and the sediment settling velocity are studied. The spatial and temporal evolutions of the TMZ position in the cases of various upstream river discharges with different monthly, seasonal and interannual variations are simulated and discussed. The effects of the M2/M4-induce tidal asymmetry on the TMZ position and those of the interactions between the eight main astronomical tides (M2, S2, N2, K2, K1, O1, P1, and Q1) are compared. It is shown that the M2/M4-induce tidal asymmetry plays a critical role in the formulation of TMZ in the downstream of the South Branch of Yangtze estuary, while the interactions between the eight main astronomical tides have more significant effects on the TMZ location in other areas of Yangtze estuary such as the South and the North Passages.

How to cite: Dong, P. and Shi, H.: Influences of River Discharge Variation and Tidal Asymmetry on the Spatial Evolution of the Turbidity Maximum Zone in Yangtze Estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13642, https://doi.org/10.5194/egusphere-egu2020-13642, 2020.

Due to the impact of the Three Gorges Dam on water and sediment storage, the sediment flux into the Yangtze River Estuary has dropped sharply by 70%, and the suspended sediment concentration in the estuary has responded accordingly. From the comparison of the measured suspended sediment concentration data of the Yangtze River estuary for many years, it is known that the suspended sediment concentration in the South Passage has been reduced by about 60% recently, and that in the middle and upper reaches of the North Channel and the South Channel has been reduced by about 40%. On the other hand, A series of artificial engineering has been completed in the past 20 years, such as the 12.5m Deep-Waterway Regulation Engineering, the Nanhui Shoal Slush-enclosure Engineering, and the Hengsha Shoal Slush-enclosure Engineering, etc. These engineering have significantly changed the original water and sediment transport pattern of the Yangtze River Estuary. It resulted in a significant change of the estuarine turbidity maximum zone and the corresponding river mouth bar topography. This paper intends to discuss the impact of human activities on the dynamic sedimentation process of the maximum turbidity zone in the Yangtze River Estuary based on field measured data. Results are as follows:

(1) Compared to two decades ago, the suspended sediment concentration in the North Passage, the South Passage and the North Channel, and the middle and lower reaches of the North Branch is still high, which is related to the existence of the turbidity maximum zone and river mouth bar in these river sections.

(2) The implementation of man-made engineering such as the submerged diversion dike between the North Passage and the South Passage and the Nanhui Shoal Slush-enclosure Engineering changed the flow structure in the upper section of the South Passage, leading to the turbidity maximum zone and the corresponding river mouth bar have completely disappeared.

(3) Affected by the 12.5m Deep-Waterway Regulation Engineering, the turbidity maximum zone and the corresponding river mouth bar originally located at the upper section of the North Passage have also disappeared.

(4) The longitudinal circulation flow structure, salt water wedges, and stagnation points in the middle and lower sections of the North Passage and the South Passage still exist. The positions of the turbidity maximum zone and the corresponding river mouth bar topography are not significantly affected by the engineering. And the core area of ​​the obvious turbidity maximum zone and the river mouth bar (only in the South Passage) still exist. Due to the artificial dredging of the navigation channel in the North Passage, it actually appeared as an invisible river mouth bar that has been dredged by continuous dredging.

 (5) The drastic reduction of sediment flux from the basin has caused seabed erosion adjacent to the Yangtze River Estuary, and the corresponding eroded sediment has become one of the main sediment budget sources of the turbidity maximum zone.

How to cite: Li, W., Zhang, X., Li, Z., and Li, J.: Dynamic sedimentation process of the turbidity maximum zone in the Yangtze River Estuary under the influence of human activities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12345, https://doi.org/10.5194/egusphere-egu2020-12345, 2020.

The development of storm-induced fluid mud is an important factor to disturb the waterway transportation. Based on the observation data of fluid mud from 2010 to 2016, the basic characteristics and dynamic factors of the storm-induced fluid mud in the North Passage of the Yangtze River Estuary are analyzed. The main conclusions are as follows: (1) The sediment composition of the storm-induced fluid mud in the North Passage has little difference with the suspended sediment, which shows high correlation with the bed sediments in the middle/lower channel and the north beach of the North Passage, but the space difference of which is weak. (2) Large-thickness fluid mud in the North Passage mainly locates in the manual dredged navigation channel, and cannot stay in the steep slope beaches. It manly distributes between IIN-C and Y channel unit where is under the protection of the south and north embankments. (3) The storm-induced fluid mud in the North Passage characterizes as three stages. The primary-stage fluid mud develops during the storm surge, characterizes as low density, blurred upper and lower interfaces. It migrates quickly following the tidal current, and can be easily weaken by the peak tidal velocity. The development-stage fluid mud mainly occurs after the storm surge, characterizes as clear upper interface, "h" type density profile, with good stability and slowly migration. The dissipation-stage fluid mud characterizes as decreasing sediment amount, increasing sediment density, fuzzy lower boundary, "L" type or multi-steps type density profile, high stability and very weak flowability. (4) The cumulative wave energy during storm surge processes is the most important factor to determine the scale of the storm-induced fluid mud in the North Passage. The stronger the cumulative wave energy, the longer duration and the larger scale of the storm-induced fluid mud will develops. In addition, the weaker tidal power during the storm surge processes is favorable to the formation of the storm-induced fluid mud in the North Passage. Stronger tidal force would cause the shorter dissipation period of the storm-induced fluid mud. (5) The mechanism that up layer tidal current disturbs the fluid mud layer that make its sediment tends to dissipation and transport to the downstream and reciprocating following the tidal current, which plays the main role during the local extinction process of the storm-induced fluid mud in the North Passage. (6) The process of the high-sediment concentration gravity flow generates in the steep slope of the beach and near-bed invades to the manual dredged navigation channel during the storm surge process, is the key process mechanism for the rapid accumulation of storm-induced fluid mud in the North Passage.

How to cite: Li, J., Li, W., and Zhang, X.: Analysis On Characters And Dynamic Mechanism Of The Storm-induced Fluid Mud In the North Passage of the Yangtze Estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12649, https://doi.org/10.5194/egusphere-egu2020-12649, 2020.

Quite a number of estuaries are characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and river discharge. Examples are the Berau estuary (Indonesia), the Pearl estuary (China) and the Yangtze estuary (China). Knowledge about tides are required for construction of dikes/harbours, while knowledge about net water transport is important for agriculture, fresh water supply to cities and for quantifying transport of sediment, nutrients and etc.

In this contribution, we present a generic, weakly nonlinear 2DV estuarine network model to study tides and net water transport and to understand the dependence of their along-channel and vertical structure on geometric characteristics and sea level changes. The model will be applied to the Yangtze Estuary for different situations, such as the wet and dry season, with and without a shortcut channel and sea level rise for scenarios for the coming 50 to 100 years.

It will be demonstrated that, for the current conditions, the model results are in good agreement with observations. Both tidal amplitude and current are weaker during the wet season than that during the dry season and the reason for these changes will be explained in terms of river-tide interactions. Effects of local changes in geometry are investigated by creation of a shortcut channel between two main channels. Results show that changes in tides are merely local, but they can be rather strong. Sea level rise (up to 2 meters) causes tides to increase, mainly as a consequence of less friction resulting from larger water depth. Net water transport turns out to be less sensitive to geometry and sea level rise compared to tides.

How to cite: Wang, J. and de Swart, H.: Sensitivity of tides and net water transport in an estuarine network to river discharge, network geometry and sea level rise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11115, https://doi.org/10.5194/egusphere-egu2020-11115, 2020.

EGU2020-2544 | Displays | OS2.1

Sedimentary signals of the upwelling along the Zhejiang coast, China

Xin Zhang, Jian Liu, and Yoshiki Saito

The muddy deposits of the Zhe-Min coastal area are of great importance to understand "source to sink" processes. However, the sedimentary processes that dominate along the Zhejiang coast remain controversial. Determining sedimentation rates is an important element of our understanding of sedimentary processes and deposition patterns. Therefore, 23 vibrocores were collected from the muddy area along the Zhejiang coast to analyse their sedimentation rates using 210Pb geochronology. The spatial distribution of the sedimentation rates derived from the 23 vibrocores, as well as previously published data, demonstrated that the middle part of the study area around 29°N experienced relatively high sedimentation rates, which has never been reported in previous studies. This location of high sedimentation rates is approximately consistent with that of the Holocene maximum thickness deposition, finest surface sediments and high concentration of chlorophyll, resulting from the existence of upwelling along the Zhejiang coast besides the fluvial inputs. 

How to cite: Zhang, X., Liu, J., and Saito, Y.: Sedimentary signals of the upwelling along the Zhejiang coast, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2544, https://doi.org/10.5194/egusphere-egu2020-2544, 2020.

Unprecedented coastal upwelling in the southern coast of the Korean peninsula was reported in the summer of 2013. The offshore water temperature was 2℃ higher than that of climate (10-year mean) due to the hot summer in 2013. However, the water temperature at the coastal region was 2℃ lower. The upwelling continued for a month despite of weakening of upwelling-favorable wind. In this study, observational data and numerical model results were analyzed to investigate what caused the upwelling and sustained it for a long time. The upwelling was induced by upwelling-favorable wind in July. Coastal upwelling resulted in dynamic uplift of bottom cold water due to geostrophic adjustment. The dynamic uplift decreased sea level in the coastal region. The sea level difference between coastal and offshore regions resulted in an intensified cross-shore pressure gradient which induced geostrophic current accompanied by geostrophic adjustment along the coast. This positive feedback between dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a long time regardless of upwelling-favorable wind.

How to cite: Jung, J. and Cho, Y.-K.: Unprecedented coastal upwelling in the southern coast of the Korean peninsula during summer 2013, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4384, https://doi.org/10.5194/egusphere-egu2020-4384, 2020.

EGU2020-9333 | Displays | OS2.1

Realistic numerical simulations of upwelling and downwelling in the middle Adriatic: the May 2017 episode

Gordana Beg Paklar, Mirko Orlic, Tomislav Dzoic, Branka Grbec, Hrvoje Mihanovic, Zoran Pasaric, and Antonio Stanesic

Regional Ocean Modeling System (ROMS) is used to reproduce and analyse upwelling detected in the middle Adriatic Sea during May 2017. The ROMS domain covers the entire Adriatic, with a rectangular grid having horizontal resolution of 2.5 km and 22 unequally spaced s levels along the vertical. Surface momentum, heat and water fluxes in the ROMS simulations are calculated using atmospheric fields from the operational ALADIN model (Tudor et al., 2013; Termonia et al., 2018), having a horizontal resolution of 8 km for scalar fields (air pressure, air temperature, relative humidity, cloudiness, precipitation, and shortwave radiation) and 2 km resolution for wind fields. The ROMS model, in addition to the atmospheric agents, is forced by river inflows, tides and water mass exchange through the Strait of Otranto. Along the Adriatic coast, 41 rivers are discharging into the sea and their climatological flow rates (Raicich, 1994) are used in the simulations. Tidal forcing is applied on the open boundary taking into account seven tidal harmonics (M2, S2, N2, K2, K1, O1 and P1) crucial for the Adriatic dynamics. The open boundary conditions for the free surface, temperature, salinity, and velocity are taken from the wider Adriatic AREG model operationally run under the Adriatic Forecasting System (Oddo et al., 2006). Baseline ROMS simulation is run for the period from 1 August 2016 to 31 December 2018 and its quality is assessed with available CTD and HF radar measurements, satellite sea surface temperatures (SST) and data collected during May 2017 and June 2018 cruises by the yo-yo CTD profiler and shipborne ADCP. Numerical experiments focus on May 2017 when upwelling induced by prevailing NW wind was recorded. Baseline experiment qualitatively reproduces the upwelling but several sensitivity experiments are needed to increase agreement between model and measurements. Various intensities of horizontal viscosity and diffusivity and drag coefficient are tested in sensitivity studies. Moreover, a third order upwind advection scheme is tested as is the behaviour of solar shortwave radiation along the water column. Significant improvement in the model results is obtained using increased drag coefficient. Circulation pattern recorded by shipborne ADCP with inflowing currents in the first 10 km from the eastern middle Adriatic coast and wind-controlled two-layer flow further offshore is also reproduced by the ROMS model. Upwelling was clearly documented in the SST satellite images of 28 and 29 May 2017 by patches of cold water close to the eastern coast. Similar structure is reproduced by ROMS, although the modelled SST underestimates the measured values by approximately 1 °C. The area of upwelling is correctly located as is the cyclonic circulation indicated by ADCP measurements along the transect.

How to cite: Beg Paklar, G., Orlic, M., Dzoic, T., Grbec, B., Mihanovic, H., Pasaric, Z., and Stanesic, A.: Realistic numerical simulations of upwelling and downwelling in the middle Adriatic: the May 2017 episode, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9333, https://doi.org/10.5194/egusphere-egu2020-9333, 2020.

EGU2020-16587 | Displays | OS2.1

Towards operational NEMO model for the Baltic Sea

Tuomas Kärnä, Jonni Lehtiranta, and Laura Tuomi

We are developing a new operational circulation model for the Baltic Sea using NEMO v4.0. The model configuration is derived from the NEMO v3.6 1 nmi NemoNordic setup (Hordoir et al., Geoscientific Model Development, 2019). A pre-operational version of the model has been implemented to produce daily forecasts of water level, temperature, salinity, and currents, as well as sea ice coverage. In this poster we present model validation for a two-year hindcast simulation. The results indicate that daily and seasonal variability of water levels and sea surface salinity are well captured. Sea ice coverage is well represented, although slightly over-estimated. Comparisons at several mooring locations show realistic vertical salinity structure, and verify that the model can simulate Baltic inflow events. Overall, the model skill has significantly improved compared to previous operational models.

How to cite: Kärnä, T., Lehtiranta, J., and Tuomi, L.: Towards operational NEMO model for the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16587, https://doi.org/10.5194/egusphere-egu2020-16587, 2020.

EGU2020-15894 | Displays | OS2.1

A strategy towards estimating a sediment budget for the Baltic Sea coastline of Schleswig-Holstein, Germany

Tanita Averes, Klaus Schwarzer, Jacobus Hofstede, Arfst Hinrichsen, Hans-Christian Reimers, and Christian Winter

Sea level rise along with the changing climate leads to severe enhancement of hydrodynamic impact to coastlines worldwide. Along the Baltic Sea coast of Schleswig-Holstein (Germany), this leads to the erosion of exposed glacial cliffs (up to 30 % of the coastline) and abrasion platforms (unknown extend). Irreversible land loss and seafloor deepening are the consequences, causing socio-economic and environmental concerns in affected areas. However, the adjacent coastal sections benefit from the development as the mobilized material constitutes the main sediment source to the nearshore bar and beach systems. Here, temporal built up of nearshore bars and the deposition at sandspits and beaches functions as natural shore protection.

The heterogenous and dynamic morphology, exposition and geology of the cliff sections and their offshore continuation complicates system understanding and management of the Schleswig-Holstein coastline. The availability of coarse-grained sediments (sand, gravel, stones) from the poorly sorted glacial till, forming the cliffs, is comparatively low. This lack of obtained material suitable to build up a coastal morphology attributes a central role to the source areas and the quantification of the sediment budget regarding coastal preservation.

On this account we attempt to develop a strategy towards a classified coastal sediment budget, which is based on a comprehensive field and literature data base, addressing the highly variable character of the observed coastline described in morphological, morphodynamic, geological, sedimentological, hydrodynamic and anthropogenic parameters.

The coastline of Schleswig-Holstein is structured into 58 active cliff sections for individual description via categorized cliff profiles. Furthermore, 22 abrasion platforms are defined in the offshore region and characterized by descriptive summaries. The data summary reveals well investigated zones (e.g. Schönhagen, Stohl, Heiligenhafen, Brodten), serving as potential pilot areas for complementary studies, but also identifies study areas which require further research.

The literature values for past cliff retreat and eroded sediment volumes bear high uncertainties. This is due to the fact that former studies are based on unequal spatial extend of cliff sections, variable time intervals and differing methods. Further, computation of eroded material volumes is lacking important input parameters, e.g. the degree of compaction and the grain size distribution. This is considered for budget calculations and their confidence for individual coastal units in template form.

The current study compiles and visualizes the heterogenous data for further scientific applications. The project aims to support future studies on the sediment availability and transport in the near-shore system using hydrodynamic modelling and thus creates a sound scientific base for system understanding and new governmental regulations concerning coastal protection measures at the Schleswig-Holstein Baltic Sea.

How to cite: Averes, T., Schwarzer, K., Hofstede, J., Hinrichsen, A., Reimers, H.-C., and Winter, C.: A strategy towards estimating a sediment budget for the Baltic Sea coastline of Schleswig-Holstein, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15894, https://doi.org/10.5194/egusphere-egu2020-15894, 2020.

EGU2020-11266 | Displays | OS2.1

Advanced modelling of wave penetration in ports

Konstantina Aikaterini Maroudi and Sebastiaan Reijmerink

Wave penetration is a challenge for hydraulic engineers as it governs vessels’ sailing and mooring and regulates port operations. A complete approach to describe this phenomenon is by a physical scale model, which is time consuming and expensive. Therefore, a numerical model is a valid alternative. In this study, wave penetration is simulated with the non-hydrostatic model SWASH (Zijlema, 2011). To validate the model, part of an open benchmark dataset of physical scale model tests (Deltares, 2016) is used. This research addresses regular waves conditions and a simple harbour basin layout, in which reflection and diffraction are the main wave processes. This study assesses SWASH’s capability to model these processes, separately and in combination, in the full harbour layout.

1. Methodology

Reflection outside and inside the harbour is studied by two simplified 1D SWASH models, while diffraction inside the harbour by a simplified 2D model. The final SWASH model represents the full harbour layout. In all the models the water level time series at the output locations are compared qualitatively to the respective series measured at the wave gauges. Moreover, the measured steady state wave height is compared to the SWASH outputs. The “Difference”, Eq. (1), is computed to evaluate the model accuracy and to quantify the relative importance of each wave process.

Difference/diff.=(HSWASH,mean-Hmeasured,mean)/Hmeasured,mean  (1)

Where HSWASH,mean ; Hmeasured,mean : mean steady state wave height obtained by SWASH or measured respectively [m].

2. Results

Although the reflection trends are reproduced qualitatively in SWASH, the exact steady state wave height values may deviate significantly (diff.>30%). Moreover, the initial diffraction trends are also identified in SWASH despite their short duration in the measurements. Regarding the steady state wave height, diffraction influences considerably the total measured wave penetration inside the harbour. In the final SWASH model, the overall changes in the wave height are reproduced by SWASH. The agreement between the measured and the computed wave height is good at many output locations (diff.<10%). However, at some locations the accuracy is low (diff.>40%), owing to standing wave patterns which change fast within a short horizontal distance. Thus, the wave height can vary significantly at the area close to a specific wave gauge.  Finally, for relatively high waves and/or breaking waves, numerical instabilities are detected. Higher spatial resolution is required to capture such phenomena.

3. Conclusions

The study shows SWASH capability to reproduce qualitatively the most important reflection and diffraction trends. To a large extend, diffraction is the main process determining the wave height inside the harbour; reflection at the harbour end comes second. Outside the harbour, reflection off a quay wall is the dominant process, while reflection off a gravel slope is noteworthy. All in all, it is concluded that for non-breaking, relatively low waves, SWASH accuracy in modelling wave penetration is sufficient for engineering purposes. With further validation to guarantee the model stability, the implemented methodology can be a useful tool to understand the performance of SWASH in modeling wave penetration per wave process and in combination.

How to cite: Maroudi, K. A. and Reijmerink, S.: Advanced modelling of wave penetration in ports, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11266, https://doi.org/10.5194/egusphere-egu2020-11266, 2020.

EGU2020-12139 | Displays | OS2.1

Sill Processes in the Saguenay Fjord

Jérôme Guay, Daniel Bourgault, Cynthia Bluteau, Cédric Chavanne, Peter Galbraith, and Louis Gostiaux

The Saguenay Fjord is a 110 km long and 250 m deep (max depth) multi-silled glacial valley that connects the Saguenay River at its head with the St. Lawrence Estuary at its mouth. The bathymetry is characterized with 3 sills: a shallow 20-m deep sill at the mouth, an intermediate 60-m deep 20 km landward sill and a deep 120-m sill 35 km landward. These sills separate 3 basins, the outer, the intermediate and the inner basins. The circulation in the fjord is forced by the Saguenay River at its head that brings freshwater, large tides (up to 6 m range) at its mouth that brings salt water and by wind. The large-scale circulation has been characterized by three seasonally dependent regimes during which the deep, intermediate and subsurface waters of the inner basin are being renewed, respectively, during early winter, summer and late winter. There are indirect indications that those regimes are determined by turbulent processes occurring locally at each of these three sills. Here, we carried out a field experiment to more directly investigate the detailed dynamics of tidally-driven sill processes and water mass modifications occurring across these three sills. Our measurements provide to date the most accurate and complete description of the stratified tidal flow structures around these sills. We also found that an internal hydraulic jump seems to form every ebb tide on the seaward side of the intermediate sill but not during flood tide on the landward side. Research is ongoing to better understand this asymmetry but our hypothesis is that it is the presence of a salty pool landward of the sill that prevents the formation of a hydraulic jump, a process that may be similar to that documented in Knight Inlet (British Columbia, Canada).

How to cite: Guay, J., Bourgault, D., Bluteau, C., Chavanne, C., Galbraith, P., and Gostiaux, L.: Sill Processes in the Saguenay Fjord, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12139, https://doi.org/10.5194/egusphere-egu2020-12139, 2020.

In order to study the vertical profile of suspended sediment concentration (SSC) and its temporal variation in the partially stratified estuaries, the profile of SSC, as well as the profiles of current and salinity were measured over a neap-spring cycle for 16 tidal cycles in April 2012 in the turbidity maximum zone (TMZ) of the North Passage in the Changjiang Estuary. The observations revealed that the TMZ was characterized by high SSCs, strong current velocities and remarkable saltwater intrusion. Both salinity and SSCs could yield strong density stratification which would exert important influence on the shape of SSC profile by damping sediment diffusion. The vertical profiles of SSCs mainly exhibited three typical types, i.e., two-layer structure profile, exponential profile and linear profile, and had significant flood-ebb and neap-spring variation patterns. In a tidal cycle, the two-layer structure profile mainly occurred during the strong stratification periods, and the exponential and linear profiles mainly occurred in the weak stratification periods. About 60% observed SSC profiles belonged to the two-layer structure profile, and 40% observed SSC profiles belonged to the exponential and linear profiles. The formation of the two-layer structure profiles during the latter half of floods and early half of ebbs was attributed to the bottom lateral currents, because it could drive the higher SSC and higher salinity in the deep channel to the south shoal through the bottom water layer. Two new empirical equations for the SSC profiles are proposed. They can predict the linear and exponential profiles accurately, and predict the two-layer structure profile reasonably. Both the exponential and linear SSC profiles had constant diffusion coefficient in the water column, and they can be delineated by a unified equation. Additionally, the bottom lateral currents directed to the south flank during 87% of the survey period, and could enhance the SSC, salinity and water exchanges between the channel and the shoal.

How to cite: Li, Z.: Vertical profile of suspended sediment concentration in the turbidity maximum zone of the partially stratified Changjiang Estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13022, https://doi.org/10.5194/egusphere-egu2020-13022, 2020.

EGU2020-17014 | Displays | OS2.1

Observed seasonality in internal wave energy and associated mixing in a temperate shelf sea

Juliane Wihsgott, Matthew Palmer, Jonathan Sharples, and Jo Hopkins

Long-term observations (March’14 − July’15) of ocean density and velocity from the North West European shelf reveal a seasonality in internal wave energy linked to the seasonal cycle of stratification. Further, this seasonality extends to internal mixing associated with internal waves that can be effectively described by the buoyancy frequency (N2), with the strongest mixing associated with strongly stratified summer conditions. To better understand these results a model was used that employed three different, commonly used parameterisations of internal mixing. Each parameterisation produced some degree of seasonality in internal mixing. Contrary to observed results however, all three model scenarios produced a minimum in internal mixing during summer, with enhanced mixing observed during spring and autumn. This failure in each model was attributed to the lack of realistic levels of enhanced baroclinic energy and shear (S2) that is identified in observations to be attributable to internal waves. These observations reveal a close relationship between N2and S2, resulting in a near continuous state of marginal stability; where the gradient Richardson number is maintained at a near critical level. Due to the observed strong dependence of internal wave energy and internal mixing on stratification, a modified version of the MacKinnon and Gregg (2003a) turbulence scaling was employed. This modified parameterisation successfully replicated the observed seasonality in internal mixing. This important result implies that future parameterisations should aim to scale internal mixing on enhanced levels of S2 from internal waves, which are shown here to be suitably predicted by the seasonal cycle of stratification (N2).

How to cite: Wihsgott, J., Palmer, M., Sharples, J., and Hopkins, J.: Observed seasonality in internal wave energy and associated mixing in a temperate shelf sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17014, https://doi.org/10.5194/egusphere-egu2020-17014, 2020.

EGU2020-18201 | Displays | OS2.1

Subducting filaments at fronts in the Alboran Sea: Physical, turbulent and biological evidences.

Francesco Marcello Falcieri, Mathieu Dever, Mara Freilich, Annalisa Griffa, Katrin Schroeder, and Amala Mahadevan

Submesoscale instabilities along oceanic fronts can cause water mass intrusions from the surface mixed layer into the stratified pycnocline. These are important drivers of vertical exchange that have a potentially significant impact on the transfer of physical properties and biological tracers.

The CALYPSO (Coherent Lagrangian Pathways from the Surface Ocean to Interior) ONR research initiative focuses on observing and understanding coherent vertical pathways by which vertical exchange occurs. The Alboran Sea (located in the south-western Mediterranean, east of Gibraltar) is well known for its strong density fronts and eddies. During a research cruise, onboard R/V Pourquoi Pas? in early April 2019, we found that fronts in this area support the generation of subducting filaments. Several types of observations (using CTD, uCTD, microstructure profiles, drifters and floats) were collected along numerous cross-front transects over a period of two weeks.

The analysis of the temperature profiles highlighted the presence of several intruding filaments moving along isopycnal surfaces in the proximity of the frontal area. The intrusion signal was also clearly visible in biophysical properties with elevated Chlorophyll-a concentrations, well below the deep chlorophyll maximum, in conjunction with high dissolved oxygen values. From a microstructure point of view, the upper and lower limits of the subducting filaments exhibited high turbulent dissipation rates, with values of O(10-7) W/m2. These dissipation rates are higher than what is generally observed at such depths and point to enhanced mixing activity at the boundaries of the intrusions even along isopycnal surfaces.

How to cite: Falcieri, F. M., Dever, M., Freilich, M., Griffa, A., Schroeder, K., and Mahadevan, A.: Subducting filaments at fronts in the Alboran Sea: Physical, turbulent and biological evidences., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18201, https://doi.org/10.5194/egusphere-egu2020-18201, 2020.

EGU2020-18354 | Displays | OS2.1

AlterEco: An Alternative Framework to Assess Marine Ecosystem Functioning in Shelf Seas

Matthew R. Palmer, Charlotte Williams, Anil Akpinar, Claire Mahaffey, Tom Hull, and Matt Toberman

A recognized global increase in the extent of shelf sea and coastal oxygen deficiency calls for an urgent need to increase the spatial and temporal measurement of oxygen and a better understanding of the processes that lead to oxygen deficiency. This need is severely impeded by the natural complexity of ecosystem functioning, the impact of a changing climate, connectivity between different regions of our shelf seas and large-scale external forcing from ocean and atmosphere. Currently, methods are severely restricted in resolving this complexity due to poor resolution in observational coverage, which calls for the development of new strategies for observing and monitoring marine ecosystem and environmental status to better enable national and regional assessments.

AlterEco is a UK based project that has been jointly funded by academic and government agencies and the WWF to address this challenge using a novel monitoring framework to deliver improved understanding of key shelf sea ecosystem drivers. This framework capitalizes on recent UK investments in marine autonomous vehicles, such as ocean gliders and wave-driven surface vehicles, and state-of-the-art chemical sensors to investigate the physical and biogeochemical functioning in the North Sea from autumn 2017 to spring 2019. The chosen area is known to undergo variable physical, chemical and biological conditioning and includes areas previously identified to experience seasonal bottom layer oxygen depletion. We will present analysis of the effectiveness of the chosen framework to meet assessments of good environmental status and will discuss the global transferability of this approach.

How to cite: Palmer, M. R., Williams, C., Akpinar, A., Mahaffey, C., Hull, T., and Toberman, M.: AlterEco: An Alternative Framework to Assess Marine Ecosystem Functioning in Shelf Seas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18354, https://doi.org/10.5194/egusphere-egu2020-18354, 2020.

EGU2020-18487 | Displays | OS2.1

New Algorithm and Processor for Obtaining Maritime Information from Sentinel-1 Radar Imagery for Near Real Time Services

Andrey Pleskachevsky, Björn Tings, Sven Jacobsen, Egbert Schwarz, Detmar Krause, and Holger Daedelow

The focus of the study is analysing storm peak/center propagation, front movement and arrival of swell using newest remote sensing information, numerical models, in-situ measurements and their combination. For this purposes, a new empirical algorithm for sea state retrieval from satellite borne Sentinel-1 (S1) Synthetic Aperture Radar (SAR) imagery was developed. The algorithm is applied inside a new processor for meteo-marine parameter estimation for Near Real Time (NRT) applications. These NRT-applications include the investigation of geophysical processes using different satellite modes ranging from high resolution modes with small image coverage of ~20km in open ocean to low resolution modes with wide coverage of ~250km in shelf areas.

The quick developments in satellite techniques, processors, algorithms and ground infrastructures provide new possibilities for series of oceanographic applications in the last years. These new techniques allow estimation of a wide range of oceanographic information including properties of surface waves and internal waves, surface wind speed, sub-meso scale fronts and eddies, ice coverage, oil spills, coastal bathymetry, currents and others. Generally, the new high resolution products from different models allow verification of meteo-marine parameters more accurately. Here, a cross validation with different sea state model results using WWIII (NOAA) and CMEMS (COPERNICUS), with in situ buoy measurements and with satellite estimated parameters allowed an significant improvement of the accuracy of the derived sea state and wind fields. 

The new empirical algorithm allows estimation of total integrated sea state parameters including significant wave height Hs, first moment wave period Tm1, second moment period Tm2, mean period Tm and also partial integrated parameters like swell and windsea wave heights and windsea period. The algorithm allows processing of different S1 Synthetic Aperture Radar (SAR) modes into sea state fields: 

  • S1 Wave Mode (WV) acquires multiple vignettes with an extent of ~20km×20km and each displaced by 100 km along satellite tracks in open ocean (global). About 60 tracks around the globe have been acquired per day. The relatively high spatial resolution of ~4 m allows estimating wave height with accuracy of ~35cm. This is comparable with the accuracy of satellite altimetry and a new achievement for SAR based techniques. 
  • S1 Interferometric Wide Swath Mode (IW) covers area-strips of thousand kilometres of earth and ocean surface in coastal areas with a resolution of ~30m by sequences of multiple images with an approximate size of 200km×250km. The accuracy of ~ 70cm (Hs) for this mode is not as so high as for S1-WV, because the short waves are not visible for S1-IW mode and imaged as noise. However, the accuracy is much higher than state-of-the-art methods for this mode. 

The algorithm has been integrated into a prototype processor for Sentinel-1 SAR imagery. The DLR Ground Station Neustrelitz applies this prototype as part of a near real-time demonstrator MSA service. The presented scientific service involves daily provision of surface wind and sea state parameters estimated fully automatically from S1 IW images of North and Baltic Sea in and around German territorial waters.

How to cite: Pleskachevsky, A., Tings, B., Jacobsen, S., Schwarz, E., Krause, D., and Daedelow, H.: New Algorithm and Processor for Obtaining Maritime Information from Sentinel-1 Radar Imagery for Near Real Time Services, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18487, https://doi.org/10.5194/egusphere-egu2020-18487, 2020.

EGU2020-5902 | Displays | OS2.1

Oceanic density/pressure gradients and slope currents

John M. Huthnance, Mark Inall, and Neil Fraser

Eastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth-integrate the linearised x and y momentum and continuity equations, assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow), and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified), and then derive an expression for the slope current velocity by integrating upwards using thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses.

This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution towards that balance. There is a direct relationship between the slope current strength, friction and along-slope forcing; also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high resolution hydrodynamic numerical model.

How to cite: Huthnance, J. M., Inall, M., and Fraser, N.: Oceanic density/pressure gradients and slope currents, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5902, https://doi.org/10.5194/egusphere-egu2020-5902, 2020.

EGU2020-20515 | Displays | OS2.1

Stratification in the North Sea: response to different atmospheric forcing

Anıl Akpınar and Matthew Palmer

On-set of spring stratification is one of the physical factors that influence the productivity of the continental shelves. Atmospheric convective mixing determines the on-set of spring stratification. This is particularly important in seasonally stratified shelf seas, where stratification constrain nutrient injection to the water column. Higher productivity in stratified period relies on intermittent diapycnal mixing events. Thus, the on-set and intensity of stratification is important for the functioning of the shelf-sea ecosystem. In this study, we investigate on-set of stratification, and its relation with the atmospheric conditions as well as imprints of sub-mesoscale features. We use high resolution in-situ measurements from 10 glider deployments, spanning over 18 months in the central North Sea. Focusing on two consecutive winters, we present year to year variability in the timing and intensity of stratification. An early initiation of stratification is observed in 2018/2019, which is also intense compared to the previous year of 2017/2018. We find that reduced wind stress and net air-sea heat fluxes result in an early on-set of stratification in 2018/2019. In February 2019, intermittent increases in chlorophyll are observed, corresponding to a minimum in sea-to-air heat loss. Similarly, in 2019 an earlier spring bloom is observed. We investigate this period with NEMO model outputs at 7km resolution (AMM7) and show a similar response, emphasizing the influence of atmospheric variability on dynamics of the shelf-sea.   

How to cite: Akpınar, A. and Palmer, M.: Stratification in the North Sea: response to different atmospheric forcing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20515, https://doi.org/10.5194/egusphere-egu2020-20515, 2020.

EGU2020-2715 | Displays | OS2.1

A study on how environmental conditions affect shipping noise propagation in the north Arabian Sea

Francesco Devoto, Georgy Shapiro, and Jose M Gonzalez-Ondina

The anthropogenic underwater noise has a negative effect on the marine fauna in the form of the damage on communication and echolocation of marine mammals, body malformations of fish and invertebrates, higher mortality of eggs and zooplankton. Anthropogenic underwater noise is recognised as a form of pollution by the Convention on Migratory Species, the International Maritime Organization and the European Commission. The noise generated by ships is considered to be the main contributor to the underwater noise with the potential to impact marine ecosystems on a global scale, and this is why the shipping noise is the focus of this study. In the upper ocean the oceanographic parameters are subject to the seasonal changes, formation of the seasonal and daily thermoclines, fronts, filaments and eddies which may influence the underwater sound propagation.

One of the most popular sound propagation models is the Range-dependent Acoustic Model (RAM) [Collins, 1995] which is based on the solution of the parabolic equation. However, RAM requires a significant number of environmental data such as bathymetry, seabed characteristics and sound speed distribution in 3D, which is often difficult to obtain. Additionally, the model is relatively slow and computationally expensive in particular in the case of multiple sound sources (ships). To overcome this limitation, simplified acoustic propagation models have been developed. One of these is the energy flux model (EFM) which assumes the homogenous sound speed distribution in all 3 directions. The advantage to use the EFM is to have a fast and efficient model, which produces results in minutes on a typical desktop PC. However, the EFM may produce results which are not very accurate in the areas with significant variability of sound speed.

The purpose of this study is to compare the EFM against RAM in an area of significant spatial and temporal variability. We set up the EFM that computes shipping noise for a frequency of 50Hz in the northern Arabian Sea including the Gulf of Oman and the Persian/Arabian Gulf. The noise at source is calculated using the modified Ross formula [Erbe et al., 2012] and ships locations and velocity, are obtained from the Marine Traffic project. We also set up RAM for the same input parameters as the EFM plus the 3D sound speed distribution calculated form our operational ocean model for the Arabian Sea.

In order to assess the effect of 3D variability of sound speed in the ocean, we have compared the EFM against the reference RAM in winter and summer season, and calculated the statistics showing the differences between the models under the same input conditions. The analysis shows the seasonal and inter-annual dependence of the differences in the received level of noise at different depths, as well the role of the thermocline, eddies and fronts in modifying propagation of acoustic energy from surface ships.

How to cite: Devoto, F., Shapiro, G., and Gonzalez-Ondina, J. M.: A study on how environmental conditions affect shipping noise propagation in the north Arabian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2715, https://doi.org/10.5194/egusphere-egu2020-2715, 2020.

OS2.2 – Oceanography at coastal scales. Modelling, coupling and observations

EGU2020-9250 | Displays | OS2.2

A new tool for identifying boundary conditions in coastal oceanic models : First tryouts with a tidal model

Guillaume Koenig, Clément Aldebert, Cristèle Chevalier, and Jean-Luc Devenon

If lateral boundary conditions are crucial for physical modelling of the ocean dynamics, their estimate may lack of accuracy in coastal regions. Data-assimilation has been a long-used tool to improve accuracy, but most of the existing popular methods are difficult to implement. To solve this, we tried a new and an easy-to-implement method to estimate boundary conditions. This method uses data assimilation with a stochastic gradient descent  and successive approximations of the boundary conditions. We  tested it with twin experiments on a tidal model in the lagoon of Ouano, in New-Caledonia. The method was successful and provided robust estimation of the boundary conditions with various settings of subsampling and noise for the pseudo-data. Here we present those results and discuss about how the stochastic gradient descent and the approximations have to be tuned.

How to cite: Koenig, G., Aldebert, C., Chevalier, C., and Devenon, J.-L.: A new tool for identifying boundary conditions in coastal oceanic models : First tryouts with a tidal model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9250, https://doi.org/10.5194/egusphere-egu2020-9250, 2020.

EGU2020-10094 | Displays | OS2.2 | Highlight

Evaluating future beach reduction in a changing climate: Methodologies and uncertainties.

Miguel Agulles and Gabriel Jordà

In recent years there have been endless coastal actions that have substantially modified the equilibrium conditions of much of the coastline. This fact, along with an unprecedented coastal population growth and the projected sea level rise, make beaches a particularly vulnerable region to climate change impacts. In particular, there is a clear need to quantify the reduction of the beach area due to the combination effects of the sea level rise and changes in the waves in the swash zone, under different future climate scenarios.

In this work different methodologies are developed to estimate the retreat of the coastline and to quantify the associated uncertainties. The methodologies have been applied to three beaches of the Balearic Islands, which have been continuously monitored during the last decade. The different methodologies imply the use of models to propagate the waves from deep waters to shallow depths and to compute wave runup. The results are compared to simpler approaches based on empirical formulations that provide a cost-effective solution to cover large domains. All the different approaches are validated with coastal wave recorders (AWACs) and data from cameras from which wave runup is estimated. Furthermore, a sensitivity analysis has been performed to assess the impact of uncertainties in the beach bathymetry.

The first results show that under the RCP8.5 scenario, the expected coastline retreat under mean conditions would be of ~22 ± 5 meters at mid-century. Considering extreme waves conditions, the retreat would reach ~40 ± 5 meters.

It is worth mentioning that the three studied beaches have a very different exposure, granulometry and maritime climate, and in spite of that, the estimated uncertainty level is relatively low (~10-25%) in all of them. Therefore, the proposed methodologies along with their uncertainty analysis, might be extrapolated to any sandy beach with a reasonable high degree of accuracy. 

How to cite: Agulles, M. and Jordà, G.: Evaluating future beach reduction in a changing climate: Methodologies and uncertainties., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10094, https://doi.org/10.5194/egusphere-egu2020-10094, 2020.

EGU2020-2428 | Displays | OS2.2

Quantifying connectivity uncertainty arising from circulation modelling inaccuracy.

Elise Vissenaekens and Katell Guizien

Ocean modelling has become an increasingly important tool to study population connectivity and is our only tool to anticipate changes in dispersal routes in future climates. To estimate the uncertainties in model predictions, a comparison was made between the simulated currents and in situ observations in the Gulf of Lion over the period of 2009-2013. The uncertainties in Eulerian current values were described using several statistical parameters, like the bias, the root mean square (RMSE), the naturalised root mean square (NRMSE), the Hannah and Heinold parameter (HH) and the correlation. Another parameter that was introduced was the correctness, which states the percentage of time the model was deemed “correct”, based on low HH values (<75%) and high correlation (>0.25). So far, the model simulated the flow speed correctly 60-70% of the time and the relative deviation between observed and simulated flow speed was about 10%. Furthermore, ensembles of Lagrangian tracks were simulated accounting for uncertainties in Eulerian flow speed. These uncertainties were either correlated to speed values or chosen according to their statistical distribution. The Lagrangian tracks were analysed to construct connectivity matrices with and without these Eulerian uncertainties. Resulting deviation in retention and larval transfer arising from flow speed uncertainty were quantified.

How to cite: Vissenaekens, E. and Guizien, K.: Quantifying connectivity uncertainty arising from circulation modelling inaccuracy., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2428, https://doi.org/10.5194/egusphere-egu2020-2428, 2020.

EGU2020-15900 | Displays | OS2.2

The significance of wave–current interaction on modelled wave fields in the Baltic Sea

Hedi Kanarik, Laura Tuomi, Jan-Victor Björkqvist, Tuomas Kärnä, and Antti Westerlund

Currents in the Baltic Sea are relatively weak and are thus often expected to have a negligible effect on sea surface waves. To evaluate the magnitude of wave–current interactions in the Baltic Sea, we ran the third generation wave model WAM with and without surface currents from the 3D hydrodynamical model Nemo4. The results showed that the currents have a notable effect on wave field only on rare occasions and that the effects are largest in coastal areas of the Baltic Proper, most notably in the western Gotland Basin, and the Gulf of Finland. The simulations showed that the currents in the Baltic Sea can cause differences of significant wave height up to tens of centimeters. More notable effect was the change in the peak of the wave spectrum from swell to wind driven waves and vice versa in some occasions. In our study we mostly focus on the events of strong wave–current interactions in the northern Baltic Proper and Gulf of Finland as we have measured wave spectra available from these locations. From the comparison with wave buoy measurements we see that implementing surface currents slightly improves the modelled peak period in the Gulf of Finland. The Gulf of Finland is of special interest also because a group of ADCP’s were installed close to the wave buoy. The current measurements from these devices can therefore be used to evaluate the accuracy of the currents in the hydrodynamical model.

How to cite: Kanarik, H., Tuomi, L., Björkqvist, J.-V., Kärnä, T., and Westerlund, A.: The significance of wave–current interaction on modelled wave fields in the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15900, https://doi.org/10.5194/egusphere-egu2020-15900, 2020.

EGU2020-20429 | Displays | OS2.2

Validation of the SAMOA Operational Forecasting System in Spanish Mediterranean Ports

María Liste Muñoz, Marc Mestres Ridge, Manuel Espino Infantes, Manel Grifoll Colls, Agustín Sanchez-Arcilla, Manuel García León, Marcos García Sotillo, and Enrique Álvarez Fanjul

Working in the coastal marine environment is highly challenging, among other reasons, due to the variety of extreme, seasonal, short and long-term environmental conditions that affect the coastline, beaches, infrastructures and port operations. The maritime climate directly affects the construction and maintenance of port infrastructures, the access of ships to ports, the safety of cargo handling operations, emergency response or the environmental management of effects of port operations. Currently, the ability to predict the sea state from a few hours to days has reached levels of precision and reliability unbelievable a few years ago. And all this, in combination with numerical measurements and predictions, has enabled significant advances in knowledge about meteorological and oceanographic conditions, making possible the development of forecasting systems to provide real, accurate and safe support in decision making in ports.

SAMOA initiative (System of Meteorological and Oceanographic Support for Port Authorities), developed by Spanish Port System (Puertos del Estado), in cooperation with Spanish Port Authorities, provides high-resolution coastal operational prediction systems in domains such as harbours and nearby coastal waters.

We present a high-resolution coastal operational prediction system which simulates the hydrodynamic in the Spanish Mediterranean Ports from April to September 2019. Bathymetry was built using a combination of bathymetric data from GEBCO (General Bathymetric Chart of the Oceans), and specific local high-resolution sources provided by port authorities. Daily updated hourly winds and heat and water fluxes from the Spanish Meteorological Agency forecast services were used as a surface forcing. The Regional Ocean Modelling System, ROMS, was used to investigate the hydrodynamics.

Three-day forecast of three-dimensional currents and other oceanographic variables, such as temperature, salinity, and sea level, were produced. These results were compared with field campaigns data, displaying agreements between model outputs and in-situ observations. Finally, a look ahead to the future of the operational prediction systems is provided as a useful tool to make informed decisions around port safety and efficiency.

We would like to acknowledge financial support from ECOSISTEMA-BC Project (CTM2017-84275-R), funded by the Spanish State Research Agency.

How to cite: Liste Muñoz, M., Mestres Ridge, M., Espino Infantes, M., Grifoll Colls, M., Sanchez-Arcilla, A., García León, M., García Sotillo, M., and Álvarez Fanjul, E.: Validation of the SAMOA Operational Forecasting System in Spanish Mediterranean Ports, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20429, https://doi.org/10.5194/egusphere-egu2020-20429, 2020.

EGU2020-4960 | Displays | OS2.2

Modelling the upper ocean dynamics of the north-west European shelf during storm events with the UK Met Office ocean-wave prediction system

Diego Bruciaferri, Marina Tonani, Huw W. Lewis, John Siddorn, Robert R. King, Pete Sykes, Juan M. Castillo, Andy Saulter, Niall McConnell, Isabella Ascione, and Enda O'Dea

Accurate modelling of the surface ocean dynamics is of paramount importance for many human activities such as search-and-rescue operations and offshore oil and wind power industry. During sea storm events, large waves can have a strong control on the surface ocean currents, making wave-current interaction a leading order process in the uppermost part of the ocean. North-west (NW) European shelf seas can be affected by extremely severe storms, increasing the need for precise predictions of the surface ocean dynamics.   

In this study we assess the impact of using a coupled ocean-wave modelling system to simulate the upper ocean dynamics of the NW European shelf during five storm events occurred in Winter 2016. Two versions of the eddy-resolving (1.5 km resolution) UK Met Office ocean-wave operational prediction system are compared: the first one uses the ocean and wave models in uncoupled mode; the second one is a coupled system including three ocean-wave interactions, namely the Stokes-Coriolis force, the modification of the surface stress by wave growth and dissipation and a wave height dependent ocean surface roughness. The assessment is carried out using the ocean currents and the Stokes’ drift reproduced by the two modelling systems to simulate the lagrangian trajectories of a number of iSphere (surface) and SVP (centered at 15m) drifters affected by the storms. The simulated trajectories are then compared with the observed drifters’ tracks. Some drifter trajectories representative of offshore, near the shelf-break and near the coast regimes have also been simulated switching on only one ocean-wave interaction per time, to better understand the relative impact of the three components we considered in the ocean-wave coupling.

Numerical results show that in the case of iSphere drifters, the trajectories simulated using ocean and wave-induced currents from the coupled system are much more accurate than the one obtained with the uncoupled system, especially near the shelf and the coasts, highlighting the importance of including wave feedback in the momentum equations of the ocean model.  For SVP drifters the effect of the ocean-wave coupling is less evident. This is probably due to the fact that the wave-current interactions considered in the current implementation of the coupled system mainly act in the proximity of the ocean surface, pointing out the need of including wave-induced effects able to influence also the sub-surface dynamics of the water column. However, results also seem to indicate that the reduced impact of the coupling might be related to some difficulties experienced by the ocean and wave models in properly representing some of the physical processes characterizing extreme storm events.

In conclusion, this study proves the importance of using a coupled ocean-wave system when simulating the ocean dynamics during storm events but also indicates where research efforts must be spent for improving the skills of the UK Met Office forecasting system.

How to cite: Bruciaferri, D., Tonani, M., Lewis, H. W., Siddorn, J., King, R. R., Sykes, P., Castillo, J. M., Saulter, A., McConnell, N., Ascione, I., and O'Dea, E.: Modelling the upper ocean dynamics of the north-west European shelf during storm events with the UK Met Office ocean-wave prediction system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4960, https://doi.org/10.5194/egusphere-egu2020-4960, 2020.

EGU2020-22401 | Displays | OS2.2

Aim, activities and early outcomes of the Coastal Working Group of the European Global Ocean Observing System (EuroGOOS)

Ghada El Serafy and the EuroGOOS Coastal Working Group

The major interface between humans and the ocean occurs in the coastal seas. Marine industries thrive in this area while European citizens make daily use of the coastal ocean for tourism, leisure and recreation. Operational oceanography assists both industry and the general public to make decisions about their use of and access to the coastal ocean. The EuroGOOS community has developed, based on in-situ and satellite observations, data and modelling capacities, a wide range of products and services for such use cases. 
The EuroGOOS Coastal working group examines the entire value chain from coastal observations, satellite data, ocean forecasts and analysis, to products and services for coastal users. The working group reviews sustainability and fitness for purpose of the existing system and identifies gaps and future steps needed to secure and improve all elements of the coastal value chain. The EuroGOOS Coastal Working Group builds upon significant initiatives already completed or underway that have focused on coastal observatories. These include, but are not limited to, the work of EMODnet, SeaDataNet, CLMS, and CMEMS-In Situ Thematic Centres (INS TACs), which play significant role in making available key datasets for coastal areas, the JERICO-NEXT and JERICO-S3 EC projects, which work towards sustaining the JERICO-RI on long term, as well as activities within EuroGOOS working groups, platforms task teams, and the five regional operational oceanographic systems (ROOS). 
More specifically, the main activities of the EuroGOOS Coastal Working Group are: (i) mapping primary users of coastal products, (ii) reviewing available and potential coastal data sources with special focus on river discharges, (iii) preparing a comprehensive inventory of European operational models, (iv) reviewing coastal data assimilation frameworks for optimizing coastal sea monitoring and forecasting systems, (v) preparing an inventory of integrated coastal products and services, and (vi) formulating a EuroGOOS roadmap for the enhanced integration of coastal services into a EuroGOOS Coastal Working Group White Paper. 
The outcomes of the EuroGOOS Coastal Working Group activities are primarily designed to support four specific areas of the EuroGOOS Strategic Agenda 2020 and the short-term priority areas, namely (1) sustained fit for purpose observations, (2) data matters, (3) product development and (4) communication and outreach. On the other hand, in broader sense these activities contribute to the coastal community through peer reviewed articles such as the review of operational modelling capacity in European Seas (Capet et al., submitted) and by promoting collaboration to initiate new opportunities to effectively serve the coastal users in the industry (e.g. aquaculture sector) as in the H2020 FORCOAST project (www.forcoast.eu). 

How to cite: El Serafy, G. and the EuroGOOS Coastal Working Group: Aim, activities and early outcomes of the Coastal Working Group of the European Global Ocean Observing System (EuroGOOS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22401, https://doi.org/10.5194/egusphere-egu2020-22401, 2020.

EGU2020-6090 | Displays | OS2.2

Hydrodynamic characterization of a meso-tidal estuary: Inhambane Bay (Mozambique)

Manel Grifoll, Gorka Solana, and Manuel Espino

This contribution analyze “in-situ” data obtained in the unexplored estuary of Inhambane Bay (Mozambique). Inhambane Bay is a bar-built estuary with a length of 30.5 km and an extension of 288 km2. Synchronous measurements of sea-level, temperature/salinity and water current velocity were obtained during two intensive field campaigns covering dry and wet climatological seasons. Additional Sea Surface Temperature (SST) obtained from GHRSST project were used to charcterize the estuary. The first results reveal a meso-tidal estuary with water currents of 1.0 m/s. The water velocity profiles show an homogeneous profile in the ADCP measurements. The hydrographic surveys and GHRSST product confirms a remarkable seasonal variability. The largest SST are observed from November to May, coinciding with the warm and rainy season. Salinity profiles are almost vertical and the variability follows the direction of the tidal phase (lower salinity values are observed during low tide). In consequence, the estuary is well-mixed with salinity increasing values downward the estuary. The flushing time is estimated between 1 -3 days in function of the neap/spring tide using the Tidal Prism method.

Acknowledgments:

 ECOSISTEMA project (CTM2017-84275-R)

How to cite: Grifoll, M., Solana, G., and Espino, M.: Hydrodynamic characterization of a meso-tidal estuary: Inhambane Bay (Mozambique), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6090, https://doi.org/10.5194/egusphere-egu2020-6090, 2020.

EGU2020-3305 | Displays | OS2.2

Optimizing COSMO_REA6 reanalysis radiation flux for a high resolution coastal ocean model

Martin Vodopivec and Matjaž Ličer

When modelling coastal areas in high spatial resolution, it is also essential to obtain atmospheric forcing with suitably fine grid. The complex coastline and coastal orography exert strong influence on atmospheric fields, wind in particular, and the east Adriatic coast with numerous islands and coastal mountain ridges is a fine example. We decided to use a high resolution COSMO atmospheric reanalysis for our long term ROMS_AGRIF hindcasts, but in our initial experiments we found out that the atmospheric model significantly underestimates the short wave flux over the Mediterranean Sea, probably due to overestimation of high clouds formation and erroneous sea surface temperature used as a boundary condition. We explore different atmospheric models and different combinations of fluxes - direct, diffuse and clear sky solar radiation and combinations of fluxes from different atmospheric models (eg. ERA5). We compare them with solar irradiance observations at a coastal meteorological station and run year-long simulations to compare model sea surface temperature (SST) with satellite observations obtained from Coprenicus Marine Environment Monitoring Service.

How to cite: Vodopivec, M. and Ličer, M.: Optimizing COSMO_REA6 reanalysis radiation flux for a high resolution coastal ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3305, https://doi.org/10.5194/egusphere-egu2020-3305, 2020.

EGU2020-3372 | Displays | OS2.2 | Highlight

Modeling the growth rates of cyanobacteria and diatoms in the Baltic Sea

Malgorzata Stramska, Joanna Stoń-Egiert, Miroslawa Ostrowska, and Jaromir Jakacki

Potential influences of various environmental factors on phytoplankton growth rates in the Baltic Sea are discussed. Our focus is on quantitative comparisons of growth rates of two phytoplankton functional types, diatoms and cyanobacteria. Growth rates are calculated as a function of quanta absorbed by phytoplankton. This in turn depends on phytoplankton exposition to light, which was simulated to represent realistic conditions encountered in the Baltic Sea in summer. In addition, phytoplankton absorption capability was characterized by absorption coefficients derived from measurements on phytoplankton mono-cultures isolated from the Baltic Sea. Estimated exposition of phytoplankton to photosynthetically available radiation (PAR) in surface waters can change about five times in case of the same solar surface insolation and water turbidity, solely due to changes in the mixed layer depth from 2 to 20 meters. When additionally changes in water turbidity are considered, phytoplankton PAR exposition can change by one order of magnitude. Light exposition and absorption properties of phytoplankton determine the effectiveness of light absorption. In our simulations for the same species of phytoplankton, changes in light exposition resulted in differences of an order of magnitude of absorbed quanta. The importance of accounting for absorptive properties is underlined through comparisons of the number of quanta absorbed by different phytoplankton types in the same environmental conditions. The effectiveness of light absorption translates to different growth rates achieved by each phytoplankton type. Our results support the notion that knowledge about phytoplankton absorption properties and light exposition is crucial when modeling phytoplankton in the Baltic Sea. Further progress is currently hindered by a lack of systematic information about maximum phytoplankton growth rates and their responses to specific environmental conditions for different functional types. Such information should be inferred in the future in specially designed laboratory experiments, that encompass realistic ranges of phytoplankton exposition to light, nutrients, temperatures and other conditions.


This work has been funded by the National Science Centre (contract number: 2017/25/B/ST10/00159 entitled: “Numerical simulations of biological-physical interactions and phytoplankton cycles in the Baltic Sea”) and by the statutory funds of IOPAN.

How to cite: Stramska, M., Stoń-Egiert, J., Ostrowska, M., and Jakacki, J.: Modeling the growth rates of cyanobacteria and diatoms in the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3372, https://doi.org/10.5194/egusphere-egu2020-3372, 2020.

EGU2020-3883 | Displays | OS2.2

The Simulation of Drifting Dead Pigs in the Sea

Yong-Jun Lin, Chih-Chung Wen, Kai-Yuan Ke, and Yih-Chi Tan

In 2017, dead pigs infected with African swine fever were found on the beach of Tianpu, Kinmen, an offshore island of Taiwan. After the event, the Kinmen government carried out a thorough pig farm quarantine. However, none infected pig was found in any of the pig farms. This study aims to identify where the dead infected pig came from. Affected by ocean currents, marine drifts can often reach hundreds or thousands of kilometers away from their origins. During the winter, ocean currents across the north of the Taiwan Strait from west to east may transport the pigs from the coast of Fujian and Zhejiang, China, to the coast of north-central Taiwan. Another possible driven force is the near-shore current of western China. In order to analyze the possible drifting path of pigs, the hydrodynamic model and the particle tracking model were applied. Pigs were simulated as mass particles. The simulation domain includes sea area nearby Kinmen and China where pigs may originate. Considering the effect of the currents and wind from 2018/12/26 to 2019/1/3, three possible drift scenarios were set for analysis, including (S1): originated from Weitou Bay; (S2): originated from Jiulong River estuary; (S3) originated from the coast of Quanzhou. The results showed that the most possible scenario is S3.

How to cite: Lin, Y.-J., Wen, C.-C., Ke, K.-Y., and Tan, Y.-C.: The Simulation of Drifting Dead Pigs in the Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3883, https://doi.org/10.5194/egusphere-egu2020-3883, 2020.

EGU2020-5817 | Displays | OS2.2

Observing Sea Level Changes Using Satellite Altimetry and In Situ Data

Nikos Flokos and Maria Tsakiri

corresponding author: N.Flokos

flksnik@gmail.com

ABSTRACT

Sea level change is one of the key indicators of climate change with numerous effects such as flooding, erosion of beaches, salt intrusion.  The detailed global picture of sea level and the monitoring of its spatial-temporal changes is performed by Satellite Altimetry (SA). Nowadays, SA data compare well with measurements from the global tide gauge network, but the aim of 0.3 mm/year accuracy in the altimeter derived rate of global mean sea level rise is still not fully met. 

Whilst the precise determination of global and regional sea level rise from SA data is promising, there is however an observational gap in our knowledge regarding the coastal zone. While Tide Gauges (TG) are usually located at the coast, therefore providing coastal sea level measurements, altimeters have difficulties there. Filling this gap becomes important when considering that the impact of sea level rise can be devastating on the coast with effects on society and ecosystems. This makes it even more significant knowing that there are many stretches of the world’s coast that still do not possess in situ level measuring devices.  

This work aims to discuss the available data and methods that link the SA measurements of sea level rise with TG measurements. Whilst there is rich literature on relevant applications, it is important to have a clear and concise methodology on this.

Tide gauge data

Several post processing steps need to be applied to the raw TG data to enrich the raw Sea Surface Heights (SSH) values and make them comparable with SA data. There are several geophysical corrections, such as pressure and wind effects, which can be applied to TG data in order to deduce  Sea Level (SL) and be consistent with altimeter data. High frequency atmospheric effects on TG data are corrected using the Dynamic Atmospheric Correction (DAC) provided by AVISO. One other large uncertainty is the vertical stability of the TG benchmark over time. TG data must be corrected for the Vertical Land Motion (VLM) to enable the comparison of two sea level measurements (TG and SA) and their later integration within the surfaces of the absolute sea heights. The main VLM dataset can be obtained from SONEL database (SONEL 2016) which provides crustal velocities from the continuous GNSS measurements at sites collocated to the TG.

Satellite altimetry data

Whilst Satellite Altimetry over the open ocean is a mature discipline, global altimetry data collected over the coastal ocean remain still largely unexploited. This is because of intrinsic difficulties in the corrections and issues of land contamination in the footprint that have so far resulted in systematic flagging and rejection of these data. In this work, the relevant methodology to overcome these problems and extend the capabilities of current and future altimeters to the coastal zone (coastal altimetry) will be discussed and a number of coastal altimetry data sets will be used (eg SARvatore, X-TRACK, RADS etc). Finally, a practical example using real data sets over the Aegean Sea will be presented. 

 

 

How to cite: Flokos, N. and Tsakiri, M.: Observing Sea Level Changes Using Satellite Altimetry and In Situ Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5817, https://doi.org/10.5194/egusphere-egu2020-5817, 2020.

EGU2020-6605 | Displays | OS2.2

Comparison of optical characteristics in non-red tide and red tide areas in the South Sea of Korea

Eunkyung Lee, Jeong-Eon Moon, Young-Je Park, and Tai-Hyun Han

Red tide, which occurs off the southern coast of the Korean Peninsula, is a maritime phenomenon that usually occurs between June and September every year, mostly by Cochlodinium polykrikoides single species. There are very few studies using the analytical methods of the inherent and apparent optical properties for these red tide. Ahn et al.(2009) analyzed the inherent optical properties of 26 species of red tide organisms occurring off the southern coast of the Korean Peninsula. Kim et al.(2016) distinguished the optical characteristics for Cochlodinium polykrikoides using field data and Hydrolight simulator. Using these analytical methods, we will understand the ocean optical properties of red tide and use the remote sensing reflectance simulator in the future to produce the input data necessary for developing the red tide analysis technology based on machine learning. Therefore, in this study, as an initial analysis, we will compare the in-situ data of red tide and non-red tide waters off the southern coast of the Korean Peninsula in September 2014 and August 2017 to identify differences in the spectral form and compare the ability of the remote sensing reflectance spectrum with the field data using a remote sensing reflectance simulator.

How to cite: Lee, E., Moon, J.-E., Park, Y.-J., and Han, T.-H.: Comparison of optical characteristics in non-red tide and red tide areas in the South Sea of Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6605, https://doi.org/10.5194/egusphere-egu2020-6605, 2020.

EGU2020-6658 | Displays | OS2.2

Ensemble Approach to Deep Learning and Numerical Ocean Modelling of Adriatic Storm Surges

Matjaz Licer, Lojze Žust, and Matej Kristan

Storm surges are among the most serious threats to Venice, Chioggia, Piran and other historic coastal towns in Northern Adriatic. Adriatic Sea has a well defined lowest seiche period of approximately 22 hours and its amplitude decays on the scale of several days, reinforcing (or diminishing) the tidal signal, depending on the relative phase lag between tides and surges. This makes prediction of Adriatic sea level extremely difficult using conventional deterministic models. The current state-of-the-art predictions of sea surface height (SSH) hence involve numerical ocean models using ensemble forcing. These simulations are computationally-demanding and time consuming, making the method unsuitable for operational or civil rescue services with limited access to dedicated high-performance computing facilities.

Ensemble approach to deep learning offers a possible solution to the challenges described above. Even though training a deep network may involve substantial computational resources, the subsequent forecasting -- even ensemble forecasting -- is fast and delivers near-realtime SSH predictions (and associated error variances) on a personal computer. In this work we present an ensemble SSH forecast using new deep convolutional neural network for sea-level prediction in the Adriatic basin and compare it to the standard approach using state-of-the-art publicly available modelling components (NEMO ocean circulation model and TensorFlow libraries for deep learning).

How to cite: Licer, M., Žust, L., and Kristan, M.: Ensemble Approach to Deep Learning and Numerical Ocean Modelling of Adriatic Storm Surges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6658, https://doi.org/10.5194/egusphere-egu2020-6658, 2020.

Most coral reef islands in the South China Sea are primary influenced by monsoons from the northeast in winter to the southwest in summer. Because the length scale of the islands ranges between O(100 m) and O(1000 m), nearshore waves and currents on the windward and leeward sides are modulated by seasonal wave directions as well as reef morphology. It is well known that long-period swell and infragravity waves play a vital role in dominating hydrodynamics and sediment dynamics on fringing reefs. However, spatial characteristics of long waves on the entire islands have not been well studied. In the present study, a phase-resolving Boussinesq-type wave model, is employed to investigate wave distributions around Taiping Island in the South China Sea during the monsoon. Wave transformations of refraction, diffraction, reflection and nonlinear wave interactions on the complex reef geometry are simulated using a high resolution grid. Model results will display spatial variability of significant wave height, infragravity waves and nearshore currents around the reef platform. The importance of long wave diffraction and resulting wave frequency components at the leeward side of the island will be examined. A small harbor located at the southern island often experiences harbor oscillations during the winter monsoon, even it located at the leeward side. Harbor oscillations forced by periods of wind-wave, swell and infragravity waves will be explored to provide critical indicators for the harbor management.

How to cite: Su, S.-F. and Weng, W.-K.: Spatial variations of long-period waves and harbor oscillations around Taiping Island in the South China Sea , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7364, https://doi.org/10.5194/egusphere-egu2020-7364, 2020.

Impacts of climate change on heat budget in the Eastern China Seas (ECSs) are estimated under the historical, RCP4.5 and RCP8.5 scenarios using an atmosphere-ocean coupled regional climate model system (REMO/MPIOM). Solar radiation contributes the largest heat source of the ECSs. The heat gain achieved by solar radiation is overruled by thermal radiation, latent heat flux and sensible heat flux released at the ocean surface. The air-sea heat exchange thus cools the ECSs, whereas an overall warming is found for the ECSs. An increased oceanic heat transport by ocean currents balances this reduced heat supply by the sea surface heat fluxes. In particular, the water transport through Taiwan Strait brings the largest amount of heat into the ECSs. Despite of an inward heat transport onto the ECS shelf caused by the Kuroshio intrusion occurring northeast of Taiwan, overall, the shelf break section acts as a heat sink for the ECSs. The net heat gain/loss by the Tsushima Strait is marginal. Under the climate projection scenarios, the net heat loss from the shelf break section reduces, probably associated with the change in surface wind. Thus the net heat transported into the ECSs through the lateral boundaries increases slightly under these scenarios, leading to an overall warming of the ECSs, relative to 20C run. Noteworthy, the warmer SST, along with strengthened wind, further enhances the surface evaporation, providing a negative feedback onto the net effect of oceanic transport.

How to cite: Tian, D.: The modeling responses of heat budget in the Eastern China Seas to global warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8649, https://doi.org/10.5194/egusphere-egu2020-8649, 2020.

EGU2020-8902 | Displays | OS2.2

Impact of sea level data assimilation on the storm surge and seiche forecasts

Marco Bajo, Iva Međugorac, Georg Umgiesser, and Mirko Orlić

This work assesses the impact of assimilating the sea level data, with an Ensemble Kalman Filter, on storm surge and seiche modelling. The study area is the Adriatic Sea, where seiches are always present after a storm surge, and often overlap on a new storm surge with a possible amplification of the total sea level. Due to errors in the wind and pressure forcing, the forecast of such extreme events is rather challenging in the Adriatic Sea, and a wrong reproduction of such pre-existing seiches reflects on a bad sea-level forecast. Here we show, by two case studies, that the assimilation of sea-level data along the coasts of the Adriatic basin is able to correct the initial state of the hydrodynamic model. Since the initial state is particularly important in the case of pre-existing seiches, the reduction of the initial error propagates several days into the forecast. The two cases here presented were between the most extreme storm surge events in the last years and they both included the pre-existing seiches. The initial forecast was very poor, due to the fact that the wind was underestimated. The assimilation of 3-day long hourly sea level data at eleven stations distributed along the Adriatic coasts produces a better forecast in both cases. Moreover, the ensemble spread allows the uncertainty of the forecast to be estimated, even if the estimate should be calibrated over time in order to be more reliable.

How to cite: Bajo, M., Međugorac, I., Umgiesser, G., and Orlić, M.: Impact of sea level data assimilation on the storm surge and seiche forecasts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8902, https://doi.org/10.5194/egusphere-egu2020-8902, 2020.

EGU2020-9525 | Displays | OS2.2 | Highlight

The influence of bottom water intrusion events on the biogeochemistry of a coastal fjord

Subhadeep Rakshit, Andrew Cogswell, Sebastian Haas, Emmanuel Devred, Richard Davis, Kate Patterson, Douglas Wallace, and Christopher Algar

Lack of bottom water exchange in fjord-like estuaries can result in low oxygen conditions and creating sites of redox-sensitive biogeochemical processes, such as denitrification. In many of these systems, occasional intrusions of well-oxygenated bottom water may temporarily alter redox gradients and sediment-water biogeochemistry. Quantifying the magnitude and importance of these changes is a challenge due to the short timescales over which these events can occur. Here we present results from Bedford Basin, a 71 m deep coastal fjord in eastern Canada, where a 20-year, weekly timeseries of bottom water conditions indicates that autumn wind-driven intrusion events are a common, but infrequent, feature of its circulation. To examine the impact of these intrusions on biogeochemistry, we deployed a benthic instrument pod at 60 m depth to record high-resolution measurements of temperature, salinity, nitrate, oxygen, and fluorescence over a 4-month period during the fall of 2018.  During this time we captured two intrusion events, one in mid-Oct and another in mid-Nov. Both intrusion events occurred on a timescale of hours and resulted in sharp changes in temperature, salinity, oxygen, and nitrate.  We used these measurements to constrain a coupled sediment-water column reactive transport model to examine the immediate and annual impacts of these intrusion events on oxygen and nitrogen dynamics in the basin bottom waters and across the sediment-water interface.

How to cite: Rakshit, S., Cogswell, A., Haas, S., Devred, E., Davis, R., Patterson, K., Wallace, D., and Algar, C.: The influence of bottom water intrusion events on the biogeochemistry of a coastal fjord, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9525, https://doi.org/10.5194/egusphere-egu2020-9525, 2020.

EGU2020-12210 | Displays | OS2.2

Improving the uncertainty of ocean surface wind and wave parameters estimated by a single HF radar system

Duy-Toan Dao, Hwa Chien, and Pierre Flament

A 16 Rx element linear array HF radar system (LERA III), working at 27.75 MHz and 300kHz in bandwidth, was installed at the north of Taichung harbor at the western coast of Taiwan in November 2018. This LERA system is low-cost, compact in size, easy to set-up, and maintain. The purpose of present studies is to implement algorithms for retrieving wave and wind fields and assess the system performance in terms of operational mode. For inter-comparison, the long-term in-situ wave data measured by an AWAC was adopted. Wind data were measured from a coastal wind gauge. The inter-comparisons between radar data and in-situ data were carried out on seasonal basis, including severe sea states during winter monsoon and passage of typhoon as well as calm seas during spring.

For the data processing, the Doppler-range spectrum for each azimuth direction was extracted by using the classical beam-forming technique and then provided as level 1 product for further analysis. Regarding the method for retrieving wave parameters, formulations directly derived from Barrick’s assumption was implemented. In those formulas, wave parameters are calculated based on the ratio of the 2nd order component multiplied by the coupling coefficient function to the 1st order component in the Doppler spectrum. It means that no empirical constants were included. Initially, Wyatt’s (1999, 2011) and Walsh & Howell’s (1993) methods were applied to determine the lower and higher bounds that separate the 1st and 2nd order component. For wind speed inversion, Dexter & Theodorides’s (1982) method was adopted. The Bragg wave direction was used as a proxy to the direction of the wind field.

It is found that when using Wyatt’s (1999, 2011) method, the wave height and period results often lead to bias estimations for severe sea-state, and with the presence of highly variable surface current. In order to improve the accuracy, adaptive methods for the identification of spectra component areas is crucial. In this study, an alternative method is proposed. This method is developed based on the concept proposed by Kirincich (2017), which includes the pretreatment of Doppler-rang spectrums, marker-controlled watershed segmentation, and an image processing technique. In this research, we will demonstrate the advantages of using the new method for wave and wind field retrieval. From comparative studies, the error indexes based on the sea truth data are discussed. It is found that the accuracy would be improved using the proposed method, especially for the cases of varying current fields, severe sea state, and noisy radio background.

Key words: high-frequency surface wave radar; phased array antennas; significant wave height, wave period, marker-controlled watershed segmentation (MCWS) techniques.

How to cite: Dao, D.-T., Chien, H., and Flament, P.: Improving the uncertainty of ocean surface wind and wave parameters estimated by a single HF radar system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12210, https://doi.org/10.5194/egusphere-egu2020-12210, 2020.

The seasonal variability of the M2 tide exerts a major influence on the coastal ocean environment. In this study, a novel method, namely modified enhanced harmonic analysis (MEHA), is developed to synchronously extract the temporally varying amplitudes and phase lags of significant tides and the constant values of the other constituents. The seasonal variability of the M2 tide in the Bohai Bay, China, is investigated by analyzing one-year sea level observations at two stations with MEHA.

In ideal twin experiments, the artificial sea level observations were analyzed. Both the estimated temporally varying amplitude and phase lag of the M2 tide and constant values of the S2, K1 and O1 tides using MEHA were much closer to the prescribed values than those obtained using the other methods, indicating the capability and efficacy of MEHA. When the real sea level observations were analyzed using MEHA, the estimated M2 tidal amplitude has significant seasonal variability, with large values in summer and small values in winter, which is robust and not affected by experimental settings. The results of numerical experiments indicate that the seasonality of vertical eddy viscosity induces seasonal variations of the M2 tide in the Bohai Bay.

How to cite: Wang, D., Pan, H., and Mu, L.: Seasonal variability of the M2 tide in the Bohai Bay: development and application of modified enhanced harmonic analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12217, https://doi.org/10.5194/egusphere-egu2020-12217, 2020.

EGU2020-13845 | Displays | OS2.2

The spreading of suspended matter formed during construction works of offshore wind farms

Anna Przyborska, Maciej Muzyka, Jan Andrzejewski, Jaromir Jakacki, and Daniel Rak

Offshore Wind Farms play a significant role in ensuring Europe's energy security. In Poland, a significant increase in the  number  of  newly-constructed  wind turbines  has  been  observed  in  recent  years. Wind power is "clean", but not completely free of environmental impacts. Their impact on the environment is difficult to assess. The impact of dredging works on suspended matter created during construction works will be presented  based on numerical model results. A model has been built based on MIKE, tools supplied by the Danish Hydraulic Institute (DHI) and the Weather Research and Forecasting model (WRF)- mesoscale numerical weather prediction system. Based on the numerical model created, analysis of the temporal and spatial variability of the spreading suspension generated from dredging works carried out during construction works aimed at positioning the wind turbines in appropriate places.

The advective-diffusive character of transported matter is the most natural clean, their direction, turbulent viscosity and sedimentation rate. Authors evaluated that during the construction phase, concentration of the suspended sediment will exceed 10 mg/l, which is denerous value for cod larvae, for short time (no longer than 48 hours) and will cover small area. The deposition will be irrelevant, the largest rate will occur in the immediate vicinity of the construction works and it will decrease with the distance. The range of the transferable suspension will not exceed 15 km. Cumulative factors significantly increase the concentration of the suspension (in a critical case they may even double it). They may occur when simultaneous actions are taken to prepare the foundation for more than one wind turbine at the same time, which is rather technical impossible because there is not a lot of special purpose sea vessel that are able to work on construction of basements of the offshore wind generators.

 

How to cite: Przyborska, A., Muzyka, M., Andrzejewski, J., Jakacki, J., and Rak, D.: The spreading of suspended matter formed during construction works of offshore wind farms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13845, https://doi.org/10.5194/egusphere-egu2020-13845, 2020.

EGU2020-15045 | Displays | OS2.2

Setting up the NEMO (Nucleus for European Modeling of the Ocean) for Baltic Sea region - open boundary conditions

Jan Andrzejewski, Jaromir Jakacki, Maciej Muzyka, and Anna Przyborska

The Baltic Sea is inland, Shelf Sea in northern part of Europe. It is shallow with average depth of 52 meters and deepest point 459 meters located at Landsort Deep. Baltic Sea is connected with North Sea via the Danish Straits (comprising of Great Belt, Little Belt and Øresund). These systems ensure only limited exchange between oceanic waters and seawaters, which affect the low salinity in Baltic reservoir. Runoff from surrounding lands (approximately 200 rivers) and positive difference of precipitation minus evaporation additionally refreshes water and makes Baltic a brackish sea. The only charge of salt comes from the North Sea with so-called inflows or less frequent occurring Major Baltic Inflows (MBI). This exchange between Danish Straits is the key for properly working simulation. In this work the tool, well known as NEMO, was used to perform the numerical simulation for the Baltic Sea area. This presentation is focused on the first stage of validation of the model results for the Baltic Sea region where influence of open boundary conditions is noticeable as soon as possible. The main change in the model is the assimilation of sea surface height in Kattegat area. Also water outflow mass controlling from the Baltic Sea has been introduced. The properly working open boundary conditions affect the water exchange between Baltic Sea and North Sea, thus the MBI and minor salty inflows are well represented. This is very important part in modeling the Baltic
Sea, where narrow Danish Straits limits the water exchange which controls the salt budget, adding the salt with inflows and receiving brackish outflow out to the Ocean. This work presents comparison between model output with results measured in situ and from other validated model, the period which is compared is the Major Baltic Inflow in the beginning of 1993.

How to cite: Andrzejewski, J., Jakacki, J., Muzyka, M., and Przyborska, A.: Setting up the NEMO (Nucleus for European Modeling of the Ocean) for Baltic Sea region - open boundary conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15045, https://doi.org/10.5194/egusphere-egu2020-15045, 2020.

EGU2020-246 | Displays | OS2.2 | Highlight

Are artificial reefs an appropriate solution to protect the Danube Delta coast?

Irina Dinu, Vicente Gràcia, Manuel García-León, and Adrian Stanica

The Danube Delta coast is part of the Danube Delta Biosphere Reserve, thus being aimed to preserve its typical natural habitats. Over the last decades, human interventions along the Danube River, as well as coastal navigation and harbour protection works on the Romanian coast have determined the reduction of sediment supply along the Danube Delta coast, which is nowadays affected by erosion on its widest part.

Sustainable management plans for the Danube Delta coast include the use of working-with-nature solutions.

In this work, the effect of artificial reefs on the wave heights along the Danube Delta coast is studied. The results of a previous wave climate study and a wave model have been used for this purpose. Simulations have been performed for different setup of artificial reefs and for extreme storms with various return periods. The effect of sea level rise has also been taken into account.

Our results show that artificial reefs are significantly effective in reducing the wave heights along the Danube Delta coast. However, further detailed analysis concerning the impact of such a coastal protection solution is still needed.  

How to cite: Dinu, I., Gràcia, V., García-León, M., and Stanica, A.: Are artificial reefs an appropriate solution to protect the Danube Delta coast?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-246, https://doi.org/10.5194/egusphere-egu2020-246, 2020.

EGU2020-2338 | Displays | OS2.2

Drivers of water exchange in the ORUST fjord system

Sandra-Esther Brunnabend, Lars Axell, Maximo Garcia-Jove, and Lars Arneborg

The Orust fjord system, located on the west coast of Sweden, has openings on both ends and consists of several fjords that are connected by narrow and shallow channels. The fjord system includes the islands Orust and Tjörn as well as various smaller islands. The water exchange between the Kattegat and the different fjords is influenced by different factors, such as winds, tides, and density gradients. However, advection between the open sea and the complex fjord system are not yet well understood as lower resolution ocean models cannot resolve the small scale structures of the fjords and their connections. In addition, observations are rather sparse.

Therefore, the water exchange in the Orust fjord system is simulated using a high resolution (50 meter) NEMO3.6 ocean model setup, forced with the UERRA atmospheric reanalysis dataset. The lateral open boundary conditions for temperature, salinity, sea levels and velocities are provided by a low resolution (1.85 km) NEMO3.6 simulation, which spans the Baltic Sea and North Sea regions.

The model results are validated by comparison of modelled temperature, salinity, velocities and sea surface height with in-situ measurements. A detailed analysis of the different drivers of modelled water exchange between the Kattegat and the fjord system as well as between the different basins is presented. In general, the modelled water properties of the near surface layer in the fjord system are influenced by the Skagerrak surface water, which is controlled by the prevailing northward flowing Baltic Sea water. However, the residence times of water masses below the sill level are longer than the ones of the surface water masses as dense inflows of Skagerrak water in the basins create a strong stratification leading to weak vertical exchange.

How to cite: Brunnabend, S.-E., Axell, L., Garcia-Jove, M., and Arneborg, L.: Drivers of water exchange in the ORUST fjord system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2338, https://doi.org/10.5194/egusphere-egu2020-2338, 2020.

EGU2020-3572 | Displays | OS2.2

Observed coastal sea level changes in Southeast Asia from retracked altimetry over 2002-present

Yvan Gouzènes, Fabien Léger, Anny Cazenave, Florence Birol, Marcello Passaro, Fernando Nino, Christian Schwatke, Jean-François Legeais, and Jérome Benveniste

We present results of contemporary coastal sea level changes along the coasts of different
regions of Southeast Asia derived from a dedicated reprocessing of satellite altimetry data.
This work is performed in the context of the ESA ‘Climate Change Initiative’ sea level project
dedicated to provide altimetry-based sea level time series in the world coastal zones. Here is
focus on Southeast Asian Seas. High-frequency (20 Hz) sea level data from the Jason-1,
Jason-2 and Jason-3 missions are considered. The data are first retracked using the ALES
adaptive leading edge subwaveform retracker and further combined with the X-TRACK
processing system developed to optimize the accuracy of the sea level time series in coastal
oceans. Rates of sea level change are estimated over the period 2002-present along the Jasontracks,
from the open ocean to the coast. Different coastal sea level trend behaviors are
observed over the study period: constant trends from open ocean to the coast, sometimes
decreasing trends, or increasing trends within the last few km to the coast. We compare the
computed coastal trends in Southeast Asia with results we previously obtained in other
regions (Mediterranean Sea, Western Africa, Northeastern Europe). We further discuss the
various small-scale processes able to explain departure of the coastal sea level rate from the
offshore (open ocean) rate.

How to cite: Gouzènes, Y., Léger, F., Cazenave, A., Birol, F., Passaro, M., Nino, F., Schwatke, C., Legeais, J.-F., and Benveniste, J.: Observed coastal sea level changes in Southeast Asia from retracked altimetry over 2002-present, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3572, https://doi.org/10.5194/egusphere-egu2020-3572, 2020.

EGU2020-6287 | Displays | OS2.2 | Highlight

A Rarely Witnessed Summertime Upwelling Event northwest off the Hainan Island

Ai-Jun Pan, Fang-fang Kuang, Kai Li, and Xu Dong

A field survey revealed a rare realization of upwelling event in the northwestern Hainan Island (UNWHI) on July 24, 2015. Model experiments suggest that the UNWHI is not locally generated, but can be treated as northward extension of the upwelling southwest off Hainan Island (USWHI) under favorable wind conditions. Therefore, presence of the USWHI is vital for the UNWHI occurrence. Tidal mixing is testified to be the primary driving force for the USWHI, whilst southerly winds plays an essential role in the induction of the UNWHI. Moreover, it is demonstrated that the UNWHI is not a stable, but intermittent coastal upwelling system. Shallow basin of the Beibu Gulf makes the interior circulation vulnerable to local monsoon changes. Given the favorable southerly winds, a cyclonic gyre northwest off Hainan Island will be induced and which, leads to northward coastal current and consequently, the UNWHI is to be formed due to the northward transport of the USWHI. Conversely, the UNWHI vanishes during northerly winds period, because the basin-scale anticyclonic gyre results in a southward current west off the Hainan Island and which, acts to push the upwelled water of the USWHI offshore and away from the northwestern Hainan Island. In addition, our diagnostics indicates that contributions from surface heat fluxes to the UNWHI occurrence is negligible. Besides, it also reminds us that application of a high-frequency, much closer to reality wind field is necessary for the coastal upwelling simulation. 

How to cite: Pan, A.-J., Kuang, F., Li, K., and Dong, X.: A Rarely Witnessed Summertime Upwelling Event northwest off the Hainan Island, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6287, https://doi.org/10.5194/egusphere-egu2020-6287, 2020.

EGU2020-6353 | Displays | OS2.2

Seasonal Evolution Features of Water Transport in the Lembeh strait of North Sulawesi

Junpeng Zhang, Aijun Pan, Fangfang Kuang, Chunsheng Jing, Muh Hasanudin, Edi Kusmanto, and Deny Sutisna

A three-dimensional baroclinic nonlinear numerical model is employed to investigate the seasonal evolution features of water transport in the Lembeh Strait of north Sulawesi. In general, the direction of water flow in the strait is oriented northward with a maximum volume of about 4x10-3Sv in August. Interestingly, the volume transport will decrease to nearly zero from November to January. Water transport is mainly controlled by seasonally changed monsoon forcing. The large-scale ocean circulation nearby North Sulawesi is weaker during November-January and stronger from July to September. In addition, the source of water bypasses the Lembeh Strait in different seasons is diagnosed and testified by tracer-release experiments. It suggests that most of the water through the narrow channel is emanated from the southern off the Strait, not only the surface water, but also the deep water brought by the upwelling and the internal tides.

How to cite: Zhang, J., Pan, A., Kuang, F., Jing, C., Hasanudin, M., Kusmanto, E., and Sutisna, D.: Seasonal Evolution Features of Water Transport in the Lembeh strait of North Sulawesi, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6353, https://doi.org/10.5194/egusphere-egu2020-6353, 2020.

EGU2020-8535 | Displays | OS2.2

Salt intrusion in an estuarine network, a study with an exploratory model

Huib E. de Swart and Inge van Tongeren

Many estuarine systems experience increased salt intrusion, which is harmful for ecology and agriculture and may cause problems for fresh water supply to cities. Some causes of salt intrusion are extraction of fresh water in the upper reaches of the estuary and climate change. Besides, anthropogenic measures, like deepening of channels, are known to have a strong impact on the salt balance.

This contribution focuses on salt intrusion in estuarine networks, which consist of multiple connected channels. The motivation of the study arose from observations in the Yangtze estuary that reveal frequent overspill of salt between its different channels. To understand the underlying physics of such behaviour, an exploratory, width- and tidally averaged model has been developed and analysed. This model describes the competition between export of salt by river flow and import of salt by density-driven flow and horizontal diffusion. Its key new aspect is that it generalises an earlier model MacCready (2004) from a single channel to estuarine networks. The new model calculates the distribution of salt in, and salt exchange between the channels, as well as the distribution of river water over the different channels.  

Here, results will be presented for a simplified estuarine network consisting of the South Channel, South Passage and North Passage of the Yangtze Estuary. It will be shown that, for the present-day situation, dry season and spring tide, salt intrusion is larger in the South Passage than in the North Passage. As will be explained, this is mainly due to the different geometry of the two channels. Furthermore, it will be shown that there is slightly more river water transport through the South Passage than through the North Passage, except during high river discharge and neap tide. These results agree with field data and results from numerical studies.

Other results that will be presented are the sensitivity of salinity intrusion length and distribution of river water over the different channels to changes in, respectively, upstream river discharge, tidal currents and human interventions. Specifically, the effects of the creation of a Deepwater Navigation Channel in the North Passage on salt dynamics will be shown and discussed.

Reference:
MacCready, P. 2004. Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries 27, 561-570.

How to cite: de Swart, H. E. and van Tongeren, I.: Salt intrusion in an estuarine network, a study with an exploratory model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8535, https://doi.org/10.5194/egusphere-egu2020-8535, 2020.

EGU2020-11158 | Displays | OS2.2

Estimating wave-induced bottom shear stresses in the Gulf of Bothnia

Laura Tuomi, Aarno Kotilainen, Hedi Kanarik, Joonas Virtasalo, Olga Vähä-Piikkiö, and Heidi Pettersson

Sea floor erosion can be induced by waves, bottom currents and ice. Although the Gulf of Bothnia in the Baltic Sea is a relatively small basin, the record value of significant wave height is 8.1 m with highest individual wave of 15 m. In the present climate the seasonal ice cover limits the wave growth during winter time, but in the future climate it is estimated that ice extent will reduce which can lead to more severe wave climate. Thus, the effect of waves on the bottom sediment erosion is expected to increase. We used wave model WAVEWATCH III to do a 30-year high-resolution hindcast for the Gulf of Bothnia. The hindcast wave parameters were validated against wave buoy and altimeter wave measurements to ensure good quality of the wave hindcast. The hindcast near-bottom orbital velocities and amplitudes were used to estimate wave-induced bottom shear stress. These calculations are based on the wave spectra, taking into account the effect of different wave heights and wave lengths. The results were used to evaluate the extent of areas that experience significant wave-induced bottom stress under the present climate. Furthermore, the results show how often and for how long periods the wave-induced stress exceeds the critical values for sediment resuspension to take place. The estimates of the critical values for resuspension are calculated utilising the seabed sediment data available for the Gulf of Bothnia. The adequacy of the results is evaluated by comparing the known erosional seafloor areas to the ones estmated based on the hindcast values. This study is part of the SmartSea project of the Strategic Research Council of the Academy of Finland (grant no. 292 985).

How to cite: Tuomi, L., Kotilainen, A., Kanarik, H., Virtasalo, J., Vähä-Piikkiö, O., and Pettersson, H.: Estimating wave-induced bottom shear stresses in the Gulf of Bothnia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11158, https://doi.org/10.5194/egusphere-egu2020-11158, 2020.

EGU2020-18546 | Displays | OS2.2

Water exchange in a coastal archipelago in the northern Baltic Sea

Elina Miettunen, Laura Tuomi, and Kai Myrberg

The Archipelago Sea, situated in the northern Baltic Sea, consists of over 40 000 small islands and islets. It is a vulnerable area and suffers from continuous nutrient loading from the catchment and also from the background loading from the surrounding open sea areas. We studied water exchange in this complex coastal archipelago by simulating the water age and currents with a 3D hydrodynamic model COHERENS. The Archipelago Sea model setup has a horizontal resolution of c. 460 m and its boundary conditions are from a model setup that covers the whole Baltic Sea with a resolution of 3.7 km. The current fields produced with the hydrodynamic model were used to simulate the transport patterns of passive tracers through the archipelago with a Lagrangian particle model OpenDrift.

The mean water age was up to three months in the outer archipelago and up to seven months in the narrowest waterways in the inner archipelago. The effect of rivers on the water age was seen mostly only in the inner archipelago. The transport of passive tracer particles from the open sea areas, across the Archipelago Sea, mostly took place through the outer archipelago. The transport of particles from the outer archipelago towards the inner parts of the archipelago was very sensitive to the geometry and number of islands i.e. density of islands in the area. The prevailing wind direction in the area is from SW, this not being optimal for transport from the outer archipelago to the middle archipelago. For example, with the tracer particles from the southern open sea boundary, most transport to the middle archipelago was seen with SE winds. Transport further to the inner archipelago was limited only to few cases. The results show that the inner archipelago areas are relatively sheltered from transport from the open sea areas and the environmental problems there are in a high extent from local origin.

How to cite: Miettunen, E., Tuomi, L., and Myrberg, K.: Water exchange in a coastal archipelago in the northern Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18546, https://doi.org/10.5194/egusphere-egu2020-18546, 2020.

EGU2020-18926 | Displays | OS2.2

Coastal circulation from along-track satellite altimetry along the northern inner shelf of the Gulf of Cadiz

Erwan Garel, Luciano Júnior, Paulo Relvas, Irene Laiz, and Jesús Gómez-Enri

Satellite-derived sea level records are generally known to be unreliable in coastal zones. However, major progresses have been made during the last decade based on improved corrections and reprocessing of along-track altimeter data. In this study, altimeter-derived CryoSat-2 products are used to study the coastal circulation along the inner northern shelf of the Gulf of Cadiz, at the southern extremity of the western Iberian upwelling system. Coastal upwelling in this region results from coastal divergence due offshore Ekman transport under westerly favourable wind, and gives theoretically origin to a cross-shore pressure gradient. Upwelling activity is usually identified based on sea temperature cooling at the coast and the development of upwelling jets produced by geostrophic balance. These eastward alongshore flows alternate with westward flows (Coastal Counter Currents, CCCs) which main driver (e.g., local wind stress, geostrophic balance or alongshore pressure variations) is not definitively identified yet.

The present research proposes to get insights into the factors that drive the coastal circulation, based on the Absolute Dynamic Topography (ADT) obtained from CryoSat-2 along-track products (in SAR mode). The studied coastal stretch, about 200 km in length, is broadly oriented E-W, allowing the use of (the meridional) satellite tracks for the determination of cross-shore sea level variations. For validation, sea level oscillations from tidal gauges are compared with sea level anomalies from nearby tracks. In general, the satellite-derived sea level data reproduce adequately the temporal trends of water level variations at the coast. However, the CryoSat-2 data obtained at less than 3/5 km from the coast was discarded to reduce potential error in the magnitude of the variations.

The coastal circulation along the coast is diagnosed based on Sea Surface Temperature (SST) satellite images and in situ Acoustic Doppler Current Profilers (ADCP) observations on the inner shelf. Remarkably, the validated cross-shore sea level data show that the water level is systematically lower near the coast during periods of active coastal upwelling. The width of the sea level gradient varies between 10 and 25 km, and closely corresponds to the cold water area identified from SST images. The corresponding geostrophic flow is estimated about 0.5 m/s, similar to the observed upwelling jets near the surface. By contrast, periods with CCCs generally correspond to a flat cross-shore slope, discarding geostrophic balance as their main driver. On-going work analyses jointly CryoSat-2 tracks which are temporally closed for the determination of sea level and slope variations along the coast associated to the development of strong alongshore flows.

How to cite: Garel, E., Júnior, L., Relvas, P., Laiz, I., and Gómez-Enri, J.: Coastal circulation from along-track satellite altimetry along the northern inner shelf of the Gulf of Cadiz, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18926, https://doi.org/10.5194/egusphere-egu2020-18926, 2020.

EGU2020-19573 | Displays | OS2.2

Comparison of the main sea ice parameters received from satellite measurements and based on numerical simulation for the Baltic Sea region

Jaromir Jakacki, Maciej Muzyka, Marta Konik, Anna Przyborska, and Jan Andrzejewski

During the last decades remote sensing observations as well as modelling tools has been developed and become key elements of oceanographic research. One of the main advantages of both tools is a possibility of measuring large-scale areas.

The remote sensing measurements deliver only snapshots of the ice situation with no information about backgroundconditions. Moreover, providing picture of the whole area requires sometimes combining various datasets that increases uncertainties.  Modelling simulations provide full history of external conditions, but they also introduce errors that are the result of parameterizations. Also, an inaccuracy provided by forcing fields at the top and bottom boundaries are accumulated in the model.

In this work sea ice parameters such as sea ice concentration, thickness and volume obtained from both – satellite measurements and modelling has been compared. Numerical simulations were performed using standalone Community Ice Code (CICE) model (v. 6.0). It is a descendant of the basin scale dynamic-thermodynamic and thickness distribution sea ice model. The model is well known by scientific community and was widely used in a global as well as regional research, even operationally. The satellite derived ice thickness products were based on the C band HH-polarized SAR measurements originating from the satellites Sentinel-1 and RADARSAT-2. The sea ice concentration maps contain also visual and infrared information from MODIS and NOAA.

The ice extent, thickness and volume were compared in several regions within the Baltic Sea.  Seasonal changes were analyzed with a particular attention to ice formation and melting time. The sea ice extent datasets were compatible. Inconsistencies were observed for the sea ice thickness delivered by satellite measurements, especially during the ice melt. The work presents direction for ignoring satellite data with an error related to ice melting that allows for excluding erroneous satellite maps and obtain reliable intercalibration.

 

This work was partly funded by Polish National Science Centre, project number 2017/25/B/ST10/00159

How to cite: Jakacki, J., Muzyka, M., Konik, M., Przyborska, A., and Andrzejewski, J.: Comparison of the main sea ice parameters received from satellite measurements and based on numerical simulation for the Baltic Sea region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19573, https://doi.org/10.5194/egusphere-egu2020-19573, 2020.

EGU2020-22001 | Displays | OS2.2 | Highlight

On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones

Antonio Ricchi, Davide Bonaldo, Mario Marcello Miglietta, and Sandro Carniel

The Mediterranean basin is the formation site of a vast number and type of cyclones. Among these, we can occasionally identify intense vortices showing tropical characteristics, called Tropical-Like Cyclones (TLC) or MEDIcanes (Mediterranean Hurricane). Their development has been studied in several case studies, showing the influence of synoptic scale upper level forcings and mesoscale features, such as the sea surface temperature and the characteristics of the air masses on the formation area. The importance of Sea Surface Temperature (SST) consists in modulating the intense latent and sensible heat fluxes, which control the development of the TLC. For tropical cyclones, one of the most studied factors in recent years is the ocean heat content in the formation basin of these storms. We plan here to extend this analysis to TLC. Besides innovative studies with coupled atmosphere-waves-ocean numerical models, a simpler approach for investigating the sole effect of the ocean heat content consists of adopting a simplified ocean (1-Dimensional) description by varying the local characteristics of the Ocean Mixed Layer (OML). In this work we use the WRF (Weather Research and Forecasting system) model, in standalone (atmospheric) mode, with 3 km grid spacing, forced with GFS-GDAL (0.25°x0.25° horizontal resolution) and SST initialization provided by the MFS-CMEMs Copernicus dataset. Three case studies of TLC are examined here, namely ROLF (06-09/11/2011), ILONA (19-21/01/2014) and NUMA (11-20/11/2017). The ocean is simulated with an OML approach, with SST updated at each iteration as a function of the atmospheric heat fluxes and with an average mixed layer deph (MDL) provided by the MFS-CMEMS dataset. For each TLC studied, the MDL is modified by increasing and decreasing its depth by 50% and increasing and decreasing its lapse rate by 50%. The results show how the structure of the MDL influences not only the intensity of the cyclone but also the structure and precipitation both in terms of quantity and location. These outcomes suggest that, as for hurricanes, also for MEDICANES the heat content of the mass of seawater plays a fundamental role in their intensification, suggesting further studies also in a climate change perspective.

How to cite: Ricchi, A., Bonaldo, D., Miglietta, M. M., and Carniel, S.: On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22001, https://doi.org/10.5194/egusphere-egu2020-22001, 2020.

OS2.5 – Advances in understanding of the multi-scale and multi-disciplinary dynamics of the Southern European Seas (Mediterranean and Black Sea)

EGU2020-2607 | Displays | OS2.5

Wind and wave effects on surface currents in the Mediterranean Sea

Verónica Morales Márquez, Ismael Hernández Carrasco, Vincent Rossi, and Alejandro Orfila

The knowledge of Lagrangian motion is of a great importance due to their impact on the properties of transported material like the Essential Ocean Variables (phytoplankton, temperature, pCO2, etc), or other material like plastics debris, oil spill pollution, etc. In this study we analyze the influence of the wind and waves in the transport and mixing properties at the upper layers of the Mediterranean Sea. In this context, we propose a new approach for current velocity where we take into account the wind-wave interaction and the variability that it inserts into the current velocity through Ekman and Stokes components.


Surface currents, Ekman, Stokes, Lyapunov exponent

How to cite: Morales Márquez, V., Hernández Carrasco, I., Rossi, V., and Orfila, A.: Wind and wave effects on surface currents in the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2607, https://doi.org/10.5194/egusphere-egu2020-2607, 2020.

EGU2020-15463 | Displays | OS2.5

The analytical model of McCartney applied in the eastern Levantine basin: formation of the Cyprus eddy and associated sub-basin and mesoscale features

Annunziata Pirro, Riccardo Gerin, Elena Mauri, Dan Hayes, and Pierre-Marie Poulain

The circulation in the Levantine Eastern Mediterranean basin is characterized by a complex system of sub-basin and mesoscale features. The most predominant and persistent eddy is an anticyclonic feature located south of Cyprus and identified as the Cyprus Eddy (CE). Previous studies based on in-situ data, satellite and model outputs confirm the presence of this warm core eddy centered around 33°E, 33.5°N. Although the center of the CE might be found slightly shifted to the west /east for different years of analysis, the anticyclonic eddy appears always above the Eratosthenes seamount whose peak lies at the depth of ~ 690 m and it rises ~ 2500 m above the surrounding seafloor. The presence of a cyclonic and anticyclonic eddy named South Shikmona (SSE) and North Shikmona (NSE) eddy, respectively has been also detected east of the CE and west of the Lebanese and Israeli coasts. Here, we present a hypothesis that attempts to explain the formation mechanism of the three eddies described above. Specifically, for an eastward stratified current on a b plane, the isolated seamount forces a Taylor column above it which can be identified as the CE. A standing wake downstream is also formed, and embedded eddies are associated with the SSE and NSE. These sub-basin features are probably part of a Rossby wave system. The analytical model of McCartney 1976 supports this hypothesis. Reanalysis and sea glider data collected during the CINEL project sponsored by the U.S. Office of Naval Research (ONR) are used to investigate McCartney’s solution. Preliminary results confirm the presence of a series of eddies above and downstream the sea mountain supporting therefore, the advanced hypothesis.

How to cite: Pirro, A., Gerin, R., Mauri, E., Hayes, D., and Poulain, P.-M.: The analytical model of McCartney applied in the eastern Levantine basin: formation of the Cyprus eddy and associated sub-basin and mesoscale features , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15463, https://doi.org/10.5194/egusphere-egu2020-15463, 2020.

EGU2020-13744 | Displays | OS2.5

Multiplatform analysis of a large anticyclonic eddy in the Algero-Provencal basin in 2019

Aida Alvera-Azcárate, Alexander Barth, Charles Troupin, Jean-Marie Beckers, Hayley Evers-King, Ananda Pascual, Eva Aguiar, and Joaquin Tintoré

A large anticyclonic eddy formed in April 2019 in the Algero-Provencal basin between Mallorca and Sardinia, and lasted until November 2019. While mesoscale activity is usually high in this part of the Mediterranean basin, the formation of such large (about 150 km in diameter) and long-lived eddies is not common. The eddy formed from a filament originated in the Algerian coast and was visible in multiple sources of satellite data, including sea surface temperature and ocean colour from Sentinel-3, until summer. Because of the warming of the surface layer, during summer months the eddy remained as a subsurface structure, evidenced by the sea level anomaly derived from altimetry data. A surface signal developed again in November, and the eddy finally dissipated in December 2019. According to CMEMS model data, in its strongest period the eddy reached about 300 m in depth, and during its sub-surface period the center was located at about 100 m depth. While at the surface the temperature signal was very clear, model data suggest the salinity anomaly was stronger than temperature, especially at depth. Such large and long-lived eddies have an impact in the basin currents, specifically in the transport of cold water from the northern to the southern part of the western Mediterranean basin, influencing the ecosystem there. The impact of the presence of this eddy, its long duration and the additional mesoscale and submesoscale activity that originated in its surroundings are investigated using a combination of remote sensing data, in situ data and model data.

How to cite: Alvera-Azcárate, A., Barth, A., Troupin, C., Beckers, J.-M., Evers-King, H., Pascual, A., Aguiar, E., and Tintoré, J.: Multiplatform analysis of a large anticyclonic eddy in the Algero-Provencal basin in 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13744, https://doi.org/10.5194/egusphere-egu2020-13744, 2020.

EGU2020-21636 | Displays | OS2.5

Sea Surface Salinity trends in the Mediterranean and Black Seas from 10 years of SMOS measurements

Estrella Olmedo, Verónica González-Gambau, Antonio Turiel, Cristina González-Haro, Justino Martínez, Carolina Gabarró, Aida Alvera-Azcárate, Marilaure Grégoire, and Marie-Helene Rio

The monitoring of the sea surface salinity (SSS) in the semi-enclosed seas has a significant impact in the study of the climate change. In those basins the oceanographic processes occur at higher temporal scales than in the open ocean, and therefore, trends and anomalies can be detected before. The Mediterranean Sea is a strongly evaporative basin (evaporation exceeds the precipitation and river run-off). Converserly, in the Black Sea the river run-off and precipitation exceeds the evaporation. Based on a 4-year time series (2015-2019) of SMAP SSS, a recent study has shown that there is an increase of the salinity in the Eastern Mediterranean [Grodsky, et al. 2019]. On the other hand, the Black Sea exhibits a rich variability in space and time from (sub)mesoscale to larger scales (interannual and larger)  that needs to be appropriately taken into account when trying to identify long-term trends.

We present new estimates of SSS trends in the Mediterranean and Black Seas. These estimations are based on 10-year series obtained from the European Soil Moisture and Ocean Salinity (SMOS) mission. Two new SMOS SSS regional products have been generated. On the one hand, we have generated a new realease of SMOS SSS regional product for the Mediterranean Sea. The new release of SMOS SSS regional product for the Mediterranean Sea provides better coverage in the Eastern Mediterranean than the previous version of this product (see [Olmedo et al 2018]). The new dedicated SMOS SSS product for the Black Sea has been developed under the currently on-going ESA EO4BIS contract (An Earth Observation Data for Science and Innovation in the Black Sea). The Black Sea and the Eastern Mediterranean are strongly affected by Radio Frequency Interferences (RFI) sources, which hamper the salinity retrieval. We have applied specific methodologies to diminish the strong RFI effects in these two basins [González-Gambau et al 2017].  The new realase of these two SMOS SSS regional products will be available soon in the Barcelona Expert Center website (http://bec.icm.csic.es ).

At this conference we will present the methodologies that we have used for the generation of both regional SMOS SSS products. We will also present a quality assessment over the two regions consisting of comparing with in situ salinity measurements. Finally, we will show the SSS trends that are obtained in the different basin (and sub-basins) as well as the significance of the results with respect to the accuracy of the new SMOS SSS products.

[Grodsky, et al. 2019] Grodsky S., et al. (2019), “Eastern Mediterranean salinification observed in satellite salinity from SMAP mission”, Journal of Marine Systems,  198
[Olmedo et al 2018] Olmedo, E, et al. , (2018) “Improving SMOS Sea Surface Salinity in the Western Mediterranean Sea through Multivariate and Multifractal Analysis,” Remote sensing,  10(3), 485.
[González-Gambau et al 2017] González-Gambau, V. et. al, (2017), "Improvements on calibration and image reconstruction of SMOS for salinity retrievals in coastal regions," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 10, 7, 3064-3078

How to cite: Olmedo, E., González-Gambau, V., Turiel, A., González-Haro, C., Martínez, J., Gabarró, C., Alvera-Azcárate, A., Grégoire, M., and Rio, M.-H.: Sea Surface Salinity trends in the Mediterranean and Black Seas from 10 years of SMOS measurements , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21636, https://doi.org/10.5194/egusphere-egu2020-21636, 2020.

EGU2020-9520 | Displays | OS2.5

New insights into nutrients dynamics and the carbonate system using a neural network approach in the Mediterranean Sea

Marine Fourrier, Laurent Coppola, and Fabrizio D'Ortenzio

The semi-enclosed nature of the Mediterranean Sea, together with its small inertia which is due to the relatively short residence time of its water masses, make it highly reactive to external forcings and anthropogenic pressure. In this context, several rapid changes have been observed in physical and biogeochemical processes in recent decades, partly masked by episodic events and high regional variability. To better understand the underlying processes driving the Mediterranean evolution and, anticipate changes, the measurement, and integration of many biogeochemical variables are mandatory.

The development of new BGC sensors implemented on in situ autonomous platforms allows to increase the acquisition of essential biogeochemical variables. However, the measurements carried out by in situ autonomous platforms (e.g. profiling floats, gliders, moorings) are not exhaustive.

Recently, deep learning techniques and in particular neural networks have been developed. The CANYON-MED (for Carbonate system and Nutrients concentration from hYdrological properties and Oxygen using a Neural-network in the MEDiterranean Sea) neural network-based method provides estimations of nutrients (i.e. nitrates, phosphates, and silicates) and carbonate system variables (i.e. total alkalinity, dissolved inorganic carbon, pHT) from systematically measured oceanographic variables such as in situ measurements of pressure, temperature, salinity, and oxygen together with geolocation and date of sampling.

This regional approach, therefore, using quality-controlled in situ measurements from more than 35 cruises. CANYON-MED obtains satisfactory results: accuracies of 0.73, 0.045, and 0.70 µmol.kg-1 for the nitrates, phosphates and silicates concentrations respectively, and 0.016, 11 µmol.kg-1 and 10 µmol.kg-1 for pHT, total alkalinity and dissolved organic carbon respectively. CANYON-MED thus generates “virtual” data of parameters not yet measured by autonomous platforms, while ably reproducing the data already sampled, emphasizing its ability to fill the gaps in time-series.

Hence, by applying it to the large and growing network of autonomous platforms in the Mediterranean Sea, this method allows us to gain new insights into nutrients and carbonate system dynamics in targeted areas. In particular, in the northwestern Mediterranean Sea, the impact of deep convection on biogeochemistry (e.g., nutrient replenishment and pHT variability) is highly variable over time and poorly covered by observing networks. In this case, CANYON-MED would improve our observations and understanding of the dynamic and coupled system.

How to cite: Fourrier, M., Coppola, L., and D'Ortenzio, F.: New insights into nutrients dynamics and the carbonate system using a neural network approach in the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9520, https://doi.org/10.5194/egusphere-egu2020-9520, 2020.

   Runoff from rivers and coastal plains delivers significant amounts of nutrients to the Mediterranean Sea from the agricultural activities and urban waste waters. Several recent studies show that variations in rivers inputs may play a significant role on the marine biogeochemical cycles and planktonic food web in the entire basin. The aim of this study is to estimate the release of nutrients (N & P) to the Mediterranean Sea from basin-wide agriculture and urbanization through the implementation of the biogeochemical land-sea nutrient transfer processes within the agro-ecosystem model LPJmL. This is a contribution to the LaSeR-Med project (Towards an integrated prediction of Land & Sea Responses to global change in the Mediterranean Basin).

A compilation of a new input data set of fertilizer, manure and wastewater nutrients content [1961-2005] has been added to the LPJmL forcing data set, with a new land use patterns produced by an econometric model. The representation of the nutrient transfer from land to sea has been introduced into LPJmL by considering the following processes: mineralization, denitrification, adsorption, remineralization, nitrification, and phytoplankton dynamics.

First basin-wide LPJmL simulation at 1/12°, indicates that the model succeeds in simulating the temporal variations of water discharge for the main rivers flowing to the Mediterranean Sea, and shows a good consistency between the simulated nutrients concentration (NO3 and PO4) and available in-situ data. Preliminary results show that wastewater strongly contribute to the phosphorus fluxes (as PO4), while both agriculture and wastewater control the nitrogen fluxes (mainly as NO3). Alternative scenarios for land-use will allow to explore the future amounts of terrestrial nutrients that will reach the sea through rivers discharge and water runoffs and impact the marine ecosystems.

How to cite: Ayache, M., Bondeau, A., Pagès, R., and Baklouti, M.: A new estimation of water and nutrients (N & P) discharge to the Mediterranean Sea from the LPJmL model: modelling the dynamics of the land-sea nutrient transfer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9138, https://doi.org/10.5194/egusphere-egu2020-9138, 2020.

EGU2020-12104 | Displays | OS2.5 | Highlight

Mediterranean Marine heatwaves: On the comparison of the physical drivers behind the 2003 and 2015 events

Sofia Darmaraki, Samuel Somot, Robin Waldman, Florence Sevault, Pierre Nabat, and Eric Oliver

Over the last decade, an intensification of extreme warm temperature events, termed as marine heatwaves (MHWs), has been reported in the Mediterranean Sea, itself a “Hot Spot” region for climate change. In the summer of 2003, a major MHW occurred in the Mediterranean with abnormal surface temperature anomalies of 2-3 Cº persisting for over a month. In 2015, an undocumented but more intense summer MHW affected almost the entire Mediterranean Sea with regional temperatures anomalies reaching 4-5 Cº. Here, we apply a MHW detection algorithm for long-lasting and large-scale summer events, on the hindcast output of a fully-coupled regional climate model (RCSM). We first examine the spatial variability and temporal evolution of both the 2003 and 2015 events. Then a basin-scale analysis of the mixed layer heat budget during each MHW is performed. The ocean and atmospheric components’ contribution is investigated separately during the onset, peak, and decay phases of both events, in order to disentangle the dominant physical processes behind each event. On the large-scale, our results indicate a key role of the wind forcing and the air-sea heat fluxes, while advection processes become more important at local scales. This study provides a comparison of the underlying mechanisms behind the two most intense MHW detected in the Mediterranean Sea during the last decade, constituting key information for the marine ecosystems of the region.

How to cite: Darmaraki, S., Somot, S., Waldman, R., Sevault, F., Nabat, P., and Oliver, E.: Mediterranean Marine heatwaves: On the comparison of the physical drivers behind the 2003 and 2015 events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12104, https://doi.org/10.5194/egusphere-egu2020-12104, 2020.

EGU2020-1864 | Displays | OS2.5

Fe isotope and Fe speciation study of water column redox dynamics during Eastern Mediterranean sapropel events S5 and S7

Alan Matthews, Ayelet Benkovitz, Nadya Teutsch, Simon Poulton, Miryam Bar-Matthews, and Ahuva Almogi-Labin

Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea  during peak interglacial periods MIS5e and MIS7a, respectively. This study investigates the dynamics of  water column redox change during their formation, through Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967 south of Cyprus. Both sapropels show an inverse correlation between δ56Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in highly euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak stages of S7, where the negative δ56Fe - FeT/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (FeHR) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. FeHR/FeT plots show increased strengthening of anoxic water conditions during the transformation from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for highly euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropels, Fepy/FeHR ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters.  Thus, the operationally defined ferruginous/euxinic boundary for Eastern Mediterranean Sea sapropels is better placed at Fepy/FeHR = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ56Fe - FeT/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile into the Eastern Mediterranean basin. The euxinic water column conditions that developed in sapropels S5 and S7 are interpreted to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of Fe (oxyhydr)oxide minerals. Both of these parameters in turn depend on the extent to which water overturn times are reduced during sapropel formation. Water overturn rate is therefore considered to define the strength of euxinic water column conditions during these periods of organic carbon-rich sedimentation.

How to cite: Matthews, A., Benkovitz, A., Teutsch, N., Poulton, S., Bar-Matthews, M., and Almogi-Labin, A.: Fe isotope and Fe speciation study of water column redox dynamics during Eastern Mediterranean sapropel events S5 and S7, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1864, https://doi.org/10.5194/egusphere-egu2020-1864, 2020.

EGU2020-6722 | Displays | OS2.5

Small scale transport processes from HF-Radar

Ismael Hernández-Carrasco, Alejandro Orfila, Vincent Rossi, and Veronique Garçon

Coastal ocean ecosystems are major contributor to the global biogeochemical cycles and biological productivity. Physical
factors induced by the turbulent flow play a crucial role in regulating marine ecosystem. However, while large scale dynamics
in the open ocean is well described by geostrophy, the role of small scale transport processes in coastal regions is still
poorly understood due to lack of continuous high-resolution observations. Here, the influence of small-scale coastal dynamics
on surface phytoplankton structuring is studied using Lagrangian metrics computed from HF Radar currents and satellite
chlorophyll-a (Chl). The combination of complementary Lagrangian diagnostics, including the accumulated divergence of the
flow along fluid trajectories, provides an improved description of the 3D flow geometry which facilitates the interpretation of two
non-exclusive physical mechanisms affecting phytoplankton patchiness. Attracting submesoscale fronts, unveiled by backwards
Lagrangian Coherent Structures, are associated to negative Lagrangian divergence where particles and Chl standing stocks
cluster. Filaments of positive Lagrangian divergence, representing large accumulated upward vertical velocities and suggesting
accrued injection of subsurface nutrients, match areas with large Chl concentrations. Our findings demonstrate that an accurate
description of small-scale transport processes is necessary to comprehend bio-physical interactions in coastal seas and to
estimate biological productivity.

How to cite: Hernández-Carrasco, I., Orfila, A., Rossi, V., and Garçon, V.: Small scale transport processes from HF-Radar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6722, https://doi.org/10.5194/egusphere-egu2020-6722, 2020.

EGU2020-7700 | Displays | OS2.5

Coastal groundwater stable isotope composition as predictor and measure of marine pollution

Diana Mance, Davor Mance, and Darija Vukić Lušić

There are numerous health hazards arising from recreational exposures to microbiologically polluted marine environments. Microbial contaminants from catchment areas of coastal and submarine springs (due to leakages of private septic tanks and/or faults in sewage systems) could be a cause of microbial marine quality worsening after heavy rainfalls. Before testing this hypothesis groundwater dynamics should be known. Stable isotopes of water have been proven to be a very useful tool in karst hydrology and we used them as a mediator variable in predicting marine coastal water microbial contamination. 
We refer to the problem of the pollution from the position of environmental economics and economic institutional mechanism design, where such ecological problems are described as either stock or flow problems. Stock pollution is strongly dependent on the concentration potentials of the pollutant in the medium. Flow pollution depends on the speed of emission of the pollutant in the medium, as well as on the rate of its depletion by natural causes. On the example of fecal indicator bacteria Escherichia coli and enterococci propagating through karstic underground and finally ending in seawater we show how stable isotope composition of coastal springs’ water can be used to differentiate marine pollution into stock or flow. 
We tested the approach on two close coastal locations located at the Kvarner Bay (the Northern part of the Croatian part of the Adriatic Sea). Locations differ in terms of the open and closed sea as well as anthropogenic pressure. Groundwater and marine samples were collected during two consecutive bathing seasons (mid-May – mid-September). The Panel Data Pairwise Granger Causality test was used to test for statistical associations. Static Estimated General Least Squares (EGLS) and dynamic Generalised Method of Moments (GMM) statistical methods were used to distinguish between stock and flow pollution. 

This work was partially supported by the University of Rijeka as part of the research projects uniri-pr-prirod-19-24 and uniri-biomed-18-292.

How to cite: Mance, D., Mance, D., and Vukić Lušić, D.: Coastal groundwater stable isotope composition as predictor and measure of marine pollution , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7700, https://doi.org/10.5194/egusphere-egu2020-7700, 2020.

EGU2020-9066 | Displays | OS2.5

On the relationship between the vertical distribution-migration of zooplankton and the organic carbon flux, before, during and after convective events, in the open southern Adriatic Sea

Mirna Batistić, Rade Garić, Stefano Miserocchi, Leonardo Langone, Laura Ursella, and Vanessa Cardin

The Southern Adriatic (SA) is the deepest part of the Adriatic Sea (1242 m) and one of three sites of open-sea deep convection in the Mediterranean. Due to winter convection events, the dense water formation processes in the open SA result in a homogenization of the water column, which determines the nutrient input into the euphotic zone, enhances phytoplankton growth and consequently, the abundance of zooplankton.

By analyzing zooplankton samples, together with acoustic data (ADCP) and data from sediment traps (at 125 m and 1150 m) taken in the SA from November 2015 to June 2016, we investigated the relationship between the distribution of zooplankton abundance and the flow of organic carbon in the deep open southern Adriatic Sea. During the pre-convection period (November 2015), the highest organic carbon flux (C org flux) was found at both depth (125 m, 1150 m), which is probably related to the autumn phytoplankton bloom and consequently an increase in zooplankton abundance. During the winter mixing phase, a lower C org flux was recorded in the upper trap samples which was a consequence of the reduced growth of phytoplankton and the transport of the cells to the deeper aphotic layers; where some increase of org C flux in the lower trap was recorded. Thus, the deepest layers were enriched leading to a minimum vertical zooplankton-migration (DVM). In spring, during the post-convection period (March, April), high abundance of mesozooplankton, mostly copepods, was registered in the upper layer, as well as an evident increase of C org flux. Other species than copepods (which remain at the food rich surface), probably ostracods and euphausiids, played a significant role in the DVM because they are more abundant in the deeper layers. The increase in C org flux in the upper samples in May is in accordance with a recorded salp bloom (also evident through a strong backscatter signal). Salp fecal pellets were observed to contribute significantly to vertical carbon flux in various ocean regions.

The relationship between vertical zooplankton distribution, zooplankton migration and carbon export has generally been poorly studied in the Adriatic Sea. Preliminary results for the open SA are presented here, but for more accurate knowledge of this topic, a long term study is needed.

How to cite: Batistić, M., Garić, R., Miserocchi, S., Langone, L., Ursella, L., and Cardin, V.: On the relationship between the vertical distribution-migration of zooplankton and the organic carbon flux, before, during and after convective events, in the open southern Adriatic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9066, https://doi.org/10.5194/egusphere-egu2020-9066, 2020.

EGU2020-9874 | Displays | OS2.5

Recent changes of the salinity distribution in the South Adriatic

Elena Mauri, Milena Menna, Giulio Notarstefano, Riccardo Gerin, Riccardo Martellucci, and Pierre-Marie Poulain

The South Adriatic is one of the dense water formation site in the Mediterranean Sea. The variations of its thermohaline properties are relevant not only from an oceanographic and climatic point of view but also for the local impact on the vertical distribution of the biogeochemical parameters.

The South Adriatic Pit has been extensively sampled during the last forty years by traditional shipboard techniques. Float and glider measurements became part of the investigation only in the last ten years, providing a more detailed and more uniform spatio-temporal dataset. From the analysis, evidences of important changes of the South Adriatic Pit salinity vertical distribution emerge in the last 5 years. In the past, the Levantine Intermediate Water (LIW) entered the South Adriatic at a depth between 100 and 300 m, highlighted by a maximum in the salinity. The recent findings suggest that the LIW is no longer characterized by the highest salinity along the vertical profiles, which is present instead in shallower subsurface layers. In addition, in most of the seasons a thick low salinity layer is evident in the top 50-100 m. Among those changes, some peculiar haline characteristics occur in 2012 and 2017; they will be analyzed in concert with auxiliary data and model outputs.

How to cite: Mauri, E., Menna, M., Notarstefano, G., Gerin, R., Martellucci, R., and Poulain, P.-M.: Recent changes of the salinity distribution in the South Adriatic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9874, https://doi.org/10.5194/egusphere-egu2020-9874, 2020.

EGU2020-11665 | Displays | OS2.5

The South Adriatic observatory: towards a multidisciplinary seafloor and water column research infrastructure

Leonardo Langone, Fabio Brunetti, Ilaria Conese, Patrizia Giordano, Stefano Miserocchi, Giuseppe Siena, Laura Ursella, and Vanessa Cardin

Continuous measurements are essential to assess the interannual variability of the thermohaline circulation, water masses properties and transports, and biochemical contents. The need for high-frequency sampling to resolve events and rapid processes (on different time scale) and the long-sustained measurements of multiple interrelated variables from the sea surface to the seafloor is provided by Southern Adriatic Node. It is formed by the observatory E2M3A located in the area of the Southern Adriatic Pit (Eastern Mediterranean) at 41°32'N, 18°04'E together with a system of moorings positioned along the Bari Canyon (mooring BB lat. 41°20.49’N long. 17°11.64’E at 605 m depth; mooring FF lat 41°48.35’N long 17°02.29’E at 751 m depth) and the open-slope. The Canyon is generally assumed to play an important role in dense water sinking and sediment transfer to the deep Southern Adriatic basin.

The dense waters of North Adriatic origin flow southwards, mostly intermittently, along the Adriatic shelf and sink into the basin, both along the open slope and, more markedly, through the canyon of Bari. Thus, the basin due to its morphology, is considered as a reservoir that collects these waters together with those formed in-situ by open deep water formation (DWF) processes, exiting the Adriatic as the ADW that feeds the thermohaline circulation of the Eastern Mediterranean.

Signals of transport through the canyon to the deep pit layer are evident, in particular environmental conditions as for winter 2012, from the physical and biogeochemical data measured simultaneously at high frequency by the various system components (E2M3A and BB and FF moorings). From BB’s mooring data after this event until 2018 do not show us very significant events but are episodes of lower intensity that are not clearly identified in the E2M3A time series.

The intrusion of very dense waters of North Adriatic origin (cascading) evidenced at the E2M3A, occurred in late march 2014, January 2015 and winter 2016 is remarked from salinity homogenization at the 900 -1000 m depth. This has most likely contributed to enhance the lithogenic material fluxes at the bottom trap.  However, this intrusion has not been clearly detected by BB mooring but might have sunk across the open-slope.

 
What do you want to do ?
New mail
 
What do you want to do ?
New mail

How to cite: Langone, L., Brunetti, F., Conese, I., Giordano, P., Miserocchi, S., Siena, G., Ursella, L., and Cardin, V.: The South Adriatic observatory: towards a multidisciplinary seafloor and water column research infrastructure , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11665, https://doi.org/10.5194/egusphere-egu2020-11665, 2020.

EGU2020-12004 | Displays | OS2.5

A neural-based bio-regionalization of the Mediterranean Sea using satellite and Argo-float records

Roy El Hourany, Chris Bowler, Carlos Mejia, Michel Crépon, and Sylvie Thiria

The regionalization of the Mediterranean Sea has been the subject of many studies. It is a miniature ocean where most of the processes of the global ocean are encountered (Lejeusne et al., 2010). Several features of the Mediterranean (near-tropical ocean in summer with a well-formed thermocline, near-polar ocean in winter with deep convection, multiple basins with different characteristics) make it a hotspot of marine biodiversity (Coll and al., 2010) and consequently vulnerable to climate change. It is therefore important to characterize the present state of the Mediterranean Sea with robust estimators in order to study the long-term evolution of this mesocosm.

We present a partitioning of the Mediterranean Sea in regions having well defined characteristics with respect to Sea Surface Temperature and surface chlorophyll observed by satellite, and Argo mixed layer depth. This regionalization was performed by using an innovative classification based on neural networks, the so-called 2S-SOM. Its major advantage is to consider the specificity of the variables by adding automatically, through machine learning, specific weights to each of them, which facilitates the classification and consequently highlights the regional correlations. The 2S-SOM provided a well differentiated regionalization of the Mediterranean Sea waters into seven bioregions governed by specific physical and biogeochemical processes such as Intermediate-water formation in the Aegean Sea, large surface currents in the Adriatic and the Alboran, deep winter convection phenomena in the Balearic and stratification phenomena during summer in the eastern part of the Mediterranean Sea.

Besides, in order to highlight the phytoplankton diversity in these regions, we processed the satellite ocean color observations with a specific neural network approach (SOM-PFT, El Hourany et al., 2019). As a result, specific phytoplankton communities characterized by their seasonal variability are associated with the obtained Mediterranean bioregions; the dominance of the Nanophytoplankton groups is largely observed in the western basin during the period ranging from autumn to spring. While the dominance of different types of cyanobacteria Synechococcus and Prochlorococcus is highlighted in summer and more precisely in the waters of the eastern basin. Diatoms dominate throughout the year in the coastal and shallow regions, which can be explained by the presence of terrigenous input necessary for the development of this type of phytoplankton. Diatoms also largely benefit from the strong deep convection in the Balearic Sea marked by a large bloom at the end of winter convection in March.

This work will be further extended to study the phytoplankton diversity at global scale using various data set from the Tara Oceans.

How to cite: El Hourany, R., Bowler, C., Mejia, C., Crépon, M., and Thiria, S.: A neural-based bio-regionalization of the Mediterranean Sea using satellite and Argo-float records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12004, https://doi.org/10.5194/egusphere-egu2020-12004, 2020.

EGU2020-15189 | Displays | OS2.5 | Highlight

Basin scale dissolved oxygen interannual variability of the Mediterranean Sea: Analysis of long-term observations

Apostolia-Maria Mavropoulou, Vassilios Vervatis, and Sarantis Sofianos

The Mediterranean Sea is characterized by a combination of long-term trends and climatic shifts known in the literature as “transients”, that impact the biogeochemical processes.  We focus on the dissolved oxygen (DO) concentration, as it is an essential oceanic parameter for the marine ecosystem functioning. Dissolved oxygen distribution in the ocean interior is controlled by air-sea interaction processes, ocean circulation patterns, and biological effects. Understanding the related mechanisms and the variability of the above processes requires systematic oceanographic measurements over long periods and at high spatial resolution. Taking advantage of the Mediterranean monitoring systems, we can examine the sensitive physical and biogeochemical processes in the Mediterranean ecosystem. In this study, we investigate and combine all available data of temperature, salinity and dissolved oxygen over the period 1960-2011 (taking into consideration the scarcity of the available DO observations during the last years). In order to receive a direct and accurate evaluation of the interannual changes in the Mediterranean Sea, we constructed a gridded dataset interpolated into 1/8ο x 1/8ο grid using Data-Interpolating Variational Analysis (DIVA). At the surface layer, the solubility-driven changes determine the dissolved oxygen concentration. In deeper layers, the interannual variability is more related to dynamical processes that may involve dense-water convection, biological consumption or mixing, rather than temperature trends. The observed changes in minimum/maximum oxygen zones are mostly related to abrupt shifts. The attribution of the observed variability involves complex physical and biogeochemical processes as well as anthropogenic activities and requires further analysis using modeling techniques and available operational tools.

How to cite: Mavropoulou, A.-M., Vervatis, V., and Sofianos, S.: Basin scale dissolved oxygen interannual variability of the Mediterranean Sea: Analysis of long-term observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15189, https://doi.org/10.5194/egusphere-egu2020-15189, 2020.

EGU2020-16533 | Displays | OS2.5

A new multidisciplinary observatory in the Eastern Ligurian Sea (NW Mediterranean Sea): a combination of deep-sea and coastal measurements

Tiziana Ciuffardi, Maristella Berta, Andrea Bordone, Mireno Borghini, Paolo Celentano, Luca Cocchi, Letizia De Fabritiis, Ivana Delbono, Roberta Delfanti, Maurizio Demarte, Roberta Ivaldi, Zoi Kokkini, Marina Locritani, Davide Marini, Simone Marini, Filippo Muccini, Federica Pannacciulli, Andrea Peirano, Giancarlo Raiteri, and Anna Vetrano

Climate change investigation, protection of marine ecosystems and mitigation of natural risks are the main research objectives of the Levante Canyon Mooring (LCM), a deep submarine multidisciplinary observatory, installed in September 2019 in the Eastern Ligurian Sea (Lat 44°05.443'N, Long 009°29.900'E at 608 m depth), inside the Pelagos Sanctuary. The observatory consists of a stand-alone station, with an instrumented mooring line ending with a submerged buoy. It operates in delayed-mode and is equipped with sensors that measure physical and biogeochemical parameters continuously and it is expected to provide data in the long-term. Temperature and salinity monitoring is carried out at three depth levels (about 80, 335 and 580 m depth), while turbidity is recorded at 580 m depth. LCM is also equipped with a sediment trap and two acoustic current profilers, able to measure direction and speed of currents in nearly the entire water column.

Data will be used to measure flux of sediments, nutrients and organic matter and to better understand the hydrodynamic and physical conditions of the Levante Canyon, which hosts valuable and vulnerable ecosystems, such as the deep-living cold-water corals, identified by IIM and ENEA in 2014, near the LCM mooring site. The LCM site is also located in an area where surface currents are monitored in near-real time by the CNR’s High Frequency Radar network, allowing data integration from multiplatform observations.

The project, co-financed by the Liguria Region, is coordinated by the DLTM in strict collaboration, in terms of human resources, infrastructures and instruments with the associated public research bodies (CNR, ENEA, INGV) and with the IIM. The project also includes the next deployment of a cabled station in the Gulf of La Spezia (10 m depth, less than 100 m far from the coast) that will monitor the gravimetric field, temperature and marine current. The main objective of the coastal station is to provide a test site for new instruments and sensors.

How to cite: Ciuffardi, T., Berta, M., Bordone, A., Borghini, M., Celentano, P., Cocchi, L., De Fabritiis, L., Delbono, I., Delfanti, R., Demarte, M., Ivaldi, R., Kokkini, Z., Locritani, M., Marini, D., Marini, S., Muccini, F., Pannacciulli, F., Peirano, A., Raiteri, G., and Vetrano, A.: A new multidisciplinary observatory in the Eastern Ligurian Sea (NW Mediterranean Sea): a combination of deep-sea and coastal measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16533, https://doi.org/10.5194/egusphere-egu2020-16533, 2020.

An important deoxygenation trend has been described in the Black Sea over the five past decades from in-situ observations [1]. While the implications for basin-scale biogeochemistry and possible future trends of this dynamics are unclear, it is important to consolidate our means to resolve the dynamics of the Black Sea oxygen content in order to assess the likelihood of future evolution scenario, and the possible morphology of low-oxygen events. 

Also, it is known that current global models simulate only about half the observed oceanic O2 loss and fail in reproducing its vertical distribution[2]. In parts, unexplained O2 losses could be attributed to illy parameterized biogeochemical processes within 3D models used to integrate those multi-elemental dynamics.

Biogeochemical processes involved in O2 dynamics are structured vertically and well separated in the stratified Black Sea. O2 sources proceed from air-sea fluxes and photosynthesis in the
photic zone. Organic matter (OM) is respired over a depth determined by its composition and
sinking, via succeeding redox reactions. Those intricate dynamics leave unknowns as regards the biogeochemical impacts of future deoxygenation on associated cycles, for instance on the oceanic carbon pump. Here we use the Black Sea scene to derive model-observation strategies to best address the global deoxygenation concern.

First, we decipher components of the O2 dynamics in the open basin, and discuss the way in which O2-based indicators informs on the relative importance of processes involved. Using 1D biogeochemical model set-up, we then conduct a sensitivity analysis to pin-point model parameters, ie. biogeochemical processes, that bears the largest part in the uncertainty of simulated results for those diagnostics. Finally, we identify among the most impacting parameters the ones that can most efficiently be constrained on the basis of modern observational infrastructure, and Bio-Argo in particular. 

The whole procedure aims at orienting the development of observations networks and data assimilation approaches in order to consolidate our means to anticipate the marine deoxygenation challenge. 

[1] Capet A et al., 2016, Biogeoscience, 13:1287-1297
[2] Oschlies A et al., 2018, Nature Geosci, 11(7):467–473

How to cite: Capet, A., Luc, V., and Marilaure, G.: Biogeochemical controls on the Black Sea oxygen dynamics : relevant diagnostics, key processes and adequacy of the monitoring infrastructure., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17933, https://doi.org/10.5194/egusphere-egu2020-17933, 2020.

EGU2020-18508 | Displays | OS2.5

Acidification in the Mediterranean Sea following a transient climate change scenario simulated with a high-resolution regional model

Jean-Claude Dutay, James orr, briac le-vu, julien palmieri, camille richon, and samuel somot

Oceans contribute to the removal of 25%-30% of the atmospheric anthropogenic CO2, which increase sea water CO2 concentration and acidity, and decrease the Aragonite saturation state that may cause problems for calcium carbonate skeletons of marine species. The Mediterranean Sea is a specific environment with a higher alkalinity and a fast ventilation that is in favor of a more important uptake of  anthropogenic CO2 relatively to global ocean, and an acidification process impacting the whole water. The future acidification of the Mediterranean Sea has not been investigated by regional model yet.

In this study, we used an eddy-permitting regional model of the Mediterranean Sea (NEMO_MED8) coupled to an oceanic biogeochemical model (PISCES) to evaluate how climate and anthropogenic CO2 changes will modify the acidification and its annual cycle from the 1850 period to the end of the 21st century according to the future IPCC SRES-A2.  Evolution of boundary conditions from Rivers and exchange at the Gibraltar strait are considered. We analyse the relative influence of temperature, salinity, DIC and alkalinity on the mean and the seasonal amplitude of acidity (H+) and aragonite saturation sate (ΩA) and their evolution following a changing climate scenario SRES-A2.

How to cite: Dutay, J.-C., orr, J., le-vu, B., palmieri, J., richon, C., and somot, S.: Acidification in the Mediterranean Sea following a transient climate change scenario simulated with a high-resolution regional model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18508, https://doi.org/10.5194/egusphere-egu2020-18508, 2020.

EGU2020-19256 | Displays | OS2.5

Deep winter ventilation dynamics in the south Adriatic convection area from in situ glider observations and model output

Riccardo Martellucci, Elena Mauri, Riccardo Gerin, Giulio Notarstefano, and Gianpiero Cossarini

Dissolved oxygen dynamics in the south Adriatic pit have been investigated between 2015 and 2019 through in situ measurements and numerical models. This area is characterized by a frequent occurrence of deep water convection phenomena during winter time. Such convection phenomena represent the main source of dense waters for the Eastern Mediterranean basin modulating the oxygen advection in the deep water.

In situ glider measurements in the south Adriatic pit were performed by the OGS Glider Team since 2013. Typically, these missions covered the transect from Bari to Dubrovnik. The glider missions aim to investigate the water masses before, during and after the convection period. Pre-convection missions were carried out between the end of November and the beginning of December. Convection missions were performed between the end of January and the beginning of May.

Over 3000 profiles from the surface to 950m depth were collected and used to better understand the physical and biogeochemical highly variable processes in the southern Adriatic pit.

During the pre-convection period the water column is generally stratified; recorded data show an inverse correlation between dissolved oxygen and salinity. The pre-convection periods in 2015 and 2016 present the highest variability; the water column is mainly characterized by vertical profiles with a double oxygen minimum, which corresponds to the highest salinity concentrations. During the 2017 pre-convex mission the water column is characterized by a vertical salinity gradient, whereas dissolved oxygen profiles show a double dissolved oxygen maximum both on the surface and at 300-400 m depth. The 2018 pre-convex mission shows a thin surface layer of low salinity and high dissolved oxygen, which extends from the surface down to 50 m depth. A nucleus of high salinity and low oxygen is present close to the Italian coast at about 80-200m depth.

The 2016 convex mission revealed an inverse correlation of oxygen and salinity profiles and a double oxygen minimum with slightly different characteristics with respect to the previous pre convection period. During 2018 and 2019 the missions occurred during the convection phenomenon. The water column is well mixed from the surface down to 600 m depth, suggesting the occurrence of deep winter convection, also confirmed by the increase in oxygen and salinity concentrations along the water column.

In order to fully understand the process development in the south Adriatic Pit, which are the combinatorial result of coastal and open ocean processes, we integrated our observations with numerical model outputs provided by the Copernicus Marine Environment Monitoring Services. As the sea glider allows us to observe a high degree of variability from mesoscale to sub-mesoscale, the model output was used to evaluate mesoscale and sub basin scale phenomena.

Such an integration of different datasets provide information at different temporal and spatial scales of water mass dynamics, thus underlying the fundamental role of integrating multi-platform contributions to gain knowledge of the ocean processes.

How to cite: Martellucci, R., Mauri, E., Gerin, R., Notarstefano, G., and Cossarini, G.: Deep winter ventilation dynamics in the south Adriatic convection area from in situ glider observations and model output, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19256, https://doi.org/10.5194/egusphere-egu2020-19256, 2020.

EGU2020-20514 | Displays | OS2.5

Trends and interconnections of physical parameters in the upper layer of the Mediterranean Sea

Milena Menna, Giulio Notarstefano, Elena Mauri, Miroslav Gačić, Giuseppe Civitarese, Riccardo Gerin, and Pierre-Marie Poulain

The Mediterranean Sea is considered a hot spot of the global warming since it is changing faster than the global ocean, with a strong impact on the marine environment. Recent studies agree on the increase of the sea level, of the Sea Surface Temperature (SST), and of the Sea Surface Salinity (SSS) in the Mediterranean Sea over the last two decade, but no one has yet come to interconnect these and other parameters that contribute to the regulatory effect of the sea on the climate.

In this study, interannual variability and decadal climatic trends in the upper-layer of the Mediterranean Sea are estimated in the last 26 years using in-situ data (Argo float), satellite (altimetry, SST, wind vorticity, freshwater fluxes, mixed layer depth) and model (SSS) products.

Spatio-temporal variability is studied performing the Empirical Orthogonal Function analysis on the gridded, monthly, de-seasonalized maps of all satellite and model data. The contribution of the western, central and eastern regions of the Mediterranean Sea to the total trends is assessed. SSS distribution and trends derived from model reanalysis are compared with those derived from Argo float data in the upper layer.

Possible relationships between the trends in different datasets are delineated and described, i.e. the connection between the sea level rise and the SST, between the freshwater fluxes and the SSS, between the SSS and the ocean dynamics, including Ekman and geostrophic transports as well as vertical entrainment.

How to cite: Menna, M., Notarstefano, G., Mauri, E., Gačić, M., Civitarese, G., Gerin, R., and Poulain, P.-M.: Trends and interconnections of physical parameters in the upper layer of the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20514, https://doi.org/10.5194/egusphere-egu2020-20514, 2020.

EGU2020-21979 | Displays | OS2.5

Analysis of specific water masses transports in the Western Mediterranean in the MEDRYS1V2 twenty-one-year reanalysis.

Quentin-Boris Barral, Bruno Zakardjian, Franck Dumas, Pierre Garreau, and Jonathan Beuvier

We present an analysis of specific water masses fluxes in the Western Mediterranean Sea issued from a twenty years (1992-2013) reanalysis (MEDRYS1V2). Water masses are identified on the base of salinity and potential density properties and computes; the fractions of each water mass involved in total flux are computed under the hypothesis assumptions of mixing lines schemes. It was first designed in order to avoid rough truncations between water masses on the T-S diagram when using fixed thermo-haline properties thresholds. The method does not use the temperature marker due to its high seasonal variability in near surface waters (0-200 m) and we consider that potential density is a better marker to discriminate deep and intermediate water masses. The algorithm discriminates successively five different water masses : the Atlantic Water (AW) incoming from the Gibraltar strait (salinity between 36,1 and 38,45 PSU), the Levantine Intermediate Waters (LIW) incoming from the Tunisia-Sicily strait (salinity between 38,45 and 39.1 PSU), the Modified Atlantic Waters (MAW) defined as near-surface waters (potential density less than 28,9 kg m-3) that are neither AW or LIW, while Western Intermediate Waters (WIW) are those remaining until the σθ = 29,10 kg m-3 threshold for Western Mediterranean Deep Waters (WMDW) is reached. Such computed fractions of each water mass, whose sum is constrained to unity, are then used to compute their water masses transports all along over twenty years of the reanalysis. The transport are assessed across computed on key transects delimiting known sub-basin entities (Ligurian Sea, Gulf of Lion, Balearic Sea...), with total transports showing balanced mass budget. The such computed total transport reveal marked differences in their seasonal to interannual variability, while the analysis of the water mass transports allows to identify those which mainly implied induced these variability. The results first show a low seasonal and no significant interannual variability at the exit of the Alboran Sea that results from the balance between the eastward AW/MAW outflow and the westward WIW and WMDW inflows. The Corsican strait, the Ligurian Sea line and Tunisia-Sardinia straits show a marked seasonal variability (0,37-0,39 Sv) mainly driven by the AW/MAW. By contrast, a strong interannual variability dominates the seasonal one (-2 to 1 Sv) between the Algerian Basin and the northern basin, correlated to the WMDW formation. The analysis of each specific water masses transport pointed out that shows this marked variability to be first driven by the intermediate and deep water masses transports. Similarly the interannual variability of the AW and MAW transports in the central part of the Western Mediterranean suggests some coupling between the deep, intermediate and surface water masses, even through the shallower Balearic Sea.

How to cite: Barral, Q.-B., Zakardjian, B., Dumas, F., Garreau, P., and Beuvier, J.: Analysis of specific water masses transports in the Western Mediterranean in the MEDRYS1V2 twenty-one-year reanalysis., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21979, https://doi.org/10.5194/egusphere-egu2020-21979, 2020.

EGU2020-20407 | Displays | OS2.5

Mesoscale Activity in the Eastern Mediterranean: Blending Altimetry with in situ observations

Georges Baaklini, Leila Issa, Julien Brajard, Milad Fakhri, Milena Menna, Isabelle Taupier-Letage, and Laurent Mortier

Mesoscale to sub-mesoscale surface dynamics in the ocean is a key parameter, driving, for instance, the dispersion of pollutants emanating from heavily populated coastal areas for example. Estimating the surface velocity can be challenging especially when data is sparse. In [1], the authors developed a near real-time 3D-Var assimilation algorithm that blends in-situ Lagrangian drifters’ positions with altimetry data to improve the estimation of the surface velocity in the Eastern Levantine Mediterranean. The algorithm was tested near the Lebanese coast and in the case of an eddy between Lebanon and Cyprus. The objective of this work is to further validate the algorithm.

First, a Comparison with Ocean color satellite images shows that eddies’ shapes and location are more consistent after the assimilation of drifter data.Independent in-situ current-meter data provided from the EGYPT campaign are also used to validate the results of the algorithm in terms of velocity intensity and direction. The comparison shows an improvement of the estimated velocity, particularly in terms of direction.

We also address the question of extending the algorithm to a larger regional scale in the Eastern Levantine Mediterranean, which is subject to a high mesoscale activity but which is less densely observed than the western part.

 

[1] L. Issa, J. Brajard, M. Fakhri, D. Hayes, L. Mortier, P-M. Poulain. Modelling Surface Currents in the Eastern Levantine Mediterranean Using Surface Drifters and Satellite Altimetry. Ocean Modelling, May 2016. Doi: 10.1016/j.ocemod.2016.05.006

How to cite: Baaklini, G., Issa, L., Brajard, J., Fakhri, M., Menna, M., Taupier-Letage, I., and Mortier, L.: Mesoscale Activity in the Eastern Mediterranean: Blending Altimetry with in situ observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20407, https://doi.org/10.5194/egusphere-egu2020-20407, 2020.

EGU2020-4766 | Displays | OS2.5 | Highlight

Modelling sediment resuspension and transport induced by ships propellers in ports – the study case of the Port of Genoa

Antonio Guarnieri, Sina Saremi, Jacob H. Jensen, Andrea Pedroncini, Marco Vaccari, and Caterina Vincenzi

The action of propellers induced jets on the seabed of ports and harbors might be responsible of erosion and deposition of sediment around the port basin, potentially inducing important variations of the bottom topography in the medium to long time scales. Such dynamics constantly repeated for long periods can result in drastic reduction of ships’ clearance - in the case of deposition - or might be a threat for the stability and duration of the structures - in the case of erosion in the close vicinity of births and decks. These sediment processes are sources of problems for the port managing authorities, both for the safety of navigation and for the optimization of the management and maintenance of the ports’ bottom.

In the present work we study by means of integrated numerical modeling the erosion and sediment transport induced by naval traffic in the passenger Port of Genoa (Italy) and we propose a novel delayed-mode methodology and new science-based tools useful to optimize and efficiently plan the maintenance of the port sea bed. Fully operational real-time tools can be further developed starting from the proposed methodology in order to monitor the dynamics of the sediment on a daily basis.

How to cite: Guarnieri, A., Saremi, S., Jensen, J. H., Pedroncini, A., Vaccari, M., and Vincenzi, C.: Modelling sediment resuspension and transport induced by ships propellers in ports – the study case of the Port of Genoa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4766, https://doi.org/10.5194/egusphere-egu2020-4766, 2020.

EGU2020-5284 | Displays | OS2.5

Rare Earth Elements and Nd isotopes as tracers of modern ocean circulation in the central Mediterranean Sea

Ester Garcia-Solsona, Leopoldo D. Pena, Eduardo Paredes, José Noel Pérez-Asensio, Lucía Quirós-Collazos, Fabrizio Lirer, and Isa Cacho

Seawater Rare Earth Element (REE) concentrations and Nd isotopic composition (εNd) are increasingly applied as valuable tracers of oceanographic processes such as water mass mixing and lithogenic inputs to seawater. However, their measurements are basically lacking in the Mediterranean Sea water column. This study analyzes 9 seawater stations around the central Mediterranean Sea to clarify the relative importance of external sources, vertical (biogeochemical) processes and lateral water mass transport in controlling REE and εNd distributions. Concentrations of REE do not show nutrient-like profiles with depth, likely indicative of relatively young waters with limited accumulation of remineralized REE. Light REE (LREE) present a non-conservative behavior, which largely peak at surface waters and rapidly decrease with depth. The negative correlation of surface LREE enrichment with offshore distance highlights the influence of continental input from the western Italian coast to the Tyrrhenian surface waters. In contrast to other regions with reported boundary exchange, this process does not modify the εNd values here. On the other side, distributions of dissolved heavy REE (HREE) and εNd display a conservative behavior that can be explained by mixing of western- (MAW and WMDW) and eastern- (LIW and EMDW) originated waters. We test this hypothesis with an Optimum Multi-Parameter Analysis (OMPA) including HREE and εNd parameters. Even though the limited data set, consistent results of water mass fractions are obtained for the four main water masses although with some particularities. While LIW take on major importance when considering HREE in the model, EMDW fractions are preferentially detected with εNd. This latter finding implies a noticeable deep water flux across the Sicily Strait into the Western Mediterranean that was not clearly evidenced before.

How to cite: Garcia-Solsona, E., Pena, L. D., Paredes, E., Pérez-Asensio, J. N., Quirós-Collazos, L., Lirer, F., and Cacho, I.: Rare Earth Elements and Nd isotopes as tracers of modern ocean circulation in the central Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5284, https://doi.org/10.5194/egusphere-egu2020-5284, 2020.

We here discuss the remarkable uplift of the Eastern Mediterranean bottom waters that flow westward, over the Malta Escarpment, and cross the sill of the Channel of Sicily (Astarldi et al., 2001; Iudicone et al., 2003; Falcini & Salusti 2015); a dynamics that is rather similar to the one occurring at the Strait of Gibraltar (Mediterranean Sea) and Bab el Mandab (Red Sea) (Siddall et al., 2002). This classical uplift, which usually occurs under a three layer system dynamics, is mostly explained by the Bernoulli suction effect (Lane-Serff et al., 2000). However, the real filed analyses suggest that this dynamics are significantly perturbed by tidal effects and or large scale storms (Smeed et al., 2004). Here consider a novel, theoretical approach to obtain a rather realistic view of natural perturbations that affect these deep flow dynamics. Our insights on uplift processes, in addition, give a contribution to the general understanding of the Mediterranean Sea deep water circulation and, on climatological grounds, heat storage dynamics. We finally remark that similar phenomena happens in several marine straits and/or in semi-enclosed, peripheral basins of particular importance for local and large-scale processes.

 

References

Astraldi, M., Gasparini, G. P., Gervasio, L., & Salusti, E. (2001). Dense water dynamics along the Strait of Sicily (Mediterranean Sea). Journal of Physical Oceanography, 31(12), 3457-3475.

Falcini, F., & Salusti, E. (2015). Friction and mixing effects on potential vorticity for bottom current crossing a marine strait: an application to the Sicily Channel (central Mediterranean Sea). Ocean Science, 11(3), 391-403.

Iudicone, D., Buongiorno Nardelli, B., Santoleri, R., & Marullo, S. (2003). Distribution and mixing of intermediate water masses in the Channel of Sicily (Mediterranean Sea). Journal of Geophysical Research: Oceans, 108(C9).

Lane-Serff, G. F., Smeed, D. A., & Postlethwaite, C. R. (2000). Multi-layer hydraulic exchange flows. Journal of Fluid Mechanics, 416, 269-296.

Siddall, M., Smeed, D. A., Matthiesen, S., & Rohling, E. J. (2002). Modelling the seasonal cycle of the exchange flow in Bab el Mandab (Red Sea). Deep Sea Research Part I: Oceanographic Research Papers, 49(9), 1551-1569.

Smeed, D. A. (2004). Exchange through the Bab el Mandab. Deep Sea Research Part II: Topical Studies in Oceanography, 51(4-5), 455-474.

How to cite: Falcini, F., Di Paolantonio, M., and Salusti, E.: New insights on bottom water flows crossing a marine sill under periodic or impulsive perturbations: an application to the Sicily Channel sill (Central Mediterranean Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6046, https://doi.org/10.5194/egusphere-egu2020-6046, 2020.

EGU2020-7459 | Displays | OS2.5

Long-term assessment of surface dynamics in the Tyrrhenian Sea

Enrico Zambianchi, Naomi Krauzig, and Pierpaolo Falco

The variability of surface dynamics has been investigated extensively in the Mediterranean Sea for different temporal and spatial coverage, whereas a specific evaluation for the area of the Tyrrhenian Sea does not exist. Thus, this study is focused on the Tyrrhenian basin, a subbasin of the western Mediterranean, which is considered sensitive to climatic variations due to its small size and isolated nature. The main scope is to provide a comprehensive and up-to-date assessment of the sea surface warming, the sea level changes and the general surface circulation in the Tyrrhenian Sea, as well as to improve the understanding of the relation to large-scale teleconnection patterns and to regional air-sea interaction. The long-term spatio-temporal variability and trends were investigated using satellite-derived, in-situ and reanalysis-based datasets up to the end of 2018. Further, the possible linkage with the occurrence of extreme weather events was assessed using observations from the European Severe Weather Database. The different datasets cover multiple temporal and spatial scales and enable the investigation of the potential physical processes related to the non-homogeneous, time-depended spatial variability. The results indicate a significant increase in sea level and sea surface temperature which appears to be linked with the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO), respectively. Moreover, analysis of the basin’s surface circulation together with local air-sea exchanges of heat, freshwater and momentum indicated a significant influence of the wind-driven Ekman pumping variability.

How to cite: Zambianchi, E., Krauzig, N., and Falco, P.: Long-term assessment of surface dynamics in the Tyrrhenian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7459, https://doi.org/10.5194/egusphere-egu2020-7459, 2020.

EGU2020-11179 | Displays | OS2.5

Lagrangian dynamics in the Gulf of Trieste from high resolution HF-Radar

Nydia Catalina Reyes-Suarez, Ismael Hernandez-Carrasco, Matjaz Licer, Vanessa Cardin, Miroslav Gacic, and Alejandro Orfila

The Gulf of Trieste (GoT) is shared by Italy, Slovenia and Croatia, with most of its coasts belonging to Italy and Slovenia, along with the two main harbours; the Harbour of Trieste (Italy) and Koper (Slovenia). Both are subject to heavy marine traffic and exposed to different threats including oil spills, maritime accidents and SAR operations. The GOT High frequency radar network provides near-real time data of sea surface currents and waves since 2016. In this work we provide a statistical description of surface variability in terms of Lagrangian descriptors in order to elucidate the transport and retention in the GoT as well as to provide the seasonal evolution of the residence time. Among the most widely used Lagrangian techniques, we focus the study on Lagrangian Coherent Structures and Path-integrated topological variables like Lagrangian divergence and Lagrangian vorticity.

How to cite: Reyes-Suarez, N. C., Hernandez-Carrasco, I., Licer, M., Cardin, V., Gacic, M., and Orfila, A.: Lagrangian dynamics in the Gulf of Trieste from high resolution HF-Radar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11179, https://doi.org/10.5194/egusphere-egu2020-11179, 2020.

EGU2020-11775 | Displays | OS2.5

Circulation patterns and eutrophication phenomena in the Thermaikos Gulf

Yannis N. Krestenitis, Vasilis Kolovoyiannis, Yannis Androulidakis, Christos Makris, and Vasilis Baltikas

Thermaikos Gulf, located in the Northwestern Aegean Sea (Greece), is a marine ecosystem of major importance, not only environmentally (as an area of the deep water formation with contribution to the renewal of the North Aegean deep waters), but also due to the various socioeconomic activities associated with the area. Observational and simulated data are used to investigate the evolution of eutrophication events during the last two years in order to evaluate the current (2017-2019) quality state of the seawater in the Gulf. The quality of the marine environment of Thermaikos Gulf was appraised by measuring physical, chemical and biological parameters. Specific physical-chemical characteristics (temperature, salinity, density along with pH and dissolved oxygen) and biological parameters (chl-a and phytoplankton biomass) throughout the water column were evaluated by conducting in situ measurements during the sampling campaigns. Current fields, derived from a high-resolution 3-D ocean model, together with ADCP measurements, are used to describe the major circulation patterns, the river plume dynamics and the renewal pathways of the Gulf. The obtained results are discussed with regards to seasonal and spatial variability, and the water column stratification. Satellite ocean color data were also used to discuss the in-situ findings and confirm “Dirty” Sea and Red Tide phenomena, that were detected and analyzed based on the physical dynamics and especially the renewal patterns of the Gulf. Moreover, we compare these recent findings to respective observations from a previous period (1997 to 2007) to evaluate potential changes in the quality state of the Gulf with respect to meteorological and river discharge conditions.     

How to cite: Krestenitis, Y. N., Kolovoyiannis, V., Androulidakis, Y., Makris, C., and Baltikas, V.: Circulation patterns and eutrophication phenomena in the Thermaikos Gulf , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11775, https://doi.org/10.5194/egusphere-egu2020-11775, 2020.

EGU2020-13494 | Displays | OS2.5

​Impact of changes in rivers inputs during the last decades on the biogeochemistry of the eastern Mediterranean basin

Rémi Pagès, Melika Baklouti, Nicloas Barrier, Camille Richon, Jean-Claude Dutay, Mohamed Ayache, and Thierry Moutin

The Mediterranean Sea (MS) is a semi-enclosed sea characterized by a zonal west-east gradient of oligotrophy, where microbial growth is controlled by phosphate availability in most situations. External inputs of nutrients including Gibraltar inputs, river inputs and atmospheric deposition are therefore of major importance for the biogeochemistry of the MS. The latter has long been considered to be driven mainly by nutrient exchanges at Gibraltar. However, recent studies indicate that river inputs significantly affect nutrients concentrations in the Mediterranean Sea, although their resulting impact on its biogeochemistry remains poorly understood. In this study, our aim was to help fill this knowledge gap by addressing the large-scale and long-term impact of variations in river inputs on the biogeochemistry of the Mediterranean Sea over the last decades, using a coupled physical- biogeochemical 3D model (NEMO-MED12/Eco3M-Med). As a first result, it has been shown by the model that the strong diminution (60%) of phosphate (PO4) in river inputs into the Mediterranean Sea since the end of the 1980s induced a significant lowering of PO4 availability in the sub-surface layer of the Eastern Mediterranean Basin (EMB). One of the main consequences of PO4 diminution is the rise, never previously documented, of dissolved organic carbon (DOC) concentrations in the surface layer (by 20% on average over the EMB). Another main result concerns the gradual deepening of the top of the phosphacline during the period studied, thus generating a shift between the top of the nitracline and the top of the phosphacline in the EMB. This shift has already been observed in situ and documented in literature, but we propose here a new explanation for its occurrence in the EMB. The last main result is the evidence of the decline in abundance and the reduction of size of copepods calculated by the model over the years 1985–2010, that could partially explain the reduction in size of anchovy and sardine recently recorded in the MS. In this study, it is shown for the first time that the variations in river inputs that occurred in the last decades may have significantly altered the biogeochemical cycles of two key elements (P and C), in particular in the EMB.

How to cite: Pagès, R., Baklouti, M., Barrier, N., Richon, C., Dutay, J.-C., Ayache, M., and Moutin, T.: ​Impact of changes in rivers inputs during the last decades on the biogeochemistry of the eastern Mediterranean basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13494, https://doi.org/10.5194/egusphere-egu2020-13494, 2020.

EGU2020-19382 | Displays | OS2.5

Coastal climatology of the North-Western Mediterranean area for long-term and short-term risk assessment.

Carlo Brandini, Stefano Taddei, Valentina Vannucchi, Michele Bendoni, Bartolomeo Doronzo, Maurizio Iannuccilli, Gianni Messeri, Francesco Pasi, and Valerio Capecchi

In this work we present the results obtained through a dynamic downscaling of the ERA5 reanalysis dataset (hindcast) of ECMWF, using high-resolution meteorological and wave models defined on unstructured computation grids along the Mediterranean coasts, with a particular focus on the North-Western Mediterranean area. Downscaling of the ERA5 meteorological data is obtained through the BOLAM and MOLOCH models (up to a resolution of 2.5 km) which force an unstructured WW3 model with a resolution of up to 500 m along the coast. Models were validated through available meteorological stations, wave buoy data and X-band wave radars, the latter for the purposes of wave spectra validation.

On the one hand, this allowed, by extracting the time series of some attack parameters of the waves along the coast, and according to the type of coast (rocky coasts, sandy coasts, coastal structures etc.), to compute the return periods and to characterize the impact of any individual storm. On the other hand, it is possible to highlight some trends observed in the last 30 years, during which recent research is showing an increasing evidence  of some changes in global circulation at regional to local scales. These changes also include effects of wind rotation, wave regimes, storm surges, wave-induced coastal currents and coastal morphodynamics. For example, in the North-Western Mediterranean extreme events belonging to cyclonic weather-types circulation with stronger S-SE components (like the storm of October 28-30th 2018 and many others), rather than events associated with perturbations of Atlantic origin and zonal circulation, are becoming more frequent. These long-term wind/wave climate trends can have consequences not only in the assessment of long-term risk due to main morphodynamic variations (ie. coastal erosion), but also in the short-term risk assessment.

This work was funded by the EU MAREGOT project (2017-2020) and ECMWF Special Project spitbran  “Evaluation of coastal climate trends in the Mediterranean area by means of high-resolution and multi-model downscaling of ERA5 reanalysis” (2018-2020).

How to cite: Brandini, C., Taddei, S., Vannucchi, V., Bendoni, M., Doronzo, B., Iannuccilli, M., Messeri, G., Pasi, F., and Capecchi, V.: Coastal climatology of the North-Western Mediterranean area for long-term and short-term risk assessment., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19382, https://doi.org/10.5194/egusphere-egu2020-19382, 2020.

EGU2020-20226 | Displays | OS2.5

Offsets and inputs of natural uranium isotopes in the Mediterranean Sea

Evan Border, Norbert Frank, Pieter van Beek, Gideon Henderson, and Joseph Tamborski

High precision measurements of natural uranium isotopes in the Atlantic Ocean, Mediterranean Sea,
and Black Sea reveal isotopic makeups which differ significantly from the well-known oceanic
composition. In the Mediterranean, water masses are strongly differentiated to the extent that they
are able to be fingerprinted on the basis of δ234U. Mediterranean deep water masses show the
highest enrichment, with an offset with respect to oceanic δ234U values of just over 1 ‰. The Black
Sea shows an even higher offset of up to ~40 ‰.
This offset provides an opportunity to look into the as of yet largely unstudied uranium inputs to the
Mediterranean, in particular rivers and submarine groundwater discharge (SGD), which are thought
to play key roles in uranium input to the global ocean. A simple box model, incorporating the
Mediterranean and Black Sea data from this study is constructed to provide a first estimate of the U
concentration and δ234U signature of rivers and SGD necessary for this offset to arise. These
estimates are then compared with new measurements of various coastal and submarine springs from
along the French Mediterranean Coast as well as with existing riverine data exists to speculate on
which inputs may be most responsible for this offset.

How to cite: Border, E., Frank, N., van Beek, P., Henderson, G., and Tamborski, J.: Offsets and inputs of natural uranium isotopes in the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20226, https://doi.org/10.5194/egusphere-egu2020-20226, 2020.

OS3.1 – Deoxygenation in the marine environment: drivers, trends and challenges

EGU2020-22686 | Displays | OS3.1

Time of Emergence of anthropogenic deoxygenation and warming in the thermocline

Angélique Hameau, Thomas Frölicher, Juliette Mignot, and Fortunat Joos
Multiple lines of evidence from observation- and model-based studies show that anthropogenic greenhouse gas emissions cause ocean warming and oxygen depletion, with adverse impacts on marine organisms and ecosystems.
Temperatures increase is a primary indicator for climate change. However, in the thermocline, changes in oxygen and other biogeochemical tracers might be detectable before warming (Hameau et al., 2019a).
Here, we compare the local time of emergence (ToE) of anthropogenic temperature and oxygen changes in the thermocline within an ensemble of Earth system model simulations from the CMIP5 dataset (Hameau et al., 2019b).
Generally, warming emerges from internal variability prior to changes in oxygen.
Yet, in 35$\pm$11\% of the global thermocline, anthropogenic deoxygenation is detectable before warming.
Earlier emergence of oxygen changes is typically related to decreasing trends in ventilation, which reduce the supply of oxygen-rich surface waters to the thermocline.
In addition, reduced ventilation slows the propagation of anthropogenic warming from the surface into the ocean interior, further contributing to the delayed emergence of warming compared to deoxygenation.
As the magnitude of simulated interval variability and of simulated anthropogenic changes vary considerably across models, we introduce the relative ToE metric. This reduces the inter-model spread, allowing for a better comparison among models.
Our results underline the importance of an ocean biogeochemical observing system and that the detection of anthropogenic impacts becomes more likely when using multi-tracer observations.

How to cite: Hameau, A., Frölicher, T., Mignot, J., and Joos, F.: Time of Emergence of anthropogenic deoxygenation and warming in the thermocline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22686, https://doi.org/10.5194/egusphere-egu2020-22686, 2020.

EGU2020-7423 | Displays | OS3.1

Physical mechanisms driving the global ocean breathe

Esther Portela Rodriguez, Nicolas kolodziejczyk, and Virginie Thierry

The physical transport of dissolved oxygen across the mixed layer base is the main process to oxygenate the interior ocean. This ventilation mechanism is suspected to play a dominant role in the predicted ocean deoxygenation over the 21st century, however, it has not yet been properly quantified or described at global scale. Here we show the mean distribution of the mechanisms driving the oxygen exchanges between the mixed layer and the ocean interior and their relation with water-mass formation. Most of the oxygen uptake occurs in well-defined hot-spots located in the subpolar North Atlantic (30%) which provide oxygen to deep and waters and in the Southern Ocean (37$%) that oxygenates intermediate and bottom waters. The oxygen release is concentrated within the ACC belt (37%), in the subtropical-subpolar North Atlantic (22%) and within the equatorial strip (13%). Globally, the mode waters account for about 72% of the subducted oxygen during their formation process. The oxygen uptake by the Subantarctic and Subpolar Mode Water is driven by strong currents flowing through large mixed layer depth gradients at localized hot-spots while the spatial continuity of the wind-driven vertical velocity over broad areas in the Subtropical gyres accounts for most of the oxygen subduction during the Subtropical Mode Water formation.

How to cite: Portela Rodriguez, E., kolodziejczyk, N., and Thierry, V.: Physical mechanisms driving the global ocean breathe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7423, https://doi.org/10.5194/egusphere-egu2020-7423, 2020.

A comparative study of dissolved oxygen concentration outputs from two generations of the NorESM model, NorESM-ME (CMIP5) and NorESM-LM (CMIP6), has been carried out as part of a general model development evaluation. Model output for dissolved oxygen consist of yearly averaged historical data over the period 1850-2000. The dimensionality of this data set was reduction by computing empirical orthogonal functions (EOFs), which are eigenvectors of the spatially weighted anomaly covariance matrix defined by the spatio-temporal dissolved oxygen field. EOF analysis of the two models show similar patterns of dissolved oxygen in the upper ocean (150m depth), with pronounced anoxic conditions in the western Pacific Ocean, Indian Ocean and southern Atlantic Ocean. At 500m depth the model outputs remain mostly in agreement for the Pacific Ocean, but the EOF patterns diverge significantly for both the Indian Ocean and Atlantic Ocean, and to some extent also for the Southern Ocean. For the Indian Ocean, the EOF shift seem to reflect a general reduction of oxygen levels in NorESM-LM compared to NorESM-ME. In the Atlantic Ocean situation is more complex, with NorESM-LM showing reduced oxygen levels near the equator, and enhanced oxygen levels at higher latitudes when compared to NorESM-ME. Further studies are currently in progress to investigate to what extent the similarities and discrepancies in dissolved oxygen concentration can be attributed to ocean temperature and stratification.

How to cite: Torsvik, T. and Heinze, C.: Comparing dissolved oxygen concentration outputs from two generations of the NorESM model , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9451, https://doi.org/10.5194/egusphere-egu2020-9451, 2020.

EGU2020-18281 | Displays | OS3.1

Can ocean deoxygenation accelerate global warming via enhanced marine N2O production?

Angela Landolfi and Wolfgang Koeve

Ocean warming is projected to cause marine deoxygenation, reduce solubility, affect ocean circulation and enhance metabolic rates over this century. These changes, affecting oceanic N2O production and emissions, have been suggested to potentially rise atmospheric N2O concentrations and increase the positive feedback to anthropogenic climate change.However, current global model projections all suggest a decline in marine N2O emissions under global warming but the processes leading to this decline are poorly constrained. Here, using an Earth system model of intermediate complexity, we disentangle the contribution of ocean deoxygenation and the direct and indirect warming effects on oceanic N2O production and emissions changes under RCP8.5 emission scenario. We find that ocean deoxygenation and warming-reduced N2O solubility do in fact increase oceanic N2O emissions, however this increase is overcompensated by ocean circulation slow-down and reduced export production, suggesting a neglectable N2O-emssion feedback to climate on centennial timescales.

How to cite: Landolfi, A. and Koeve, W.: Can ocean deoxygenation accelerate global warming via enhanced marine N2O production? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18281, https://doi.org/10.5194/egusphere-egu2020-18281, 2020.

EGU2020-6151 | Displays | OS3.1

Future trends in oxygen minimum zone volume are sensitive to model representation of iron

Wanxuan Yao, Karin Kvale, Angela Landolfi, Wolfgang Koeve, Eric Achterberg, and Andreas Oschlies

Increasing the complexity of the representation of iron in an earth system model can lead to significant differences in surface ocean nutrient pathways in a pre-industrial climate. These differences persist even after automated calibration forces the models to achieve similar fit to the same observational data. We explore the impact of these nutrient pathway differences in the context of climate change by forcing the models (one without iron, one with a seasonally-cyclic iron mask, and one with a fully dynamic iron module) with the RCP8.5 business-as-usual atmospheric CO2 concentration scenario from years 1800 until 2100. We find that while the global oxygen inventory drops across all models over this period, different trends in the oxygen minimum zone (OMZ) volume arise. Models with iron represented simulate decreases between 60 and 80 percent in OMZ volume, while the model without iron simulates an OMZ volume increase of 10 percent. The difference is attributed to the role of iron limitation in regulating the low latitude primary production response to warming and stratification. We further quantify the corresponding denitrification trends and impact on ocean nitrate inventory. This study illustrates that model structural uncertainty further challenges predictions under a changing climate, and highlights the strong role of iron in regulating nutrient cycling and ocean deoxygenation.

How to cite: Yao, W., Kvale, K., Landolfi, A., Koeve, W., Achterberg, E., and Oschlies, A.: Future trends in oxygen minimum zone volume are sensitive to model representation of iron, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6151, https://doi.org/10.5194/egusphere-egu2020-6151, 2020.

EGU2020-13038 | Displays | OS3.1

Anoxic metabolism after the 21st century in oxygen minimum zones

Wolfgang Koeve and Angela Landolfi

Global models project a decrease of marine oxygen over the course of the 21th century. The future of marine oxygen becomes increasingly uncertain further into the future after yr 2100 , partly because ocean models differ in the way organic matter remineralisation continues under oxygen- and nitrate-free conditions. Using an Earth system model of intermediate complexity we found that under a business-as-usual CO2-emission scenario ocean deoxygenation further intensifies for several centuries until eventually ocean circulation re-establishes and marine oxygen increases again. (Oschlies et al. 2019, DOI 10.1038/s41467-019-10813-w).

In the Pacific Ocean the deoxygenation after yr 2100 goes along with the large scale loss of nitrate from oxygen minimum zones. Here we explore the impact on simulated ocean biogeochemistry of three different process formulation of anoxic metabolism, which have been used in other ocean models: (1) implicit sulphate reduction (organic matter degradation continues without oxidant), (2) no sulphidic metabolism (organic matter is not degraded under anoxic conditions), and (3) explicit sulphate reduction (with H2S as explicit model tracer). The model with explicit sulfphate reduction supports larger regional organic matter fluxed into the deep ocean and an increase in respired carbon storage, compared with the model applying implicit sulphate. We discuss the impact of anoxic metabolism on the coupling between export production and respired carbon stored in the ocean interior.

How to cite: Koeve, W. and Landolfi, A.: Anoxic metabolism after the 21st century in oxygen minimum zones , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13038, https://doi.org/10.5194/egusphere-egu2020-13038, 2020.

EGU2020-13393 | Displays | OS3.1

Controls on oxygen response to climate change on the Northwest European Continental Shelf

Sarah L. Wakelin, Yuri Artioli, Momme Butenschön, Jason Holt, and Jeremy Blackford

Dissolved oxygen in the ocean is an indicator of water quality and low concentrations can threaten ecosystem health. The main sources of marine oxygen are diffusion from the atmosphere and phytoplankton photosynthesis. Biological respiration and decomposition act to reduce oxygen concentrations. Under conditions of vertical stratification, the water column below the pycnocline is isolated from oxygen exchange with the atmosphere, photosynthesis may be limited by light availability and oxygen concentrations decrease. Climate change influences the oxygen cycle in two ways: 1) changing the hydrodynamic climate and 2) affecting rates of biogeochemical processes. The hydrodynamic climate affects the nutrient supply and so controls phytoplankton production while changes to water column stratification affects vertical mixing. Gas solubility decreases with increasing temperature so that oxygen uptake from the atmosphere is expected to decrease under increasing oceanic temperatures. Biological cycling rates increase with increasing temperature affecting photosynthesis, respiration and bacterial decomposition. It is not obvious whether changes in oxygen concentrations due to changing ecosystem processes will mitigate or reinforce the projected reduction from solubility changes.

The Northwest European Continental shelf (NWES) is a region of the northeast Atlantic that experiences seasonal stratification. We use the physics-biogeochemical model NEMO-ERSEM to study near-bed oxygen concentrations on the NWES under a high greenhouse gas emissions scenario (Representative Concentration Pathway (RCP) 8.5). We show that much of the NWES could experience low oxygen concentrations by 2100 and assess the relative impacts of changing temperature and ecosystem processes. Until about 2040 the impact of solubility dominates the oxygen change. The mean near-bed oxygen concentration is projected to decrease by 6.3% by 2100, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. In the oxygen-depleted region in the eastern North Sea, 77% of the near-bed oxygen reduction is due to ecosystem processes.

How to cite: Wakelin, S. L., Artioli, Y., Butenschön, M., Holt, J., and Blackford, J.: Controls on oxygen response to climate change on the Northwest European Continental Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13393, https://doi.org/10.5194/egusphere-egu2020-13393, 2020.

EGU2020-21909 | Displays | OS3.1

Disentangling oxygen depletion in the Benguela Upwelling System: A biophysical approach.

Natalia Herran and Martin Schmidt

Understanding the multilevel complexity of marine ecosystems is one of the greatest challenges on ecosystem modeling so far, due to the dualism of governing hydrodynamical processes acting on a regional scale and complex biogeochemical chain reactions that happen locally on the marine environment. A coupled hydrodynamic-ecological model based on nitrogen stoichiometry has been developed to better understand the short-term nutrient and oxygen coastal dynamics in the Benguela Upwelling System (BUS). The model shows that the effect of internal waves in the Benguela region re-shapes the benthic ecosystem due to the increased of turbulence on the ocean floor with a consequently increase of fine sediment on the water column. We show that an increase on organic-rich sediment resuspension on the water column enhance oxygen consumption and ultimately contribute to the apparent deoxygenation of the Namibian coastal shelf.

How to cite: Herran, N. and Schmidt, M.: Disentangling oxygen depletion in the Benguela Upwelling System: A biophysical approach. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21909, https://doi.org/10.5194/egusphere-egu2020-21909, 2020.

EGU2020-20993 | Displays | OS3.1

Low-oxygen subsurface eddies in the eastern South Pacific

Oscar Pizarro, Marcela Contreras, Nadín Ramírez, and Matías Pizarro-Koch

Modeling studies have shown that mesoscale eddies significantly contribute to modulate the variability of the oxygen minimum zone (OMZ) of the eastern South Pacific at seasonal and interannual time scales. Nevertheless, only few observations have shown the properties of these eddies. Particularly subsurface (intrathermocline) eddies may play an important role in the dynamics of the southern tip of this OMZ. In this work we analyze the characteristics of these eddies based on underwater glider observations, along with oceanographic cruises and satellite data. We also combine our observations with results from a high resolution numerical model to analyze the generation mechanism of these subsurface eddies. Observations show that the eddies are characterized by a core with high salinity (SA > 34.6 g kg-1), low oxygen (DO < 0.5 mL L-1)  and relatively low potential vorticity  (f PV < 10-13 s-4, where f is the Coriolis parameter). The eddy core is typically centered around σθ ~ 26.5 kg m3 (150-200 m depth) and their diameters are about 50 km, transporting typically ~0.2 Sv of very low-oxygen (< 0.5 mL L-1) waters offshore. The eddy core properties coincide with the water mass that is transported by the Peru-Chile Undercurrent. Our modeling study shows that the generation of the subsurface eddies is associated with the separation of the Undercurrent from the slope and current reversals (northward subsurface flow) close to the slope.

How to cite: Pizarro, O., Contreras, M., Ramírez, N., and Pizarro-Koch, M.: Low-oxygen subsurface eddies in the eastern South Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20993, https://doi.org/10.5194/egusphere-egu2020-20993, 2020.

EGU2020-6473 | Displays | OS3.1

Expansion of the eastern North Pacific OMZ and the associated denitrification regime
not presented

Allan Devol and Wendi Ruef

 

At this point ocean deoxygenation is well documented, including in oxygen minimum zones (OMZs).  Within the large OMZs of the Arabian Sea and eastern Pacific are imbedded areas where oxygen concentrations are so low that they are undetectable by routine CTD sensors (oxygen deficient zones, ODZs).  How do we determine if these ODZ are losing O2?  Furthermore, denitrification occurs in oxygen minimum zones (OMZs) so one might hypothesize that denitrification is likewise expanding if oxygen is decreasing.  This is important because the ocean's fixed nitrogen inventory limits the productivity over large marine areas.

We have investigated these questions in the largest OMZ, the eastern tropical North Pacific (ETNP) through an analysis of  6 repeats of a 1000 km transect along 110o West in the heart of the ETNP ODZ between 1971-2019.  We use N*, a stoichiometric parameter calculated from nitrate and phosphate, as our indicator of denitrification. The more Negative N* the more denitrification has occurred. After secondary QC the values of O2 concentration between potential density 24.75 and 1000m along with N* were integrated across the transect and over the depth of the ODZ.  

The results show a clear decrease in oxygen inventory along with an increase in N*, suggesting deoxygenation and intensification of denitrification over during the 50 year period. We discuss potential mechanisms for denitrification signal increase including ENSO, Pacific Decadal Oscillation, tropical hurricane intensity, and variations in thermocline depth.

How to cite: Devol, A. and Ruef, W.: Expansion of the eastern North Pacific OMZ and the associated denitrification regime, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6473, https://doi.org/10.5194/egusphere-egu2020-6473, 2020.

EGU2020-6612 | Displays | OS3.1

Iron, primary production and an oxygen minimum zone feedback mechanism

Mark Hopwood, Allanah Paul, Thomas Browning, Madeleine Freund, Toste Tanhua, Insa Rapp, Martha Gledhill, and Eric Achterberg

In regions bordering productive eastern-boundary upwelling systems, extensive O2 minimum zones (OMZs) develop along the ocean shelf. Enhanced sedimentary release of Fe and PO4 under these low O2 conditions could drive enhanced N2-fixation and primary production in the ocean, thus ultimately increasing O2 consumption, and potentially creating a positive feedback to ocean deoxygenation. A similar feedback loop has been identified in shallow and enclosed coastal regions such as the Baltic where primary production is controlled primarily by N and P bioavailability, but it is unclear to what extent a similar mechanism operates at a larger scale in the open ocean where productivity is proximally constrained by Fe and/or N availability. This is largely because of uncertainties in the fate of the Fe released from shelf sediments.

Here we present extensive Fe measurements from a series of 5 cruises and a mesocosm experiment on the Peruvian shelf which combined extensive measurements of Fe distribution with an inert tracer release experiment, to quantify off-shelf transport, and N2 fixation rates. As expected for a region with among the highest reported benthic Fe fluxes in the world, dissolved Fe concentrations were generally elevated along the inner-Peruvian shelf reaching >60 nM. Whilst concentrations rapidly declined across the shelf, reaching as low as 0.01 nM after the shelf break, ‘pockets’ of 1-2 nM elevated Fe concentrations were evident in some transects suggesting a significant role of eddies in off-shelf transport which was verified by the results of our CF3SF5 tracer release experiment. Never-the-less, benthic Fe release was rapidly attenuated close to the sediment interface, with the vast majority of Fe loss occurring on timescales of <1 day and spatial scales of <1 km. Evidence of Fe limitation was even found at some stations on the Peruvian shelf questioning the efficiency with which Fe released from sediments is able to positively influence marine primary production. Despite an excess of P across the region with respect to biological requirements, and extremely high inner-shelf Fe concentrations, N2 fixation rates remained consistently low across the region (0-0.8 nmol N L-1 d-1). Furthermore, the maximum lateral transfer of Fe was de-coupled spatially from maximum benthic Fe release, limiting the potential for a P/Fe-fueled positive feedback loop to OMZ expansion.

How to cite: Hopwood, M., Paul, A., Browning, T., Freund, M., Tanhua, T., Rapp, I., Gledhill, M., and Achterberg, E.: Iron, primary production and an oxygen minimum zone feedback mechanism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6612, https://doi.org/10.5194/egusphere-egu2020-6612, 2020.

EGU2020-11803 | Displays | OS3.1

Characterisation of low oxygen extreme events in the Eastern Tropical Pacific between 1979 and 2016

Eike Eduard Köhn, Matthias Münnich, Meike Vogt, and Nicolas Gruber
The oxygen minimum zones (OMZs) in the Eastern Tropical Pacific (ETP)
have expanded over the past 50 years, likely leading to more frequent and more
intense low oxygen extreme events. This has potentially far-reaching implica-
tions for e.g., the production of the climate-relevant gas nitrous oxide or the
reduction of habitat for fish and zooplankton. Yet, to date our understanding
of the distribution and characteristics of low oxygen extreme events in the ETP
remains limited.
 
To fill this gap, we study low oxygen extremes in the ETP using results from
an eddy-resolution hindcast simulation with the coupled physical-biogeochemical
model ROMS-BEC for the Pacific from 1979 to 2016. Our setup permits us to
simulate oxygen variability in the ETP affected by processes on a broad range
of scales, from climate modes down to mesoscale dynamics. We detect and
track low oxygen extreme events in the upper 500 meters of the ETP, by ap-
plying temporally constant statistical thresholds to the hindcast simulation and
requiring a minimum event duration of 5 days. While most extremes last less
than 10 days and are of small volumetric extent, about 15% of the extremes
exist for over a month. The diversity of the long-lasting extremes is dominated
by westward propagating low oxygen eddies, which are mostly generated in the
near-coastal area. Superimposed inter-annual variability associated with the El
Niño-Southern Oscillation (ENSO) leads to a decrease in mesoscale extremes
during El Niño periods. Along the boundaries of the ETP OMZs transient
shoaling events of the oxycline linked to ENSO dynamics or the seasonal cycle
contribute to the generation of further pronounced low oxygen extreme events.
 
The presented detection and tracking of low oxygen extremes is an important
step towards a better understanding of extreme event occurrences and charac-
teristics and lays the groundwork for further research such as the biogeochemical
impact of such extremes.

How to cite: Köhn, E. E., Münnich, M., Vogt, M., and Gruber, N.: Characterisation of low oxygen extreme events in the Eastern Tropical Pacific between 1979 and 2016, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11803, https://doi.org/10.5194/egusphere-egu2020-11803, 2020.

Intermediate waters (500 - 2000 m) from the equatorial- to North Pacific are currently hypoxic (oxygen concentrations below 120 µmol/kg), while deeper waters are well oxygenated. For the last ice-age, some proxy records suggested that this trend was reversed, with well-oxygenated Pacific intermediate waters, and lower oxygenated deeper waters associated with an increased deep carbon reservoir. Recent work however suggests that there was an overall expansion of oxygen depleted water in the eastern tropical North Pacific during the last glacial period (Hoogakker et al., 2018). To further assess the natural variability in intermediate water dissolved oxygen concentrations over longer time-scales we extend the bottom water oxygen record of ODP Site 1242 (1360 m depth located in the eastern tropical north Pacific), to 140,000 years, using  the benthic foraminifera carbon isotope gradient approach of Hoogakker et al. (2015).  Our reconstructions suggest that oxygen concentrations varied with an approximate 40 kyr period; with lowest concentration during cool periods of the penultimate glacial, MIS 5b, 4 and 2.

How to cite: Hoogakker, B., Day, C., and Leng, M.: Changes in oxygen concentrations of intermediate water in the eastern tropical north Pacific over the last 140,000 years., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4732, https://doi.org/10.5194/egusphere-egu2020-4732, 2020.

EGU2020-12973 | Displays | OS3.1

Sedimentary molybdenum and uranium sequestration in a non-euxinic coastal setting: role of the sulfate-methane transition zone

Sami Jokinen, Karoliina Koho, Joonas Virtasalo, and Tom Jilbert

Molybdenum (Mo) and uranium (U) contents in sedimentary records are commonly used to track past changes in seafloor oxygenation. However, inadequate understanding of Mo and U sequestration mechanisms in non-euxinic coastal areas limits their use as redox proxies in these settings. Because large areas of the coastal oceans are currently undergoing partial deoxygenation due to anthropogenic nutrient inputs and increased stratification, it is critical to improve our understanding of these proxies to allow robust assessment of the trajectory of environmental change. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to deconvolve the mechanisms of authigenic Mo and U sequestration in a shallow non-euxinic coastal setting in the northern Baltic Sea. Despite the permanently oxic bottom waters in the area, eutrophication over the past decades has led to establishment of a shallow sulfate-methane transition zone (SMTZ) in the sediment, which is typical for human-impacted coastal settings on a trajectory towards hypoxia. Our results demonstrate remarkably synchronous patterns of Mo and U sequestration, whereby their authigenic uptake is largely predicated upon the depth and intensity of the SMTZ. Based on sequential extraction analyses, the authigenic Mo pool is dominated by refractory Fe-S phases such as pyrite and nanoscale FeMoS4, signaling that authigenic Mo uptake largely proceeds through the Fe-sulfide pathway. In addition, we observe a pool of extremely labile Mo deep within the SMTZ, potentially denoting a transient phase in authigenic Mo uptake and/or partial switch in the mode of sequestration to the organic matter pathway at low levels of dissolved iron. Authigenic U is largely hosted by acid-extractable and refractory phases, reflecting sequestration into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Analogously to Mo, authigenic sequestration of U proceeds at two distinct fronts within the SMTZ, which are characterized by shifts in dissolved sulfide concentrations, providing strong evidence for a link between sulfide-producing processes and U reduction. Our results imply that both Mo and U have the potential to capture temporal shifts in bottom water oxygenation indirectly, through the connection between oxygenation and the depth of the SMTZ. Of the two elements, Mo appears a more viable redox proxy because of the substantially higher share of the authigenic pool. However, temporal resolution of these proxies is restricted by the relatively deep authigenic uptake within the sediment column and the integrated character of the signal caused by vertical migrations of the SMTZ. These findings set a framework for interpreting sedimentary Mo- and U-based paleoredox archives in other non-euxinic coastal settings.

How to cite: Jokinen, S., Koho, K., Virtasalo, J., and Jilbert, T.: Sedimentary molybdenum and uranium sequestration in a non-euxinic coastal setting: role of the sulfate-methane transition zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12973, https://doi.org/10.5194/egusphere-egu2020-12973, 2020.

The Cenomanian-Turonian (C/T) Oceanic Anoxic Event 2 (OAE 2) at ~94 million years ago was characterized by severe depletion in marine water oxygen levels and extreme perturbations in the carbon cycle at a global scale that lasted for 5 to 6-million years. However, wealth of the data comes mainly from deep marine records, hugely limiting our understanding on the contemporaneous terrestrial environmental conditions. Here, we present major and trace element concentrations, carbon isotope composition of carbonates (δ13Ccarb) and organic matter (δ13Cbulk), organic carbon content (TOC), and biomarker composition from a ~20 m thick well-preserved shallow marine sequence from the Bagh Beds in Uchad, western India in order to investigate the nutrient dynamics, productivity variations and carbon reservoir perturbations in shallow marine as well as in terrestrial environment. Based on litho-stratigraphy, the Uchad section is divided into Lower Cenomanian, Turonian and Upper Coniacian units. A total of ~5‰ increase in the δ13Ccarb and 0.07% in TOC values and a sharp 1.7‰ decrease in the δ13Cbulk values in Lower Cenomanian suggest large changes in organic carbon recycling before the advent of OAE 2. Higher terrigenous influx and micro-nutrient supply in the lower parts is also suggested from relatively higher concentrations of Al, Ti, Th, Fe, Zn, Ni and K, although their concentrations decrease rapidly above the C/T boundary. Significant correlation observed between δ13Cbulk and δ13Ccarb (r=0.51, p=0.03) supports an authigenic organic matter production in the shallow marine environment. However, minor enrichments in redox-sensitive elements like Mo, V and U observed above the C/T boundary probably suggest that the shallow marine region was relatively less affected during the initial anoxic phases. Lack of correlation between redox-sensitive elements and Al or Ti concentrations (r <0.12) suggest that there is minimal influence of detrital supply on recycling of U, V and Mo. Interestingly, Lower Turonian units show large positive excursions in redox-sensitive elements as well as increases in U/Th, Ni/Co and V/(V+Ni) values, which are succeeded by a major decrease in δ13Ccarb values (7.6‰) and increase in the TOC values by 0.15%, thereby suggesting occurrence of a more expanded episode of anoxia in Lower Turonian that perturbed the shallow marine carbon reservoir. Ba/Al ratios are variable throughout the section, although large positive spikes preceding and succeeding the anoxic phases suggest a causal link between organic matter productivity and anoxia.

How to cite: Sahu, B. R., Roy, S., and Sanyal, P.: Understanding Effects of OAE 2 in the Marginal-marine Environment: A Multi-proxy approach from Bagh Beds, Western India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-964, https://doi.org/10.5194/egusphere-egu2020-964, 2020.

OS3.5 – Effects of Anthropogenic Pressure on Marine Ecosystems and Biogeochemistry

EGU2020-14807 | Displays | OS3.5

To burn or not to burn? Impact of in-situ oil burning by-products on marine plankton: A mesocosm experimental approach

Iordanis Magiopoulos, Christos Chantzaras, Katerina Symiakaki, Eleftheria Antoniou, Christina Pavloudi, Filomena Romano, Giorgos Piperakis, Giulio Zanaroli, Nikolaos Kalogerakis, and Paraskevi Pitta

In-situ oil burning (isOB) is one of the oil-spill mitigation measures and has been used after some major oil-spill events such as the Deepwater Horizon oil spill in the Gulf of Mexico. Although the ecological impact of oil spills and mitigation measures on the marine ecosystem are of great interest, the toxicity and biodegradation potential of isOB by-products have been poorly addressed and mainly are an unknown.

We investigated the effects of burned oil residue and soot deposition on the marine plankton communities of the oligotrophic Eastern Mediterranean Sea using a mesocosm experimental approach. Coastal water (collected at 300 m from the coast, north of Crete, Greece) was incubated in 3.5 m3 mesocosms for 26 days. Three different treatments in triplicates were tested. In one treatment, 2 L of Iranian Crude Oil were added and burned (Burned treatment) while soot was collected and deposited in the form of artificial rain in more mesocosms (Soot treatment) using a custom-designed soot collection apparatus. The third treatment served as the non-contaminated control (Control treatment). Samples were collected at 9 time points (from Day 0 to Day 26) and the plankton, from viruses to micro-plankton, was studied using flow cytometry and inverted microscopy.  

Although the abundance of prokaryotes was slightly decreased in Burned and Soot treatments compared to the Control, the percentage of active heterotrophic bacteria and their relative size (based on their cytometric characteristics) was higher, which is an indication of increased bacterial activity in the contaminated treatments. Viral to Prokaryote Ratio and pico/nano-eukaryotic abundance were significantly increased in the Burned treatment compared to the Control, which could explain the lower bacterial abundance, despite the estimated increased activity, in the Burned treatment. Also, ciliate abundance was significantly lower in the Burned treatment while the opposite was observed in the Soot treatment (up to three orders of magnitude difference between Burned and Soot) compared to Control. Moreover, soot deposits seem to have had a positive effect on the abundance of dinoflagellates and diatoms.

To our knowledge, this is the first experiment to study the effects of isOB on whole marine plankton communities. It is evident that the in-situ oil burning has a significant effect on the plankton communities not only at the event site but also on distant areas through the soot deposition.     

How to cite: Magiopoulos, I., Chantzaras, C., Symiakaki, K., Antoniou, E., Pavloudi, C., Romano, F., Piperakis, G., Zanaroli, G., Kalogerakis, N., and Pitta, P.: To burn or not to burn? Impact of in-situ oil burning by-products on marine plankton: A mesocosm experimental approach , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14807, https://doi.org/10.5194/egusphere-egu2020-14807, 2020.

The coastal systems are among the most dynamic natural systems, being extreme complex zones in which chemical, physical and biological processes interact at different spatial and temporal scales. A holistic approach, based on the integration of multiple monitoring tools for data collection (i.e. satellite imagery, numerical models and in situ observations), may provide different information about coastal ecosystems, at different spatial and temporal scales. Of course, none of these tools is perfect, being each characterized by intrinsic errors and therefore specific uncertainty, the latter also considered as an important subject of investigation.

In this context, our goal is to understand the spatial and temporal distribution of phytoplanktonic biomass in coastal waters in order to evaluate the phytoplankton dynamics in a polluted coastal area located in the northern Tyrrhenian Sea. Long-term high-resolution observations (weekly sampling from 2015 to 2017) of phytoplankton biomass at a coastal site from the C-CEMS observing system (central Tyrrhenian Sea offshore Civitavecchia) are presented, discussed and integrated with the analysis data provided by the Copernicus Marine Environment Monitoring Services (CMEMS) for the Mediterranean Sea, generated by the MedBFM model system, and with satellite observations (from CMEMS Ocean Colour database). The focus of this work is twofold: on one side, to analyse the phytoplankton bloom dynamics of the Civitavecchia coastal ecosystem by adopting a multi-platform approach which integrates CMEMS products and C-CEMS in situ data, on the other side, to propose best practices to integrate multi-platform data streams that may be adopted also in other similar contexts of coastal ecosystems.

The analysis of the time series of phytoplankton provided by in situ, satellite and model data show the typical dynamics of temperate climate, characterized by spring and autumn blooms, together with a significant interannual variability. The EOF analysis has shown consistency among multi-platform datasets. Notwithstanding the incongruences, specifically related to the chlorophyll model outputs, which underestimate the in situ and satellite data and that may be related to some representativeness error (i.e. river nutrient inputs based on climatological information and grid resolution), the intercomparison is beneficial to provide information at different temporal and spatial scales of the phytoplankton dynamics.

How to cite: Salon, S., Martellucci, R., and Cossarini, G.: Proposed best practices for the integration of in situ observations and large scale analysis and forecast systems: case study of the phytoplankton blooming off a Tyrrhenian coastal site, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14041, https://doi.org/10.5194/egusphere-egu2020-14041, 2020.

Natural climate variability influences phytoplankton community both directly and indirectly by altering ocean stratification and availabilities of nutrient and light, or grazing pressure. The world’s largest ecosystem, North Pacific Subtropical Gyre (NPSG), is largely controlled by basin-scale decadal climate variability, such as the North Pacific Gyre Oscillation and the Pacific Decadal Oscillation. These indices have two phases, known as warm phase and cool phase, respectively. Previous studies reported that warm phase was related to the dominance of pico-phytoplankton induced by warm temperature anomaly (i.e., strong stratification) while cool phase was related to the dominance of nano-phytoplankton induced by cold temperature anomaly (i.e., weak stratification). Besides the impact of natural climate variability, anthropogenic global warming has accelerated in recent years and it might have abnormal impact on marine ecosystems. However, there is little information about the responses of phytoplankton community to recent climate change in the NPSG. Here, we present the temporal variations of deseasonalized and normalized NPSG phytoplankton community using phytoplankton pigment concentrations and cell densities, obtained on monthly intervals over the period 1988−2018 at Station ALOHA (22°45’N, 158°W). These variations were compared with the variations of climate indices, physical, and biogeochemical parameters from Station ALOHA. The NPSG climate indices showed five phase transitions; warm (~1997) – cool (1998−2002) – warm (2003−2006) – cool (2007−2013) – warm (2014~). Before 2006 year, the phase transitions of phytoplankton community (pico→nano→pico) were coincident with physical factors (e.g., stratification; strong→weak→strong) and biogeochemical factors (e.g., particle export; low→high→low), coupling with phases of climate indices. However, interestingly, following the recent rapid rise in greenhouse gas emission (since 2007), phytoplankton community, even under continued coupling of climate indices and physical factors, showed only dominance of pico-phytoplankton, decoupling with the phases of climate indices. These findings suggest that the contribution of pico-sized plankton to NPSG phytoplankton community will increase gradually in response to the acceleration of the global warming.

How to cite: Yoon, J.-E. and Kim, I.-N.: Climate-driven phytoplankton community shifts in the North Pacific Subtropical Gyre, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6754, https://doi.org/10.5194/egusphere-egu2020-6754, 2020.

EGU2020-2306 | Displays | OS3.5

Spatio-temporal distribution of nekton community structure and diversity change in Hangzhou Bay, CHINA

dongrong Zhang, lihong Chen, and guodong Jia

The stock of nekton resources and spatiotemporal variation of community in the gulf of estuary area are closely related to the quality of habitat and ecological environment (e.g., habitat degradation, freshwater input, eutrophication, and water exchange). However, their relative importance in shaping the spatial heterogeneity of nekton in bay of China estuary remains rarely reported. In this study, the spatiotemporal dynamics of the nekton assemblage structure were investigated in Hangzhou Bay (HZB), a semi-enclosed bay of China estuary. It was based on a comprehensive survey constituting 40 collections from 10 stations over 4 seasons within a year. A wide taxonomic diversity was encountered during the nekton survey, including 57 taxa distributed among 23 families. Average nekton biomass was significantly higher in winter which dominated by the fish biomass than in summer with the shrimp biomass had the higher proportion. The average nekton abundance was significantly higher in warm seasons with the especially high abundance of shrimp than in cold seasons which the abundance of shrimp relatively low. Nekton of HZB were mainly composed of small near-shore species. Little differences between subtropical (27 species) and warm-temperate (30 species) species of nekton were surveyed on the basis of thermal adaptability, but the seasonal variation was obvious. The number of demersal species in warm seasons (54 species) was higher than that in cold seasons (43 species). The dominant species (IRI>1000) were fishes such as Miichthys miiuy, Collichthys lucidus and shrimps such as Exopalaemon annandalei, Palaemon gravieri and crabs such as Portunus trituberculatus. The biomass diversity index (H'), evenness index (J') and richness index (d) of nekton in HZB were higher in summer and autumn (warm seasons) than in winter and spring (cold seasons). Nevertheless, the abundance diversity index (H') and evenness index (J') of nekton were showed opposite seasonal variation characteristics. The seasonal alternate index (AI) and migrate index (MI) of nekton community varied from 94 to 218 and -73 to 35 respectively, which meant that the nekton community in the survey waters were in large variation and unstable comfortably. The survey stations could be divided into 3~4 groups in the 55%~65% similarity levels by the clustering and NMDS sequencing analysis of nekton during four seasons. Important environmental correlates of assemblage structure were identified using redundancy analysis (RDA). Strong physical gradients in salinity, water temperature, dissolved oxygen and depth correlated predominantly with nekton assemblage structure, and reflected substantial spatiotemporal variation. And chemical variables like DIP and Chla were also highly correlated with nekton community structure. Estuarine embayments in the central bay of China, like Hangzhou Bay, might be viewed appropriately as landwardmost sections of the wider, highly productive spawning and nursery grounds of this region with the good fisheries monitoring and management strategies adapted to the needs of ecosystems and national conditions.

How to cite: Zhang, D., Chen, L., and Jia, G.: Spatio-temporal distribution of nekton community structure and diversity change in Hangzhou Bay, CHINA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2306, https://doi.org/10.5194/egusphere-egu2020-2306, 2020.

The Antarctic toothfish (Dissostichus mawsoni), commonly known as Chilean Sea Bass, has a critical role in Southern ecosystems as a top fish predator. Simultaneously, it represents the most lucrative Antarctic fishery.

Its fishery is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), which introduced the world’s largest Marine Protected Area (MPA) in the Ross Sea region of the Southern Ocean in 2016.

Since 2013, scientists at the Alfred Wegener Institute in Germany have been proposing the creation of an even more expansive MPA in the Weddell Sea region of the Southern Ocean, in order to protect unique ecosystems in this region, which has largely escaped the exploitation seen in the Ross Sea, due to its historic inaccessibility. However, CCAMLR, whose 25-member country composition functions by consensus, has failed to arrive at unanimous support for the various forms of a Weddell Sea MPA (WSMPA) proposed over recent years.

A remaining impediment to the design and acceptance of a WSMPA, is a near total lack of knowledge of the life history and population structure of Antarctica toothfish in the Weddell Sea. Much of the data regarding connectivity and ontogenic movement of Antarctic toothfish derive from the Ross Sea, given the presence of an active fishery there since 1997. Based on the hypotheses that have arisen from the Ross Sea (which remain contentious), a possible life cycle of Antarctic toothfish comprises juvenile development on nutrient rich continental shelf areas, followed by passive transport via gyre systems to offshore sea mounts, where spawning occurs, prior to completion of the cycle as fish are passively transported back towards the coast.

The combination of population genetics and otolith chemistry, methodologies which define population structure via metrics of relatedness and provenance respectively, offers the possibility to fill many of the existing knowledge gaps with regards to Antarctic toothfish life history connectivity in the Weddell Sea region of the Southern Ocean. The integration of hydrographic data on water mass movement which informs both the passive transport of Antarctic toothfish at various life stages, as well as the location of important prey sources, is an integral third point of consideration, completing the development of life history connectivity hypotheses testable via the aforementioned metrics.

Tissue samples from the present study derive from otoliths (fish ear bones), which are a standard tissue extract by CCAMLR observers on Antarctic fishing vessels, historically collected for age determination. Otoliths provide both a source of DNA for genetics work, via tissue traces dried on the otolith exterior, as well as a source for chemistry analysis, via trace element analysis of otolith ring layers from the nucleus (earliest) to edge (latest) elemental depositions.

The aim of the present study is to utilize this readily available tissue source (otoliths) in order to apply both aforementioned methodologies, with the ultimate aim to test between hypotheses of single or multiple populations within the Weddell Sea, while also contextualizing those Weddell Sea population(s) within the greater Southern Ocean distribution of Antarctic toothfish.

How to cite: Caccavo, J., Mazzoni, C., and Brey, T.: A multidisciplinary approach to understanding the population structure of an exploited Southern Ocean top predator, the Antarctic toothfish, to improve sustainability and marine spatial planning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-894, https://doi.org/10.5194/egusphere-egu2020-894, 2020.

EGU2020-7082 | Displays | OS3.5

The Rise And Fall Of An Oyster Bed In The German Bight (North Sea) Before The Dawn Of Industrial Fishery

H. Christian Hass, Rune Michaelis, Lasse Sander, Tanja Hausen, and Bernadette Pogoda

The European flat oyster (Ostrea edulis, EFO) has been common in the North Sea at least throughout the late Holocene. Aside from its important ecological function as a reef builder the EFO has been of high economic value ever since man invented commercial fishery. During first half of the 20th century CE the EFO became functionally extinct in the German Bight. It is commonly agreed that industry-style overexploitation using dredges and trawls destroyed the habitats and eventually caused the extirpation of the EFO. Today, the EFO is a severely endangered and protected species.

Recently, an as yet uncharted fossil EFO bed was encountered close to the island Helgoland (German Bight, SE North Sea), that does not reveal any obvious signs of the physical disturbances typical for heavily harvested sites. This suggests possible environmental forcing behind the decline of this particular EFO bed, which could also have contributed to the fall of the entire oyster population of the German Bight.

The area was surveyed using drift videos along with grab samples to locate the EFO bed, measure its perimeter and evaluate characteristics, such as the density of shells. A total of 590 shells from 17 locations were measured (length, width) and weighed. A total of 19 shell samples from 17 locations were AMS radiocarbon dated to obtain absolute age control.

The EFO bed is located on a rocky slope to the west of Helgoland, its area is about 0.6 km2 and the water depth ranges between 32 and 43 m. The shallower parts are characterized by rubble whereas the EFO bed disappears under muddy sediments in the deeper parts. A rough estimation reveals around 200,000,000 single oyster shells in this bed. The oldest shell dated to about 4000 years BP, the youngest age dates to the beginning of the 19th century CE. Most shell ages are between 2700 and 2000 years BP. A hiatus occurs between 1300 and 300 years BP, and only one measurement reveals an age younger than 1300 years.

This EFO bed had perished already 1300 years ago, hence intense fishery as a reason can be ruled out. Today, the deeper part of the EFO bed is affected by muddy sediment that temporally buries the seafloor and occasionally forms turbid clouds of suspended matter. The sediment likely originates from large rivers (e.g. Rhine, Weser, Elbe) and the mudflats of the Wadden Sea. Climatic/oceanographic fluctuations that had modified the discharge and transport of muddy sediments in combination with increased influx of sediment from fluvial sources as a result of land-use changes in early medieval central Europe may have been a major stressor for the oyster habitats, albeit biological stressors cannot be ruled out.

How to cite: Hass, H. C., Michaelis, R., Sander, L., Hausen, T., and Pogoda, B.: The Rise And Fall Of An Oyster Bed In The German Bight (North Sea) Before The Dawn Of Industrial Fishery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7082, https://doi.org/10.5194/egusphere-egu2020-7082, 2020.

EGU2020-19080 | Displays | OS3.5

Impacts of heat stress on the calcifying fluid chemistry and trace element composition of corals from thermally variable reefs

Verena Schoepf, Juan-Pablo D'Olivo, Cyrielle Rigal, Maria Jung, and Malcolm McCulloch

Coral reefs are increasingly threatened by climate change and mass bleaching events. Predicting how corals will respond to rapid ocean warming requires a better understanding of how they have responded to environmental change in the past – information that can be reconstructed from coral skeletal records. However, significant knowledge gaps remain in our understanding of how coral biomineralization and the incorporation of geochemical tracers is impacted by heat stress and bleaching, particularly since the physiological status of corals used for reconstruction of past stress events is often unknown. Using boron-based geochemical tracers (δ11B, B/Ca), we investigated how heat stress caused by a marine heatwave impacted the carbonate chemistry of the coral calcifying fluid as well as skeletal trace element composition in the branching coral Acropora aspera. Importantly, we recorded in situ temperature and coral health status during the bleaching event and after 7 months of recovery. We show that heat-stressed Acropora corals continued to upregulate the pH of their calcifying fluid (cf); however, dissolved inorganic carbon upregulation inside the cf was significantly disrupted by heat stress. Similarly, we observed suppression of the typical seasonality in the temperature proxies Sr/Ca, Mg/Ca, Li/Ca and Li/Mg, likely due to a combination of reduced growth rates, disruption of key enzymes involved in calcification and Rayleigh fractionation. Anomalies in TE/Ca ratios were still observed 7 months after peak bleaching, even though symbiont densities and chlorophyll a concentrations were fully restored at this point. Interestingly, the response to heat stress did not differ between the thermally variable intertidal and the thermally more moderate subtidal environments whose coral populations are known to have a different heat tolerance, nor between colonies with varying degrees of bleaching. Our findings suggest that coral biomineralization mechanisms are highly sensitive to heat stress, and that the biogeochemical stress response of branching Acropora corals is remarkably consistent with that of massive Porites corals.

How to cite: Schoepf, V., D'Olivo, J.-P., Rigal, C., Jung, M., and McCulloch, M.: Impacts of heat stress on the calcifying fluid chemistry and trace element composition of corals from thermally variable reefs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19080, https://doi.org/10.5194/egusphere-egu2020-19080, 2020.

EGU2020-19366 | Displays | OS3.5

Dissolved heavy metal fluxes at sediment-water interface in polluted sediments of the Adriatic Sea

Federico Spagnoli, Rocco De Marco, Giordano Giuliani, Pierluigi Penna, Alessandra Campanelli, Eleni Kaberi, Christina Zeri, Giovanni Bortoluzzi, Patrizia Giordano, Mariangela Ravaioli, and Valter Martinotti

To evaluate the anthropogenic impact in surface sediments of the Northern and Central Adriatic Sea, dissolved fluxes at the sediment water interface of heavy metals and nutrients on the sea bottom in front of the Po River mouths and along the western side of the Northern and Central Adriatic Sea have been determined. The fluxes have been measured by benthic chamber deployments and calculated by pore water modelling. Pore waters composition have been used also to understand the early diagenesis processes generating the benthic fluxes.

Benthic chamber deployments and sediment core collection for pore water extraction have been carried out in three cruises in spring and autumn 2013 and autumn 2014.

The study stations have been chosen on the base of previous research results indicating a decreasing heavy metal and organic matter surface content leaving from the Po River mouths (Pérez-Albaladejo et al., 2016). The data obtained have been compared with previous studies carried in the Adriatic Sea (Spagnoli et al. 2010).

Results of the 2013 and 2014 cruises and of previous investigations indicate a consistent and rapid dissolved benthic flux decreasing going away from the Po River mouths both southward, eastward and northward.

The decreasing regards the final electron acceptors and the organic matter degradation products and some heavy metals.

On the whole, different early diagenesis environments have been recognized in the Northern and Central Adriatic Sea: they embrace two end members: from the Po River Prodelta to the Mid Adriatic Depression (MAD) (Spagnoli et al., 2014). In front of the Po River sediments are characterized by high sedimentation rate and by high inputs of fresh marine organic matter, continental organic matter and Fe-oxyhydroxides. These inputs produce high concentrations of organic matter degradation products, strong anoxic environment in the pore waters and high dissolved benthic fluxes. In the MAD the diagenetic environments are characterized by low sedimentation rate and low inputs of reactive organic matter that produce low concentrations of pore-water organic matter degradation products with oxic conditions near the surface and weak benthic fluxes.

As regard the two major metals involved in the early diagenesis processes (Fe and Mn), they too show dissolved benthic fluxes decreases from the Po River mouths. Also in this case, this trend is attributed to the high Po River dissolved and particulate, anthropogenic and natural, metal inputs that deposit in the surface sediments of the Po Prodelta (Spagnoli and Bergamini, 1997).

The dissolved benthic fluxes of trace heavy metals (Co, Ni, Zn, Cu, Cd, Pb) indicate that some elements, such as Co and Pb, are clearly adsorbed by the sediment that act, for these two elements as sink. Other elements, such as Cu, don’t show a clear north–south trend s, while other elements, such as Cd, indicate a southward decreasing trend suggesting a behavior affected by the Po River inputs and Fe-Mn-oxyhydroxide cycle.

How to cite: Spagnoli, F., De Marco, R., Giuliani, G., Penna, P., Campanelli, A., Kaberi, E., Zeri, C., Bortoluzzi, G., Giordano, P., Ravaioli, M., and Martinotti, V.: Dissolved heavy metal fluxes at sediment-water interface in polluted sediments of the Adriatic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19366, https://doi.org/10.5194/egusphere-egu2020-19366, 2020.

EGU2020-22036 | Displays | OS3.5

Anthropic disturbance to seabed habitats in the Punta Campanella Marine Protected Area, southern Italy

Crescenzo Violante, Francesco Paolo Buonocunto, Eliana Esposito, Luciana Ferraro, Laura Giordano, and Alfonsa Milia

The main goal of this study is to investigate the anthropic influence on benthic habitats in the Punta Campanella Marine Protected Area (MPA). This area is located at the western end of the Sorrento Peninsula in the Bay of Naples, southern Italy. It is a rocky coast consisting of vertical or near vertical limestone cliffs of structural control, with marine areas characterized by suboutcropping rocky substrate. Seabed sediments are mainly coarse and biogenic in origin with skeletal grains and coralligenous bioconstructions occurring widely [1].

The Punta Campanella seabed habitats have been characterized and mapped on the base of geophysical and sedimentological data together with results from benthic communities. In addition, several environmental components both marine and terrestrial have been analysed in order to evaluate the anthropic influence on the recognized benthic habitats. Such environmental components include foraminiferal assemblages, water column features and inorganic pollutants (heavy metals) as well as terrestrial biota, fresh water supply and quality, land use and natural hazard.

First results indicate 1) anomalous values of specific heavy metals (Ni, Hg) in the marine sediments, 2) the presence of benthic foraminiferal assemblages distinctive of human-impacted environmental conditions, and 3) the occurrence of morphological deformities affecting some foraminiferal species. At present, as next step of this study, we are applying a methodology based on the Environmental Functional Analysis (EFA) in order to combine and analyse terrestrial and marine environmental components together with territorial data and selected socio-economic components of the coastal zone (i.e. human pressure, land use, etc.) [2]. This method was originally developed by Cendrero and Fischer (1997) [3] and successively employed as management tool and monitoring technique for coastal areas [4] and terrestrial protected areas [5]. Such a holistic-based approach can be used to evaluate the anthropic disturbance in the Punta Campanella MPA and to compare the potential for conservation and the potential for use of the study area.

References

[1] D'Argenio B., Violante C., Sacchi M., Budillon F., Pappone G., Casciello E., Cesarano M., 2004: Capri, Bocca Piccola and Punta Campanella (southern Italy), marine and onland geology compared. In: G. Pasquarè and C. Venturini (Eds), Mapping Geology in Italy, APAT, Roma, 35-42.

[2] Hopkins, T.S., Bailly, D., Støttrup, J.G., 2011. A Systems Approach Framework for Coastal Zones. Ecol. Soc. 16(4), 25.

[3] Cendrero A., Fischer D.W., 1997: A procedure for assessing the environmental quality of coastal areas for planning and management. Journal of Coastal Research 13(3), 732-744.

[4] Giordano L., Ferraro L., 2020. Conservation or development? An environmental function analysis assessment of the Volturno River coastal zone (central Tyrrhenian Sea - Italy). Journal of Coastal Conservation, 24(6), 5-12.

[5] Calado H., Bragagnolo C., Silva S., Vergílio M., 2016: Adapting environmental function analysis for management of protected areas in small islands e case of Pico Island (the Azores). Journal of Environmental Management 171, 231-242.

How to cite: Violante, C., Buonocunto, F. P., Esposito, E., Ferraro, L., Giordano, L., and Milia, A.: Anthropic disturbance to seabed habitats in the Punta Campanella Marine Protected Area, southern Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22036, https://doi.org/10.5194/egusphere-egu2020-22036, 2020.

Shelf sea ecosystems are amongst the most productive on Earth. A large proportion of the nutrients required to sustain this productivity is supplied via the remineralization of organic material in the underlying sediments. Industrial fisheries trawling at the seafloor, the most intense form of anthropogenic disturbance in shelf seas, has the potential to influence the release of sediment and nutrients from the seafloor and hence shelf-scale primary production. However, the absence of data on the concentrations and composition of the materials resuspended by benthic trawling hinders our ability to robustly assess the wider ecosystem effects of this industrial activity. We addressed this key knowledge gap by conducting the first in-situ experiments to quantify the concentrations of particles and nutrients released by different benthic trawling gears.

Our results demonstrate that the composition of resuspended particles and nutrients are both influenced by sampling height above the seafloor and the amount of drag exerted by the trawl gear, although the relative importance of these factors differs between the two response variables examined. These differences likely reflect that sediment particles adhere to physical laws of a solid in a fluid with associated weights, while dissolved nutrients follow the physical laws of fluids and dilution.  

Our findings demonstrate that trawl gear design strongly influences the amount of dissolved and particulate material resuspended. This suggests that gear design could be modified to reduce the impacts on shelf ecosystems. Future work will upscale our observations to assess the potential effects of benthic trawling activities on primary production at the shelf-scale.

How to cite: Breimann, S., O'Neill, B., Stinchcombe, M., and Mayor, D.: The level of hydrodynamic drag as a main component in explaining concentrations of nutrients and particle loads at different heights of a fisheries trawl plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5521, https://doi.org/10.5194/egusphere-egu2020-5521, 2020.

EGU2020-6794 | Displays | OS3.5

Polymetallic nodules are essential for food-web integrity of Pacific abyssal plains

Tanja Stratmann, David Amptmeijer, Daniel Kersken, Karline Soetaert, and Dick van Oevelen

The abyssal seafloor is at some locations covered with polymetallic nodules that provide hard substrate for sessile organisms. Extraction of these mineral-rich nodules will likely severely modify the trophic and non-trophic interactions within the abyssal food web, but the importance of nodules and their associated sessile fauna in supporting this food web remains unclear. Here, we present highly resolved interaction webs with ~200 (Peru Basin) and ~450 (Clarion-Clipperton Zone, CCZ) food-web compartments based on an extensive literature research. Compartments were connected with ~3,100 (Peru Basin) and ~8,500 (CCZ) trophic and non-trophic (e.g. substrate-providing nodules) links. The webs were used to assess how nodule extraction would modify the number of network compartments, number of links, link density and web connectance. We showed that nodule removal would reduce the number of food-web compartments and links by ~25% and ~35%, respectively, in the Peru Basin and by 21% and 20%, respectively, in the CCZ. Subsequent analysis identified stalked sponges, living attached to the nodules, as key structural species that support a high diversity of commensal and mutualistic fauna. We conclude that nodules are critical for food-web integrity and suggest the deployment of artificial sponge stalks as a potential mitigation strategy for deep-sea mining.

How to cite: Stratmann, T., Amptmeijer, D., Kersken, D., Soetaert, K., and van Oevelen, D.: Polymetallic nodules are essential for food-web integrity of Pacific abyssal plains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6794, https://doi.org/10.5194/egusphere-egu2020-6794, 2020.

EGU2020-22362 | Displays | OS3.5

Addressing the effects of bottom trawling on benthic processes using experimental and field studies in the Baltic Sea

Claudia Morys, Martin Jakobsson, Mattias Sköld, Pere Masqué, Volker Brüchert, Stefano Bonaglia, and Clare Bradshaw

Bottom trawling is one of the most important anthropogenic disturbances affecting marine ecosystems and there has been increased attention to its impacts on seabed habitats as well as the structure and functioning of benthic ecosystems. The impact of bottom trawling is well-known with regard to benthic organisms. However, we still have a poor understanding of its effects on bentho-pelagic coupling and biogeochemical cycling in the sediment. In the Baltic Sea, the study area of the present investigation, there is a particular lack of data.
Here, we present new results from field experiments to quantify changes in sediment properties, macrofauna and biogeochemical cycling after the passage of a benthic dredge. To put the results in a broader context, a field survey was conducted in six areas of different commercial trawling intensities in the Bornholm Basin. Acoustic geophysical mapping, isotope profiling, functional categorization of macrofauna and sediment-water nutrient and oxygen flux measurements were used to evaluate the physical disturbance of the seabed. Preliminary results suggest a range of ecological, biogeochemical and physical impacts of trawling in the Baltic Sea, with implications for benthic ecosystem functioning.

How to cite: Morys, C., Jakobsson, M., Sköld, M., Masqué, P., Brüchert, V., Bonaglia, S., and Bradshaw, C.: Addressing the effects of bottom trawling on benthic processes using experimental and field studies in the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22362, https://doi.org/10.5194/egusphere-egu2020-22362, 2020.

For several decades the Belgian Continental Shelf (BCS) has been exploited for its sand. As this exploitation intensified over the years, according with the environmental regulation both on national and European levels, evaluating the extraction impact on the seabed bathymetry and morphology as well as on the sediment nature itself is a legal obligation.

The impact assessment of sand extraction is based on multiple types of data: statistics derived from the extraction registers, data from the Electronic Monitoring Systems (EMS = “black-boxes”) on board the dredging vessels, bathymetric and backscatter time series derived from regular MBES surveys, and ground truth data. In this framework, the most extracted areas have been surveyed several times each year for more than 15 years, enabling the development of well-controlled highly valuable and informative time series. In addition to this local approach, regular but less frequent surveys along straight lines, parallel and perpendicular to the sandbanks and the gullies, provide valuable information on the global evolution of the bathymetry and the sediment allowing a comparison between extracted and non-extracted areas. This multi-scale approach combining various types of data provides a 4D (space and time) overview of the evolution of the extraction and leads to robust and pragmatic conclusions about the impact of the sand extraction on the bathymetry, the morphology and the seabed habitats.

The long MBES bathymetric and BS time series on several monitoring areas inside the extraction sectors demonstrate the direct and non cumulative impact of the extraction on the bathymetry of the sandbanks and the unsustainable character of the sand resource. However, several questions remain regarding how the extraction impacts the seabed morphology and sedimentology in real time. Dredging by suction generates fine sediment plumes which could, after transport and sedimentation, modify the habitats within a certain radius around the extraction sites. Although the plumes generated during dredging operations have been the subject of numerous publications, few projects have been attempted to visualize these plumes and quantify the volume of fine sediment by using MBES water column amplitude data.

Specific series of acoustic measurements using the Kongsberg EM2040 MBES installed on the RV Simon Stevin were carried out following dredging vessels on the Belgian Continental Shelf. The resulting high quality dataset allows the evaluation of the real time impact of the extraction on the seabed and the water column in a 4D visualization. Another goal of this research is to evaluate the feasibility to use the MBES water column amplitude data to characterize and quantify the sediment plumes generated by the dredging operation. In a second experiment the scope was extended to plumes from fishing vessels and an evaluation of the impact of tidal currents on the water column measurements. The results of the simultaneous measurements with several acoustic and optical instruments and water sampling will contribute to the establishment of a methodology which will extend the present monitoring program to include the important impact on the seabed from the extraction technique itself.

How to cite: Degrendele, K. and Roche, M.: Monitoring the impact of sand extraction on the bathy-morphology and the seabed sediment in the Belgian part of the North Sea: Lessons of fifteen years of MBES measurements and current innovation. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10428, https://doi.org/10.5194/egusphere-egu2020-10428, 2020.

EGU2020-3729 | Displays | OS3.5

Impact of simulated deep-sea polymetallic nodule mining on sediment and pore-water geochemistry in prospective mining areas in the NE Pacific Ocean

Jessica Volz, Laura Haffert, Matthias Haeckel, Andrea Koschinsky, and Sabine Kasten

Industrial-scale harvesting of deep-sea mineral resources, such as polymetallic nodules, is likely to have severe consequences for the natural environment. However, the effects of mining activities on deep‑sea ecosystems, sediment geochemistry and element fluxes are still poorly understood. Predicting the environmental impact is challenging due to the scarcity of environmental baseline studies and the lack of mining trials with industrial mining equipment in the deep sea. Thus, currently we have to rely on small-scale disturbances simulating deep-sea mining activities as a first-order approximation to study the expected impacts on the abyssal environment and ecosystem.

We have investigated surface sediments in disturbance tracks of seven small-scale benthic impact experiments, which have been performed in four European contract areas for the exploration of polymetallic nodules in the Clarion-Clipperton Zone (CCZ) in the NE Pacific Ocean. These small-scale disturbance experiments were performed 1 day to 37 years prior to our sampling program in the German, Polish, Belgian and French contract areas using different disturbance devices, such as dredges and epibenthic sledges. We show that the depth distribution of solid-phase Mn in the upper 20 cm of the sediments in the CCZ provides a reliable tool for the determination of the disturbance depth. We found that the upper 5–15 cm of the sediments were removed during various small‑scale disturbance experiments in the different contract areas. Transient transport‑reaction modelling for the Polish and German contract areas reveals that the removal of the surface sediments is associated with the loss of reactive labile organic carbon. As a result, oxygen consumption rates decrease significantly after the removal of the surface sediments, and consequently, oxygen penetrates up to tenfold deeper into the sediments inhibiting denitrification and Mn(IV) reduction. Our model results show that the return to steady state geochemical conditions after the disturbance is controlled by diffusion until the reactive labile TOC fraction in the surface sediments is partly re‑established and the biogeochemical processes commence. While the re-establishment of bioturbation is essential, steady state geochemical conditions are ultimately controlled by the burial rate of organic matter. Hence, under current depositional conditions, new steady state geochemical conditions in the sediments of the CCZ are reached only on a millennium-scale even for these small-scale disturbances simulating deep-sea mining activities.

How to cite: Volz, J., Haffert, L., Haeckel, M., Koschinsky, A., and Kasten, S.: Impact of simulated deep-sea polymetallic nodule mining on sediment and pore-water geochemistry in prospective mining areas in the NE Pacific Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3729, https://doi.org/10.5194/egusphere-egu2020-3729, 2020.

EGU2020-978 | Displays | OS3.5

Natural vs. trawling-derived transport of sediment and particulate organic matter in a submarine canyon

Sarah Paradis, Marta Arjona-Camas, Miguel Goñi, Pere Masqué, and Pere Puig

Deep bottom trawling often occurs in the vicinities of submarine canyons since these morphological features act as nursery areas for commercial species. Previous studies in the submarine canyons incising the NW Mediterranean margin have highlighted that bottom trawling resuspends large volumes of sediment which are partly transported downcanyon as sediment gravity flows. To assess the contribution of downward particle fluxes in La Fonera Canyon (NW Mediterranean) linked to natural sediment transport events and bottom trawling, a near-bottom mooring equipped with a 24-cup sediment trap, a current meter, and a turbidimeter was deployed during 2017 in its axis (1200 m water depth), next to a trawling ground. Temporal variations in the quantity and composition of trapped particulate organic matter were assessed through the analysis of organic carbon (OC), total nitrogen (TN) and several biomarkers (lignins, cutin acids, p-hydroxybenzenes, benzoic acids, amino acid-derived products, dicarboxylic acids, and fatty acids).

High downward particle fluxes (60-100 g·m-2·d-1) were registered in autumn and winter associated to torrential river discharges, seasonal storms and dense shelf water cascading. During these natural events, sediment transported downcanyon had high organic matter contents that were mostly terrigenous in origin. However, the highest downward particle flux (>140 g·m-2·d-1) was recorded in the onset of the bottom trawling season in March, after a 2-month seasonal trawling closure. During the following summer months no major natural sediment transport events occurred, but the high frequency of bottom trawling activities (10-26 hauls·week-1) near the sediment trap caused considerably high downward particulate fluxes (80-125 g·m-2·d-1) during this season. Compared to autumn and winter months, sediment transferred downcanyon caused by trawling had lower organic matter contents, mostly consisting in refractory compounds (i.e. lignins, p-hydroxybenzenes and benzoic acids) with similar concentrations to those observed in the bottom sediments of the trawling grounds, confirming that this material originates from these areas. During periods with less trawling activity, lower sediment fluxes (30-50 g·m-2·d-1) with higher organic matter contents enriched in labile compounds (i.e. amino acid-derived products, di-carboxylic acids, and fatty acids) were recorded. These results highlight how bottom trawling activities on the flanks of submarine canyons modify the supply of sediment and organic matter downcanyon. The low-quality of organic matter transferred by bottom trawling activities may ultimately affect the fragile ecosystems dwelling in these deep environments.

How to cite: Paradis, S., Arjona-Camas, M., Goñi, M., Masqué, P., and Puig, P.: Natural vs. trawling-derived transport of sediment and particulate organic matter in a submarine canyon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-978, https://doi.org/10.5194/egusphere-egu2020-978, 2020.

EGU2020-5084 | Displays | OS3.5

Impact of marine sand extraction on benthic communities west off Sylt (SE North Sea)

Finn Mielck, Rune Michaelis, Werner Armonies, and H. Christian Hass

Ongoing erosion at coasts, beaches and dunes accompanied by a climate change-induced sea-level rise requires extensive protection measures. At the Island of Sylt (SE North Sea) beach nourishments were conducted for almost 50 years to protect the exposed western coast against erosion. Since 1984, the materials for the sand replenishments were dredged from an offshore excavation site approx. 7 km west off Sylt in the German Bight. In this study, we investigate the long-term effects of sand extraction on the local geomorphology, the associated benthic habitats and fauna. Hydroacoustic surveys and grab sampling revealed that after more than 35 years changes in bathymetry (with dredging pits of down to ~15 m below sea floor) and also changes in habitat characteristics are still present. Additionally, the sediment and benthic faunal compositions have changed. A comparison between dredged areas and undisturbed seafloor revealed significant differences in mud content (increasing), the number of individuals and species of macrozoobenthic organisms (decreasing). This indicates that the benthic communities in the dredging areas are in a persistent successional stage. Mud-loving species (e. g. Notomastus latericeus and Kurtiella bidentate) profit from the changed habitats, however sand-preferring organisms (e.g. Pisione remota and Aonides paucibranchiata) largely disappeared. Because of the slow backfill rates, we conclude that a complete backfill of the deep dredging pits is likely to take centuries. The same is expected to apply for the regeneration of the benthic communities. However, since rather coarse-to-medium sand was removed from this area and re-accumulation of this Pleistocene material is not possible because of weak transport rates, a re-establishment of benthic communities that prefer coarser sand seems to be unlikely. Since benthic communities are strongly linked to the habitat characteristics, habitat mapping using hydroacoustic techniques is an efficient and cost-effective measure to monitor the state of regeneration in this study site.

How to cite: Mielck, F., Michaelis, R., Armonies, W., and Hass, H. C.: Impact of marine sand extraction on benthic communities west off Sylt (SE North Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5084, https://doi.org/10.5194/egusphere-egu2020-5084, 2020.

EGU2020-9804 | Displays | OS3.5

Impacts of bottom trawling on benthic trophic status and meiofauna in a deep-sea sedimentary environment (Gulf of Castellammare, southwestern Mediterranean Sea)

Antonio Pusceddu, Sarah Paradis, Davide Moccia, Pere Puig, Pere Masque, Tommaso Russo, Maria Carmela Meloni, and Claudio Lo Iacono

The impacts of bottom trawling on the structure of benthic communities can be relatively non-selective, hitting biodiversity as a whole. This holds true also in the deep sea, where the impacts of trawling can be more severe and long-lasting than in shallow-waters, due to the reduced capacity for recovery and greater vulnerability of deep-sea organisms. For years, our knowledge of the impact of trawling on deep-water ecosystems has remained limited and has focused mainly on fish stocks and hard bottom systems. More recently, a number of studies have addressed the impacts of bottom trawling in the deep-sea sedimentary environments, and very few of them have focused on the impacts on meiofauna, though it is a key faunal component of deep-sea ecosystems.

We investigated the impact of bottom trawling on the quantity, biochemical composition and nutritional value of sedimentary organic matter and meiofauna along the Sicilian Margin (Gulf of Castellammare, southwestern Mediterranean) at ca. 550 m depth, during the summer of 2016. Amount, biochemical composition and freshness of sedimentary OM, as well as the abundance and community composition of meiofauna were determined in sediment cores taken at both trawled and untrawled grounds. The continuous erosive processes in the trawled site have led, generally, to the depletion of OM contents (20-60% lower than those in the untrawled site), as well as to statistically significant differences from the untrawled site in its biochemical composition. Nevertheless, the upper 2 cm of the trawled site consisted of recently accumulated sediments, enriched in phytopigments, and bulk OM contents similar to those in the untrawled one, interpreted as a very recent input of fresh OM from the upper water column. The abundance of meiofauna in trawled grounds was significantly higher than that in untrawled ones, whereas no differences were observed between trawled and untrawled grounds deeper in the sediment. Differences in the meiofaunal community composition among sediment layers in each site were larger than those among sites.

As previously reported, deep bottom trawling in the Gulf of Castellammare erodes large volumes of sediment, exposing old compacted sediment that is depleted in OM. This erosive action generally prevents the accumulation of fresh sediment. However, the episodic short-lived deposition of fresh organic detritus between hauls can lead to a temporary accumulation of fresh and bioavailable OM which, in turn, can induce a positive response in meiofauna abundance.

These results pinpoint the need of considering the impacts of bottom trawling on the benthic communities of deep-sea sedimentary environments at temporal scales shorter than previously done.

How to cite: Pusceddu, A., Paradis, S., Moccia, D., Puig, P., Masque, P., Russo, T., Meloni, M. C., and Lo Iacono, C.: Impacts of bottom trawling on benthic trophic status and meiofauna in a deep-sea sedimentary environment (Gulf of Castellammare, southwestern Mediterranean Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9804, https://doi.org/10.5194/egusphere-egu2020-9804, 2020.

EGU2020-13789 | Displays | OS3.5

Impact of deep-sea polymetallic nodule mining on benthic microbial community and mediated biogeochemical functions

Massimiliano Molari, Tobias R. Vonnahme, Felix Janssen, Frank Wenzhöfer, Matthias Haeckel, Jürgen Tischack, and Antje Boetius

Industrial-scale mining of deep-sea polymetallic nodules will remove nodules in large areas and impact the physical integrity of the seafloor. However, environmental standards for seafloor integrity and studies of recovery from environmental impacts are still largely missing. Further we have only a poor understanding of the role of nodules in shaping benthic microbial diversity and element cycles. We revisited the deep-sea disturbance and recolonization experiment carried out with a towed plough harrow in 1989 in the Peru Basin nodule field within a circular area of approx. 3.5 km diameter (>4100 m water depth). In the experimental area, the 26 years old plough tracks were still visible and showed different types and levels of disturbance such as removal and compaction of surface sediments. Microbial communities and their diversity were studied in disturbance tracks and undisturbed sites and related to habitat integrity, remineralization rates, and carbon flow. Locally, microbial activity was reduced up to 4 times in the impacted areas. Microbial cell numbers were reduced by ~50% in fresh, and by <30% in the old tracks. Our data suggest that microbially-mediated biogeochemical functions need more than 50 years to return to undisturbed levels in the sediments. In areas with nodules (i.e., outside the disturbance tracks) microbial communities in the nodules themselves were studied. Nodule communities were distinct from sediments and showed a lower diversity and a higher proportion of sequences related to potential metal-cycling bacteria (i.e. Magnetospiraceae, Hyphomicrobiaceae), bacterial and archaeal nitrifiers (i.e. AqS1, unclassified Nitrosomonadaceae, Nitrosopumilus, Nitrospina, Nitrospira), as well as bacterial sequences typically found in ocean crust, hydrothermal deposits and sessile fauna. Our results confirm that nodules host specific microbial communities with potentially significant contributions to organic carbon remineralization and metal cycling. This study contributes to developing environmental standards for deep-sea mining and highlights the limits for maintaining and recovering ecological integrity and functions during large-scale nodule mining.

How to cite: Molari, M., Vonnahme, T. R., Janssen, F., Wenzhöfer, F., Haeckel, M., Tischack, J., and Boetius, A.: Impact of deep-sea polymetallic nodule mining on benthic microbial community and mediated biogeochemical functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13789, https://doi.org/10.5194/egusphere-egu2020-13789, 2020.

EGU2020-7710 | Displays | OS3.5

Monitoring environmental effects of a deep-sea mining test in shallow water

Henko de Stigter, Sabine Haalboom, Christian Mohn, Thomas Vandorpe, Marck Smit, Laurens de Jonge, and Gert-Jan Reichart

Concerns about future access to strategic raw materials for the high-tech industry have led to a renewed interest in mining of mineral resources from the deep-sea as a potential alternative for land-based mining. Polymetallic nodules, especially abundant in the eastern equatorial Pacific Ocean in water depths of 4000-6000 m, are a likely target of future deep-sea mining. However, many questions exist about the environmental sustainability of deep-sea mining, as it would involve the removal of hard substrate, disturbance of the surface sediment layer and dispersion of mobilised sediment over large areas of seabed adjacent to the mining sites. Anticipating on full-scale industrial mining tests, which are likely to start in the near future in the deep Pacific Ocean, we tried approaches for environmental monitoring of mining activities during two industry field tests in relatively shallow water offshore southern Spain, carried out in the framework of the European Blue Nodules project. The aim of these field tests was to assess technical and environmental performance of a scaled polymetallic nodule mining vehicle developed by the Dutch shipbuilder and maritime technology provider Royal IHC. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude around 5-10 cm s-1, and gently sloping seabed covered with fine muddy sediment are fairly comparable to operational conditions in the deep-sea. The plume of suspended sediment mobilised by the mining vehicle, considered to represent a major environmental pressure which may extend far beyond the actual mining area, was monitored with turbidity sensors deployed with ship-operated ROV and CTD, as well as in a static array of moored sensors. It was found that the generated sediment plume extended not more than 2 m above the seabed close to the disturbance (< 100 m), but increased in height with distance away from the disturbance site. Turbidity decreased rapidly with increasing distance from the source, but a distinct signal could still be distinguished above background turbidity at 350 m away from the source. In this near-coast setting, plume monitoring suffered significant interference by bottom trawling activities in neighbouring areas. The monitoring setup proved to be well designed and the findings on the plume size and dispersion can be significantly extrapolated to account for a more realistic mining situation. Seabed surveys with ROV-based video and scanning sonar showed that the tracks of the test vehicle, exerting an average pressure of 3 kPa on the seabed, left impressions of 4±0.8 cm deep in the surface sediment. In sediment cores collected from the path of the vehicle, geotechnical testing showed an increase in undrained shear strength and bearing capacity, as compared to undisturbed sites, indicating compaction of the surface sediment. Surveys revealed ubiquitous signs of bottom trawling, including furrows of approximately 10 cm deep produced by trawl doors. 

How to cite: de Stigter, H., Haalboom, S., Mohn, C., Vandorpe, T., Smit, M., de Jonge, L., and Reichart, G.-J.: Monitoring environmental effects of a deep-sea mining test in shallow water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7710, https://doi.org/10.5194/egusphere-egu2020-7710, 2020.

EGU2020-4653 | Displays | OS3.5

Time series of near-bottom currents and particle fluxes from Ocean Bottom Moorings in the NE equatorial Pacific

Martina Hollstein, Annemiek Vink, Katja Schmidt, Niko Lahajnar, Andreas Lückge, and Kaveh Purkiani

The globally increasing demand for metals and rare earth elements has raised the interest for potential mining of deep-sea mineral resources such as polymetallic nodules. One important field of polymetallic nodules is located within the Clarion-Clipperton Fracture Zone (CCZ) in the northeastern equatorial Pacific. To date, the International Seabed Authority (ISA) has granted 25 licenses for the exploration of polymetallic nodules in the CCZ. However, the impact of potential future mining activities on the deep-sea environment is only insufficiently known. To assess the environmental impacts of potential future mining activities, a nodule pre-prototype collector test is scheduled to occur in the German license area within the CCZ in autumn 2020, and will be accompanied by an extensive environmental monitoring program (joint effort between BGR and the European research project JPI-Oceans “MiningImpact2”). However, to assess the environmental impact of mining activities, for example due to the development of an operational sediment plume on the seafloor, prior knowledge on the bottom current regime and variability of particle flux and composition within the CCZ under natural conditions is a prerequisite. In order to analyze the bottom current regime and background particle fluxes, BGR deployed Ocean Bottom Moorings (OBM) equipped with current and turbidity meters (4 years between 2013 and 2019), and a sediment trap (2018-2019). Here, we present preliminary results and analyses of these oceanographic and sedimentological time-series data, and compare the results with other available information deriving from the region.

How to cite: Hollstein, M., Vink, A., Schmidt, K., Lahajnar, N., Lückge, A., and Purkiani, K.: Time series of near-bottom currents and particle fluxes from Ocean Bottom Moorings in the NE equatorial Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4653, https://doi.org/10.5194/egusphere-egu2020-4653, 2020.

EGU2020-5682 | Displays | OS3.5

Assessing the Influences of Large-Scale Disturbance on Sedimentary Marine Carbon Stores

Kirsty Black, William Austin, and Joanna Norkko

Shelf and coastal seas hold vast quantities of sedimentary carbon which contribute to atmospheric carbon dioxide removal and long-term carbon storage. However, the stability and resilience of this key component of global natural capital remains poorly quantified, particularly under anthropogenic stressors. Demersal trawling activity is the most significant cause of widespread anthropogenic disturbance to the seabed, leading to massive sediment resuspension events and wide scale impact to benthic communities. The impacts of trawling on benthic ecosystems and biodiversity are well reported and understood within the literature (e.g. Jones, 1992; Rijnsdorp et al., 2016); however, a knowledge gap remains regarding the post-trawl fate of sedimentary carbon (van de Velde et al., 2018).

In order to gain a better understanding of the post-disturbance effects of carbon cycling in marine sediments, an experimental trial to mimic fishing impacts was created. Over a 21-day period, a series of closed-tank incubation experiments investigating the impact of simulated benthic fishing gear penetration depth in soft sediments was conducted. Here, marine sediments underwent an artificial disturbance event every 24 hours, with a series of varying depth regimes used. We hypothesise that the large-scale resuspension events caused by trawling may contribute towards an enhancement in localised carbon cycling, and thus a reduction in the net carbon storage within these sediments. The aim of this experiment was to better understand the biogeochemical processes which occur in marine sediments during massive resuspension events, with a particular emphasis on the fate of resuspended organic carbon matter and its potential vulnerability. Dissolved organic carbon and various macronutrients of interest (e.g. PO4, SiO2, NH4, NO2, NO3) were also measured.

Jones, J.B., 1992. Environmental impact of trawling on the sea bed: a review. New Zeal. J. Mar. Freshw. Res. 26, 59–67. https://doi.org/10.1080/00288330.1992.9516500org/10.1080/00288330.1992.9516500

Rijnsdorp, A.D., Bastardie, F., Bolam, S.G., Buhl-Mortensen, L., Eigaard, O.R., Hamon, K.G., Hiddink, J.G., Hintzen, N.T., Ivanović, A., Kenny, A., Laffargue, P., Nielsen, J.R., O’Neill, F.G., Piet, G.J., Polet, H., Sala, A., Smith, C., Van Denderen, P.D., Van Kooten, T., Zengin, M., 2016. Towards a framework for the quantitative assessment of trawling impact on the seabed and benthic ecosystem. ICES J. Mar. Sci. 73, i127–i138. https://doi.org/10.1093/icesjms/fsv207

van de Velde, S., Van Lancker, V., Hidalgo-Martinez, S., Berelson, W.M., Meysman, F.J.R., 2018. Anthropogenic disturbance keeps the coastal seafloor biogeochemistry in a transient state. Sci. Rep. 8. https://doi.org/10.1038/s41598-018-23925-y

How to cite: Black, K., Austin, W., and Norkko, J.: Assessing the Influences of Large-Scale Disturbance on Sedimentary Marine Carbon Stores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5682, https://doi.org/10.5194/egusphere-egu2020-5682, 2020.

EGU2020-1946 | Displays | OS3.5

Monitoring coastal pollution by using an integrated low-cost device

Daniele Piazzolla, Andrea Terribili, Simone Bonamano, Sergio Scanu, Marco Marcelli, Giancarlo Della Ventura, Alessandra Conte, Federico Lucci, and Augusto Marcelli

Coasts are extremely sensitive areas and are internationally considered “hotspot” of environmental contamination. The presence of multiple human activities in these areas frequently lead to the potential increase in organic and inorganic pollutants. In particular, industrial and maritime activities, tourism, recreational activities, aquaculture and fishing contribute to the pollutants release in the coastal environments. In this context, northern Latium coastal area (northern Thyrrenian Sea, Italy) hosts several industrial activities of national and international relevance, located in a very restricted seaside area: the Port of Civitavecchia, one of the most important hub for cruise and commercial traffic in the Mediterranean Sea, the Torrevaldaliga Nord coal-fired power plant of the national energy company (ENEL), and the Tirreno Power combined cycle (gas-fueled) power plant. All these activities strongly contribute to the increase of pollutant load to the land as well as marine coastal environment. For this reason, a research project aimed at understanding the main source for the pollution has been undertaken in the last years. The project is particularly aimed at designing and testing of reliable low-cost devices (Gozzi et al., 2015, 2017) able to provide both the amount and typology of solid particles spread in the environment.

As a first step, the air quality inside the Civitavecchia harbor has been monitored for six months by measuring the content of PM1, PM2.5, and PM10 simultaneously to environmental parameters such as air temperature and humidity. The sensing station (Della Ventura et al., 2017) was equipped with a filtering set-up able to collect the solid load in the atmosphere with dimension > 400 nm. The filters were periodically removed from the station and studied by combining microscopic (optical and electron), spectroscopic (IR, Raman) and microchemical (SEM-EDS) techniques for a full characterization of microparticles typologies. Collected information, augmented by environmental (wind, rain) data from local broadcasting stations provides a valuable tool for assessing the contribution of anthropic (industrial and maritime) activities to the pollution in this coastal area.

 

References

Gozzi, F., Della Ventura, G., Marcelli, A. (2015) Mobile monitoring of particulate matter: State of art and perspectives. Atmospheric Pollution Research, 7, 228-234. DOI:10.1016/j.apr.2015.09.007.

Gozzi, F., Della Ventura, G., Marcelli, A., Lucci, F. (2017) Current status of particulate matter pollution in Europe and future perspectives: a review. Journal of Materials and Environmental Science, 8, 1901-1909. ISSN: 2028-2508

Della Ventura, G., Gozzi, F., Marcelli, A. (2017) The MIAMI project: design and testing of an IoT low-cost device for mobile monitoring of PM and gaseous pollutants. Superstripe Press, Science Series, 12, 41-44, ISBN 9788866830764.

How to cite: Piazzolla, D., Terribili, A., Bonamano, S., Scanu, S., Marcelli, M., Della Ventura, G., Conte, A., Lucci, F., and Marcelli, A.: Monitoring coastal pollution by using an integrated low-cost device, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1946, https://doi.org/10.5194/egusphere-egu2020-1946, 2020.

EGU2020-6746 | Displays | OS3.5

Low-Cost Underwater Sound Recorder for coastal applications.

Valentina Cafaro, Andrea Terribili, Lorenzo Pasculli, Viviana Piermattei, Marco Marcelli, and Walter MX Zimmer

Sound is the most widespread and pervasive kind of anthropogenic energy that human activities introduce into the marine environment. Sound energy input can be highly variable both in time and space, becoming an important part of the total ocean acoustic background. Moreover, the underwater sound plays an ecologically important role in marine ecosystems, being a critical sensory modality for many marine organisms that can be useful for both sensing the environment and communication. With the Marine Strategy Framework Directive (MSFD) (2008/56/EC, EU 2008), underwater noise has been recognized as pollution and included in the qualitative high-level descriptors to achieve good environmental status, GES.

During recent years, passive acoustic monitoring in the ocean has become a standard technique across the oceanographic community and is used to address biological, geological and meteorological issues. Due to the highly spatio-temporal variability of the ocean noise, a large number of the observing systems would be needed. Extended marine monitoring would require a reduction in the cost of platforms and instruments, without compromising data quality. Despite, a significant effort has been invested by the scientific community in the development of low-cost PAM recorders, much work still remains. Most of the problems are related to the pressure to which the devices are exposed, the battery pack limits, storage memory limits, and sensibility of the sound sensor once waterproofing and so on.

Here, we present a low-cost underwater sound recorder for coastal applications developed to be applied in both background noise monitoring and bioacoustic monitoring. This recorder consists of a high-performance USB-based microcontroller development system with an audio adapter that guarantees high audio quality. Additionally, test were conducted using both an ECM (Electret Condenser Microphone) and a MEMS microphone (Micro-Electro-Mechanical System) for a wide frequency range recordings to find the better solutions for good data quality. Compact and small in size, it can be easily installed on various oceanographic platforms for different types of sampling.

Here we present the first results of the laboratory and field tests, comparing our assembled device with a commercial recorder and a pre-calibrated hydrophone.  

How to cite: Cafaro, V., Terribili, A., Pasculli, L., Piermattei, V., Marcelli, M., and Zimmer, W. M.: Low-Cost Underwater Sound Recorder for coastal applications., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6746, https://doi.org/10.5194/egusphere-egu2020-6746, 2020.

EGU2020-10405 | Displays | OS3.5 | Highlight

TT-MARINE: a new open modular cost-effective board to monitor coastal ecosystems

Viviana Piermattei, Marco Marcelli, Valentina Cafaro, Alice Madonia, Andrea Terribili, and Riccardo Valentini

The coastal marine system is characterized by multiple uses and it represents a vulnerable area highly subjected to anthropogenic pressures. Coastal marine ecosystems monitoring therefore requires an integrated multidisciplinary approach. The modeling of marine coastal dynamics and processes and the development of new observational technologies are fundamental in order to increase the available amount of data needed for the application of integrated approaches. New technologies and coastal observation networks are therefore a priority of the Global Ocean Observation System (GOOS) and of the Agenda 2030 strategy to improve the sustainable management of marine ecosystems and to contribute to future climate change scenarios. In this context a big effort is carried out by existing observing programs (ARGO, DPCP, GO-SHIP, OceanSITES, SOOP), which focus on open ocean waters and do not cover coastal areas. To do this, a significant reduction in the costs of platforms and instruments is necessary while maintaining sufficient measurement precision and consequently data quality. To face this issue, an Arduino based technology has been developed starting from the Tree-Talker-Cloud Technology (TT-Cloud board), a data acquisition and transmission system to monitor the health of trees and the impacts of climate change. From this technology, a new low-cost board, TT-Marine, has been developed, characterized by a high modularity allowing to manage the sensors by different types of communication protocols: RS232, UART, i2c, RS485; analog sensors can be managed by 16 and 24 bit AD converters. Depending on the characteristics and opportunities of the site, the system can manage LoRa, WiFi, gprs/gsm or cable data transmission systems. The TT-Marine is designed to be used in different modes: autonomous, ship-like as a profiler, on buoys and other measuring platforms.Here we present several operating modalities, with different missions and instrumental configurations.

How to cite: Piermattei, V., Marcelli, M., Cafaro, V., Madonia, A., Terribili, A., and Valentini, R.: TT-MARINE: a new open modular cost-effective board to monitor coastal ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10405, https://doi.org/10.5194/egusphere-egu2020-10405, 2020.

EGU2020-11920 | Displays | OS3.5 | Highlight

An innovative ASV for the monitoring of anthropogenic pressure on Wetlands

Angelo Odetti, Gabriele Bruzzone, Marco Bibuli, Roberta Ferretti, Enrica Zereik, and Massimo Caccia

This paper describes the technological development of an innovative Autonomous Surface Vehicle designed to meet the requirement of accessing the extremely shallow waters peculiar of the Wetlands. Wetlands are those geographic areas where water meets the earth and cover between 5 and 8 % of the Earth's surface. Wetlands include mangrove zones, swamps, bogs and marshes, rivers and lakes, alluvial plains and flooded forests, shallow coasts and coral reefs. In recent years, their importance is becoming more and more recognized and various international conventions, directives and projects work on the protection of these fundamental ecosystems. Wetlands are considered among the world’s most productive environments for their biological diversity. Nevertheless the number, quality and spacial resolution of surveys in these peculiar environments is reduced due to the absence of suitable tools expressly addressed to work in these areas. This reduces the possibilities of monitoring and protecting the Wetlands. In this paper the first prototype of an innovative class of reliable modular re-configurable lightweight ASVs for extremely shallow water and remote areas applications is presented. SWAMP (Shallow Water Autonomous Multipurpose Platform) is a fully electric, modular, portable, lightweight, and highly-controllable Autonomous Surface Vehicle (ASV). It is a catamaran equipped with azimuth pump-jet actuators, and it is characterized by small draft soft-foam, unsinkable hull structure with high modularity and a flexible hardware/software architecture. The main advantage of pumpjet motors is that they are flush with the hull, thus minimizing the risks of damages due to possible grounding. This system is used to increase the manoeuvrability in narrow spaces and to increase the special resolution also in extremely shallow waters. The introduction of a soft hull structure is suitable for working in coastal areas and in riverine environment where impacts can affect the survival of the robot. The foam of the hulls is drilled in order to make SWAMP a completely modular catamaran that is able to host various types of tools, such as intelligent systems, samplers, and sensors, together with thrusters that are, in this way, protected by the foam. A preliminary study is shown related to the bathymetry carried on with a single beam sonar in a Ligurian river after a series of floods. Liguria is one of the European regions where extreme events related to anthropic changes have had the greatest number of negative effects. In this kind of areas the use of suitable robots can improve the assessment and monitoring the impact of anthropogenic pressure in wetland ecosystems.

How to cite: Odetti, A., Bruzzone, G., Bibuli, M., Ferretti, R., Zereik, E., and Caccia, M.: An innovative ASV for the monitoring of anthropogenic pressure on Wetlands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11920, https://doi.org/10.5194/egusphere-egu2020-11920, 2020.

Plastic pollution is a globally concerning issue in marine environments. There is currently little research about the seasonal changes in microplastics in coastal areas. Here, we report a seasonal study on the concentrations and characteristics of microplastics in the surface seawater and zooplankton of Jiaozhou Bay, a typical bay in the west Yellow Sea. The concentrations of microplastics in the surface water of Jiaozhou Bay were 0.063, 0.174, 0.094, and 0.050 pieces/m3 in February, May, August and November, respectively, with an annual average concentration of 0.095 pieces/m3, a low value compared with the plastic concentrations of other coastal areas. The size of the collected microplastics ranged from 346 to 155200 μm, with an average of 5093 μm. The overall percentages of fibers, fragments and plastic foams were 29%, 55% and 16%, respectively. Fragments were the most dominant shape in four seasons. Nine plastic polymers were detected from the surface water of Jiaozhou Bay. The dominant chemical composition was polypropylene (PP), accounting for 51.04% of polymers, followed by polyethylene (PE), accounting for 26.04% of polymers. The seasonal variation of plastic characteristics in Jiaozhou Bay, including the shape, color and chemical composition, was significant. The highest concentration of plastics occurred in May and the lowest concentration of plastics occurred in November. Strong rainfall resulted in an increase in the plastic concentration in May, and winds and eddies affected the spatial distribution of plastics in Jiaozhou Bay. Focused on the dominant zooplankton groups in Jiaozhou Bay, the morphology, color, size, chemical composition and quantity of MPs in zooplankton were investigated in Jiaozhou Bay. The results showed that the MPs in zooplankton of the Jiaozhou Bay were dominated by fibers. The proportions of fiber in February, May, August and November were 91%, 88%, 89% and 88%, respectively. The average size of MPs in zooplankton was 441±2, 468±2, 576±2, and 379±4μm in the four seasons. For the 2 common zooplankton groups in the 4 seasons, the MP/zooplankton was 0.3, 0.26, 0.17, 0.19 for copepod, and 0.22, 0.19, 0.17, 0.45 for chaetognath, respectively.

How to cite: Sun, X.: Seasonal variation of microplastics in seawater and zooplankton in Jiaozhou Bay, the Yellow Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3892, https://doi.org/10.5194/egusphere-egu2020-3892, 2020.

EGU2020-7438 | Displays | OS3.5

Development and application of Phyto-VFP model (Variable Fluorescence Phytoplankton Production) to estimate primary production in highly vulnerable marine pelagic ecosystems

Simone Bonamano, Alice Madonia, Antonio De Luca, Luigi Lazzara, Silvia Becagli, Viviana Piermattei, and Marco Marcelli

How to cite: Bonamano, S., Madonia, A., De Luca, A., Lazzara, L., Becagli, S., Piermattei, V., and Marcelli, M.: Development and application of Phyto-VFP model (Variable Fluorescence Phytoplankton Production) to estimate primary production in highly vulnerable marine pelagic ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7438, https://doi.org/10.5194/egusphere-egu2020-7438, 2020.

EGU2020-15329 | Displays | OS3.5

Synoptic representation of P.oceanica ecosystem services in the Italian seas

Sergio Scanu, Simone Mellini, Daniele Piazzolla, Simone Bonamano, Emanuele Mancini, and Francesco Manfredi Frattarelli

This work analyzes and quantifies the value of ecosystem services in the P.oceanica meadows of the Italian seas, defining methodological approaches and creating synoptic maps through the use of GIS. Ecosystem Services can be defined as benefits provided to mankind by natural ecosystems. Their contribution is essential for human progress and of fundamental importance in the long run.

Posidonia oceanica was chosen as the object of study because its meadows represent one of the Mediterranean "climax community". P. oceanica is, therefore, one of the most important ecosystem in the Mediterranean and has been indicated as "priority habitat" according to the Habitat Directive (Dir. N. 92/43 / EEC), which groups together all the Sites of Community Importance (SCI) that need to be protected.

The method of evaluating the ecosystem services for P.oceanica is derived from what reported in Costanza et al. (1997) applying the specific site approach for the definition of benefits and services (Marcelli et al. 2018).

The identified benefits for P.oceanica are carbon sequestration, oxygen production, erosion protection, bioremediation and food production.

P.oceanica data are organized from the dataset collected by the Italian Institute for the Protection of the Environment and Research (ISPRA) for the Marine Strategy Framework Directive and include parameters such as coverage and shoots number (m2), average leaf area, leaf area index, average number of leaves, average height of the rhizomes, average foliar and rhizomes production. The data were used for the calculation of the benefits of P. oceanica which are represented in synoptic maps through GIS with the creation of the Atlas of the values ​​of ecosystem services in the Italian seas.

How to cite: Scanu, S., Mellini, S., Piazzolla, D., Bonamano, S., Mancini, E., and Frattarelli, F. M.: Synoptic representation of P.oceanica ecosystem services in the Italian seas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15329, https://doi.org/10.5194/egusphere-egu2020-15329, 2020.

EGU2020-17439 | Displays | OS3.5

Evaluation of the effects of Caulerpa cylindracea on Posidonia oceanica through the analysis of primary production and morphometric characteristics

Marco Boschi, Giulia Caporale, Lorenzo Pasculli, Daniele Piazzolla, and Emanuele Mancini

Today, biological invasions represent a threat to endemic animal and plant communities and a major cause of biodiversity loss worldwide. In the Mediterranean Sea, there are about 100 species of macrophytes, introduced intentionally or accidentally, most of which are highly invasive. Among these, the macroalga Caulerpa cylindracea Sonder, 1845, entered in the Mediterranean basin since 1990 through the Suez Canal and now it is widespread along the Italian coasts. This species is able to colonize a high number of coastal substrates and it can affect the density of some seagrasses, such as Cymodocea nodosa (Ucria) Ascherson, 1870 and Posidonia oceanica (L.) Delile, 1813. Its colonization ability is enhanced in environments with a high concentration of nutrients and its growth can modify the redox potential of the substrate making it unsuitable for the establishment of other seagrasses and algae. This work aimed to analyse and describe the potential interaction between the C. racemosa and P. oceanica in the coastal area of Civitavecchia. The potential effects of this interaction were studied inside of two different P. oceanica patches, located at a depth of 3-5 m and characterized by the presence/absence of the invasive alga, through the morphostructural analysis of the two species. In particular, the seagrass growth and primary production were analysed using some direct and indirect techniques (phenology and lepidochronology), while for the alga were analysed the phenological characteristics and the percentage of coverage of the substrate. The sampling campaigns were carried out in two different months of the same year, June and October 2019, in order to observe both the growth phase and the maximum bloom phase of the C. racemosa.

How to cite: Boschi, M., Caporale, G., Pasculli, L., Piazzolla, D., and Mancini, E.: Evaluation of the effects of Caulerpa cylindracea on Posidonia oceanica through the analysis of primary production and morphometric characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17439, https://doi.org/10.5194/egusphere-egu2020-17439, 2020.

Adélie penguin (Pygoscelis adeliae) are known as an eco-indicator species of the Southern Ocean marine ecosystem. Environmental variability drives penguin population dynamics through its effects on vital rates (e.g., survival, dispersal, or breeding success). Recent studies suggested that inter-annual variation strongly affected Adélie penguin abundance in individual colonies, and aggregating abundance across space provided more reliable trends of population dynamics. By considering the similarity of the regional sea-ice concentration, we divided the Ross Sea region into six areas to investigate the effects of environmental changes on penguin population dynamics from 1982–2013. Time lagged analysis of 0–6 years between penguin abundance and environmental factors were conducted in our study. We found that penguin abundance was significantly correlated with environmental factors at different lag times (p <0.05). In the western Ross Sea region, penguin abundance was positively correlated with temperature over the past 5 years (p <0.05), and positively correlated with sea-ice concentration at a lag of 4–6 years (p <0.05). In the northernmost region of the Ross Sea, penguin abundance was significantly correlated with chlorophyll concentration 4 years earlier (p <0.01). Generalized additive model (GAM) results showed that in mid-Victoria Land, the relationship between sea-ice concentration and penguin abundance was quadratic. Penguin abundance peaked when sea-ice concentration was approximately 40%. On Franklin Island, temperature positively affected penguin abundance when temperature was lower than -3°C, and the contribution decreased considerably when temperature was higher than -2.5°C. Optimal ranges of environmental factors for Adélie penguin population might exist and differ spatially in the Ross Sea. Our study highlighted the lagged response of penguin population to environmental factors to further understand the effects of climate changes on the Antarctic biosphere.

How to cite: Chen, X. and Li, X.: Lagged response of Adélie penguin population dynamics to environmental variability in the Ross Sea, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21001, https://doi.org/10.5194/egusphere-egu2020-21001, 2020.

The barrier reef lagoon of Noumea (New Caledonia) is potentially influenced by Ni and black carbon (BC) rich aerosols (the two latter originating from a Ni power plant).  Experiments were performed with lagoon water collected close to the reef crest to assess the effect of anthropogenic influences on bacterial abundance (BA), production (BP), respiration (BR), growth efficiency (BGE) and community composition (BCC) in artificial produced aggregates. In both treatments, BA were not affected or enhanced compared to the unamended control (up to 40% for BA and 180% for VA). BP was not or negatively affected (up to 70%). No clear trend was found for BR in the Ni treatment; in the BC treatment BR was enhanced by 63-69%. BGE was reduced in both treatments. The strongest effects on BCC profiles as determined by 16S rDNA denaturing gradient gel electrophoresis were due to incubation time treatment particularly for BC. A phylotype corresponding to a specific BC band was closely related to Acinetobacter oleivorans DR1. Thus, this experimental study confirms potential anthropogenic influences on the bacterial community on aggregates in the Bay of Noumea.

How to cite: Weinbauer, M.: Ni and black carbon influence bacterial production, respiration and community composition in the barrier reef Lagoon of Noumea (New Caledonia): an experimental study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8867, https://doi.org/10.5194/egusphere-egu2020-8867, 2020.

Heavy metals are considered to be among the most potent environmental contaminants, and their release into the environment is increasing rapidly since the last decades from various sources and activities and may enter into the environment by a wide range of pathways and processes. They can be translocated, disperse in the environment, and bio-concentrate in aquatic organisms, causing increase ecosystem degradation and leading to biodiversity loss. Furthermore, they may enter the food chain, creating health risks for both humans and animals. This study aimed to evaluate the ecotoxicological effects of anthropogenic pressure in semi-enclosed artificial lagoon ecosystems in the Gulf of Aqaba, Red Sea. Our findings with regard to heavy metal contamination showed that the area is contaminated by significant amounts of several potentially toxic heavy metals (such as Cd, Cr, Cu, and Fe). The anthropic intervention in the area impacted heavily the natural environment. We found that the biological test (lipid peroxidation) was a useful assay for assessing the overall health condition and response (stress level) towards natural and anthropogenic forces in the studied areas. The selected marine organisms (Holothuria atra, Tripneustes gratilla, Ulva lactuca and Halophila stipulacea) have the ability to accumulate several levels of heavy metals in their tissue with different trends of bioaccumulation. Therefore, they can be used as promising bioindicators for such research. The results obtained permit to assess the environmental effects of anthropogenic pressure and can be a useful basis for planning possible remediation projects.

How to cite: Wahsha, M. A. and Al-Najjar, T.: Ecotoxicological effects of anthropogenic pressure in semi-enclosed artificial lagoon ecosystems in the Gulf of Aqaba, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16480, https://doi.org/10.5194/egusphere-egu2020-16480, 2020.

EGU2020-21954 | Displays | OS3.5

Multivariate tools to investigate the occurrence of pollutants in a highly anthropised marine area

francesco paolo buonocunto, nicola cardellicchio, antonella di leo, eliana esposito, luciana ferraro, santina giandomenico, alfonsa milia, lucia spada, crescenzo violante, and matilda mali

Environmental monitoring of highly anthropised areas demands for deep survey of different environmental compartments (water, sediment, soil and biota) and determination of numerous bio-geo-chemical parameters, due to the huge impact of natural and anthropogenic organic substances constantly released into these environments. However, the interpretation of the large amount of data is not straightforward task due to their complexity that require a very tricky elaboration especially for the decision making processes. Chemo-metrics tools based on the multivariate statistical data analysis seems to be a powerful tool in addressing such complexity (Mali et al., 2017). In this work they are performed to get insight the occurrence of organic pollutants within a highly populated area such as the Gulf of Naples.

The Gulfs of Naples, located along the Eastern Tyrrhenian Sea, is semi-enclosed by the Ischia-Procida Islands to NW, Campi Flegrei and the Campanian Plain in NE, the Sorrento peninsula in SE, and Capri island in W. The Campania region has one of the highest population densities in Italy. The Gulfs of Naples are the receiving environment for persistent toxic substances from the Campania Plain (Albanese et al., 2010; Arienzo et al., 2017). Montuori and Triassi (2012) reported that the discharges of PAHs from the Sarno River to the Gulf of Naples is approximately 8530 g/d. The main goal of this work is to establish the influence of the Campania Plain on the present sedimentation in the Naples bay continental shelf by evaluating organic matter contribution and pollution. For this purpose, superficial sediment samples collected from 158 sites located offshore the Gulf of Naples between Sarno River and Capri Island, were analyzed for total nitrogen and phosphorus, total organic carbon (TOC), grain size, metals, priority polycyclic aromatic hydrocarbons (PAHs), total petroleum hydrocarbon (TPHs), volatile organic compounds (VOCs), polychlorinated biphenyls (PCBs), pesticides and organotin compounds (OTs). The adopted multivariate approach allowed, through a clear spatial representation of score plots, a deep dive into the large dataset generated by the investigation campaign, highlighting the influence of some main factors controlling the contamination pattern, such as organic matter content and depositional environment.

References

Albanese, S., De Vivo, B., Lima, A., Cicchella, D., Civitillo, D., Cosenza, A., 2010. Geochemical baselines and risk assessment of the Bagnoli brownfield site coastal sea sediments (Naples, Italy). J. Geochem. Explor. 105, 19–33.

Arienzo, M., Donadio, C., Mangoni, O., Bolinesi, F., Stanislao, C., Trifuoggi, M., Toscanesi, M., Di Natale, G., Ferrara, L., 2017. Characterization and source apportionment of polycyclic aromatic hydrocarbons (pahs) in the sediments of gulf of Pozzuoli (Campania, Italy). Mar. Pollut. Bull. 124, 480–487.

Mali, M., Dell'Anna, M.M., Notarnicola, M., Damiani, L., Mastrorilli, P., 2017. Combining chemometric tools for assessing hazard sources and factors acting simultaneously in contaminated areas. Case study: "Mar Piccolo" Taranto (South Italy). Chemosphere 184, 784-794.

Montuori, P., Triassi, M., 2012. Polycyclic aromatic hydrocarbons loads into the Mediterranean Sea: estimate of Sarno River inputs. Mar. Pollut. Bull. 64, 512–520.

How to cite: buonocunto, F. P., cardellicchio, N., di leo, A., esposito, E., ferraro, L., giandomenico, S., milia, A., spada, L., violante, C., and mali, M.: Multivariate tools to investigate the occurrence of pollutants in a highly anthropised marine area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21954, https://doi.org/10.5194/egusphere-egu2020-21954, 2020.

EGU2020-22276 | Displays | OS3.5

Assessment of cumulative impacts and coastal ecosystem health in the JiaoZhou Bay

Ziyuan Hu, Murray Logan, XiaoXia Sun, Richard Brinkman, and Song Sun

Coastal areas are under compounding pressures from urbanization, industrialization, infrastructure growth, and aquaculture.  There is hence an urgent need for developing solutions for coastal pollution and ecosystem safety. In this work, based on our long-term and multidisciplinary ecosystem monitoring data, we develop tools to translate the observing data into management information to sustainable coastal use and development. From the existing data and experimental studies, we develop approaches to understand key processes and factors controlling coastal ecosystems and to define thresholds and guidelines values of ecological parameters to determine. With focus on marine ecosystem health assessment, we use the integrating data to describe ecosystem condition, its potential trend and the impact of existing pressures. This present study initially focused on the coastal area of the Yellow Sea, in the JIAOZHOU Bay. The technique of the structured research can be applied to other coastal regions as well to understand how these ecosystems respond to local and global pressures.

How to cite: Hu, Z., Logan, M., Sun, X., Brinkman, R., and Sun, S.: Assessment of cumulative impacts and coastal ecosystem health in the JiaoZhou Bay, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22276, https://doi.org/10.5194/egusphere-egu2020-22276, 2020.

EGU2020-22316 | Displays | OS3.5

Major threats of pollution and climate change to global coastal ecosystems and enhanced management for sustainability

Yonglong Lu, Jingjing Yuan, Xiaotian Lu, Chao Su, Yueqing Zhang, Yuan Quan, and Wushuang Xie

Coastal zone is of great importance in the provision of various valuable ecosystem services. However, it is also sensitive and vulnerable to environmental changes due to high human populations and interactions between the land and ocean. Major threats of pollution from over enrichment of nutrients, increasing metals and persistent organic pollutants (POPs), and climate change have led to severe ecological degradation in the coastal zone, while few studies have focused on the combined impacts of pollution and climate change on the coastal ecosystems at the global level. A global overview of nutrients, metals, POPs, and major environmental changes due to climate change and their impacts on coastal ecosystems was carried out in this study. Coasts of the Eastern Atlantic and Western Pacific were hotspots of concentrations of several pollutants, and mostly affected by warming climate. These hotspots shared the same features of large populations, heavy industry and (semi-) closed sea. Estimation of coastal ocean capital, integrated management of land-ocean interaction in the coastal zone, enhancement of integrated global observation system, and coastal ecosystem-based management can play effective roles in promoting sustainable management of coastal marine ecosystems. Enhanced management from the perspective of mitigating pollution and climate change was proposed.

How to cite: Lu, Y., Yuan, J., Lu, X., Su, C., Zhang, Y., Quan, Y., and Xie, W.: Major threats of pollution and climate change to global coastal ecosystems and enhanced management for sustainability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22316, https://doi.org/10.5194/egusphere-egu2020-22316, 2020.

EGU2020-19136 | Displays | OS3.5

Will climate change impacts the biogeochemical cycles of essential micronutrients?

Camille Richon and Alessandro Tagliabue

Trace metals are important micronutrient for marine microbial life. They support many metabolic reactions necessary for bacteria, phytoplankton and zooplankton, however, some trace metals such as Cu or Zn may have toxic effects at high concentrations. The processes shaping trace metals distribution and cycling in the ocean are still poorly understood. Furthermore, the evolution of trace metals distributions as a consequence of climate change has never been investigated.

This work aims at characterizing the potential impacts of climate change on the global biogeochemical cycling of essential trace metals. We use a state-of-the-art global ocean biogeochemical model representing a suite of trace elements (Fe, Cu, Zn, Co, Mn) to investigate how their distribution and inventories may change in response to the RCP8.5 climate change scenario. The changing ocean circulation in this scenario may result in surface stratification, resulting in changes in biological pump strength, oxygen distribution and particle loadings. In regions affected by external inputs, surface concentration of micronutrient may increase as a result of surface stratification. On the other hand, surface micronutrient concentrations may decrease in the open ocean as a result of the decline in vertical supply of nutrients. We use an extended Redfield ratio calculation in order to characterise the changes in trace metal concentrations and highlight the regions where trace metal cycling is the most affected by climate change. Results show that cobalt and manganese concentrations display important evolutions in the Arctic whereas zinc is mostly impacted in the Southern Ocean. Our results suggest that changes to the ocean physics and productivity will impact trace metals differently depending on their role and biogeochemical drivers.

 

 

How to cite: Richon, C. and Tagliabue, A.: Will climate change impacts the biogeochemical cycles of essential micronutrients?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19136, https://doi.org/10.5194/egusphere-egu2020-19136, 2020.

Artificial fish reefs are the underwater structures placed on the ocean floor to simulate some characteristics of natural reefs. The onshore current can be transformed into upwelling under the influence of artificial fish reefs, thus the nutrient at the bottom of the near shore can be raised, which increases the prey of plankton and fish yield. In order to investigate this phenomenon, a 3D large eddy simulation (LES) of the ocean boundary layer was combined with four different types of artificial fish reef terrains (square, convex-fan, isosceles right triangle, concave-fan). In the near surface, almost only the square terrain can uplift the nutrient, which brings about the most uniform nutrient distribution. Based on the size of integral values of nutrient concentration in the upper part of the four reefs, they are listed as follows: square terrain, convex-fan terrain, isosceles right triangle terrain, concave-fan terrain decreases (from largest to smallest). What is more, the integral values of the four terrains reduce exponentially. Because the nutrient flow encounters the square terrain’s vertical plane, it has a larger vertical velocity. Nevertheless, for convex-fan terrain and isosceles right triangle terrain, their slopes are smoothly, resulting in poor lifting effect. Meanwhile, compared with the other three types of terrains, the concave-fan terrain can prevent the overflow of nutrients better. Among those four reefs, it can be found the square-shaped artificial fish reef is the best one for uplifting the nutrient.

How to cite: Wang, Z. and Li, S.: A large eddy simulation study of the inshore nutrient uplift process: The role of topography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12913, https://doi.org/10.5194/egusphere-egu2020-12913, 2020.

OS3.8 – Exploring marine biogeochemical cycles: organic matter and pollutants

EGU2020-10736 | Displays | OS3.8 | Highlight | OS Division Outstanding ECS Lecture

Controls over the export flux of marine snow into the deep ocean

Frederic Le Moigne

The oceanic biological carbon pump (BCP) regulates the Earth carbon cycle by transporting part of the photosynthetically fixed CO2 into the deep ocean. Suppressing this mechanism would result in an important increase of atmospheric CO2 level. The BCP occurs mainly in the form of organic carbon particles (POC) sinking out the surface ocean. Various types of particles are produced in surface ocean. They all differ in production, sinking and decomposition rates, vertically and horizontally. The amount of POC transported to depths via these various export pathways as well as their decomposition pathways all have different ecological origins and therefore may response differently to climate change. Here I will briefly review some of the processes driving both particle export out of the euphotic zone (0-100m) as well as particles transport within the mesopelagic zone (100-1000m). In the early 2000s, strong correlations between POC and mineral (calcite an opal) fluxes observed in the deep ocean have inspired the inclusion of “ballast effect” parameterizations in carbon cycle models. These relationships were first considered as being universal. However global analysis of POC and mineral ballast fluxes showed that mineral ballasting is important in regions like the high-latitude North Atlantic but that in most places (some of which efficiently exporting) the unballasted fraction often dominates the export flux. In such regions, we later on showed that zooplankton-mediated export (presence of faecal pellets) and surface microbial abundance were important drivers of the efficiency of particles export. Similar trends were found globally by including bacteria and zooplankton abundances to a global reanalysis of the global variations of the POC export efficiency. This implies that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting the strength of the biological carbon pump. Further down in the water column (mesopelagic zone), processes impacting the transport of particles are less clear. Sinking particles experience a number of biotic and abiotic transformations during their descent. These includes solubilization, remineralisation, fragmentation, ingestion/active transport, break down among others. While some potential factors such as O2 concentration and temperature have been proposed as powerful controls, globally evidences are often inconsistent. Current challenges related to the role of particles consumption by zooplankton and fishes as well as the role of particles attached prokaryotes (bacteria and archaea) in setting the efficiency of the carbon transport in the mesopelagic zone will be discussed.

How to cite: Le Moigne, F.: Controls over the export flux of marine snow into the deep ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10736, https://doi.org/10.5194/egusphere-egu2020-10736, 2020.

EGU2020-19041 | Displays | OS3.8

Phytoplankton communities influence the dissolved organic matter composition of the sea-surface microlayer

Lea Oeljeschlaeger, Nils Hintz, Jutta Niggemann, Oliver Wurl, and Thorsten Dittmar

The sea surface microlayer (SML) is the boundary layer at the ocean and atmosphere interface and plays a crucial role in air-sea gas exchange processes and global climate. It is enriched in dissolved organic matter (DOM) compared to the underlying water, but the chemical composition of this material has been insufficiently studied. For improved understanding of the exchange processes it is of utmost importance knowing the molecular composition of the SML. Studying the microlayer is very challenging due to its thinness and strong influence of external forces as wind, UV light and atmospheric deposition on the chemical and microbial composition. The complex and dynamic nature of the microlayer and the enrichment of hydrophobic substances led to the assumption that we find unique chemical composition and distinct compound groups. SML samples of the Indo-Pacific Ocean from R/V Falkor cruise FK161010 (October 2016) were studied with respect to molecular composition of DOM. We analyzed solid-phase extracted DOM with high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The results were compared to the underlying water (ULW, 1m depth). We found similar molecular DOM composition in the ULW, whereas microlayer extracts were more variable and diverse. This can be related to the influence of changing weather conditions during the cruise on the SML. To reveal molecular changes without interfering external forces, a 5-week indoor mesocosm experiment with induced marine phytoplankton blooms was conducted. A modified solid-phase extraction approach was used to chemically fractionate the microlayer DOM prior to molecular analysis. Our experiment showed that the DOM enrichment in the SML is linked to different phytoplankton communities. In addition, it revealed that depending on the predominant community the DOM concentration can be even depleted in the SML compared to the ULW. Based on the outcome of our field and laboratory studies we conclude that molecular level analysis of surface microlayers is essential to understand the chemical diversity of this highly dynamic boundary layer.

How to cite: Oeljeschlaeger, L., Hintz, N., Niggemann, J., Wurl, O., and Dittmar, T.: Phytoplankton communities influence the dissolved organic matter composition of the sea-surface microlayer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19041, https://doi.org/10.5194/egusphere-egu2020-19041, 2020.

EGU2020-12621 | Displays | OS3.8

Parsing the DOM sources using calibrated biomarkers in the San Francisco Bay Estuary

Chia-Ying Chuang, Francois Guillemette, Jennifer Harfmann, Karl Kaiser, Robert Spencer, Brian Bergamaschi, and Peter Hernes

The San Francisco Bay Estuary (SFBE) together with the Sacramento–San Joaquin River Delta is the second largest estuary in the United States and represents a highly dynamic ecosystem. From 2014 to 2016, we conducted three transects across a salinity gradient to investigate the roles of sources, hydrologic and seasonal changes on the DOM composition. Sampling started with a riverine endmember, through a vast area of marshes, wetlands, to the Golden Gate, the largest estuary in western North America. The winter transect at its maximum discharge allowed the study of DOM dynamics largely in the absence of photodegradation processes and low levels of algal production; the summer transect captured significant photodegradation and algal production; the spring transect revealed the signal of stored DOM from the snowmelt cold water flows. Multiple studies indicated that algal primary production alone cannot support the SFBE foodweb, and the wetlands could also serve to reduce DOM loadings coming off of the delta.  Hence, other sources of organic matter must be considered, including autochthonous and allochthonous DOM. Terrestrial DOM export in SFBE were revealed by dissolved lignin dynamics. Optical proxies (UV-vis and fluorescence) were also used to study the photochemical and biological transformations of DOM.

How to cite: Chuang, C.-Y., Guillemette, F., Harfmann, J., Kaiser, K., Spencer, R., Bergamaschi, B., and Hernes, P.: Parsing the DOM sources using calibrated biomarkers in the San Francisco Bay Estuary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12621, https://doi.org/10.5194/egusphere-egu2020-12621, 2020.

EGU2020-4496 | Displays | OS3.8

Modelling the Microbial Carbon Pump in a changing ocean: current state and future directions

Luca Polimene, Sevrine Sailley, Darren Clark, and Susan Kimmance

Circa 624 gigatons of carbon are locked in the ocean as dissolved organic matter (DOM), an amount comparable with the entire CO2 content of the extant atmosphere. This DOM is operationally defined as refractory, meaning that it is resistant to bacterial degradation and persists in the ocean for millennia. Refractory DOM is considered primarily a residual product of heterotrophic bacterial activity after the bacterial consumption of more labile (i.e. easily degradable) DOM produced by marine autotrophs through photosynthesis. The process through which bacteria form refractory-DOM is termed the ‘Microbial Carbon Pump’ (MCP). Abiotic degradation (e.g. photo-degradation) is thought to balance refractory DOM production, thus maintaining its current pool in steady state. However, recent studies suggest that changes in surface ocean inorganic nutrient availability, due to climate change related increases in thermal stratification, could modify MCP activity, increasing refractory-DOM production with respect to its consumption. Marine bacteria thus have the potential to mitigate increases in atmospheric CO2 by shunting more photosynthesised carbon into refractory-DOM. This hypothesis can only be tested by including the MCP in numerical models used for climate prediction. However, the lack of mechanistic understanding of the process (due, in turn, to the lack of experimental data) has hitherto prevented the development of adequate model formulations. In this talk, I will discuss the potential (and limitations) of existing process models to simulate (at least partially) the MCP and highlight future research directions (and related challenges) to develop new model formulations describing this process.

How to cite: Polimene, L., Sailley, S., Clark, D., and Kimmance, S.: Modelling the Microbial Carbon Pump in a changing ocean: current state and future directions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4496, https://doi.org/10.5194/egusphere-egu2020-4496, 2020.

EGU2020-8929 | Displays | OS3.8

Influence of salinity gradient on Ni complexation by Fluorescent Dissolved Organic Matter (FDOM) and dispersal across estuaries in New Caledonia

Cécile Dupouy, Farid Juillot, Hugues Lemonnier, Marie Bessard, Leocadie Jamet, Laura Boher, and Stephane Mounier

Dissolved organic matter (DOM) is ubiquitous in the environment. Its composition and properties depend on water type (freshwater, estuarine, brackish, marine) and are influenced by the geological nature (ultramafic, volcano-sedimentary, metamorphic) and occupancy (mangrove, forest, agriculture, urbanized) of the upstream catchment. Due to its capacity  to form complexes with dissolved trace metals, DOM can render them hardly available to living organisms, and thus limit their toxicity. Considering its capacity to be transported on large distances, DOM can significantly contribute to the dispersal of trace metals in aquatic ecosystems. In this study, we used 3D fluorescence spectroscopy to characterize the actual nature of FDOM (a fraction of DOM that shows specific fluorescence properties) across estuaries downstream of two contrasted catchments (ultramafic vs. volcano-sedimentary) in New Caledonia. In a first step, Excitation-Emission Matrix (EEM) were obtained on 0.2 µm filtered water samples and a parallel factor analysis (PARAFAC) allowed to identify the different FDOM components in the two catchments. These data indicated a dramatic decrease of all components as a function of increasing salinity, with a threshold value around 25 g/L whatever the catchment. This trend is considered to reflect an aggregation-flocculation behavior of FDOM across the salinity gradient of the studied estuaries. In a second step, fluorescence quenching experiments emphasized the complexing capacity of the different components toward dissolved Ni. FDOM might thus play a significant role on Ni dispersal in aquatic ecosystems through the formation of FDOM-Ni complexes. However, this dispersal capacity might be hampered in estuaries due to the suspected aggregation-flocculation behavior of FDOM across the salinity gradient. Rather than the geological setting of the upstream catchment, salinity appears thus as the major driver of Ni dynamics across estuaries through FDOM-Ni complexes.

How to cite: Dupouy, C., Juillot, F., Lemonnier, H., Bessard, M., Jamet, L., Boher, L., and Mounier, S.: Influence of salinity gradient on Ni complexation by Fluorescent Dissolved Organic Matter (FDOM) and dispersal across estuaries in New Caledonia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8929, https://doi.org/10.5194/egusphere-egu2020-8929, 2020.

EGU2020-7342 | Displays | OS3.8

Towards the estimation of DOC from space in the open ocean

Ana Gabriela Bonelli, Hubert Loisel, Vincent Vantrepotte, Daniel Jorge, Antoine Mangin, and Julien Demaria

The Dissolved Organic Carbon (DOC) represents the largest pool of organic carbon and the most active carbon compartment in the ocean. Describing the spatio-temporal dynamics of the oceanic DOC in response to variation in the physical of biological forcings is therefore crucial for better understanding the global carbon cycle. The DOC distribution and its temporal dynamics is however currently not well known.

In the recent years several works have demonstrated the possibility to assess from space the DOC distribution in the coastal ocean thanks to direct relationships between DOC and the optical properties of colored dissolved organic matter (CDOM). Such CDOM-DOC relationships are not applicable for the open ocean water due making more complex the DOC estimation from space in the latter environments. Here we present first results documenting an alternative method for estimating DOC from satellite imagery which rely on the use of a neural network which combines different physical and biogeochemical input variables (SST, SSS, PAR, aCDOM and Chl-a).

How to cite: Bonelli, A. G., Loisel, H., Vantrepotte, V., Jorge, D., Mangin, A., and Demaria, J.: Towards the estimation of DOC from space in the open ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7342, https://doi.org/10.5194/egusphere-egu2020-7342, 2020.

EGU2020-10101 | Displays | OS3.8

CDOM Absorption Properties of Natural Water Bodies Along Extreme Environmental Gradients

Jakob J. Stamnes, Ciren Nima, Børge Hamre, Øyvind Frette, Yi-Chun Chen, Kai Sørensen, Marit Norli, Daren Lu, Qiangguo Xing, Dennis Muyiimbwa, Taddeo Ssenyyonga, Knut Stamnes, and Svein Rune Erga

We present absorption properties of colored dissolved organic matter (CDOM) sampled in six different water bodies along extreme altitudinal, latitudinal, and trophic state gradients. Three sites are in Norway: the mesotrophic Lysefjord (LF), Samnangerfjord (SF), and Røst Coastal Water (RCW); two sites are in China: the oligotrophic Lake Namtso (LN) and the eutrophic Bohai Sea (BS); and one site is in Uganda: the eutrophic Lake Victoria (LV).

 

The site locations ranged from equatorial to subarctic regions, and they included water types from oligotrophic to eutrophic and altitudes from 0 m to 4700 m. The mean CDOM absorption coefficients at 440 nm [aCDOM(440)] and 320 nm [aCDOM(320)] varied in the ranges 0.063–0.35 m-1 and 0.34–2.28 m-1, respectively, with highest values in LV, Uganda and the lowest in the high-altitude LN, Tibet.

 

The mean spectral slopes S280-500 and S350-500 were found to vary in the ranges of 0.017–0.032 nm-1 and 0.013–0.015 nm-1, respectively. The highest mean value for S280-500 as well as the lowest mean value for S350-500 were found in LN. Scatter plots of S280-500 versus aCDOM(440) and aCDOM(320) values ranges revealed a close connection between RCW, LF, and SF on one side, and BS and LV on the other side.

 

CDOM seems to originate from terrestrial sources in LF, SF, BS, and LV, while RCW is characterized by autochthonous-oceanic CDOM, and LN by autochthonous CDOM. Photobleaching of CDOM is prominent in LN, demonstrated by absorption towards lower wavelengths in the UV spectrum. We conclude that high altitudes, implying high levels of UV radiation and oligotrophic water conditions are most important for making a significant change in CDOM absorption properties.

Considering all study sites, we find a strong negative linear relationship between the base-10 logarithm of aCDOM(440) and the spectral slope S280-500, and also between the base-10 logarithm of aCDOM(320) and the spectral slope S280-500.

How to cite: Stamnes, J. J., Nima, C., Hamre, B., Frette, Ø., Chen, Y.-C., Sørensen, K., Norli, M., Lu, D., Xing, Q., Muyiimbwa, D., Ssenyyonga, T., Stamnes, K., and Erga, S. R.: CDOM Absorption Properties of Natural Water Bodies Along Extreme Environmental Gradients, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10101, https://doi.org/10.5194/egusphere-egu2020-10101, 2020.

Phytoplankton growth in the Indian Ocean is generally limited by macronutrients (nitrogen: N and phosphorus: P) in the north and by micronutrient (iron: Fe) in the south. Increasing anthropogenic atmospheric deposition of N and dissolved Fe (dFe) into the ocean can thus lead to significant responses from marine ecosystems in this ocean basin. Previous modeling studies investigated the impacts of anthropogenic nutrient deposition on the ocean, but their results are uncertain due to incomplete representations of Fe cycling. We use a state-of-the-art ocean ecosystem and Fe cycling model to evaluate the transient responses of ocean productivity and carbon uptake in the Indian Ocean, focusing on the centennial time scale. The model incorporates all major external sources and represents a complicated internal cycling process of Fe, thus showing significant improvements in reproducing observations. Sensitivity simulations show that after a century of anthropogenic deposition, increased dFe stimulates diatoms productivity in the southern Indian Ocean poleward of 50⁰S and the southeastern tropics. Diatoms production weakens in the south of the Arabian Sea due to the P limitation, and diatoms are outcompeted there by coccolithophores and picoplankton, which have a lower P demand. These changes in diatoms and coccolithophores productions alter the balance between the organic and carbonate pumps in the Indian Ocean, increasing the carbon uptake in the south of 50⁰S and the southeastern tropics while decreasing it in the Arabian Sea. Our results reveal the important role of ecosystem dynamics in controlling the sensitivity of carbon fluxes in the Indian Ocean under the impact of anthropogenic nutrient deposition over a centennial timescale.

How to cite: Pham, A. and Ito, T.: Anthropogenic iron deposition alters the ecosystem and carbon balance of the Indian Ocean over a centennial timescale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21916, https://doi.org/10.5194/egusphere-egu2020-21916, 2020.

EGU2020-20458 | Displays | OS3.8

Influence of organic carbon cycling on the fate of persistent organic pollutants in marine environments

Anna Sobek, Inna Nybom, Hans Peter Arp, Naiara Berrojalbiz, Nathan Charlton, Mari Forsell, Dorothea Gilbert, Gisela Horlitz, and Janneke van Grimbergen

Persistent organic pollutants (POPs) are ubiquitously present in the aquatic environment. They are hydrophobic substances that sorb to organic carbon (OC), and thus their environmental fate is closely linked to OC fluxes and pools. In this project, we test the hypothesis that future changes in the OC cycle can influence POP flux from air to sediment and reduce the POP sink in Baltic Sea sediments. The hypothesis relies on the assumption that the OC sorption capacity is affected by the relative contribution of terrestrial OC as well as the trophic status (oligotrophic versus eutrophic) of the area. Four different coastal sites were sampled, to capture different carbon regimes in terms of nutrient status and level of terrestrial OC influence. Concentrations of POPs were analysed along high-resolution sediment porewater- bottom water interface profiles, in total sediment, suspended matter collected in sediment traps and plankton, in the water column and in air. Stable carbon isotope signatures (δ13C) showed that the sites are different in terms of the influence of terrestrial organic matter, and the sites differ in nutrient conditions.

Preliminary results demonstrate that in general, sediments (three sites analysed) act as a source of PAHs to overlying water, whereas sediment and water more often are in equilibrium for PCBs, although there are variations for individual compounds. At the high nutrient-low terrestrial site, which was sampled at two different seasons, both air and water concentrations were higher for PAHs and PCBs in the autumn compared to the summer, indicating the importance of air as source of these contaminants to the water column. Accordingly, air seems an important source of PAHs to the water column in the low terrestrial-low nutrient site, as concentrations in the water column are increasing towards the water surface, whereas this was not observed for PCBs at the same site. Preliminary results from two contrasting sites in the Gulf of Finland, both with high nutrient levels but with different relative contribution of terrestrial OC, demonstrate higher total sediment concentrations of PAHs in the sediment with more marine OC, which was not observed as clearly for PCBs. Data from the water column indicate that marine OC is more efficient in sorbing POPs as air and water concentrations were similar at both sites, even though the OC vertical export at the high terrestrial site was more than double compared to the low terrestrial site. The full data set, will allow for further evaluation of hypotheses on the links between OC cycling and contaminant fate in the Baltic Sea.

How to cite: Sobek, A., Nybom, I., Arp, H. P., Berrojalbiz, N., Charlton, N., Forsell, M., Gilbert, D., Horlitz, G., and van Grimbergen, J.: Influence of organic carbon cycling on the fate of persistent organic pollutants in marine environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20458, https://doi.org/10.5194/egusphere-egu2020-20458, 2020.

A subsurface enrichment of methylmercury (MeHg) has been observed at shallow depths (~100–300 m) in several regions of the Arctic Ocean. The spatial distribution of this subsurface seawater MeHg has been suggested to be responsible for the spatial variability in mercury concentrations of marine animals in the Canadian Arctic. The origin of the sub-surface MeHg in seawater, however, remains a subject of debate. In most other ocean basins, seawater MeHg typically peaks at deeper depths with a much lower dissolved oxygen content, and is thought to be produced in situ and in association with organic matter remineralization. In contrast, our water mass analysis suggests that MeHg enrichment in the shallow, well-oxygenated waters in the Canadian Arctic bears a signature that is most consistent with long-range transport of the Upper Halocline Pacific Water (UHW). Our results further show that seawater MeHg concentrations exhibit a significant correlation with a denitrification tracer, N*, and that the MeHg-to-N* slope decreases progressively from west to east across the Canadian Arctic. The negative N* values in the Canadian Arctic are known to have originated from denitrification in the productive Chukchi Sea shelf sediments, where anaerobic mercury methylation could also be favored. As MeHg- and N*-carrying UHW advects eastwards toward the Canadian Arctic, MeHg is progressively lost through demethylation, resulting in the observed decreasing trend in the MeHg-to-N* slope. The long-distance transport implies that the half-life of MeHg in Arctic seawater below the euphotic zone must be much longer than previously reported. This is supported by a critical literature review, which casts doubt on mercury methylation and demethylation rates previously determined from a seawater incubation approach due to unexplainable methylation and demethylation at time zero and poor fitting of the experimental data to first-order kinetics. Our results thus call for a better understanding of productive shelf sediments as potential MeHg “hotspots” in the Arctic and more reliable measurements of mercury methylation and demethlyation rates in the ocean.

How to cite: Wang, F. and Wang, K.: On the origin of seawater methylmercury in the Canadian Arctic: in-situ production vs. long-range advection , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6183, https://doi.org/10.5194/egusphere-egu2020-6183, 2020.

EGU2020-19646 | Displays | OS3.8

Adsorption of Dimethylmercury onto Natural Particles

Sofi Jonsson, Johannes West, and Van Liem Nguyen

Dimethylmercury (DMeHg), a highly toxic form of mercury (Hg), appears as a dissolved gas in marine waters as well as some terrestrial environments. Although DMeHg does not reach concentrations in natural environments that are of direct concern for human and wildlife health, it has been suggested that DMeHg could play a role in controlling the amount of monomethylmercury (MMeHg) available. As MMeHg bioaccumulates in aquatic food webs to concentrations of concern, it links the occurrence of DMeHg in marine systems with the negative consequences of Hg pollution for human and wildlife health. Our understanding of the biogeochemical cycle of DMeHg is however scant. The potential for adsorption of DMeHg onto natural particles has so far not been addressed. This, despite the fact that adsorption is recognised to be an important process controlling the distribution of other chemical forms of Hg in the environment, including ionic forms of inorganic divalent Hg and monomethylmercury as well as gaseous elemental Hg.  

Here, we will present the data from the first adsorption experiments with dimethylmercury using model compounds (including FeS-minerals) as well as natural sediments and soils. For FeS(s), DMeHg readily adsorbed onto the mineral surface reaching equilibrium within 1-2 h. Observed partitioning between the solid and aqueous phase for FeS were also close to observed partitioning of e.g. MMeHg. Preliminary data also suggests DMeHg to readily adsorb onto natural particles, including sediments and soils.

How to cite: Jonsson, S., West, J., and Nguyen, V. L.: Adsorption of Dimethylmercury onto Natural Particles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19646, https://doi.org/10.5194/egusphere-egu2020-19646, 2020.

EGU2020-13828 | Displays | OS3.8

Temperate, macrotidal, turbid estuarine behavior of mercury species. The case of the Loire river

Joel Knœry, Bastien Thomas, Christophe Brach-Papa, Nicolas Briant, Sandrine Bruzac, Tiphaine Chouvelon, Sylvette Crochet, pauline Le Monier, Emmanuel Ponzevera, and Teddy Sireau

We have examined the distribution and spectiation mercury and trace metals under contrasted season, river flow conditions and tidal amplitudes with a series of  cruises taking place between 2012 and 2015. Our laboratory had alredy undertaken a similar study in the early 1990's. The latest data acquisition efforts are an opportunity to examine the changes that occurred within that time span for mercury and its species, and to discuss the causes of the observed changes. In particular, we will replace the  observations in the perspectives of environmental regulation, and of field and laboratory practices observed for environmental chemical characterization.

Early 1980's estuarine transects show total dissolved Hg levels near 5nM in the Loire river (Frenet, 1981). In the early 1990's, a careful and detailed study shows estuarine levels between 1 and 6pM (Coquery, 1994) with a strong dependency on the hydrological regime. New results confirm the riverine clevels in the low pM range. Concentrations decrease progressively during mixing with seawater and the oceanic end-member concentrations are near 0.5pM. Monitoring of mercury levels and speciation in the Loire river and the upper reaches of the estuary show that these Hg levels and species fluctuate with water flow and areal flooding upstream, AND on the history of that hydrological regime.

How to cite: Knœry, J., Thomas, B., Brach-Papa, C., Briant, N., Bruzac, S., Chouvelon, T., Crochet, S., Le Monier, P., Ponzevera, E., and Sireau, T.: Temperate, macrotidal, turbid estuarine behavior of mercury species. The case of the Loire river, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13828, https://doi.org/10.5194/egusphere-egu2020-13828, 2020.

EGU2020-14875 | Displays | OS3.8

Modeling mercury cycling in the marine environment

Johannes Bieser, Ute Daewel, and Corinna Schrum

Five decades of Hg science have shown the tremendous complexity of the global Hg cycle. Yet, the pathways that lead from anthropogenic Hg emissions to MeHg exposure through sea food are not fully comprehended. Moreover, the observed amount of MeHg in fish exhibits a large temporal and spatial variability that we cannot predict yet. A key issue is that fully speciated Hg measurements in the ocean are difficult to perform and thus we will never be able to achieve a comprehensive spatial and temporal coverage.

Therefore, we need complex modeling tools that allow us to fill the gaps in the observations and to predict future changes in the system under changing external drivers (emissions, climate change, ecosystem changes). Numerical models have a long history in Hg research, but so far have virtually only addressed inorganic Hg cycling in atmosphere and oceans.

Here we present a novel 3d-hydrodynamic mercury modeling framework based on fully coupled compartmental models including atmosphere, ocean, and ecosystem. The generalized high resolution model has been set up for European shelf seas and was used to model the transition zone from estuaries to the open ocean. Based on this model we present our findings on intra- and inter-annual dynamics and variability of mercury speciation and distribution in a coastal ocean. Moreover, we present the first results on the dynamics of mercury bio-accumulation from a fully coupled marine ecosystem model. Most importantly, the model is able to reproduce the large variability in methylmercury accumulation in higher trophic levels.

How to cite: Bieser, J., Daewel, U., and Schrum, C.: Modeling mercury cycling in the marine environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14875, https://doi.org/10.5194/egusphere-egu2020-14875, 2020.

EGU2020-1695 | Displays | OS3.8

Biomagnification of methylmercury in a marine plankton ecosystem

Peipei Wu, Emily Zakem, Stephanie Dutkiewicz, and Yanxu Zhang

Methylmercury is greatly bioconcentrated and biomagnified in marine plankton ecosystems, and these communities form the basis of marine food webs. Therefore, evaluating the potential exposure of methylmercury to higher trophic levels, including humans, requires a better understanding of its distribution in the ocean and the factors that control its biomagnification. In this study, a coupled physical/ecological model was used to simulate the trophic transfer of monomethylmercury (MMHg) in a marine plankton ecosystem. The model includes phytoplankton, a microbial community, herbivorous zooplankton (HZ), and carnivorous zooplankton (CZ). The model captured both shorter food chains in oligotrophic regions, with small HZ feeding on small phytoplankton, and longer chains in higher nutrient conditions, with larger HZ feeding on larger phytoplankton and larger CZ feeding on larger HZ. In the model, trophic dilution occurred in the food webs that involved small zooplankton, as the grazing fluxes of small zooplankton were insufficient to accumulate more MMHg in themselves than in their prey. The model suggested that biomagnification was more prominent in large zooplankton and that the microbial community played an important role in the trophic transfer of MMHg. Sensitivity analyses showed that with increasing body size, the sensitivity of the trophic magnification ratio to grazing, mortality rates, and food assimilation efficiency (AEC) increased, while the sensitivity to excretion rates decreased. More predation or a longer zooplankton lifespan may lead to more prominent biomagnification, especially for large species. Because lower AEC resulted in more predation, modeled ratios of MMHg concentrations between large CZ and HZ doubled when the AEC decreased from 40% to 10%. This suggested that the biomagnification of large zooplankton was particularly sensitive to food assimilation efficiency.

How to cite: Wu, P., Zakem, E., Dutkiewicz, S., and Zhang, Y.: Biomagnification of methylmercury in a marine plankton ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1695, https://doi.org/10.5194/egusphere-egu2020-1695, 2020.

EGU2020-669 | Displays | OS3.8

Biological lability of Dissolved Organic Matter released by phytoplankton

Giancarlo Bachi, Elisabetta Morelli, Margherita Gonnelli, Raffaella Casotti, Stefano Vestri, Valter Evangelista, Cecilia Balestra, and Chiara Santinelli

Phytoplankton is the primary source of Dissolved Organic Matter (DOM) to the oceans. DOM is mainly released by extracellular exudation and used by heterotrophic prokaryotes to synthesise biomass and recycle inorganic nutrients. DOM released by phytoplankton is mainly composed by carbohydrates, proteins and lipids that are thought to be labile and by humic substances that are thought to be recalcitrant and thus resistant to bacterial degradation. There are a lot of uncertainties regarding the biological lability of exudates and the role of DOM released by phytoplankton in the marine carbon cycle. In this study, cultures of the diatom P. tricornutum were produced under axenic conditions and Dissolved Organic Carbon (DOC) concentration, Excitation-Emission matrices (EEMs) and cell density were measured with time in order to follow the release of DOM during the different growth phases. Exudates were then inoculated with a marine microbial community for 24 days, DOC removal and FDOM transformation were followed with time in the exudates and in the permeate (< 3k Da; Low Molecular Weight, LMW) and retentate (> 3k Da; High Molecular Weight, HMW) fractions. Heterotrophic prokaryotes abundance was also followed during the incubations. Our results show that ~75% of the total DOC pool was LMW. After 24 days, 28% of the initial DOC pool was removed. Fluorescence indicate high lability of protein-like molecules and degradation of bigger proteins into smaller peptides before their removal. The production of humic-like and flavin-like substances was also observed.

 

How to cite: Bachi, G., Morelli, E., Gonnelli, M., Casotti, R., Vestri, S., Evangelista, V., Balestra, C., and Santinelli, C.: Biological lability of Dissolved Organic Matter released by phytoplankton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-669, https://doi.org/10.5194/egusphere-egu2020-669, 2020.

EGU2020-19058 | Displays | OS3.8

Retrieval of hyperspectral CDOM absorption with integrating cavity sphere

Vincenzo Vellucci, Melek Golbol, and David Antoine

Typical measurements of the absorption coefficient of the chromophoric fraction of dissolved organic carbon (CDOM) are performed on filtered seawaters samples. Though samples could be stored in the dark at 4° for up to 6 months, it is preferable to analyse them within 24 hours from collection as variation of absorption might occur depending on the nature of the sample, and to minimize the effect of possible contamination.

As it is not always practical to analyse samples on board, techniques have been proposed to measure the CDOM with in situ deployed reflective tube absorption meters (i.e. SeaBird ac-9 and ac-s). These techniques allow time effective measurements of CDOM at high vertical resolution. However the typical path-length of the cavity containing the water sample is of 0.25 m, i.e. one eighth of most common protocols used in laboratory analyses, thus limiting the accuracy of the measurements at lower signals.

Integrating cavity absorption meters (ICAM) represent an alternative to reflective tube absorption meters. They have been primarily developed to reduce the effect of scattering of particulate onto absorption measurements. Nonetheless this technique presents also the advantage to increase the effective optical path-length of the light beam due to multiple reflections into the reflecting cavity (up to 2 m for a 10 cm sphere diameter). This peculiarity make ICAM suitable for applications with lower signal such as open ocean case I waters.

Here we present some advances toward the definition of a protocol for the use of a hyperspectral integrating cavity sphere (Hobilabs a-sphere) for the in situ measurement of CDOM. In particular we address aspects related to cleaning, blank measurements, water flow into the cavity and pressure and we present data from the BOUSSOLE bio-optical time series (NW Mediterranean Sea).

How to cite: Vellucci, V., Golbol, M., and Antoine, D.: Retrieval of hyperspectral CDOM absorption with integrating cavity sphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19058, https://doi.org/10.5194/egusphere-egu2020-19058, 2020.

EGU2020-20832 | Displays | OS3.8

Optical properties of chromophoric and fluorescent dissolved organic matter in the coastal Jeju Island: Impact of the anthropogenic sources

Jeonghyun Kim, Yeseul Kim, Tae-Hoon Kim, Sung Eun Park, Dong-Jin Kang, and TaeKeun Rho

The hydrological properties (temperature, salinity, pH, and dissolved oxygen), dissolved organic carbon (DOC) and nitrogen (DON), and optical absorption and fluorescence signals were measured in Jeju Island, Korea, during 2016–2018, especially in potential point-sources (e.g. coastal aquafarms, a sewage treatment facility, and coastal artesian springs). The water samples were filtered through 0.2 μm polycarbonate syringe filters. The optical analysis was conducted using a spectrophotometer (Aqualog, Horiba, USA). Absorbance spectra were converted into the absorption coefficient, and fluorescence intensities were conducted by the parallel factor analysis (PARAFAC) model. The fluorescent components were compared with previous studies through the web-based OpenFluor database.

The absorption coefficient at 350 nm ranged from 0.05 to 7.63 m −1, and it was up to 150 times higher near the point-sources than in the normal coastal ocean. In addition, a350 was observed to be exponentially increased as the reduced distance from the aquafarm outlet. Similarly, the concentration of DOC was 89 ± 29 μM near the point-sources and 78 ± 13 μM in the normal coastal area. They were also observed to be high fluorescence near the point-sources. Principal component analysis (PCA) was applied to illustrate the relationship among the five PARAFAC components, DOC, DON, a350, and fluorescence indexes (HIX, BIX, FI, TC ratio, and RI). The PCA results separated allochthonous, terrestrial components from autochthonous, microbial components, as explained 71.3% of the variance in the data. Based on the HIX (1.26 – 55.70) and BIX (0.52 – 2.87) values in this study, the organic matter around the coastal Jeju Island seem to be highly affected by the coastal groundwater. Here, we used multiple optical properties of organic matter near the coastal area to identify the key factor contributing its distribution and water qualities and to determine the significant influence of the point-sources.

How to cite: Kim, J., Kim, Y., Kim, T.-H., Park, S. E., Kang, D.-J., and Rho, T.: Optical properties of chromophoric and fluorescent dissolved organic matter in the coastal Jeju Island: Impact of the anthropogenic sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20832, https://doi.org/10.5194/egusphere-egu2020-20832, 2020.

EGU2020-18207 | Displays | OS3.8

DOM DYNAMICS IN THE MEDITERRANEAN SEA. Can a new fluorescence SENSOR contribute to its understanding?

Simona Retelletti Brogi, Marta Furia, Giancarlo Bachi, Vanessa Cardin, Giuseppe Civitarese, Bruno Tiribilli, Toste Tanhua, Massimo Vassalli, and Chiara Santinelli

The Mediterranean Sea (Med Sea) can be considered as a natural laboratory for the study of dissolved organic matter (DOM) dynamics. Despite its small size, it is characterized by the same physical processes and dissolved organic carbon (DOC) concentration and distribution as the global ocean. The Med Sea deep water DOC pool is however older than the Atlantic one and differences in the microbial loop and in DOM dynamics have been observed between the eastern (EMED) and western (WMED) basins. Fluorescence is a fast, cheap and highly sensitive tool to study DOM dynamics, it can therefor give useful information about the main processes affecting DOM distribution.

The main aims of this study were: (i) to investigate DOM dynamics in both Med Sea basins, in relation to the physical processes (e.g. vertical stratification, irradiation); and (ii) to validate the use of a new fluorescence sensor, developed in the framework of the SENSOR project (POR FESR, Tuscany Region), for the rapid, in-situ measurements of open-sea fluorescent DOM (FDOM). DOM dynamics was investigated by measuring dissolved organic carbon (DOC) and the fluorescence of FDOM. Samples were collected from surface to bottom in 26 stations during the trans-Mediterranean cruise “MSM72”, carried out on board the R/V MARIA S.MERIAN (Institut für Meereskunde der Universität Hamburg). The stations cover both the EMED and the WMED, from Gibraltar to the Crete Island.

Six fluorescent components were identified by applying the parallel factorial analysis (PARAFAC) to the measured excitation-emission matrices (EEMs). Two components were identified as marine humic-like, two as terrestrial humic-like, one as protein-like and one as polycyclic aromatic hydrocarbon-like (PAH-like).

Temperature and salinity increased moving from the WMED to the EMED. A surface minimum in salinity, was observed in the WMED, indicating the occurrence of the Atlantic Water (AW), whereas the presence of the Levantine Intermediate Water (LIW) was observed south of Crete. The vertical distribution of both DOC and humic-like FDOM was strongly affected by the water masses circulation and water column stratification. In the upper 200 m, DOC markedly increased from 50 to 80 μM moving eastward, likewise the protein-like component dominates the upper layer and increased moving from Gibraltar to Crete. In contrast, the humic-like components showed a minimum in the surface layer, and a decreasing moving eastward, probably due to photobleaching. The PAH-like component showed its maximum in correspondence with the areas characterized by intensive naval traffic. The accumulation of DOC, observed in the EMED, could be explained by a change in DOM quality, supported by the differences in FDOM.

In 2 selected stations, the fluorescence of humic-like and protein-like compounds was also measured along the water column by using the new fluorescence sensor and compared with PARAFAC results, in order to evaluate its performance for open sea waters.

How to cite: Retelletti Brogi, S., Furia, M., Bachi, G., Cardin, V., Civitarese, G., Tiribilli, B., Tanhua, T., Vassalli, M., and Santinelli, C.: DOM DYNAMICS IN THE MEDITERRANEAN SEA. Can a new fluorescence SENSOR contribute to its understanding?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18207, https://doi.org/10.5194/egusphere-egu2020-18207, 2020.

EGU2020-10870 | Displays | OS3.8

Organic Matter Characterization from sediments of the Tietê and Piracicaba rivers dam (Brazil).

Carla Morais, Benjamin Oursel, Francisco Edouardo Gontijo Guimaraes, Debora Marcondes Bastos Pereira Milori, and Stéphane Mounier

The sediments are complex and heterogeneous environments, thus, besides determining the concentration of potentially toxic metals present in sediments, it is necessary to understand the sediment's ability to accumulate or release contaminants, because many biogeochemical processes are involved, influencing the fate of these metals. The main modes of dispersion, which can lead to remobilization of contaminants are (i) early diagenesis, (ii) natural or anthropogenic resuspension of the sediment and (iii) the diffusive flow at the water-sediment interface. In this context, it is important to understand in how sedimentary organic matter (SedOM) acts in the retention and remobilization of metals, what environmental risks and how climate change influences the flow of rivers and causes remobilization of sedimentation, resulting in the release of these metals. As a result, it will be possible to evaluate if SedOM is a danger or protection against contaminants. In this work were used 69 samples collected at different depths at 3 points on the Tietê river, at 3 points on the Piracicaba river and at one point in the confluence region. The samples were freeze-dried, crushed, and sieved through a 100 mesh sieve. Two SedOM extraction methods were conducted in this work: NaOH extraction and deionized water extraction. Approximately 1.0 g of each sediment was placed in polypropylene flasks with 45.0 mL of 0.1 mol L-1 NaOH and 45.0 mL of deionized water, and then shaken for 24 h in an overhead shakerat 10 rpm. Then the samples were centrifuged at 3,000 g for 10 min and filtered over a 0.45 μm syringe filters. To study the fluorescence mode in EEM, 1.0 mL of each diluted with same absorbance sample was placed in quartz cells with 1.0 mL of 0.3 mol L-1 HEPES and 1.5 mL of 0.1 mol L-1 NaClO4. The fluorescence spectra were acquired in scan speed of 2,400 nm min-1 from 250 to 700 nm in emission and from 200 to 500 nm in excitation. The steps and slits of emission and excitation were fixed at 5 nm, and the detector voltage was 700 V. EEM data were processed using the method of Parallel Factor Analysis to determine the contribution of each component using homemade PROGMEEF software. SedOM samples extracted with NaOH and deionized water from Tietê and Piracicaba rivers presented COre CONsistency DIAgnostic of 83.3% with the contribution of four components or fluorophores. According the components obtained by PARAFAC, the component 2 is noise and it was excluded. The emission wavelength of fluorophore 1 is approximately 450 nm, fluorophore 3 is 550 nm and fluorophore 4 is 400 nm. Therefore, the fluorophore 4 refers to OM fresher, simpler and less humidified. Whereas fluorophore 3 refers to OM older, more complex and more humidified, that is, it is from the terrestrial environments. According to data obtained by EEM and treated with PARAFAC was possible to determine the presence of three fluorophores and the complexity of their structure.

How to cite: Morais, C., Oursel, B., Guimaraes, F. E. G., Marcondes Bastos Pereira Milori, D., and Mounier, S.: Organic Matter Characterization from sediments of the Tietê and Piracicaba rivers dam (Brazil)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10870, https://doi.org/10.5194/egusphere-egu2020-10870, 2020.

EGU2020-11065 | Displays | OS3.8

Dissolved organic matter fate in coastal Mediterranean site: Toulon bay case - France

Amonda El Houssainy, Gaël Durrieu, Huy Duc Dang, Cédric Garnier, and Stéphane Mounier

Dissolved organic matter (DOM) is a complex continuum of molecular species and plays an important role in biogeochemical processes in the aquatic ecosystem, waters and sediments (Aiken et al., 2011; Bolan et al., 2011; Burdige et al., 2004; Chen and Hur, 2015; Fu et al., 2006; Jiang et al., 2018; Stedmon et al., 2003). Once settled into marine sediments, organic matter undergoes biogeochemical transformations (Chen and Hur, 2015). These biogeochemical conditions are dependent on different parameters such as redox condition and microbial activities controlling its dynamics. Studies on pore water organic matter (PW-OM) and extracted organic matter (EOM) give understanding on the fate of sedimentary organic matter. Several studies (Burdige, 2001; Burdige et al., 2004; Chen and Hur, 2015; Dang et al., 2014; Hur et al., 2014; Murphy et al., 2008; Stedmon et al., 2003; Stedmon and Bro, 2008) have used Excitation Emission Matrices of fluorescence coupled to PARAFAC and UV-Vis spectroscopy to characterize DOM among several technics. As it is rapid and gives information on the dynamics, the aromatic structure and even the degree of humification of DOM. Toulon bay is a semi-enclosed bay located in the N-W basin of the Mediterranean Sea and in the S-E of the French coast. This bay is exposed to numerous pollution sources. The origins of organic matter in the bay is the input of two urbanized rivers (Las and Eygoutier), aquaculture, treated sewage outlets and planktonic activities (Boge et al., 2006; Nicolau et al., 2012). Sediment cores and column seawaters were collected at four points in the bay in front of Las River. Each core was sliced within a 2cm resolution under inert atmosphere (N2). Then, pore water was retrieved and filtered (0.2 µm) and solid fraction was freeze-dried, 2mm sieved, crushed. DOC and nutrients concentrations in water samples and total carbon, organic carbon (POC) and total nitrogen were measured on solid. Extraction with alkaline solution was performed and extracted organic carbon and extracted nitrogen were measured. 3D fluorescence measurements were done for all samples from 200 to 800 nm for both excitation and emission. The results showed that the degradation of OM was more intense in front of Las River than outside of the bay. Pore water and extracted OM are influenced by fresh biomass input and the latter is strongly humified. Pore water OM in the superficial layers of sediments comes from autochthonous origin and less humified with a weak aromaticity. However, in deep layers, it shows a terrestrial origin, a regain of aromaticity and an important humic character based on HIX index. Moreover, it is controlled by the production and degradation of nutrients and POC. Extracted OM derives from terrestrial origin and is strongly humified based on HIX index. The state of FDOM in superficial sediments is different from the one in deep sediments because of the reducing environment. There is a contradiction of FDOM fate/behavior between the dissolved and solid phase in deep sediments because the production of FDOM isn’t from the POC pool.

How to cite: Houssainy, A. E., Durrieu, G., Dang, H. D., Garnier, C., and Mounier, S.: Dissolved organic matter fate in coastal Mediterranean site: Toulon bay case - France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11065, https://doi.org/10.5194/egusphere-egu2020-11065, 2020.

Denitrifier communities differ in mangrove wetlands across China

Ruili Li*, Sijie Wu, Minwei Chai, Xiaoxue Shen, Lingyun Yu

Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, Guangdong, PR China

*Corresponding author. E-mail address: liruili@pkusz.edu.cn

Abstract:

Denitrification plays an important role in the removal of nitrogen from coastal wetlands. However, knowledge regarding the dynamics of denitrifier communities in mangrove wetlands is limited, and the corresponding influential factors lack quantitative analysis. To explore the geographical variations in the nosZ-denitrifier community and the underlying influential factors, surface sediments in mangrove forest and adjacent mudflat were collected from six mangrove wetlands across China, including high-latitude Yunxiao (YX) and Futian (FT) mangroves, middle-latitude Fangchenggang (FCG), Zhanjiang (ZJ) and Dongzhaigang (DZG) mangroves, and low-latitude Dongfang (DF) mangrove. The nosZ gene abundance in mangrove sediments varied from 1.60×105–1.17×106 copies g-1 dry sediment, with a higher density in Avicennia marina forest than the mudflat. Denitrifier community richness and diversity increased with decreasing latitude based on the Chao1 richness and Shannon diversity index, with the highest diversity being observed in the DF mangrove. The denitrifier communities could be classified into three groups including low-latitude DF mangrove, middle-latitude FCG, ZJ and DZG mangroves, and high-latitude YX and FT mangroves based on HCA and PCA analysis. The nosZ OTUs could be divided into seven distinct clusters with different proportionality characteristics among mangroves. Environmental factors (TN, TOC, and salinity) could collectively shape denitrifier communities in mangrove sediments.

How to cite: Li, R.: Denitrifier communities differ in mangrove wetlands across China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9059, https://doi.org/10.5194/egusphere-egu2020-9059, 2020.

EGU2020-12861 | Displays | OS3.8 | Highlight

Level up ocean carbon observations: Successful implementation of a novel autonomous total alkalinity analyzer on a commercial Ship of Opportunity

Katharina Seelmann, Tobias Steinhoff, and Arne Körtzinger

The observation and documentation of the marine carbon cycle is of utmost importance because of probable future changes such as ocean acidification, warming or deoxygenation. Over decades, ship-based observatories (Ships of Opportunity – SOOP) equipped with sensors measuring the CO2 partial pressure (pCO2) in the surface seawater form the backbone of the global ocean carbon observation system. However, one severe shortcoming of the current carbon-SOOP observatory is the fact that it mostly only measures pCO2 which is required to calculate the net air-sea CO2 flux. Full insight into the marine CO2 system for important aspects such as net biological production, ocean acidification, and marine calcification requires the measurement of two out of the four measurable variables of the marine CO2 system which are pCO2, total alkalinity (AT), dissolved inorganic carbon (CT) and pH. The so far common workaround is the calculation of AT from sea surface temperature and sea surface salinity using established parameterizations. Unfortunately, this procedure leads to high uncertainties and is particularly prone to regional bias. Therefore, autonomous AT measurements are necessary. Our study describes the implementation of a novel autonomous analyzer for seawater AT, the CONTROS HydroFIA® TA system (Kongsberg Maritime Contros GmbH, Kiel, Germany) on a North Atlantic SOOP line based on the merchant vessel M/V Atlantic Sail (Atlantic Container Line). The first main part of this work deals with the installation of the analyzer, for which several circumstances must be taken into account: 1) The system’s typical drift behavior, 2) stabilization measurements and cleaning procedures, and 3) the waste handling. We present our installation in detail and how we handle the named issues. Another major problem during automated long-term campaigns is the provision of sufficient reference seawater for regular quality assurance measurements and subsequent drift correction. We tested ten different container types and materials with minimum 5L volume (e.g. gas sampling bags) for their suitability as long-term seawater storage. As a result, only one gas sampling bag based on polyvinylidene fluoride (PVDF) featured the high-quality requirements and was chosen as reference seawater storage. The second main part focusses on the measured sea surface AT data from the first four unattended measurement campaigns. In order to prove the success of the installation, we compared the measurements with 1) discrete samples (taken manually only during the first two transits), and 2) calculated AT values based on established parameterization. The gained results show very promising consistency between the measured values and the AT range and variability of the monitored region. We conclude that the implementation of the CONTROS HydroFIA® TA system on a SOOP line was successful and brings ocean carbon observations to a new level.

How to cite: Seelmann, K., Steinhoff, T., and Körtzinger, A.: Level up ocean carbon observations: Successful implementation of a novel autonomous total alkalinity analyzer on a commercial Ship of Opportunity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12861, https://doi.org/10.5194/egusphere-egu2020-12861, 2020.

EGU2020-478 | Displays | OS3.8

Linkages between the occurrence of persistent organic pollutants and biogeochemical characteristics of deep-sea trenches

Gisela Horlitz, Stefano Bonaglia, Igor Eulaers, Ronnie N. Glud, and Anna Sobek

The biogeochemistry of deep-sea trenches is strongly influenced by their V-shape topography and tectonic position in the ocean, leading to a focusing effect of sediment and organic matter into the trench centre. Recent findings showed elevated mineralization rates in trench sediments, suggesting both high carbon turnover and organic matter degradation rates. As persistent organic pollutants (POPs) favourably partition to organic matter, deep-sea trenches act as a sink for these substances. Composition, source and age of the organic matter have been shown to have a significant influence on contaminant dynamics in sediment from more shallow regions. Also, the trophic status of marine systems plays a significant role in transport of POPs from air to water and to sediment. However, knowledge about organic pollutants in deep-sea environments is scarce. In the present study, sediment samples from two deep-sea trenches with different trophic states and deposition regimes are analysed for POPs with a wide range of physicochemical properties. Concentrations will be compared between the semi-eutrophic Atacama and the oligotrophic Kermadec Trench. Sampling of sediment cores was performed at the slope, abyssal plain and trench at Atacama (depth between 2,500 and 8,000m) and at the abyssal plain and trench at Kermadec (depth of 6,000 and 9,600m). The total organic carbon content largely varied between 0.3 and 2.1% at different sites at the Atacama Trench, while values were more homogeneous at the Kermadec Trench (around 0.3%). Preliminary results from the Atacama samples demonstrate concentrations of PCBs at the pg g-1 dw level, and indicate highest concentrations to occur at the highest depth in the trench. Low sedimentation- and high mineralization rates in the trench centre, as well as the funnel-effect from the topology may explain these differences.

How to cite: Horlitz, G., Bonaglia, S., Eulaers, I., Glud, R. N., and Sobek, A.: Linkages between the occurrence of persistent organic pollutants and biogeochemical characteristics of deep-sea trenches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-478, https://doi.org/10.5194/egusphere-egu2020-478, 2020.

Submarine groundwater discharge (SGD) is a possible source for nutrients and anthropogenic pollutants that flow from the land to the ocean. The coastal zone of southwest (SW) India is capped with Tertiary sandstone-limestone-clay intercalations, Quaternary sediments, and laterites up to 600 m thickness above bedrock, which are considered as productive aquifer belts. The signatures of freshwater discharge to sea are not entirely vivid on the SW coast of India due to different constraints on investigation techniques and coastal dynamics. Hence, an onshore and offshore sampling and monitoring were carried out from Kanyakumari to Mangalore (∼640 km) along the SW coast of India to understand the groundwater discharge from the coastal aquifer system. The combined techniques used make it possible to identify groundwater outflows using satellite thermal infrared images to monitor physico-chemical anomalies in the sea (from 7 October – 5 November 2019 onboard the Sakar Kanya research vessel). Surface-to-bottom CTD (conductivity, temperature, depth) profiling and sampling of radium and nutrients were performed during fieldwork. The conventional water balance method and radium isotopic analyses were used to quantify the SGD. The findings of the water balance method show that the average of all fresh SGD is 790 m3/y/m with a minimum of 72 m3/y/m and a maximum of 2070 m3/y/m exported by SW coast to the sea. Regional precipitation patterns and coastal drainage geometry control local variation in fresh SGD. Nutrient concentrations have apparently followed conservative and non-concentrative mixing between fresh, high nutrient groundwater and saline, low-nutrient seawater at coastal ocean sites. Further investigations are in progress for flux estimation using radium isotopes in offshore and deployment of seepage meters in specific known areas along the shore.

How to cite: Ramasamy, M., Babu, S., and Srinivas, R.: Understanding the regional submarine groundwater discharge and the associated nutrient inputs - an assessment from the southwest coast of India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1159, https://doi.org/10.5194/egusphere-egu2020-1159, 2020.

Progress has been made towards optimizing extracting dissolved organic matter (DOM) using a styrene-divinylbenzene copolymer (PPL) sorbent, which is a widely used solid phase extraction (SPE) method to separate DOM from different aquatic samples. To establish the suitable extraction conditions, the effects of critical SPE variables such as loading mass, concentration, flow rate, as well as the extraction selectivity of the PPL sorbent have been systematically studied. Tens liters of water samples were collected from various aquatic environments, including headwater, downstream river water, coastal seawater, surface seawater from open ocean, seawater from open ocean at the depth of fluorescence maximum, and deep ocean water. 5g-Bond Elut PPL columns were used and the extraction kinetics of DOM were monitored liter-by-liter while extraction. Fluorescence spectrum were decomposed into their underlying chemical components resolved by PARAllel FACtor analysis (PARAFAC). Extraction selectivity of the PPL sorbent among different types of waters was verified through those fluorescence excitation emission matrices (EEMs) and chromophoric dissolved organic matter (CDOM) measurements.

How to cite: Ho, Y.-H., Lee, J. Y., and Chuang, C.-Y.: Extraction kinetics of solid phase extraction of dissolved organic matter in environmental samples from various aquatic system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10016, https://doi.org/10.5194/egusphere-egu2020-10016, 2020.

X-ray fluorescence core scanners (XRF-CS) allow rapid, non-destructive and continuous high-resolution analyses of the elemental composition of sediment cores. Since XRF-CS analyses are usually performed in fresh untreated materials, elemental intensities can be affected by the physical properties of the sediment (e.g. pore water content, grain size, sediment irregularities and changes in matrix) and the selected excitation parameters. Accordingly, the records of the measured elemental intensity cannot be considered quantitative. Nonetheless, these data can be converted to quantitative data through a linear regression approach using a relatively small number of discrete samples analyzed by techniques providing absolute concentrations. Such conversion constitutes a powerful tool to determine pollution levels in sediments at very high resolution. However, a precise characterization of the errors associated with the linear function is required to evaluate the quality of the calibrated element concentrations.

Here we present a novel calibration of high-resolution XRF-CS for Ti, Mn, Fe, Zn, Pb and As measured in heavily contaminated marine deposits. Three widely applied regression methods have been tested to determine the best linear function for XRF data conversion, which are: the ordinary least-squares (OLS) method, which does not consider the standard error in any variable (x and y), the weighted ordinary least-squares (WOLS) method, which considers the weighted standard error of the vertical variable (y), and the weighted least-squares (WLS) method, which incorporates the standard error in both x and y variables.

The results, derived from the analysis of metal-polluted sediments from offshore Portmán Bay and Barcelona, in the Mediterranean Sea off Spain, demonstrate that the applied calibration procedure improves the quality of the linear regression for any of the three regression methods (OLS, WOLS, and WLS), thus increasing correlation coefficients, which are higher than r2=0.94, and reducing data deviation from the linear function. Nonetheless, the WLS appears as the best regression method to minimize errors in the calibrated element concentrations. Our results open the door to use calibrated XRF-CS data to evaluate marine sediment pollution according to the sediment quality guidelines (SQG) with errors lower than 0.4% to 2% for Fe, 1% to 7% for Zn, 3 to 14% for Pb, and 5% to 16% for Mn, which highlight the robustness of the calibration procedure here presented. Our study incorporates and evaluates for the first time the analytical and statistical errors of XRF-CS data calibration, and evidences that the errors of the calibrated element concentrations must be properly assessed in future calibration efforts.

How to cite: Cerdà-Domènech, M., Frigola, J., Sanchez-Vidal, A., and Canals, M.: Absolute metal concentrations after calibrating high resolution XRF core scanner data from highly polluted marine deposits offshore Portmán Bay and Barcelona, Spain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11163, https://doi.org/10.5194/egusphere-egu2020-11163, 2020.

EGU2020-22062 | Displays | OS3.8

Modelling of detailed transformations of PCB 153 and PCB 28 in coastal regions

Elena Mikheeva, Johannes Bieser, and Corinna Schrum

Polychlorinated biphenyls (PCBs) are persistent organic pollutants (POPs) that have a long residence time in the marine environment. Due to processes of biomagnification in food chains, their presence in aquatic matrices is especially harmful for living organisms (incl. humans). The process of photolysis in the upper layers of the water column leads to a decreasing concentrations of higher chlorinated congeners (HCC) and increases the concentration of lower chlorinated congeners via the process of HCC declorination. This impacts the environmental fate of pollutants (e.g. sedimentation, accumulation, air-sea exchange), especially in coastal areas. Additionally, depending on the pollutant, degradation products can be even more harmful than the originally emitted species.

To estimate the environmental fate of the chosen contaminants in the marine ecosystems, a Framework for Aquatic Biogeochemical Models (FABM) has been implemented into a high-resolution numerical model. The first results of modeling PCBs concentrations will be presented for 1d simulations including factors such as light penetration, mixing, remineralisation and resuspension and the biological pump. This study aims to compare results for different regions and regimes with different conditions.

How to cite: Mikheeva, E., Bieser, J., and Schrum, C.: Modelling of detailed transformations of PCB 153 and PCB 28 in coastal regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22062, https://doi.org/10.5194/egusphere-egu2020-22062, 2020.

EGU2020-5545 | Displays | OS3.8

Mercury dynamics in a changing coastal area over industrial and post-industrial phases

Ginevra Rosati, Cosimo Solidoro, and Donata Melaku Canu

The Venice Lagoon (Mediterranean Sea) is a shallow coastal lagoon that have been subjected to several anthropogenic pressures, including significant Hg loadings from industrial activities. Inorganic Hg is methylated to neurotoxic MeHg in lagoon water and sediment, posing the ecosystem wealth at risk.

Here, we use a biogeochemical model to investigate the long-term dynamics of Hg species in the Venice Lagoon from the pre-industrial period to the post-industrial period (1900-2100), also taking into account environmental changes occurred in the lagoon such as eutrophication, and the increase of sediment resuspension driven by manila clam harvesting.

Time-variable Hg emissions from industries were estimated from available information about industrial production and technology-dependent emissions factors, while Hg loading species from other sources (river, atmospheric deposition, urban wastes) where estimated through downscaling from global studies, using observations from previous field studies (1970 - 2010) as constraints. The impacts of future trends of Hg atmospheric deposition are explored through scenario analysis. 

Modeled Hg species are in a satisfactory agreement with the available observations. In the current postindustrial phase, HgT in the lagoon waters comes mostly from sediments, while MeHg comes primarily from the watershed.

We estimate in ∼56 kg y-1 the HgT export for 2019 to the Adriatic Sea, which includes ∼0.13 kg y-1 of MeHg. Both Hg and MeHg concentrations are decreasing since outputs slightly exceed inputs. The analysis of Hg and MeHg reservoirs and fluxes reveals the impacts of the changes in environmental conditions on Hg fluxes. On the one hand, eutrophication has enhanced sediment deposition to the seabed, causing a maximum in sediment Hg concentrations when Hg inputs were already declining; on the other hand, the enhanced sediment resuspension due to clam harvesting led to increased Hg fluxes from the sediment to the water, also causing a redistribution of Hg from the central lagoon to the northern and southern areas, as reported by observational studies. These results emphasize the importance of adopting an ecosystem approach when investigating Hg dynamics, considering the different uses of the ecosystem.

How to cite: Rosati, G., Solidoro, C., and Melaku Canu, D.: Mercury dynamics in a changing coastal area over industrial and post-industrial phases, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5545, https://doi.org/10.5194/egusphere-egu2020-5545, 2020.

In this study, we analyzed the distribution and bioaccumulation of six heavy metals (Cu, Pb, Zn, Cr, Cd, Hg) in marine organisms from China’s Hainan and Zhoushan coastal regions. Across all marine organism samples, as well as sediment and seawater samples, Zn and Hg ranked highest and lowest in concentration, respectively. Heavy metal distributions in the marine organisms varied by region and species; concentrations were higher (except for Zn) in Zhoushan than in Hainan and in crab than in fish. A marine organism’s ability to digest and eliminate heavy metals (bioaccumulation ability), based on bioaccumulation factors, was significantly higher for heavy metals in seawater than in sediment; higher sediment background values may explain the higher heavy metal concentrations in crab. In general, a marine organism’s bioaccumulation ability was higher for Cu and Zn and lower for Pb in China.

How to cite: Hao, Z., Wang, C., and Zou, X.: Heavy metal distribution and bioaccumulation ability in marine organisms from coastal regions of Hainan and Zhoushan, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2246, https://doi.org/10.5194/egusphere-egu2020-2246, 2020.

EGU2020-9204 | Displays | OS3.8 | Highlight

Post medieval cargo - contemporary problem source of mercury in pristine seawater environment (Gnalić, Biograd na Moru, Croatia)

Vlado Cuculić, Neven Cukrov, Irena Radić Rossi, and Željko Kwokal

Near the Gnalić islet at the southeast entrance of the Pašman Channel, Croatian Eastern Adriatic there is one of the most important "post-medieval shipwreck known". In November 1583 during sailing popular sea route from Venice (Italy) to Constantinople (Istanbul, Turkey) Venetian merchantman “Gagliana Grossa” with capacity of 1200 Venetian barrels (700 tonnes) and length of 40 meters sank at this site, two nautical miles of the town of Biograd na Moru. A very varied ship's cargo consisted of large quantities of semi finished and finished products manufactured in various part of Europe. However, the ship also carried raw materials such as tin, brass, white lead and especially mercury in various forms: elemental mercury, ore cinnabar (HgS), and vermillion powder (HgS opaque red pigment). Elemental sulfur found in the cargo also indicates possibility of its use in the production of vermillion by chemical coupling of Hg and S. It is assumed that the mercury was meant for medical (elemental Hg), cosmetic and painting (vermillion) purposes. The ship with the full cargo sank at twenty-five meters of depth and wreck was discovered in 1967, while first detailed and systematic sampling and measurements of mercury at a sinking site and its vicinity began in 2013. Seawater sampling was performed eight times in six years (2013-2019). Individual samples were taken by scuba diving at eight positions 1 to 1.5 m above excavating area (60x20 m) as well as in vicinity and on the sea surface above the site. Measurements of mercury species (total, reactive and dissolved gaseous) were performed 24 hours after sampling using CVAAS method.

The Gnalić shipwreck is located in the Middle Adriatic coastal waters. According to the "A long-term survey (1984-2017) of the spatial and temporal trends of the total mercury in seawater of the Adriatic Sea" (Kwokal and Cuculić, in preparation) the mean concentration of total mercury based on over 600 samples is 1.4 ng L-1 for the Middle Adriatic and 1.6 ng L-1 for the coastal water.

During archaeological activities on the excavation site all three measured mercury species, total, reactive and dissolved gaseous appeared in concentrations up to three orders of magnitude higher in comparison with the averages found in the Middle Adriatic seawater.

There is a difference between the results obtained during recovering of the artefacts, cleaning of the hull at the shipwreck site and during the idle state when workspace is conserved. Nevertheless, with no activity on the site, concentrations of mercury species are more than one order of magnitude higher compared to surrounding pristine environment. Data indicates the need of removal of all forms of mercury, especially elemental (roughly estimated 500-1000 kg) from the seabed in order to stop damaging impact on seawater and sediment, consequently on marine life.

How to cite: Cuculić, V., Cukrov, N., Radić Rossi, I., and Kwokal, Ž.: Post medieval cargo - contemporary problem source of mercury in pristine seawater environment (Gnalić, Biograd na Moru, Croatia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9204, https://doi.org/10.5194/egusphere-egu2020-9204, 2020.

EGU2020-21887 | Displays | OS3.8 | Highlight

World War II Munitions as a source of Mercury to the Southwest Baltic Sea

Kathleen Gosnell, Aaron Beck, and Eric Achterberg

The Second World War (WWII) resulted in many humanitarian, cultural and environmental impacts throughout Europe and the world. During WWII anti-aircraft ammunition was used extensively in the Baltic Sea region, and the legacy of WWII munitions are present throughout the area. For example, up to 1.5 million anti-aircraft grenades were shot down in a 10 km2 region along the Dänisch-Nienhof (DN) training center of northern Germany near Kiel. Anti-aircraft grenades contain toxic explosive chemicals such as trinitrotoluene (TNT) and mercury fulminate. It has been estimated that the detonation of WWII bombs released up to 2 tons of mercury (Hg) species into the coastal environment of Germany in the surrounding Kiel area. The DN and greater Kiel Bay (KB) region additionally have non-detonated and partial bombs which could also yield a critical source of Hg to the area. Until now very little research has been done into how much of this Hg might be stored in the sediment, or moving through the waters and food chains of the region.

Water, sediment and plankton samples were collected from around DN and KB in order to quantify and investigate potential impacts and magnitudes of Hg contamination from munition sites and bombs. These Hg levels are compared to available TNT values, and other potential munition-sourced pollutants. Water samples were collected using ‘trace metal clean’ techniques at surface and depth for each station. Plankton samples were gathered at each water station using a vertically towed net in order to assess Hg in the lower food chain. While sediment samples were carefully collected from stations surrounding the KB bomb dumps. These results provide an initial assessment into how much of an impact Hg sourced from anti-aircraft munitions might have on the environment and food chain health within the southern Baltic and KB region.

How to cite: Gosnell, K., Beck, A., and Achterberg, E.: World War II Munitions as a source of Mercury to the Southwest Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21887, https://doi.org/10.5194/egusphere-egu2020-21887, 2020.

OS4.1 – Open session on ocean processes and techniques

EGU2020-183 | Displays | OS4.1 | Highlight

Listening to the Oceans - Effective Techniques for Acoustic Imaging of Oceanic Structure

Tobias Ehmen, Katy Sheen, Andrew Watson, Alexander Brearley, Matthew Palmer, and Daniel Roper

Sub-mesoscale variability in the ocean is poorly understood. The challenge is to observe oceanic thermohaline structures on sufficiently fine space and time scales. One promising approach is seismic oceanography, which applies acoustic reflection techniques to image temperature and salinity gradients within the water column. Here, we present three acoustic oceanographic datasets showing different survey methods and assess their advantages and disadvantages.

Firstly, seismic images from a 2D seismic survey in the Sub-Antarctic southwest Atlantic Ocean are shown. The study site is located northeast of Drake Passage, through which Pacific and Antarctic waters flow to merge with those from the Atlantic, making it a key confluence region. The data show detailed images of sub-mesoscale structures in the upper ocean. Future work will make use of coincident XBT measurements to invert the acoustic data and create detailed 2D maps of temperature distribution.

The second dataset contains samples from a 3D seismic survey in the narrowest part of the Mozambique Channel, which is affected by eddies and changes in transport direction and volume. It acts as a pacemaker for the Agulhas Current system, which plays an important role in global heat transport. The dataset, courtesy of Schlumberger Ltd, combines a high signal to noise ratio with a dense data grid, where locations are sampled several times over several days. These data were used to create time lapse images of the area, providing an invaluable insight into the variabilities in the Mozambique Channel.

Despite a lot of advantages, seismic surveys are generally expensive and lack mobility and versatility. Therefore, another acoustic dataset using a hull-mounted EK80 scientific echo sounder, as part of the ICEBERGS project, is presented as a cheaper and more readily available alternative. Images from the West Antarctic Peninsula show thermohaline structures recorded along the actively de-glaciating margin, contributing to the understanding of underlying physical processes that modulate the flux of oceanic heat to the Antarctic cryosphere, and how and where glacial meltwater is discharged, transported and modified. Furthermore, recording and processing difficulties are discussed.

Lastly, acoustic forward modelling work is discussed, which will wrap up the analysis of the methods presented earlier. Based on the three datasets, the use of different acoustic sources will be forward modelled. Through this ideal sources of different sizes and configurations for seismic oceanography can be analysed. Is it feasible to use acoustic sources small enough to be attached to autonomous vehicles, in order to overcome the current difficulties in extracting temporal variability of oceanic fine structure?

How to cite: Ehmen, T., Sheen, K., Watson, A., Brearley, A., Palmer, M., and Roper, D.: Listening to the Oceans - Effective Techniques for Acoustic Imaging of Oceanic Structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-183, https://doi.org/10.5194/egusphere-egu2020-183, 2020.

EGU2020-10499 | Displays | OS4.1 | Highlight

Release of a Seaglider from an AutoNaut surface vehicle: demonstration mission in the Eurec4a project

Karen J. Heywood, Elizabeth Siddle, Callum Rollo, Ben Webber, Rob Hall, Gillian Damerell, Philip Leadbitter, Gareth Lee, Peter Bromley, Paul Stafford, Alastair Nichol, and Hugh Maclean

This PICO presentation describes a recent demonstration mission of our wave-propelled autonomous vehicle, an AutoNaut named Caravela. Caravela has been designed and built to carry and deploy a profiling ocean glider at a specified location and time. This has applications for example in transporting an ocean glider to a remote location without use of a research vessel, or initiating a glider campaign at a particular time such as prior to a hurricane or the spring bloom.

 

In January-February 2020 we participated in the international Eurec4a field campaign in the tropical Atlantic to the east of Barbados. Caravela was deployed from Barbados and carried a Seaglider to release at the study site. The observational campaign was designed to occupy a time series site with three Seagliders (making intensive measurements of upper ocean properties) and the AutoNaut (making continuous measurements of surface meteorology, radiation and surface ocean currents).  Here we describe the technological challenges, the field campaign and the preliminary results of the scientific observations from Caravela and the Seagliders. The aim is to use the observations to calculate the air-sea fluxes and ultimately to close a mixed layer heat budget for the observation site.

How to cite: Heywood, K. J., Siddle, E., Rollo, C., Webber, B., Hall, R., Damerell, G., Leadbitter, P., Lee, G., Bromley, P., Stafford, P., Nichol, A., and Maclean, H.: Release of a Seaglider from an AutoNaut surface vehicle: demonstration mission in the Eurec4a project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10499, https://doi.org/10.5194/egusphere-egu2020-10499, 2020.

EGU2020-7026 | Displays | OS4.1

Observations of turbulence at a near-surface temperature front in the Arctic Ocean

Zoé Koenig, Ilker Fer, Eivind Kolås, Trygve Fossum, Petter Norgren, and Martin Ludvigsen

Measurements made at an Arctic thermohaline front show turbulence production through convection and forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross-front Ekman transport by down-front winds reduce the potential vorticity in the upper ocean. The front, located in the Nansen Basin close to the sea ice edge, separates the cold and fresh surface melt water from the warm and saline mixed layer. High resolution temperature, salinity, current and turbulence data were collected in the upper 100 m, on 18 September 2018 across the front from a research vessel and an autonomous underwater vehicle. The AUV was deployed to autonomously collect high resolution data across the front using adaptive sampling. Both front detection and sampling location were decided by a state-based autonomous agent running onboard the AUV, optimizing data collection across and along the front.

In addition to convection by heat loss to atmosphere and mechanical forcing by moderate wind in the mixed layer, forced symmetric instability contributed with comparable magnitude in generation of turbulence at the front location down to 40 m depth. This turbulence was associated with turbulent heat fluxes of up to 10 W.m-2, eroding the warm and cold intrusions observed at respectively 35 and 55 m depth. A similar frontal structure has been crossed by a Seaglider in the same region 10 days after our survey. The submesoscale-to-turbulence scale transitions and resulting mixing can be widespread and important in the Atlantic sector of the Arctic Ocean.

How to cite: Koenig, Z., Fer, I., Kolås, E., Fossum, T., Norgren, P., and Ludvigsen, M.: Observations of turbulence at a near-surface temperature front in the Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7026, https://doi.org/10.5194/egusphere-egu2020-7026, 2020.

The physical dynamics of the Sea of Oman are well resolved on meso- and basin-scales. The most prominent features are the slope current of the Persian Gulf Water (PGW), an energetic field of persistent eddies and circulation driven by seasonal monsoon wind regimes. Past work has shown that both oxygenation of the deep oxygen minimum zone and stimulation of local surface primary production are driven by submesoscale processes. In contrast to the pronounced summer-monsoon upwelling in the Arabian Sea, upwelling at the northern Omani shelf appears in the form of short irregular events. The main drivers for local upwelling and the exchange of water and its properties across the shelf break are not fully resolved. In particular, the relative importance of the two dominant causes of upwelling (ekman dynamics and eddy/topography interactions) and their interactions with the PGW slope-current are not known. Cross-shelf coupling is strongly determined by processes on the sub-mesoscale with weak surface signatures preventing analysis through remote sensing. The high system complexity and the lack of adequate observations explain past difficulties in resolving cross-shelf transport and local upwelling responsible for increased primary productivity and OMZ oxygenation.

Here we present new results identifying the submesoscale processes which control productivity and oxygenation in the region at a scale not previously described. These observations build on past work and illustrate how autonomous underwater vehicles can bring forward a full system understanding from basin-wide circulation and description of large ocean currents to submesoscale processes responsible for controlling biogeochemical cycling from a single campaign using standard ocean sensors and utilising the vehicles' inherent ability to measure upwelling and currents. We hope to illustrate the multidisciplinarity and flexibility of autonomous platforms in situations where vessels may not easily survey.

How to cite: Queste, B.: Resolving submesoscale processes and cross-scale interactions in the Gulf of Oman, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1119, https://doi.org/10.5194/egusphere-egu2020-1119, 2020.

EGU2020-1591 | Displays | OS4.1

Optimal 3D Time-Energy Trajectory Planning for AUVs using Ocean General Circulation Models

Sultan Albarakati, Ricardo Lima, thomas Theussl, Ibrahim Hoteit, and Omar Knio

We develop a new approach for solving optimal time and energy-trajectory planning problems for Autonomous Underwater Vehicles (AUVs) in transient, 3D ocean currents. Realistic forecasts using an Ocean General Circulation Model (OGCM) are used for this purpose. The approach is based on decomposing the problem into a minimal time problem, followed by minimal energy subproblems. In both cases, a Non-Linear Programming (NLP) formulation is adopted. The methodology is first tested in idealized, steady, 2D settings, to verify the effectiveness of the method in addressing the multi-objective optimization problem. The scheme is then demonstrated for time-energy trajectory planning problems in the Gulf of Aden. In particular, the numerical experiments illustrate the capability of generating Pareto optimal solutions in a broad range of mission durations. In addition, the analysis also highlights how the methodology effectively exploits both the vertical structure of the current field, as well as its unsteadiness, namely, to minimize travel time and energy consumption.

How to cite: Albarakati, S., Lima, R., Theussl, T., Hoteit, I., and Knio, O.: Optimal 3D Time-Energy Trajectory Planning for AUVs using Ocean General Circulation Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1591, https://doi.org/10.5194/egusphere-egu2020-1591, 2020.

EGU2020-3086 | Displays | OS4.1

Observing Microstructure with Seagliders

Gillian Damerell, Rob Hall, and Peter Sheehan

In recent years the UEA glider group worked with Kongsberg Maritime and Rockland Scientific Inc. (RSI) to develop and test an integrated microstructure system which could be mounted on Seagliders.   Existing RSI microstructure packages such as the MicroRider could not be used on Seagliders because of a geometry mismatch with the shape of the Seaglider’s hull that made mounting difficult, and because the size of those packages added an unacceptable drag.  We also required a more sophisticated software interface so that the RSI system could be controlled by the standard Seaglider scientific logger payload control software.

 

Since the initial sea trials in 2015, microstructure Seagliders have been deployed on missions in the Bay of Bengal, the Faroe Shetland Channel, east of the Bahamas, and the Weddell Sea.  Early successes were followed by a number of technical problems which have now been resolved.  Improvements in the glider flight model have led to improvements in microstructure estimates.  The most recent deployment in the Weddell Sea allowed comparison between Seaglider estimates of dissipation and those from an established microstructure profiler (MSS90).  Dissipation estimates measured by the Seaglider system varied between 10-10 and 10-5 W kg-1, with higher values generally closer to the surface.  Those observed by the MSS were similar at depth but slightly higher in the top 200 m.  Further work will aim to understand these differences.

How to cite: Damerell, G., Hall, R., and Sheehan, P.: Observing Microstructure with Seagliders, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3086, https://doi.org/10.5194/egusphere-egu2020-3086, 2020.

EGU2020-3546 | Displays | OS4.1

Consistency of observed sea surface height changes, bottom pressure changes and temperature, salinity variations in a South Atlantic transect of the Antarctic Circumpolar Current

Alisa Yakhontova, Roelof Rietbroek, Jens Schröter, Nadja Jonas, Christina Lück, and Bernd Uebbing

Improved estimates of temperature, salinity, and sea surface height changes are computed from radar altimetry, satellite gravimetry and Argo profiles, and validated by the in situ ocean bottom pressure measurements in a South Atlantic transect of the Antarctic Circumpolar current. Using satellite gravimetry and altimetry observations, separate contributions to the global sea level can be estimated, but a regional solution is more challenging. Furthermore, Argo derived steric sea level change suffers from spatio-temporal sampling problems, and some signals are not well captured, e.g. in the deeper ocean below 2000m, around the boundary currents, in the Arctic or in the shelf/coastal regions. Jointly processing radar altimetry, Argo and data from the Gravity Recovery and Climate Experiment (GRACE), would allow to correct the deficiencies of the individual datasets, and produce observation based estimates of consistent temperature, salinity and sea surface height changes. In order to pave the way for an advanced joint inversion scheme that additionally resolves for temperature and salinity, the observation equations are formulated which link the satellite observations to temperature and salinity at depth. Observations in the South Atlantic region are compared with simulations from the FESOM model in terms of variability and the model data is used to find the spatial coherence of the signals at the sites with the surrounding ocean. The experiment is performed in the Southern Atlantic Ocean, where the estimates can be validated using an array of in situ ocean bottom pressure observations.

How to cite: Yakhontova, A., Rietbroek, R., Schröter, J., Jonas, N., Lück, C., and Uebbing, B.: Consistency of observed sea surface height changes, bottom pressure changes and temperature, salinity variations in a South Atlantic transect of the Antarctic Circumpolar Current, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3546, https://doi.org/10.5194/egusphere-egu2020-3546, 2020.

EGU2020-22115 | Displays | OS4.1 | Highlight

Evaluating the signature of oceanic striations on the distribution of biogeochemical properties in the Eastern Pacific Ocean off Chile

Pierre-Amaël Auger, Valerie Villegas, Ali Belmadani, David Donoso, Tomas Berger, and Samuel Hormazabal

In recent years, quasi-zonal mesoscale jet-like features, also called “striations”, have been ubiquitously detected in the time-mean circulation of the world ocean using satellite altimetry and in situ data. These alternating bands of eastward/westward flow are able to advect and mix physical properties. Yet, their impact on biogeochemistry, and potentially marine ecosystems, has not been assessed yet.

In eastern boundary upwelling systems, a mesoscale structuration of biogeochemical properties may be associated to striations through the interaction of zonal flows with sharp coastal-offshore background gradients (stirring). Transport patterns by mesoscale eddies (trapping) may also be involved as striations were noticeably suggested to result from the organization of the mesoscale eddy field as preferred eddy tracks in the eastern South Pacific upwelling system (off central Chile).

In this region, we evaluate the expression of striations in satellite records of ocean color and in a set of numerically simulated biogeochemical tracers (chlorophyll, carbon, primary production, oxygen, nutrients). A multi-decadal hindcast simulation of the physical-biogeochemical dynamics was run over the period 1984-2013 using the ROMS-PISCES (for Regional Oceanic Modeling System - Pelagic Interactions Scheme for Carbon and Ecosystem Studies) platform at an eddy-resolving resolution. High-pass spatial filtering is used to remove the large-scale signal in time-averaged satellite data and model outputs, and subsequently evaluate the match between striations and biogeochemical tracer anomalies in the model and observations. The relation between striations and the shape of the coastal-offshore gradient of the phytoplankton biomass and the oxygen-minimum zone is then deduced in the CTZ, and further in the open ocean region. The fraction of tracer anomalies associated to striations is quantified, and the respective potential role of stirring and eddy trapping is explored by matching quasi-zonal bands of sea level anomaly, geostrophic currents and biogeochemical tracers on moving frames of variable widths from 3 months to several years. The role played by eddy trapping is then confirmed by a composite analysis based upon automated eddy tracking.

How to cite: Auger, P.-A., Villegas, V., Belmadani, A., Donoso, D., Berger, T., and Hormazabal, S.: Evaluating the signature of oceanic striations on the distribution of biogeochemical properties in the Eastern Pacific Ocean off Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22115, https://doi.org/10.5194/egusphere-egu2020-22115, 2020.

Sampling and analysis of trace elements in open seawater and in sediment pore water in the deep sea is challenging due to small sample volumes and matrix effects. Here we evaluate an alternative method using the technique of diffusive gradients in thin films (DGT passive samplers), focussing on rare earth elements and yttrium (REY). DGT measures the labile fraction of metals in situ by fixing them on a Chelex resin after diffusion through a gel layer, providing a diffusive flux and averaged in situ concentrations of elements during the time of deployment. As the accumulated element concentrations increase with exposure time to solution, long-term deployment times overcome low trace metal concentrations in seawater and pore water. So far, no deep-sea applications of passive samplers are yet reported.

Sampling was performed in bottom seawater and surface sediments in the German licence area for manganese nodule exploration in the Clarion Clipperton Zone (CCZ, research cruise SO268 in April/May 2019), deployment times ranged from 12 hours in sediments to 4 weeks in open seawater.

Seawater DGT’s were deployed 0.5 m to 8 m above the seafloor. PAAS-normalized REY show the typical seawater pattern, with increase from LREE to HREE, a strong negative Ce anomaly, a kink from Gd to Tb, and a pronounced positive Y/Ho anomaly. The pattern and calculated concentrations agree very well with reported dissolved REY (<0.2 µm) for Pacific deep water (Alibo and Nozaki, 1999). Sediment DGT sticks were deployed in cores taken with a multicorer and cover the first 15 cm of the sediment. They provide in situ high-resolution profiles of trace metal fluxes and were analysed in 0.5 cm to 2 cm segments. We observe smooth PAAS-normalized patterns with negative Ce anomaly, an increase from LREE to MREE, and a slight decrease from Tb to Lu, sometimes accompanied by a small positive Y/Ho anomaly. The calculated concentrations generally increase with depth. Paul et al (2019) previously described very similar distribution patterns for CCZ sediment pore water and suggested Mn and Fe phases as the REY source. The pore water REY patterns clearly differ from bottom seawater already in the first centimetres of surface sediment– this sharp shift demonstrates that the dissolved pore water REY pool in the sediment surface is controlled by fluid-mineral equilibria.

The DGT passive sampling method may provide an additional tool to investigate biogeochemical processes at the deep-sea sediment-water interface and in the open ocean, and to monitor effects of anthropogenic disturbances at the seafloor on benthic trace element fluxes. We will discuss uncertainties of concentration calculation resulting from diffusion coefficients and from non-steady state conditions in pore water, and the comparability of DGT-derived distribution pattern and concentrations with results from physically filtered water. The DGT labile fraction is thought to represent the bioavailable fraction of trace elements and may also include colloids and nanoparticles (NPCs).

 

Alibo and Nozaki, 1999: Geochimica et Cosmochimica Acta 63, pp. 363-372.

Paul et al, 2019: Geochimica et Cosmochimica Acta 251, pp. 56-72.

How to cite: Schmidt, K., Paul, S. A. L., and Kriete, C.: REY distribution and concentration in bottom seawater and oxic pore water in the CCZ, NE Pacific: pilot study on the application of a DGT passive sampling method in deep sea environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19128, https://doi.org/10.5194/egusphere-egu2020-19128, 2020.

EGU2020-6215 | Displays | OS4.1

Assessment of long-term stability of inductive conductivity sensors at Argo floats

Nikolay Nezlin, Mark Halverson, Jean-Michel Leconte, Igor Shkvorets, Eric Siegel, Rui Zhang, and Greg Johnson

This study analyses accuracy and stability of salinity measurements collected by four Argo autonomous drifters with RBR Ltd. inductive conductivity sensors operating in the Pacific Ocean during the recent 2-4 years. Inductive sensors have advantages over traditionally used electrode-type cells due to their better resistance to surface contamination and low power requirements, resulting in more robust and accurate measurements and extended float lifetimes.  Proper assessment of the quality of the data collected by autonomous drifters is challenging due to lack of reference information. An important part of Argo program is the Delayed-Mode Quality Control process including salinity drift analysis and correction using the ‘Owens-Wong Calibration’ (OWC) method based on objective mapping of available reference data. This method, however, can misinterpret imperfect reference data as sensor drift. In this study, analyzing OWC output we introduce a combination of visualization methods focused on the locations where reference data can be treated as problematic. These methods include the analysis of spatial locations of the ‘profile correction factor’ along the float trajectory, comparing reference salinity fields calculated by the OWC method to additional reference sources (climatologies) and comparative analysis of different floats operating in the same area using the same reference datasets. The results demonstrate high level of stability of inductive conductivity cells on Argo floats, making them promising alternative for traditionally used Argo float CTDs equipped with electrode-type conductivity sensors.

 

How to cite: Nezlin, N., Halverson, M., Leconte, J.-M., Shkvorets, I., Siegel, E., Zhang, R., and Johnson, G.: Assessment of long-term stability of inductive conductivity sensors at Argo floats, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6215, https://doi.org/10.5194/egusphere-egu2020-6215, 2020.

EGU2020-17071 | Displays | OS4.1

Exploiting SeaDataCloud Temperature and Salinity time series data collections and comparing with Copernicus - a novel approach with SOURCE tool

Paolo Oliveri, SImona Simoncelli, Pierluigi DI Pietro, and Sara Durante

One of the main challenges for the present and future in ocean observations is to find best practices for data management: infrastructures like Copernicus and SeaDataCloud already take responsibility for assembly, archive, update and publish data. Here we present the strengths and weaknesses in a SeaDataCloud Temperature and Salinity time series data collections, in particular a tool able to recognize the different devices and platforms and to merge them with processed Copernicus platforms.

While Copernicus has the main target to quickly acquire and publish data, SeaDataNet aims to publish data with the best quality available. This two data repository should be considered together, since the originator can ingest the data in both the infrastructures or only in one, or partially in both. This results sometimes in data partially available in Copernicus or SeaDataCloud, with great impact for the researcher who wants to access as much data as possible. The data reprocessing should not be loaded on researchers' shoulders, since only skilled users in all data management plan know how merge the data.

The SeaDataCloud time series data collections is a Global Ocean soon-to-be-published dataset that will represent a reference for ocean researchers, released in binary, user friendly Ocean Data View format. The database management plan was originally for profiles, but had been adapted for time series, resolving several issues like the uniqueness of the identifiers (ID).

Here we present an extension of the SOURCE (Sea Observations Utility for Reprocessing. Calibration and Evaluation) Python package, able to enhance the data quality with redundant sophisticated methods and simplify their usage. 

SOURCE increases quality control (Q/C) performances on observations using statistical quality check procedures that follows the ocean best practices guidelines, exploiting the following  issues:

  1. Find and aggregate all broken time series using likeness in ID parameter strings;
  2. Find and organize in a dictionary all different metadata variables;
  3. Correct time series time to match simpler measure units;
  4. Filter devices that are outside of a selected horizontal rectangle;
  5. Give some information on original Q/C scheme by SeaDataCloud infrastructure;
  6. Give information tables on platforms and on the merged ID string duplicates together with an errors log file (missing time, depth, data, wrong Q/C variables, etc.).

In particular, the duplicates table and the log file may be helpful to SeaDataCloud partners in order to update the data collection and make it finally available for the users.

The reconstructed SeaDataCloud time series data, divided by parameter and stored in a more flexible dataset, give the possibility to ingest it in the main part of the software, allowing to compare it with Copernicus time series, find the same platform using horizontal and vertical surroundings (without looking to ID) find and cleanup  duplicated data, merge the two databases to extend the data coverage.

This allow researchers to have the most wide and the best quality possible data for the final users release and to to use these data to calibrate and validate models, in order to reach an idea of a whole area sea conditions.

How to cite: Oliveri, P., Simoncelli, S., DI Pietro, P., and Durante, S.: Exploiting SeaDataCloud Temperature and Salinity time series data collections and comparing with Copernicus - a novel approach with SOURCE tool, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17071, https://doi.org/10.5194/egusphere-egu2020-17071, 2020.

EGU2020-18477 | Displays | OS4.1

Using and acquiring time-series data with the EMSO ERIC DataLab

Raul Bardaji, Jaume Piera, Juanjo Dañobeitia, and Ivan Rodero

In marine sciences, the way in which many research groups work is changing as scientists use published data to complement their field campaign data online, thanks to the large increase in the number of open access observations. Many institutions are making great efforts to provide the data following FAIR principles (findability, accessibility, interoperability, and reusability) and are bringing together interdisciplinary teams of data scientists and data engineers.

There are different platforms for downloading marine and oceanographic data and many libraries to analyze data. However, the reality is that scientists continue to have difficulty finding the data they need. On many occasions, data platforms provide information about the metadata, but they do not show any underlying graph of the data that can be downloaded. Sometimes, scientists cannot download only the data parameters of interest and have to download huge amounts of data with other not useful parameters for their studies. On other occasions, the platform allows to download the data parameters of interest but offers the time-series data as many files, and it is the scientist who has to join the pieces of data into a single dataset to be analyzed correctly. EMSO ERIC is developing a data service that helps reduce the burden of scientists to search and acquire data as much as possible.

 

We present the EMSO ERIC DataLab web application, which provides users with capabilities to preview harmonized data from the EMSO ERIC observatories, perform some basic data analyses, create or modify datasets, and download them. Use case scenarios of the DataLab include the creation of a NetCDF file with time-series information across EMSO ERIC observatories.

The DataLab has been developed using engineering best practices and trend technologies for big data management, including specialized Python libraries for web environments and oceanographic data analysis, such as Plotly, Dash, Flask, and the Module for Ocean Observatory Data Analysis (MOODA).

How to cite: Bardaji, R., Piera, J., Dañobeitia, J., and Rodero, I.: Using and acquiring time-series data with the EMSO ERIC DataLab, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18477, https://doi.org/10.5194/egusphere-egu2020-18477, 2020.

EGU2020-6737 | Displays | OS4.1

CRDS-based dissolved N₂O & CH₄ measurement system

Il-Nam Kim, Seong-Su Kim, Ki-Tae Park, and Jae-Hyun Lim

 Gas chromatography (GC) is the most commonly used analytical equipment for tracer gas measurements. However, high performance equipment such as cavity ring-down spectrometer (CRDS) has been developed and currently become commercially available (G2308, PICARRO). CRDS is optical spectrometer to measure tracer gas, and its principal is that determines the gas concentration through the rate of decay of the optical signal. The great advantage of using CRDS is that not required too many material, time, and easy to handle than GC system. In general, CRDS is used for continuous measurement, which requires a large amount of gas for quantification. So, we have modified CRDS system to measure small amount of N2O/CH4 gases which is extracted from seawater samples using headspace method, and in turn have tested in various marine environments from coastal regions to open oceans. As a result, we have obtained highly accurate concentrations of dissolved N2O & CH4 gases, suggesting that the system would be useful to study dynamics of climate-relevant trace gases.

How to cite: Kim, I.-N., Kim, S.-S., Park, K.-T., and Lim, J.-H.: CRDS-based dissolved N₂O & CH₄ measurement system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6737, https://doi.org/10.5194/egusphere-egu2020-6737, 2020.

EGU2020-22606 | Displays | OS4.1

Using Ship-Deployed High-Endurance Unmanned Aerial Vehicles for the Study of Ocean Surface and Atmospheric Boundary Layer Processes

Scott M. Brown, Christopher J. Zappa, Nathan J. M. Laxague, Tejendra Dhakal, Ryan A. Harris, Aaron Farber, and Ajit Subramaniam

Unmanned aerial vehicles (UAVs) are proving to be an important modern sensing platform that supplement the sensing capabilities from platforms such as satellites, aircraft, research vessels, moorings, and gliders. UAVs, like satellites and aircraft can provide a synoptic view of a relatively large area. However, the coarse resolution provided by satellites and the operational limitations of manned aircraft has motivated the development of unmanned systems. UAVs offer unparalleled flexibility of tasking; for example, low altitude flight and slow airspeed allow for the characterization of a wide variety of geophysical phenomena at the ocean surface and in the marine atmospheric boundary layer. Here, we present the development of cutting-edge payload instrumentation for UAVs that provides a new capability for ship-deployed operations to capture a unique, high resolution spatial and temporal variability of the changing air-sea interaction processes than was previously possible. The modular design of the base payload means that new instruments can be incorporated into new research proposals that may include new instruments for expanded use of the payloads as a long-term research facility. Additionally, we implement a novel capability for vertical take-off and landing (VTOL) from research vessels. This VTOL capability is safer and requires less logistical support than previous ship-deployed systems. Furthermore, these VTOL UAV systems have 15-hour endurance with 15-lb payloads, fully autonomous take-off, flight, and landing from ships, and high-bandwidth data telemetry (100 Mbits/s over 50+ nm range) for real-time mission control and provide for our “eyes over the horizon.” The payloads developed include thermal infrared, visible broadband and hyperspectral, and near-infrared hyperspectral high-resolution imaging. Additional capabilities include quantification of the longwave and shortwave hemispheric radiation budget (up- and down-welling) as well as direct air-sea turbulent fluxes. Finally, a UAV-deployed dropsonde-microbuoy was developed in order to profile the temperature, pressure and humidity of the atmosphere and the temperature and salinity of the near-surface ocean. These technological advancements provide the next generation of instrumentation capability for UAVs. We present on the results of 3 case studies in the South Pacific near Fiji, including measurements characterizing meso-scale ocean SST fronts, trichodesmium blooms, and floating pumice rafts from a recent undersea volcanic eruption near Tonga. When deployed from research vessels, these UAVs will provide a transformational science prism unequaled using 1-D data snapshots from ships or moorings alone.

How to cite: Brown, S. M., Zappa, C. J., Laxague, N. J. M., Dhakal, T., Harris, R. A., Farber, A., and Subramaniam, A.: Using Ship-Deployed High-Endurance Unmanned Aerial Vehicles for the Study of Ocean Surface and Atmospheric Boundary Layer Processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22606, https://doi.org/10.5194/egusphere-egu2020-22606, 2020.

OS4.2 – Surface Waves and Wave-Coupled Effects in Lower Atmosphere and Upper Ocean

EGU2020-3204 | Displays | OS4.2

A nonlinear Schrödinger equation for gravity waves slowly modulated by linear shear flow

Shaofeng Li, Juan Chen, Anzhou Cao, and Jinbao Song

Assume that a fluid is inviscid, incompressible, and irrotational. A nonlinear Schrödinger equation (NLSE) describing the evolution of gravity waves in finite water depth is derived using the multiple scale analysis method. The gravity waves are influenced by a linear shear flow, which is composed of a uniform flow and a shear flow with constant vorticity. The modulational instability (MI) of the NLSE was analyzed in this paper, and the region of MI for gravity waves (the necessary condition for the existence of freak waves) was identified. In this paper, the uniform background flows along or against wave propagation are referred to as down-flow and up-flow, respectively. Uniform up-flow enhances the MI, whereas uniform down-flow reduces it. Positive vorticity enhances the MI, while negative vorticity reduces it. Hence, the influence of positive (negative) vorticity on MI can be balanced out by that of uniform down- (up-)flow. Furthermore, the Peregrine breather (PB) solution of the NLSE is applied to freak waves. Uniform up-flow increases the steepness of free surface elevation, while uniform down-flow decreases it. Positive vorticity increases the steepness of free surface elevation, whereas negative vorticity decreases it.

How to cite: Li, S., Chen, J., Cao, A., and Song, J.: A nonlinear Schrödinger equation for gravity waves slowly modulated by linear shear flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3204, https://doi.org/10.5194/egusphere-egu2020-3204, 2020.

EGU2020-7149 | Displays | OS4.2

Microphysics of the air-sea coupling at high winds and its role in the dynamics and thermodynamics of severe sea storms

Yuliya Troitskaya, Alexander Kandaurov, Daniil Sergeev, Olga Ermakova, Dmitrii Kozlov, Maxim Vdovin, and Oleg Druzhinin

Showing the record strengths and growth-rates, a number of recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea coupling. Especially challenging is the nature and effect of the very small-scale phenomena, the sea-spray and foam, supposed to strongly affecting the momentum- and heat- air-sea fluxes at strong winds. This talk will focus on our progress in understanding and describing these "micro-scale" processes, their physical properties, the spray and foam mediated air-sea fluxes and the impact on the development of marine storms.

The starting points for this study were two laboratory experiments. The first one was designed for investigation of the spray generation mechanisms at high winds. We found out 3 dominant spray generating mechanisms: stretching liquid ligaments, bursting bubbles, splashing of the falling droplets and "bag-breakup". We investigated the efficiency spray-production mechanisms and developed the empirical statistics of the numbers of the spray generating events of each type. Basing on the "white-cap method" we found out the dependence of the spray-generating events on the wind fetch. The main attention was paid to the "bag-breakup" mechanism. Here we studied in detail the statistics of spray produced from one "bag-breakup" event. Basing on these developments, we estimated heat and momentum fluxes from the spray-generating events of different types and found out the dominant role of the "bag-breakup" mechanism.

To estimate the direct heat and momentum fluxes from the ocean surface to the atmosphere, we studied in the special experiment the foam impact on the short-wave part of the surface waves and the heat momentum exchange in the atmospheric boundary layer at high winds. Based on these results, we suggest a simple model for the aerodynamic and temperature roughness and the eddy viscosity in the turbulent boundary layer over a fractionally foam-covered water surface.

The synergetic effect of foam at the water surface and spray in the marine atmospheric boundary layer on ocean surface resistance at high winds is estimated so as to be able to explain the observed peculiarities of the air-sea fluxes at stormy conditions. Calculations within the nonhydrostatic axisymmetric model show, that the "microphysics" of the air-sea coupling significantly accelerate development of the ocean storm.

This work was supported by RFBR grant 19-05-00249 and RSF grant 19-17-00209.

How to cite: Troitskaya, Y., Kandaurov, A., Sergeev, D., Ermakova, O., Kozlov, D., Vdovin, M., and Druzhinin, O.: Microphysics of the air-sea coupling at high winds and its role in the dynamics and thermodynamics of severe sea storms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7149, https://doi.org/10.5194/egusphere-egu2020-7149, 2020.

EGU2020-12109 | Displays | OS4.2 | Highlight

Air-sea Interface Exchanges in Rapidly Intensifying Tropical Cyclones

Alexander Soloviev, Breanna Vanderplow, and Roger Lukas

Rapid intensification of tropical cyclones is a challenge for forecasters. In 2017, Hurricane Maria intensified to a Category 5 storm within 24 hours and devastated Puerto Rico. The official forecast and all computer models were unable to predict it. Hurricane Dorian had been predicted as a tropical storm; unexpectedly, it intensified into a Category 5 storm and destroyed the Bahamas. Soloviev et al. (2017) suggested that under the assumption of constant enthalpy exchange coefficient, rapid cyclone intensification and decay can be related to the drag coefficient dependence on wind speed including an “aerodynamic drag well” around 60 m/s. This concept is in general terms consistent with Emanuel’s (1988) theory of maximum potential intensity of a tropical cyclone and its extension by Lee et al. (2019). The influence of sea spray is still a significant uncertainty. In order to study the effect of spray on dynamics of tropical cyclones, we have implemented a Volume of Fluid to Discrete-Phase Model (VOF to DPM). This model re-meshes the areas with increased gradients or curvature, which are suspicious for the interface instability. The generated water particles that satisfy the condition of asphericity are converted into Lagrangian particles. The size distribution of spray measured in air-sea interaction facilities is used for the model verification. Due to dynamic remeshing, VOF to DPM resolves spray particle radius from ten micrometers to a few millimeters, which correspond to spume. Results of the numerical simulation show a dramatic increase of spume generation under major tropical cyclones. Though sub-micrometer and micrometer scale spray particles are not resolved in this simulation, they are likely less significant in the momentum exchange at the air-sea interface than spume. These results are expected to contribute to the parameterization and proper treatment of spray in forecasting models, including cases of rapid intensification and rapid decline of tropical cyclones.
References:
Emanuel, K. A. (1988). The maximum intensity of hurricanes. JAS 45, 1143–1155.
Soloviev, A. V., Lukas, R., Donelan, M.A., Haus, B. K., Ginis, I. (2017). Is the state of the air-sea interface a factor in rapid intensification and rapid decline of tropical cyclones? JGR - Oceans 122, 10174-10183.
Lee, W., Kim, S.‐H., Chu, P.‐S., Moon,I.‐J., and Soloviev, A. V. (2019). An index to better estimate tropical cyclone intensity change in the western North Pacific. GRL 46, 8960-8968.

How to cite: Soloviev, A., Vanderplow, B., and Lukas, R.: Air-sea Interface Exchanges in Rapidly Intensifying Tropical Cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12109, https://doi.org/10.5194/egusphere-egu2020-12109, 2020.

EGU2020-9311 | Displays | OS4.2

Modeling of Sea Sea Surface Waves In Hurricane Basing On Self-Similarity Concept

Mariya Yurovskaya, Vladimir Kudryavtsev, and Bertrand Chapron

The study is based on a simple parametric model, which is an extension of the self-similarity theory for surface waves generated by a wind field. According to the original similarity concept, the development of wind waves can be fully described using the scale of the fetch length (or time) and wind velocity. The aim of the work is to develop a parametric model to describe the wave generation in arbitrary spatio-temporal wind field. We assume that in this case similarity laws are also fulfilled, i.e., the rate of spectral the peak frequency and wave energy change is completely determined by the wave age. The source function is written in a form providing the stationary solution that corresponds to the well-known fetch law, confirmed in numerous experiments.

In order to extend the equations to the two-dimensional case, when the wind change occurs in both directions, it is assumed that the relations stay valid if the wind speed is replaced by its component in spectral peak direction. In this case, the system of equations should be supplemented by an expression for the evolution of spectral peak direction, describing its adaptating to the direction of non-uniform wind.

The algorithm for solving the complete system of equations describing the evolution of wave height, spectral peak frequency, its propagation direction and focusing/defocusing of wave energy, is based on the method of characteristics. To simulate the evolution of waves in a hurricane, we use the calculation in a non-stationary reference system associated with the hurricane. Coordinates, wave peak frequency, energy and direction are calculated along ray trajectory at every discrete time moment. To increase the stability of the numerical scheme, an implicit 4th-order Runge-Kutta method is used.

Test calculations were carried out for the case of the wave development from the coast with a uniform wind and then for an inhomogeneous cyclonic wind field for different hurricane speeds. The calculations reproduce the anisotropy of the energy distribution inside the hurricane and the effect of wave trapping by a moving cyclone. A comparison of the results with available field measurements of wave parameters in tropical cyclones showed their good agreement. The proposed algorithm can be used in wave forecast models and can serve for deeper understanding the wave field formation in extreme conditions.

The work was supported by Russian Science Foundation via grant 17-77-30019 and the Ministry of Education and Science of the Russian Federation under the State Assignment No. 0827-2018-0003.

How to cite: Yurovskaya, M., Kudryavtsev, V., and Chapron, B.: Modeling of Sea Sea Surface Waves In Hurricane Basing On Self-Similarity Concept, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9311, https://doi.org/10.5194/egusphere-egu2020-9311, 2020.

EGU2020-22561 | Displays | OS4.2

The Impact of Wind Gusts on the Ocean Thermal Skin Layer by Capillary-Gravity Waves

Christopher J Zappa, Nathan Laxague, Sophia Brumer, and Steven Anderson

The thermodynamic and emissive properties of the ocean thermal skin layer are crucial contributors to air-sea heat flux. In order to properly observe ocean surface temperature without disturbing any delicate fluid mechanical processes, thermal infrared imaging is often used. However, wind impacting the ocean surface complicates the extraction of meaningful information from thermal imagery; this is especially true for transient forcing phenomena such as wind gusts. Here, we describe wind gust-water surface interaction through its impact on skin layer thermal and emissive properties. Two key physical processes are identified: (1) the growth of centimeter-scale wind waves, which increases interfacial emissivity and (2) microscale wave breaking and shear, which mix the cool skin layer with warmer millimeter-depth water and increase the skin temperature. As more observations are made of air-sea interaction under transient forcing, the full consideration of these processes becomes increasingly important.

How to cite: Zappa, C. J., Laxague, N., Brumer, S., and Anderson, S.: The Impact of Wind Gusts on the Ocean Thermal Skin Layer by Capillary-Gravity Waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22561, https://doi.org/10.5194/egusphere-egu2020-22561, 2020.

Numerous observations show that in spite of relative motions of floes caused by wave propagation in marginal ice zone (MIZ) direct contacts between them don’t occur. Nevertheless, relative motions of floes may influence formation of oscillating water currents between them which take and dissipate the energy of incoming waves. Full-scale and laboratory experiments were performed to investigate characteristics of water currents between interacting floes. The experiments included the investigation of vertical and horizontal oscillating motions of floes in ice environment. During the experiments we recorded floe accelerations, water pressure and water velocity. Main goal of the experiments was to estimate effective viscosity of water in gapes between interacting floes, describe floe-floe forces caused by the floe accelerations, and estimate the influence of slush formation on the effective viscosity of water. The floe motion was initiated by mechanical pooling, towing with a rope and by original pendulum rig. The experiments were performed in the Van-Mijen Fjord of Spitsbergen in winter seasons of 2018 and 2019, and in the wave flume at UNIS. A lubrication theory was used to describe the dependence of water pressure between interacting floes from their relative speed and the distance between approaching surfaces. Comparison of numerical simulations with experimental records showed that the action of water pressure and the formation of flow jets can prevent direct collision of approaching floes. Obtained analytical formulas are used for the formulation of rheological equations describing the behavior of broken ice in MIZ.                  

How to cite: Marchenko, A.: Influence of floe-floe interactions on wave damping in marginal ice zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4214, https://doi.org/10.5194/egusphere-egu2020-4214, 2020.

EGU2020-6378 | Displays | OS4.2 | Highlight

Wave groups and spectral shape in ice

Johannes Gemmrich, Jim Thomson, and Todd Mudge

The reduction of arctic summer ice coverage has prompted a renewed interest in the physics of wave-ice interactions. Progress has been made on the observational, theoretical and modeling aspects of waves in the presence of ice.

This presentation will address our recent observational studies of spectral wave properties in the marginal ice zone, and in ice covered seas. Waves propagating through ice are attenuated and scattered, resulting in a pronounced change of the shape of the wave spectrum. In particular, the strong attenuation of the high frequency components affects wave steepness and the spectral bandwidth, and thus wave groupiness and the crest height distribution.

We present data for various ice conditions, obtained from drifting SWIFT buoys, a moored ADCP and moored inverted echosounders. All observations show well developed group structures of the waves. However, for different datasets we obtain opposite dependencies between wave groupiness and wave spectral characteristics. This suggests that depending on ice condition, both, the linear mechanism of wave superposition, or wave nonlinarites can be responsible for the wave group enhancement in ice. These mechanisms will be discussed.

How to cite: Gemmrich, J., Thomson, J., and Mudge, T.: Wave groups and spectral shape in ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6378, https://doi.org/10.5194/egusphere-egu2020-6378, 2020.

The global analyses and medium range forecasts from the European Centre for Medium range Weather Forecasts rely on a state-of-the-art Numerical Weather Prediction (NWP) system. To best represent the air-sea exchanges, it is tightly coupled to an ocean wave model.  As part of ECMWF approach to Earth System Model, it is also coupled to a global ocean model for all its forecasting systems from the medium range up to the seasonal time scale.

Because the feedback from and to the ocean can be significant, it is only in the fully coupled system that parameterisation for air-sea processes should be revisited. For instance, it is now accepted that the drag coefficient should generally attained maximum values for storm winds but should level or even decrease for very strong winds, namely in tropical cyclones or intense mid-latitude wind storms.

A modification of the wind input source was tested, whereby the Charnock coefficient estimated by the wave model and therefore the drag coefficient sharply reduce for large winds (> 30 m/s). As a consequence, ECMWF tendency to under predict strong tropical cyclones was sharply alleviated, in better agreement with observational evidence. This change is now planned for operational implementation with the next model cycle (CY47R1, June 2020).

Experimental evidences also point to a sea state/wind dependency of the heat and moisture fluxes.  Following an extension of the wind wave generation theory, a sea state dependent parameterisation for the roughness length scales for heat and humidity has been tested. Again, a proper assessment of the different parameterisations warrants the fully coupled system. Experimentations so far indicate the benefit of such change. Ongoing work aims at future operational implementation.

How to cite: Bidlot, J.: Sea-state dependency of air-sea fluxes for high winds in ECMWF Earth System Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6074, https://doi.org/10.5194/egusphere-egu2020-6074, 2020.

EGU2020-21965 | Displays | OS4.2 | Highlight

In situ measurements of coupled wind-wave dynamics using PIV

Marc Buckley and Jochen Horstmann

Small-scale turbulent dynamics within the coupled atmospheric and oceanic wave boundary layers control air-sea fluxes of momentum and scalars. However measuring and understanding small-scale dynamics very close to the rapidly moving ocean surface remains technically challenging.

We present novel in situ measurements of small-scale motions in the airflow above, and in the water below the wavy air-water interface. A high resolution, large field of view PIV system (Particle Image Velocimetry) was developed for in situ air-water measurements within the first millimeters to meters above and below the wavy surface. The system was recently deployed on a single pile platform in the Szczecin lagoon (Baltic Sea coast, Germany). We will show first results and we will discuss the influence of waves on the partitioning of momentum flux within the coupled air-water wave boundary layers.

How to cite: Buckley, M. and Horstmann, J.: In situ measurements of coupled wind-wave dynamics using PIV, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21965, https://doi.org/10.5194/egusphere-egu2020-21965, 2020.

EGU2020-18553 | Displays | OS4.2

Observation of high-resolution fetch-limited wave growth using CFOSAT near-nadir measurements

Alexey Mironov and Bertrand Chapron

CFOSAT is the joint space mission of the French (CNES) and Chinese (CNSA) space agencies dedicated to the observation of ocean surface wind and waves. Two main on-board payloads, the Ku-band near-nadir wave scatterometer SWIM and dual-polarization Ku-band wind scatterometer SCAT for the first time provide regular synchronized surface wind vector and sea state observations on a global scale. 
 
After the first year of the mission, SWIM innovative wave scatterometer conception proved to be suitable for space-born directional wave spectra measurements. The overall performance of the instrument and the quality of inverted data are close to the planned specifications. Moreover, in particular cases, precise wave parameters can be estimated even in limited coastal seas with varying wave and wind conditions.
 
In this work, we will show examples of high-resolution directional wave spectrum fetch evolution as observed by CFOSAT mission. The present analysis was performed for areas with fetch-determined conditions, during periods when waves were generated by strong coastal winds. The dataset includes spectra starting from young wind waves (~20 km fetch distance) to mature almost developed sea state (>450 km fetch). The unique multi-beam configuration of SWIM provides multiple estimates of wave spectral parameters all over the sensor footprint along the satellite track. This allows the capturing of the very fine details of spectral variability with distance from the coast. It will be shown, the observed deviation of direction and wavelength of the measured spectral peak from a wave growth fetch law can be attributed to time-varying or topography-induced coastal wind field inhomogeneities. In some cases, this can be explained by local surface current configuration. The obtained results can be directly compared with third party remote sensing observations, numerical model outputs, classical wave fetch-growth laws, and existing empirical parametrizations. 

How to cite: Mironov, A. and Chapron, B.: Observation of high-resolution fetch-limited wave growth using CFOSAT near-nadir measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18553, https://doi.org/10.5194/egusphere-egu2020-18553, 2020.

EGU2020-19804 | Displays | OS4.2

Global wave height trends and variability from new multi-mission satellite altimeter products, reanalyses and wave buoys

Christine Gommenginger, Ben Timmermans, Guillaume Dodet, and Jean-Raymond Bidlot

Accurate knowledge and understanding of the sea state and its variability is crucial to numerous oceanic and coastal engineering applications, but also to climate change and related impacts including coastal inundation from extreme weather and ice-shelf break-up. The increasing duration of the satellite altimeter record for sea state motivates a range of global analyses, including the examination of changes in ocean climate. For ocean surface waves in particular, the recent development and release of new products providing observations of altimeter-derived significant wave height make long term analyses fairly straightforward.

In this study, significant wave height climatologies and trends over 1992-2017 are intercompared in four recent high-quality global datasets using a consistent methodology. In particular, we make use of products presented by Ribal et al. (2019), and the recently released product developed through the European Space Agency Climate Change Initiative (CCI) for Sea State (Dodet et al. 2020, ESSD, in review). Regional differences in mean climatology are identified and linked to low and high sea states, while temporal trends from the altimetry products, and two reanalysis and hindcast datasets, show general similarity in spatial variation and magnitude but with major differences in equatorial regions and the Indian Ocean. Discrepancies between altimetry products likely arise from differences in calibration and quality control. However, multidecadal observations at buoy stations also highlight issues with wave buoy data, raising questions about their unqualified use, and more fundamentally about uncertainty in all sea state products. We discuss these results in the context of both the current state of knowledge of the changing wave climate, and the on-going development of CCI Sea State altimetry products.

How to cite: Gommenginger, C., Timmermans, B., Dodet, G., and Bidlot, J.-R.: Global wave height trends and variability from new multi-mission satellite altimeter products, reanalyses and wave buoys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19804, https://doi.org/10.5194/egusphere-egu2020-19804, 2020.

EGU2020-38 | Displays | OS4.2 | Highlight

Wave Climate of the Southern Ocean

Ian Young

Although the Southern Ocean is often viewed as a very remote area, it plays a critical role in global climate. Waves generated in intense Southern Ocean storms propagate across the Indian, Pacific and Atlantic Oceans and define the wave climate for many areas of these oceans. In addition, the wind and wave climate of the Southern Ocean plays an important role in determining the rate of decay of Antarctic glaciers which are an important element in global sea level change. Despite this important role, little is known about the wind and wave climate of this vast region. This paper will bring together a series of unique datasets to provide a comprehensive view of this wind and wave climate. These datasets include: the long duration model reanalysis dataset ERA-I, a 33-year calibrated and validated altimeter dataset and buoy data from four deployments. These buoys have been located at: Macquarie Island (540S), Campbell Island (520S), a site west of South America (550S) and the Southern Ocean Flux Buoy south of Tasmania (460S). Data from these buoy deployments spans a total of approximately 7 years and provides directional spectra in unique long fetch environments. In addition to providing valuable data for model validation, engineering design and Naval Architecture, these combined datasets provide new insights into air-sea interaction under extremely long fetch conditions. The paper will also use the satellite datasets to investigate changes in wave conditions in recent decades and the role that climate variability plays in such changes. This analysis will examine: long-term trends, annual variability and multi-year oscillations.

How to cite: Young, I.: Wave Climate of the Southern Ocean , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-38, https://doi.org/10.5194/egusphere-egu2020-38, 2020.

EGU2020-11567 | Displays | OS4.2

Interannual variability of ocean surface waves in the Southeast Pacific

Catalina Aguirre, Diego Becerra, Marcelo Godoy, and Diego Silva

Ocean surface (wind-driven) waves continuously shape the coastal environment and play a relevant role in ocean-atmosphere interaction processes. They are also important in operational aspects of ports and have significant energy potential. This research is focused on the interannual variability of the wind waves in the Southeast Pacific, particularly its relationship with the Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO). We used a 38-year wave simulation (1979-2016) performed using the Wavewatch III model forced with surface winds and ice concentration from the ERA-Interim reanalysis. Additionally, a cyclone tracking software was used to analyze the trajectories of the extratropical storms which generate the wind waves that reach the coast of western South America. Time series statistics, such as correlation and composites analysis, have been applied to both wave parameters (such as significant wave height and mean period) and directional spectra. Results show a significant and positive correlation between the SAM and the significant wave height and the mean period of the wind waves. However, local storms in central Chile, which are the most damaging extreme wave events for coastal infrastructure, are less frequent during the positive phase of the SAM. Furthermore, a trend analysis shows an increase of the significant wave height during the last decades, which is consistent with the trend toward the positive phase experienced by the SAM. On the other hand, the wave energy of remote origin that travels from the North Pacific toward the Southeast Pacific, which is maximum during the austral summer, shows a significant relationship with the extreme El Niño events. These energetic swells events that reach the coast of western South America during the austral summer are more intense and frequent during the warm phase of ENSO.

How to cite: Aguirre, C., Becerra, D., Godoy, M., and Silva, D.: Interannual variability of ocean surface waves in the Southeast Pacific, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11567, https://doi.org/10.5194/egusphere-egu2020-11567, 2020.

EGU2020-2781 | Displays | OS4.2

Projected 21st Century changes in extreme wind-wave events

Alberto Meucci

Extreme ocean waves shape world coastlines and significantly impact offshore operations. Climate change may further exacerbate these effects increasing losses in human lives and economic activities. Studies generally agree on the trends in the mean values, yet there is no consensus on the extreme events, and whether their magnitude and/or frequency are changing. The present work applies an innovative extreme value analysis approach to a multi-model ensemble wind-wave climate dataset, derived from seven global climate models, to evaluate projected extreme wave height changes towards the end of the 21st century. Under two greenhouse gas emission scenarios, we find that at the end of the 21st century, the one in 100-year wave height event increases across the scenarios by 5 to 15 % over the Southern Ocean. The North Atlantic shows a decrease at low to mid-latitudes (5 to 15 %) and an increase at the high latitudes (10 %). The extreme wave heights in the North Pacific increase at the high latitudes by 5 to 10 %. The present work suggests that pooling an ensemble of future projected ocean storms from different GCMs might significantly improve uncertainty estimates connected to future coastal and offshore wave extremes, thereby improving climate adaptation strategies.

How to cite: Meucci, A.: Projected 21st Century changes in extreme wind-wave events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2781, https://doi.org/10.5194/egusphere-egu2020-2781, 2020.

The research we perform has important engineering applications since a lot of marine activities and offshore engineering activities are in shallow water areas where phenomena like bottom and white-capping dissipation and wind growth take place. The physical parametrization of such forcing/dissipation has become an important issue in the improvement of the performance of models in order to provide accurate sea-state information. In this regard, we perform a sensitivity analysis of dissipation parameterizations in the third-generation spectral wave model WAVEWATCH III using the ST6 source term packages, proposed by Zieger-Babanin 2015, to describe wind generation and dissipation due to white-capping and bottom friction.

A system of nested grids is used to model long distance swells generated in the North Atlantic Ocean and propagating all the way to the west coast of Ireland. We used a 30-minute coarse resolution for the North Atlantic grid, a 6-minute intermediate resolution for the North-East Atlantic, and a 3-minute fine resolution in coastal areas closer to Ireland.

The sensitivity analysis in the parameterization is based on the effect of the model performance by varying the adjustable parameters in the wind input source, swell dissipation in terms of the interaction of waves with oceanic turbulence and the drag coefficient to potentially eliminate a bias in the wind field. The results of the model for the coast of Ireland are discussed in terms of various parametrization schemes.

How to cite: Fernández, L., Calvino, C., and Dias, F.: Sensitivity analysis of dissipation parameterizations in the WAVEWATCH III spectral wave model using the ST6 source term packages for Ireland Coast., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-547, https://doi.org/10.5194/egusphere-egu2020-547, 2020.

EGU2020-3605 | Displays | OS4.2

Implementation of the wave boundary layer model in the OpenIFS model

Nefeli Makrygianni, Jean R. Bidlot, Michaela Bray, and Shunqi Pan

For more than 30 years, many studies have been carried out to improve the understanding of the air-sea interaction and its impact on the predictions of atmospheric and the oceanic processes. It is well understood that the accuracy in predictions of the wind-driven waves is highly dependent on the source input and dissipation terms. The Wave Boundary Layer (WBL) approach for the estimation of surface stress has previously been used to improve the wind and wave simulations under extreme conditions. However, until recently the WBL was only used to determine the roughness length (z0) and drag coefficient (Cd), but not to alter the wind input source function in wave models. In this study, the wave boundary layer model (WBLM) was implemented in the OpenIFS coupled model as source functions as suggested by Du et al. (2017, 2019). The new wind input and dissipation terms are then tested using numerical model simulations, with a particular focus on the contribution of the high frequency tail in the source input function.  The comparison of the results of this study with published results hints at better performance of the model on the estimation of the roughness length and drag coefficient. This should improve predictions of the significant wave height and wind speeds, especially under extreme conditions.

Corresponding Author: Nefeli Makrygianni (makrygiannin@cardiff.ac.uk)

How to cite: Makrygianni, N., Bidlot, J. R., Bray, M., and Pan, S.: Implementation of the wave boundary layer model in the OpenIFS model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3605, https://doi.org/10.5194/egusphere-egu2020-3605, 2020.

EGU2020-3207 | Displays | OS4.2 | Highlight

Observational evidence of surface wave-generated strong ocean turbulence

Hongyu Ma, Dejun Dai, Jingsong Guo, and Fangli Qiao

By using an Acoustic Doppler Velocimeter (ADV) mounted on the seabed of the continental shelf of the northern South China Sea, high frequency velocity fluctuations were measured for 4.5 days. The turbulent kinetic energy dissipation rate was estimated. During the observation, the strong ocean response to Typhoon Rammasun was recorded to compare the turbulent characteristics before and during the typhoon. The results show that the turbulence near the seabed is mainly generated by the tidal current shear and exhibits a quarter diurnal variation during the period before the typhoon. During the typhoon period, the dissipation rate dramatically increased from 1×10-6 m2 s-3 to 1×10-2 m2 s-3 within a short time, and the significant wave height and the surface wave orbital velocity showed the same tendency. This finding suggests that the turbulence is dominantly generated by the surface waves near the seabed.

How to cite: Ma, H., Dai, D., Guo, J., and Qiao, F.: Observational evidence of surface wave-generated strong ocean turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3207, https://doi.org/10.5194/egusphere-egu2020-3207, 2020.

EGU2020-4945 | Displays | OS4.2

Large Eddy Simulation Coupling Model For Small-scale Air-sea Interaction

Danyi Sun and Shuang Li

Atmosphere and ocean are two important factors that affect the earth's climate system, and their interaction is an important topic in the study. In view of the lack of turbulence scale analysis in the traditional large-scale air-sea coupling model, this paper uses the Parallelized Large-Eddy Simulation Model (PALM) to explore the effect of Langmuir circulation on air-sea flux and turbulent kinetic energy budget at a small scale, and conducts air-sea coupled simulation for atmospheric boundary layer (ABL) and ocean mixed layer (OML). The results show that the distribution of air-sea flux near the surface is greatly influenced by the Langmuir circulation, thus strengthening the ocean mixing. The pressure term in the turbulent kinetic energy budget of the ocean is greatly affected by the Langmuir circulation near the sea surface and weakens rapidly as the depth deepens. This study shows the application of the small-scale air-sea coupling model in the study of air-sea flux, which has certain significance for the study of small-scale air-sea interaction.

How to cite: Sun, D. and Li, S.: Large Eddy Simulation Coupling Model For Small-scale Air-sea Interaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4945, https://doi.org/10.5194/egusphere-egu2020-4945, 2020.

EGU2020-6316 | Displays | OS4.2

Could the cloud cover in future climate models be improved by incorporating the role of sea spray in surface heat fluxes?

Yajuan Song, Fangli Qiao, Qi Shu, Jiping Liu, Ying Bao, Meng Wei, Biao Zhao, and Zhenya Song

Accurate cloud cover and radiative effect simulation remains a long-standing challenge for global climate models (GCMs). The Southern Ocean (SO) cloud cover is substantially underestimated by most GCMs. Therefore, too much shortwave radiation is absorbed by oceans, which causes an overly warm sea surface temperature (SST) bias over the SO. For the first time, sea spray effects on latent and sensible heat fluxes are considered in a climate model. The most notable sea spray impacts on heat fluxes occur over the SO, with anomalous latent heat fluxes up to -7.74 W m-2. Enhanced latent heat release lead to SST cooling. In addition, more clouds are formed over the SO to reflect excessive downward shortwave radiation, especially low-level clouds at 1.51% increments. Our results provide a feasible solution to mitigate the lack of low-level clouds and overly warm SST biases over the SO in GCMs.

How to cite: Song, Y., Qiao, F., Shu, Q., Liu, J., Bao, Y., Wei, M., Zhao, B., and Song, Z.: Could the cloud cover in future climate models be improved by incorporating the role of sea spray in surface heat fluxes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6316, https://doi.org/10.5194/egusphere-egu2020-6316, 2020.

EGU2020-6557 | Displays | OS4.2

Growth of Wave Height with Retreating Ice Cover in the Arctic

Changlong Guan and Jingkai Li

For the Arctic surface waves, one of the most uncontroversial viewpoints is that their escalation in the past few years is mainly caused by the ice extent reduction. Ice retreat enlarges the open water area, i.e., the effective fetch, and thus allows more wind input energy and available distance for wave evolution. This knowledge has been supported by a few previous studies on the Arctic waves which analyzed the correlation between time-series variations in wave height and ice coverage. However, from the perspective of space, the detailed relationship between retreating ice cover and increasing surface waves is not well studied. Hence, we performed such a study for the whole Arctic and its subregions, which will be helpful for a better understanding of the wave climate and for forecasting waves in the Arctic Ocean.

Wave data are produced by twelve-year (2007-2018) hindcasts of summer melt seasons (May-Sept.) and numerical tests with WAVEWATCH III. When a viscoelastic wave-ice model and a spherical multiple-cell grid are applied, simulated wave heights agree with available buoy data and previous research. After the validations, simulated significant wave heights over twelve-year summer melt seasons are used to demonstrate the detailed relationship between the escalation of wave height and reduction of ice extent for the whole Arctic and seven subregions. Through least square regression, we find that the mean wave height in the Arctic Ocean will increase by 0.071m (106km2)-1 when the ice extent is smaller than 9.4×106km2, and roughly 51% is contributed by the enlarged fetch. By analyzing the nondimensional wave energy and comparing the simulated wave height with Wilson IV, we prove the swell is widespread during the summertime in the current Arctic Ocean. Furthermore, we also display the variations in probabilities of occurrence of large waves as ice-edge retreats in seven subregions. Assuming that an ice free period occurs in the Arctic in September, the model results show that the simulated mean wave height is approximately 1.6m and the large waves occur much more frequently, which mean that the growth rate of wave height will be higher if the minimum ice extent keeps reducing in the future.

How to cite: Guan, C. and Li, J.: Growth of Wave Height with Retreating Ice Cover in the Arctic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6557, https://doi.org/10.5194/egusphere-egu2020-6557, 2020.

EGU2020-7752 | Displays | OS4.2

The Changed Sign of Swell Coherent Stress During Swell Conditions

Wei Pan, Zhongshui Zou, and Jinbao Song

Data measured by ultrasonic anemometer moored at a fixed platform located in South China Sea has been used to analyze turbulence within wave boundary layer. Compared to wind seas, it can be found a dominant area in power spectra, cospectra and Ogive curves under swell conditions from the measurements at 8 m height above sea surface, which is consistent with earlier studies. Our result also shows that the cospectra have both negative and positive regions, which represent the upward and downward momentum induced by swell waves. The wave coherent stress derived from the cospectra shows that it is more larger than the traditional method, implying earlier studies have greatly underestimated the stress of swell. Our study also found that the change of sign of swell coherent stress was related to the wave age, i.e. for cp/u* ≥ 60, swell induced the upward momentum, for cp/u* < 60, vice versa.  

How to cite: Pan, W., Zou, Z., and Song, J.: The Changed Sign of Swell Coherent Stress During Swell Conditions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7752, https://doi.org/10.5194/egusphere-egu2020-7752, 2020.

The Stokes drift in the marginal ice zones (MIZ) of the Arctic Ocean is modelled by WAVEWATCH III. Applying two viscoelastic and one empirical frequency-dependent wave-ice models, the modelled wave parameters and spectrum are compared with field observations in the Beaufort-Chukchi Sea. Three wave-ice parameterizations show similar abilities to produce the surface Stokes drift estimated from buoy measurements. By using five-year (2015-2019) hindcasted directional spectra of the autumn Arctic, we present and discuss the monthly mean surface Stokes drift (1-10 cm/s), e-folding depth (1-14 m) and vertically integrated transport (0.1-0.4 m2/s) in the marginal ice zones, which are stronger in October than in September. When bulk wave parameters are adopted to estimate the Stokes drift fields, the surface Stokes drift will be underestimated by about 44-59% with mean ice concentration smaller than 60%, and the Stokes e-folding depth will be overestimated by about 1.4 to 5.0 times increasing from the interior to the edge of the ice cover. Since the Stokes drift may be an important component of the total surface current, we compare the modelled surface Stokes drift with the Eulerian current from reanalysis data, which shows that the mean surface Stokes drift is typically about 30% of the Eulerian current over large parts of the MIZ in Arctic Ocean, and is of the same order or even larger in some sea areas of the Chukchi, E. Siberian and Laptev Seas. It indicates that the Stokes drift is necessary to be considered to better model the dynamic processes of the sea ice, especially for the drift of ice floes.

How to cite: Li, J.: Modelled Stokes drift in the Marginal ice zones of the Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8125, https://doi.org/10.5194/egusphere-egu2020-8125, 2020.

EGU2020-20763 | Displays | OS4.2

A Strategy for Scaling ERA5 Sea Winds Using Climate Model Data

Silvio Davison, Francesco Barbariol, Alvise Benetazzo, Luigi Cavaleri, and Paola Mercogliano

Over the past decade, reanalysis data products have found widespread application in many areas of research and have often been used for the assessment of the past and present climate. They produce reliable atmospheric fields at high temporal resolution, albeit at low-to-mid spatial resolution. On the other hand, climatological analyses, quite often down-scaled to represent conditions also in enclosed basins, lack the historical sequence of stormy events and are often provided at poor temporal resolution.

In this context, we investigated the possibility of using the ERA5 reanalysis 10-m wind (25-km and 1-hour resolution data) to assess the Mediterranean Sea wind climate (past and scenario). We propose a statistical strategy to relate ERA5 wind speeds over the sea to the past and future wind speeds produced by the COSMO-CLM (8-km and 6-hour resolution data) climatological model. In particular, the probability density function of the ERA5 wind speed at each grid point is adjusted to match that of COSMO-CLM. In this way, past ERA5 winds are corrected to account for the COSMO-CLM energy, while ERA5 scaled wind sequence can be projected in the future with COSMO-CLM scenario energy. Comparison with past observations confirms the validity of the adopted method.

In the Venezia2021 project, we have applied this strategy for the assessment of the changing wind and, after WAVEWATCH III model runs, also the wave climate in the Northern Adriatic Sea, especially in front of Venice and the MOSE barriers, under two IPCC (RCP 4.5 and 8.5) scenarios.

In general, this strategy may be applied to produce a scaled wind dataset in enclosed basins and improve past wave modeling applications based on any reanalysis wind data.

How to cite: Davison, S., Barbariol, F., Benetazzo, A., Cavaleri, L., and Mercogliano, P.: A Strategy for Scaling ERA5 Sea Winds Using Climate Model Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20763, https://doi.org/10.5194/egusphere-egu2020-20763, 2020.

EGU2020-22643 | Displays | OS4.2

Forty years of global wave hindcasts using the observation-based source terms: validation and geophysical applications

Qingxiang Liu, Alexander Babanin, Erick Rogers, and Stefan Zieger

Forty years (1979-2019) of global wave hindcasts are developed with the third generation spectral wave model WAVEWATCH III® using the state-of-the-art observation-based source term parameterizations (i.e., ST6) and the advanced irregular-regular-irregular (IRI) 1/4 grid system. The wave model has been forced with two distinct wind databases sourced from the latest NCEP Climate Forecast System (CFS) and the fifth generation of the ECMWF climate reanalyses (ERA5), together with the ice concentration available from the EUMETSAT OSI SAF (version 2). The hindcasts not only include traditional integral wave parameters (e.g., wave height, period) but also provide various novel parameters such as the dominant wave breaking probability, wave-induced mixed layer depth and whitecap coverage that are derived from wave spectrum based on previous theoretical and empirical studies. Wave parameters are extensively validated against observations from in-situ buoys and satellite altimeters on a global scale. Possible applications of these hindcasts in the fields of freak waves, sea spray and air-sea gas transfer will also be discussed.

How to cite: Liu, Q., Babanin, A., Rogers, E., and Zieger, S.: Forty years of global wave hindcasts using the observation-based source terms: validation and geophysical applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22643, https://doi.org/10.5194/egusphere-egu2020-22643, 2020.

EGU2020-12145 | Displays | OS4.2

Global Wave Data Assimilation Model using Optimal Interpolation Method

Kyeong Ok Kim, Hanna Kim, Kyung Tae Jung, and Young Ho Kim

To construct a reanalyzed global ocean wave data set with improved accuracy, which is important for the better understanding and simulation of various near-surface ocean dynamics, a data assimilation method has been embedded to the global spectral wave model based on WW3. The major factors controlling the wave simulation accuracy are the wind condition and the parameterization on the wave energy development, dissipation and nonlinear processes between wave components. However, the atmospheric prediction accuracy is still not sufficient, and the parameterization cannot be generalized due to the local geographic conditions.

In detail, the data assimilation using the optimal interpolation method has been applied, verification through the comparison with satellite altimeters and buoy observations has been made with examination of the data assimilation effects. The significant wave heights computed by the integration of wave energy spectra are showed to be quite similar with observed results. However, the wave periods and directions related to the shape of wave energy spectra are not sufficiently comparable. Generally there have been difficulties in predicting the propagation of long period waves such as swells.

The wave energy spectra on wave number and direction domains was multiplied by optimal interpolation method with the ratio of observed significant wave heights on first guessed simulated results. The energy spectra was thereafter shifted by the difference between simulated and observed peak wave periods and directions. From then examination of the reanalysis simulation during 1 year, it could be seen that the accuracy of the model with the data assimilation shows better results than that without data assimilation.

How to cite: Kim, K. O., Kim, H., Jung, K. T., and Kim, Y. H.: Global Wave Data Assimilation Model using Optimal Interpolation Method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12145, https://doi.org/10.5194/egusphere-egu2020-12145, 2020.

EGU2020-12912 | Displays | OS4.2

Ocean surface wave dynamics, energy and momentum air-sea transfer under a variety of wind and waves conditions

Francisco J. Ocampo-Torres, Pedro Osuna, Nicolas Rascle, Hector Garcia-Nava, Carlos F. Herrera-Vazquez, Daniel Pelaez-Zapata, and Lucia Robles-Diaz

Direct measurements have been conducted from a spar buoys deployed in the Gulf of Mexico, and in the vicinity of Todos Santos Island, offshore Ensenada BC, Mexico, in order to better understand ocean surface wave modulated processes under a variety of oceanographic and meteorological conditions. Full ocean surface wave directional spectrum is estimated from sea surface elevation data acquired with an array of capacitance wires, to represent directional spectrum as a function of frequency and direction, as well as a function of the wave number components Kx and Ky. Momentum transfer between ocean and the atmosphere is calculated directly through the eddy correlation method applied to wind velocity components acquired with a sonic anemometer. Momentum transfer variability is analysed to study its dependance on the surface wave conditions, with special emphasis on mixed sea states. Comparison between single peak spectra results with those cases where bi-modal spectra were present are performed in order to detect wind stress variability effects. Ocean-atmosphere transfer of momentum is studied and explained in terms of the shape and evolution of the surface wave spectrum. This research is funded by SENER-CONACYT 249795 and 201441 projects.

How to cite: Ocampo-Torres, F. J., Osuna, P., Rascle, N., Garcia-Nava, H., Herrera-Vazquez, C. F., Pelaez-Zapata, D., and Robles-Diaz, L.: Ocean surface wave dynamics, energy and momentum air-sea transfer under a variety of wind and waves conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12912, https://doi.org/10.5194/egusphere-egu2020-12912, 2020.

EGU2020-1171 | Displays | OS4.2

Effects of the sea state in the momentum flux over the ocean atmosphere interface

Diego Larios, Francisco J. Ocampo-Torres, and Pedro Osuna

The sea surface wind stress is relevant in processes of different scales of space and time such as the exchange of gases and heat, the surface currents, the depth of the mixed layer, the turbulence injection into the ocean. The wind waves are the key component in the coupling of the lower layer of the the atmosphere and the surface layer of the ocean, and various studies have shown the direct and indirect effects on the surface wind stress. In the present study, we present the measurements of the momentum flux and the results meteorological variables at the interface between the ocean and the atmosphere, by using and Oceanographic and Marine Meteorology Buoy (BOMM1) between November 2017 and February 2018. The analysis of the results during moderate wind conditions (U10N > 8 ms-1) in which mixed sea state conditions occur (swell that interacts with locally generated wind waves) we found a decrease of the roughness length (z0), related to developing waves with higher steepness (ak), the data suggest that the presence of swell alters the wind sea part of the spectrum, which leads a reduction of the energy level of the wind-generated waves, hence reducing the wind sea associated roughness. For well developed waves conditions, the roughness length is greater than the parametrization proposed by Drennan al., (2003) for pure wind sea conditions, the data suggest that this is due direct interaction of the wind airflow and swell with higher steepness.  The data of this work suggests that during these conditions (U10N > 8 ms-1) , the mechanism of reduction of the drag of the wind sea due to the presence of swell, and the increase of the wind stress by direct interaction of swell with the airflow causes the net effect of wave field to behave as expected under pure wind sea conditions, and there seems to be no swell effect.

How to cite: Larios, D., Ocampo-Torres, F. J., and Osuna, P.: Effects of the sea state in the momentum flux over the ocean atmosphere interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1171, https://doi.org/10.5194/egusphere-egu2020-1171, 2020.

EGU2020-2293 | Displays | OS4.2

Impact of Surface Waves on Wind Stress under Low to Moderate Wind Conditions

Sheng Chen, Fangli Qiao, Wenzheng Jiang, Jingsong Guo, and Dejun Dai

The impact of ocean surface waves on wind stress at the air–sea interface under low to moderate wind
conditions was systematically investigated based on a simple constant flux model and flux measurements
obtained from two coastal towers in the East China Sea and South China Sea. It is first revealed that the
swell-induced perturbations can reach a height of nearly 30m above the mean sea surface, and these perturbations
disturb the overlying airflow under low wind and strong swell conditions. The wind profiles severely
depart from the classical logarithmic profiles, and the deviations increase with the peak wave phase speeds. At
wind speeds of less than 4 m/s, an upward momentumtransfer from the wave to the atmosphere is predicted,
which is consistent with previous studies. A comparison between the observations and model indicates that
the wind stress calculated by the model is largely consistent with the observational wind stress when considering
the effects of surface waves, which provides a solution for accurately calculating wind stress in ocean
and climate models. Furthermore, the surface waves at the air–sea interface invalidate the traditional
Monin–Obukhov similarity theory (MOST), and this invalidity decreases as observational height increases.

How to cite: Chen, S., Qiao, F., Jiang, W., Guo, J., and Dai, D.: Impact of Surface Waves on Wind Stress under Low to Moderate Wind Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2293, https://doi.org/10.5194/egusphere-egu2020-2293, 2020.

EGU2020-4970 | Displays | OS4.2

Comparing the effect of shortwave penetration and mixing induced by non-breaking surface waves in an ocean climate model

Shizhu Wang, Qiang Wang, Qi Shu, Patrick Scholz, Gerrit Lohmann, and Fangli Qiao

Numerical models have been widely utilized to simulate the ocean and climate system. Parameterizations of some important processes, however, including the vertical mixing induced by surface waves, are still missing in many ocean models. In this work we incorporate the vertical mixing induced by non-breaking surface waves derived from a wave model into the multi-resolution Finite Element Sea ice-Ocean Model (FESOM), and compare its effect with that of shortwave penetration, another key process to vertically redistribute the heat in the upper ocean. Numerical experiments reveal that both processes ameliorate the simulation of upper-ocean temperature in mid and low latitudes mainly on the summer hemisphere. The regions where nonbreaking wave generates stronger improvement are where large temperature bias exists. The non-breaking surface waves plays a more significant role in decreasing the mean cold biases at 50 m (by 1.0 °C, in comparison to 0.5 °C achieved by applying shortwave penetration). We conclude that the incorporation of mixing induced by non-breaking surface waves into FESOM is practically very helpful, and suggest that it needs to be considered in other ocean climate models as well.

How to cite: Wang, S., Wang, Q., Shu, Q., Scholz, P., Lohmann, G., and Qiao, F.: Comparing the effect of shortwave penetration and mixing induced by non-breaking surface waves in an ocean climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4970, https://doi.org/10.5194/egusphere-egu2020-4970, 2020.

EGU2020-12168 | Displays | OS4.2

Accelerated wind conditions: spectral shape evolution and turbulent kinetic energy dissipation from laboratory and field measurements

Lucia Robles-Diaz, Francisco J. Ocampo-Torres, and Hubert Branger

A determined shape of the energy wave spectrum can be estimated from a given fetch and wind speed. Also, several studies have characterized the balance of the turbulent kinetic energy under the effect of waves and currents under constant wind conditions. However, deeper research is needed in order to characterize the wind-wave generation processes under non-stationary wind conditions. In this way, to be able to determine the uncertainty on not considering accelerated wind events in the air-sea momentum exchange estimations.

Periods of accelerated winds were analyzed from experimental and field data. On one hand, several laboratory experiments were carried out in a large wind-wave facility at the Institut Pytheas (Marseille-France). Momentum fluxes were estimated from hot wire anemometry and, the free surface displacement was measured along the wave tank by resistance and capacitance wire probes. Also, the surface drift current was measured from a profiling acoustic velocimeter. During these experiments, the wind speed goes from 2 m/s to reach the maximum wind speed of 13 m/s. A constant wind acceleration characterizes each test. On the other hand, the field data were obtained from an Oceanographic and Marine Meteorology Buoy (BOMM) located in the Gulf of Mexico, from July 2018 to February 2019. The BOMM was equipped with a sonic anemometer, capacitance wires, and an inertial motion unit. Both sets of data are characterized by a high sampling rate that allows us to directly estimate the wind stress over the sea surface. Also, provide us with useful information about the evolution of the wave spectra and enable us to determine the dissipation rate of turbulent kinetic energy. It was observed that the wind acceleration has a direct effect on the momentum transfer efficiency from the wind to the wave field and that the momentum transfer is reduced as wind acceleration increases.

How to cite: Robles-Diaz, L., Ocampo-Torres, F. J., and Branger, H.: Accelerated wind conditions: spectral shape evolution and turbulent kinetic energy dissipation from laboratory and field measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12168, https://doi.org/10.5194/egusphere-egu2020-12168, 2020.

EGU2020-12436 | Displays | OS4.2

Study on air-sea CO2 exchange with wave breaking

Shuo Li, Alexander Babanin, Fangli Qiao, Dejun Dai, Shumin Jiang, and Changlong Guan

Hydrodynamic processes at air-sea interface play a significant role on air-sea CO2 gas exchange, which further affects global carbon cycle and climate change. CO2 gas transfer velocity (KCO2) is generally parameterized with wind speed but ocean surface waves have direct impact on the gas exchange. Thus, the relationship between wave breaking and CO2 gas exchange was studied through laboratory experiments and by utilizing field campaign data. The results from laboratory show that wave breaking plays a significant role in CO2 gas exchange in all experiments while wind forcing can also influence KCO2. A non-dimensional empirical formula is established in which KCO2 is expressed as the product of wave breaking probability, transformed Reynolds number and an enhancement factor of wind speed. The parameterization is then improved by considering the bubble-mediated gas transfer based on both laboratory and ship campaign data sets. In the end, the formula is employed in the estimation of global CO2 uptake by ocean and the result is found consistent with reported values.

How to cite: Li, S., Babanin, A., Qiao, F., Dai, D., Jiang, S., and Guan, C.: Study on air-sea CO2 exchange with wave breaking, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12436, https://doi.org/10.5194/egusphere-egu2020-12436, 2020.

High-quality wave prediction with a numerical wave model is of societal value. To initialize the wave model, wave data assimilation (WDA) is necessary to combine the model and observations. Due to inaccurate wind forcing, imperfect numerical schemes, and approximated physical processes, a wave model is always biased in relation to the real world. In this study, two assimilation systems are first developed using two nearly independent wave models; then, “perfect” and “biased” assimilation frameworks based on the two assimilation systems are designed to reveal the uncertainties of WDA. A series of “biased” assimilation experiments is conducted to systematically examine the adverse impact of initial condition, boundary forcing, and model bias on WDA, then model bias play a strongest role among them . A statistical approach based on the results from multiple assimilation systems is explored to carry out bias correction, by which the final wave analysis is significantly improved with the merits of individual assimilation systems. The framework with multiple assimilation systems provides an effective platform to improve wave analyses and predictions and help identify model deficits, thereby improving the model.

How to cite: li, J. and zhang, S.: Mitigation of Model Bias Influences on Wave Data Assimilation with Multiple Assimilation Systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1550, https://doi.org/10.5194/egusphere-egu2020-1550, 2020.

Langmuir turbulence (LT) can have a significant impact on the ocean mixing in the ocean surface boundary layer. In this study, using the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COWAST) modeling system, a 3D wave-current coupled regional model of the South China Sea (SCS) was developed. And the second-moment closure model developed recently by Harcourt, which includes the effects of LT, was introduced into the circulation model to investigate its influence on the coupled models’ performance. Comparisons between the model results with and without LT effects and between the model results and the observations are analyzed. Overall, with the LT effects, the simulated results are more consistent with the observations. When the LT effects are not considered, the mean annual deviation between the model simulated sea surface temperature (SST) and the observed data in the SCS is 1.09℃, as the LT effects introduced, the mean annual SST deviation decrease to -0.06℃ and the deviation almost disappear. Compared with the huge improvement in SST simulation, the introduction of LT has less influence on the simulation of the upper mixing layer depth (MLD). Nevertheless, the model results have also been improved to some extent, with the model performance is improved by 12.7%. Moreover, the eastern and southern parts of the SCS show a more pronounced amelioration than other regions. From the intercomparisons among the experiments, it is also found that the LT exerts more significant impacts on both SST and MLD during spring and summer.

How to cite: Yu, C., Song, J., and Li, S.: Impact of Langmuir Turbulence on the Ocean Upper Mixed Layer in a 3D Coupled Regional Model of the South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3249, https://doi.org/10.5194/egusphere-egu2020-3249, 2020.

Turbulence over the mobile ocean surface has distinct properties compared to turbulence over land. This raises the issue of whether functions such as the turbulent kinetic energy (TKE) budget and Monin-Obukhov similarity theory (MOST) determined over land are directly applicable to ocean surfaces because of the existence of a wave boundary layer (the lower part of atmospheric boundary layer including effects of surface waves. We used the term “WBL” in this article for convenience), where the total stress can be separated into turbulent stress and wave coherent stress. Here the turbulent stress is defined as the stress generated by wind shear and buoyancy, and wave coherent stress accounts for the momentum transfer between ocean waves and atmosphere. In this study, applications of the turbulent kinetic energy (TKE) budget and the inertial dissipation method (IDM) in the context of the Monin-Obukhov similarity theory (MOST) within the WBL are examined. It was found that turbulent transport terms in the TKE budget should not be neglected when calculating the total stress under swell conditions. This was confirmed by observations made on a fixed platform. The results also suggested that turbulent stress, rather than total stress should be used when applying the MOST within the WBL. By combing the TKE budget and MOST, our study showed that the stress computed by the traditional IDM corresponds to turbulent stress rather than total stress. The swell wave coherent stress should be considered when applying the IDM to calculate the stress in the WBL.

How to cite: Zou, Z.: Atmospheric boundary layer turbulence in the presence of swell: turbulent kinetic energy budget, Monin-Obukhov similarity theory and inertial dissipation method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6593, https://doi.org/10.5194/egusphere-egu2020-6593, 2020.

EGU2020-20360 | Displays | OS4.2

A depth-resolved framework for the coupling of sub-surface flows and surface gravity water waves

Simen Ådnøy Ellingsen, Stefan Weichert, and Yan Li

This work aims to develop a new framework for the interaction of a subsurface flow and surface gravity water waves, based on a perturbation and multiple-scales expansion.  Surface waves are assumed of a narrow band δ (δ ), indicating they can be expressed as a carrier wave whose amplitude varies slowly in space and time relative to its phase. Using the Direct Integration Method proposed in Li & Ellingsen (2019), the effects of the vertical gradient of a subsurface flow are taken into account on the linear wave properties in an implicit fashion. At the second order in wave steepness ϵ, the forcing of the sub-harmonic bound waves is considered that plays a role in the primary equations for a subsurface flow.

The novel framework derives the continuity and momentum equations for a subsurface flow in two different formats, including both the depth integrated as well as the depth resolved version. The former compares with Smith (2006) to examine the roles of the rotationality of wave motions in the subsurface flow equations. The latter employs the sigma coordinate system proposed in Mellor (2003, 2008, 2015) and extends the framework therein to allow for quasi-monochromatic surface waves and the effects of the shear of a current on linear surface waves. Compared to Mellor (2003, 2008, 2015), the vertical flux/vertical radiation stress term in the proposed framework is approximated to one order of magnitude higher, i.e. O(ϵ2δ2).

References

Li, Y., Ellingsen, S. Å. A framework for modeling linear surface waves on shear currents in slowly varying waters. J. Geophys. Res. C: Oceans, (2019) 124(4), 2527-2545.

Mellor, G. L. The three-dimensional current and surface wave equations. J. Phys. Oceanogr., (2003) 33, 1978–1989.

Mellor, G. L. The depth-dependent current and wave interaction equations: a revision. J. Phys. Oceanogr., (2008) 38(11), 2587-2596.

Smith, J. A. Wave–current interactions in finite depth. Journal of Physical Oceanography, (2006) 36(7), 1403-1419.

How to cite: Ellingsen, S. Å., Weichert, S., and Li, Y.: A depth-resolved framework for the coupling of sub-surface flows and surface gravity water waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20360, https://doi.org/10.5194/egusphere-egu2020-20360, 2020.

OS4.3 – Internal Gravity Waves

EGU2020-6118 | Displays | OS4.3

Middle-Atmosphere Mountain Waves and Drag Near the Drake Passage: Observations, mini-MIP, and an OSSE

Christopher Kruse, Joan Alexander, Lars Hoffmann, Inna Polichtchouk, Annelize van Niekerk, Riwal Plougonven, Corwin Wrioght, Julio Bacmeister, Manfred Ern, Kaoru Sato, Ryosuke Shibuya, Catrin Meyer, Olaf Stein, Laura Holt, Petr Šácha, and Sonja Gisinger

Orographic gravity wave (OGW) drag is one of the fundamental physics parametrizations employed in every global numerical model across timescales from weather to climate. These parameterizations have significant influences, both direct and indirect, on the atmosphere’s general circulation from the troposphere at least through the mesosphere. Despite their significant influence, observational constraints on these parameterizations are still largely lacking.

Presented here is a team project jointly supported by SPARC and the International Space Science Institute with the overall objective of providing new quantitative constraints for OGW drag parameterizations. Specific objectives are to evaluate methods that quantify vertical fluxes of horizontal momentum (MF) from satellite observations via an observing system simulation experiment (OSSE), a validation of WRF, UKMO, ECMWF, and ICON models against satellite and balloon observations, and an inter-comparison of OGW properties (e.g. MF and drag) within these models. Evaluation of satellite-based estimates of MF and model validation/inter-comparison will help to better quantify actual MF in the stratosphere, providing the best stratospheric MF and drag estimates for parameterizations to reproduce to date.

Two unique aspects of the project are that all models involved are deep, extending up to 1 Pa. The motivations for doing so was to include entire instrument weighting functions for AIRS observations, allowing direct, quantitative comparison between AIRS (and other satellite-borne) observations and the models. The second is the effort to perform an OSSE within the simulations, allowing comparison between MF from satellite-based methods within the models to the true MF in the models.

Preliminary results show that higher-res models (dx = 3 km) compare well and produce significantly more MF than lower-res global models, but the higher-res models still underrepresent OGW amplitudes. Mesospheric tides in analyses used to force the models significantly modulate resolved GWs and their drags.

How to cite: Kruse, C., Alexander, J., Hoffmann, L., Polichtchouk, I., van Niekerk, A., Plougonven, R., Wrioght, C., Bacmeister, J., Ern, M., Sato, K., Shibuya, R., Meyer, C., Stein, O., Holt, L., Šácha, P., and Gisinger, S.: Middle-Atmosphere Mountain Waves and Drag Near the Drake Passage: Observations, mini-MIP, and an OSSE, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6118, https://doi.org/10.5194/egusphere-egu2020-6118, 2020.

Using long-term high-resolution operational radiosonde observation data from nine stations in the subtropics and mid-latitudes of Japan, this study performed statistical analysis of the dynamical characteristics of gravity waves (GWs). Wave generation by shear instability in summer was a particular focus because orographic GWs cannot propagate deep into the middle atmosphere through their critical layer in the lower stratosphere. The kinetic energy of summer stratospheric GWs is markedly large south of 37°N. Hodograph analysis revealed that GWs propagating eastward relative to the ground are dominant in summer. The percentage of GWs propagating energy upward (downward) is large above (below) the height at which the mean occurrence frequency of shear instability is high. The time series of the kinetic energy of stratospheric GWs exhibited statistically significant positive correlation with the occurrence frequency of shear instability slightly below the tropopause. These findings strongly suggest the possibility of excitation of summer stratospheric GWs by shear instability above the jet. The shear instability condition is satisfied more frequently in the region 30°–35°N. This is probably related to two characteristics of the background fields slightly below the tropopause: larger vertical shear of zonal winds at higher latitudes and lower static stability at lower latitudes.

How to cite: Okui, H. and Sato, K.: Characteristics and sources of gravity waves in the summer stratosphere based on long-term and high-resolution radiosonde observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4347, https://doi.org/10.5194/egusphere-egu2020-4347, 2020.

EGU2020-6474 | Displays | OS4.3

Inertia-Gravity Wave Generation Near Upper Jets in the Australian Region

Michael Reeder and Adam Morgan

During rapid extratropical cyclogenesis in the Australian region, distinctive striated, triangular-shaped clouds commonly form on the poleward side of the jet exit near the axis of inflection between an upstream trough and downstream ridge. These clouds are called striated deltas and the striations are shown here to be caused by radiating inertia-gravity waves. An analysis of 28 striated delta clouds shows that the striated deltas have a mean length of 1034 km and width of 537 km, and a striation wavelength of 74 km. Large parcel accelerations, nonlinear flow imbalance and active convection within the striated delta are features of the composite upper-tropospheric environment. Patterns of Q-vector illustrates the unique shape of striated delta clouds to be coincident with a delta-shaped forcing of adiabatic ascent in the poleward jet exit. One of the extratropical cyclones analysed and its associated striated delta is simulated with WRF-ARW both with and without diabatic heating. Both simulations produce pronounced gravity wave packets along the surface cold front, along the downstream upper jet axis and a delta-shaped packet in the lower stratosphere above the jet exit. Ray tracing identifies the source region of the waves in the stratosphere to be the upper jet. Vertically-radiating gravity waves originating in the vicinity of the jet during rapid extratropical cyclogenesis, propagate both upwards and downwards, imprinting striations onto the cloud in the jet exit and setting the spacing between the convective bands.

How to cite: Reeder, M. and Morgan, A.: Inertia-Gravity Wave Generation Near Upper Jets in the Australian Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6474, https://doi.org/10.5194/egusphere-egu2020-6474, 2020.

EGU2020-9876 | Displays | OS4.3

Gravity wave intermittency derived from HIRDLS and SABER satellite limb soundings

Manfred Ern, Peter Preusse, and Martin Riese

Sources of atmospheric gravity waves are mostly located in the troposphere and the lower stratosphere. Gravity waves propagate away from their sources, re-distribute energy and momentum in the atmosphere, and exert drag on the atmospheric background flow where they dissipate. Therefore they are important drivers of the atmospheric circulation. In climate models, their effect on the background circulation is usually parameterized because of their relatively short horizontal and vertical wavelengths that are of the order of 10-1000km and 1-100km, respectively. Gravity wave parametrizations are very simplified. For example, they often neglect the fact that gravity wave source processes and gravity wave propagation conditions can vary on short temporal and spatial scales. Therefore the global distribution of gravity wave activity is very intermittent, which has also important consequences where gravity waves dissipate and exert drag on the background flow, and which should be accounted for in parametrizations.
For guiding models, global observations of the gravity wave distribution and its intermittency are needed. We derive gravity wave potential energies and absolute momentum fluxes from observations of the High Resolution Dynamics Limb Sounder (HIRDLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments. As a measure of intermittency, we calculate global distributions of Gini coefficients. We find that our results are qualitatively in good agreement with previous findings from satellite, and similar in magnitude to intermittency obtained from previous superpressure balloon campaigns. In the stratosphere, strongest intermittency is found over orographic gravity wave sources, followed by gravity wave activity in the polar night jets. Intermittency in the tropical stratosphere is weakest. However, in the tropical upper mesosphere intermittency is increased, which is likely caused by the modulation of the gravity wave distribution by tides.

How to cite: Ern, M., Preusse, P., and Riese, M.: Gravity wave intermittency derived from HIRDLS and SABER satellite limb soundings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9876, https://doi.org/10.5194/egusphere-egu2020-9876, 2020.

EGU2020-19821 | Displays | OS4.3

Exploring long-term satellite observations of global 3-D gravity wave characteristics in the stratosphere

Neil Hindley, Corwin Wright, Lars Hoffmann, Tracy Moffat-Griffin, M. Joan Alexander, and Nicholas Mitchell

Atmospheric gravity waves are a fundamental component of the Earth’s dynamical system. These mesoscale waves play a key role in the coupling of different atmospheric layers, acting as crucial drivers of the middle atmospheric circulation through the transport and deposition of energy and momentum. As Global Circulation Models (GCMs) achieve ever higher resolution in the stratosphere, there is a need to ensure that the simulated gravity waves resolved in these models are well constrained by observations, which in turn ensures that modelled circulations are realistic and not over-dependent on tuning with parameterisations. However, obtaining 3-D gravity-wave measurements in the real atmosphere is notoriously difficult. Global 3-D observations of wave properties in the stratosphere are required in order to accurately estimate gravity wave fluxes that can be compared to models. Here we analyse a unique long-term satellite dataset of specialised high-resolution 3-D temperature measurements from NASA’s AIRS/Aqua instrument from 2002-2020. By analysing these data with a 3-D Stockwell transform (3DST) using high-performance computing, we can reveal global distributions of gravity-wave amplitudes, wavelengths, intermittency and directional momentum fluxes in the stratosphere across two decades - the largest such 3-D study of stratospheric gravity waves performed yet. This long-term dataset reveals solar-cycle variability of gravity-wave amplitudes in the tropics, significant reductions in gravity-wave fluxes during southern Sudden Stratospheric Warmings (SSWs) and the persistent oblique propagation of wintertime gravity waves into the southern polar vortex around 60S each year, a phenomenon that is not observed in the northern hemisphere. With these new observations we can begin to better constrain simulated gravity waves and their impacts in GCMs, ultimately leading to better forecasts of weather and climate.

How to cite: Hindley, N., Wright, C., Hoffmann, L., Moffat-Griffin, T., Alexander, M. J., and Mitchell, N.: Exploring long-term satellite observations of global 3-D gravity wave characteristics in the stratosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19821, https://doi.org/10.5194/egusphere-egu2020-19821, 2020.

EGU2020-12918 | Displays | OS4.3

First airborne gravity wave observations at the world’s hotspot in Southern Argentina

Markus Rapp, Bernd Kaifler, Andreas Dörnbrack, Sonja Gisinger, Tyler Mixa, Robert Reichert, Natalie Kaifler, Andreas Giez, Peter Preusse, Markus Geldenhuis, Lukas Krasauskas, Wolfgang Woiwode, Felix Friedl-Vallon, Alejandro de la Torre, Jose Luis Hormaechea, Martin Riese, Björn-Martin Sinnhuber, Peter Hoor, and Andreas Engel

The region around Southern Argentina and the Antarctic peninsula is known as the world’s strongest hotspot of stratospheric gravity wave activity. In this region, large tropospheric winds are perturbed by the orography of the Andes and the Antarctic peninsula resulting in the excitation of mountain waves which might propagate all the way up into the upper mesosphere when the polar night jet is intact. In addition, satellite observations also show large stratospheric wave activity in the region of the Drake passage, i.e., in between the Andes and the Antarctic peninsula, and along the corresponding latitudinal circle of 60°S. The origin of these waves is currently not entirely understood. Several hypotheses are currently being investigated, like for example the idea that the mountain waves that were originally excited over the Andes and the Antarctic peninsula propagate horizontally to 60°S and along the latitudinal circle. In order to investigate this and other hypotheses the German research aircraft HALO was deployed to Rio Grande, Tierra del Fuego, at the Southern Tip of Argentina in September and November 2019 in the frame of the SOUTHTRAC (Southern hemisphere Transport, Dynamics, and Chemistry) research mission. A total of 6 dedicated research flights with a typical length of 7000km were conducted to obtain gravity wave observations with the newly developed ALIMA (ALIMA=Airborne LIdar for Middle Atmosphere research)-instrument and the GLORIA (GLORIA=Gimballed Limb Observer for Radiance Imaging of the Atmosphere) limb sounder. While ALIMA measures temperatures and temperature perturbations in the altitude range from 20-90 km, GLORIA observations allow to characterize wave perturbations in temperatures and trace gas concentrations below flight level (<~14 km). This paper gives an overview of the mission objectives, the prevailing atmospheric conditions during the HALO deployment, and highlights some outstanding initial results of the gravity wave observations.

How to cite: Rapp, M., Kaifler, B., Dörnbrack, A., Gisinger, S., Mixa, T., Reichert, R., Kaifler, N., Giez, A., Preusse, P., Geldenhuis, M., Krasauskas, L., Woiwode, W., Friedl-Vallon, F., de la Torre, A., Hormaechea, J. L., Riese, M., Sinnhuber, B.-M., Hoor, P., and Engel, A.: First airborne gravity wave observations at the world’s hotspot in Southern Argentina, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12918, https://doi.org/10.5194/egusphere-egu2020-12918, 2020.

EGU2020-19489 | Displays | OS4.3

Numerical modeling of mesospheric mountain wave propagation observed in Southern Argentina during the SOUTHTRAC campaign

Tyler Mixa, Andreas Dörnbrack, Bernd Kaifler, and Markus Rapp

We present numerical simulations of a deep orographic gravity wave (GW) event observed by the ALIMA airborne lidar on 11-12 September 2019 over Southern Argentina. The measurements are taken from the 2019 SOUTHTRAC Campaign, employing a comprehensive suite of remote sensing and in-situ instruments onboard the HALO research aircraft to study the stratospheric GW hotspot over Tierra del Fuego and the Antarctic Peninsula. Wind conditions on 11-12 September exhibit local and large-scale directional shear from the ground to the polar night jet, creating a complex propagation environment supporting multiple orientations of GW propagation and strong potential for local GW breaking and secondary GW generation. Using high resolution numerical models, we simulate the 3D evolution of the orographic GW field to analyze the remote sensing and in-situ measurements from the event.

How to cite: Mixa, T., Dörnbrack, A., Kaifler, B., and Rapp, M.: Numerical modeling of mesospheric mountain wave propagation observed in Southern Argentina during the SOUTHTRAC campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19489, https://doi.org/10.5194/egusphere-egu2020-19489, 2020.

EGU2020-10176 | Displays | OS4.3

Can we improve gravity wave parameterizations by imposing sources at all altitudes in the atmosphere?

Bruno Ribstein, Christophe Millet, Francois Lott, and Alvaro de la Camara

A multiwave non-orographic gravity wave (GW) scheme is adapted to represent waves of small intrinsic phase speed and sources located at all altitudes in the troposphere and middle atmosphere. Using reanalysis data, these changes impose larger amplitude saturated waves everywhere in the middle atmosphere, which permits to produce more realistic GW vertical spectra than when the phase speeds are large and the sources are in the troposphere only. The same scheme, tested online in the Laboratoire de Météorologie Dynamique Zoom(LMDz) general circulation model, performs at least as well as the operational non-orographic GW scheme.  Some modest benefits are seen, for instance, in the equatorial tilt with altitude of the winter jets in the middle atmosphere. Although the scheme includes the effects of inertial waves, which are more and more often detected in the mesosphere, the configuration that gives a reasonable climatology in LMDz hinders the vertical propagation of these parameterized waves and do not generally reach mesospheric altitudes.

How to cite: Ribstein, B., Millet, C., Lott, F., and de la Camara, A.: Can we improve gravity wave parameterizations by imposing sources at all altitudes in the atmosphere?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10176, https://doi.org/10.5194/egusphere-egu2020-10176, 2020.

EGU2020-5248 | Displays | OS4.3

Intermittency of gravity waves modelled by a transient gravity-wave parametrization coupled with convective sources

Young-Ha Kim, Gergely Bölöni, Sebastian Borchert, Hye-Yeong Chun, and Ulrich Achatz

The intermittency of gravity waves (GWs) is investigated using Multi-Scale Gravity Wave Model (MS-GWaM) implemented in the upper-atmosphere extension of ICON model. The intermittency of GWs is originated from that of wave sources but altered during propagation of the waves. Conventional GW parametrization (GWP), which diagnoses vertical profiles of GW properties under the steady-state assumption, can take into account the source intermittency if the GWP employs flow-dependent sources, while it cannot present the change of intermittency by transient evolutions of GWs. MS-GWaM is a prognostic model that explicitly solves the evolution of positions of waves (as well as their wavenumbers and amplitudes) in time and thus capable of describing the intermittency change. In order to include the source intermittency and variability, we couple the convective source, as diagnosed by subgrid-scale cumulus parametrization in ICON, to MS-GWaM, based on an analytic formulation of GW response to this source. In addition to this, a spatio-temporally uniform, persistent source is prescribed in the extratropics to take into account other non-orographic sources. Orographic sources are currently not used. The GW intermittency is measured by the Gini index, and is found to be quite high in the tropics, compared to that in the extratropics. In both regions, the index has similar values to those obtained from superpressure balloon observations reported in previous studies. A control experiment is performed using GWP based on the steady-state assumption, but coupled to the same wave sources, to assess the effects of transient modelling using MS-GWaM on the simulated intermittency. From comparison to the control experiment, the intermittency is found to increase largely for GWs from the uniform source but to decrease for convective GWs by the transient modelling.

How to cite: Kim, Y.-H., Bölöni, G., Borchert, S., Chun, H.-Y., and Achatz, U.: Intermittency of gravity waves modelled by a transient gravity-wave parametrization coupled with convective sources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5248, https://doi.org/10.5194/egusphere-egu2020-5248, 2020.

EGU2020-8371 | Displays | OS4.3

A Parametrization for Triad Interactions of Internal Gravity Waves in Varying Background Flows for WKBJ Ray-Tracing Methods

Georg Sebastian Voelker, Triantaphyllos Akylas, and Ulrich Achatz

Internal gravity waves are a well known mechanism of energy transport in stratified fluids such as the atmosphere and the ocean. Their abundance and importance for various geophysical processes like ocean mixing and momentum deposition in atmospheric jets are widely accepted. While resonant wave-wave interactions of monochromatic disturbances have received intensive study, little work has been done on triad interactions between wave trains that are modulated by a variable mean flow.

Using the method of multiple scale asymptotics we consider a weakly non-linear Boussinesq WKBJ theory for interacting gravity wave trains propagating through a finite amplitude background flow. Consequently the wave trains are allowed to spectrally pass through resonance conditions and exchange energy when sufficiently close to resonance. We find a global optimal threshold for the deviation from resonance and derive a corresponding parametrization for the triad interaction applicable to ray tracing schemes.

We test the theory with idealized simulations in which two wave trains generate a third by passing through resonance in a sinusoidal background shear flow with varying vertical scales. Comparing WKBJ simulations with wave resolving large eddy simulations we find qualitative and quantitative agreement. Furthermore we assess the impact of the strength of the modulation as well as the effect of the wave amplitudes on the energy exchange between the interacting wave triad.

How to cite: Voelker, G. S., Akylas, T., and Achatz, U.: A Parametrization for Triad Interactions of Internal Gravity Waves in Varying Background Flows for WKBJ Ray-Tracing Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8371, https://doi.org/10.5194/egusphere-egu2020-8371, 2020.

EGU2020-18419 | Displays | OS4.3

Experimental internal gravity wave turbulence

Géraldine Davis, Thierry Dauxois, Sylvain Joubaud, Timothée Jamin, Nicolas Mordant, and Clément Savaro

Stratified fluids may develop simultaneously turbulence and internal wave turbulence, the latter describing a set of a large number of dispersive and weakly nonlinear interacting waves. The description and understanding of this regime for internal gravity waves (IGW) is really an open subject, in particular due to their very unusual dispersion relation. In this presentation, I will show experimental signatures of a large set of weakly interacting IGW obtained in a 2D trapezoidal tank.

Due to the peculiar linear reflexion law of IGW on inclined slopes, this setup - for given excitation frequencies - focuses all the input energy on a closed loop called attractor. If the forcing is large enough, this attractor destabilizes and the system eventually achieves a nonlinear cascade in frequencies and wavevectors via triadic resonant interactions, which results at large forcing amplitudes in a k^-3 spatial energy spectrum. I will also show some results obtained in a much larger set-up -the Coriolis facility in Grenoble- with signature of 3D internal wave turbulence.

How to cite: Davis, G., Dauxois, T., Joubaud, S., Jamin, T., Mordant, N., and Savaro, C.: Experimental internal gravity wave turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18419, https://doi.org/10.5194/egusphere-egu2020-18419, 2020.

Inertia-gravity waves (IGWs) are known to play an essential role in the terrestrial atmospheric dynamics as they can lead to energy and momentum flux when they propagate upwards. An open question is to which extent nearly linear IGWs contribute to the total energy and to flattening of the energy spectrum observed at the mesoscale.
In this work, we present an experimental investigation of the energy distribution between the large-scale balanced flow and the small-scale imbalanced flow. Weakly nonlinear IGWs emitted from baroclinic jets are observed in the differentially heated rotating annulus experiment. Similar to the atmospheric spectra, the experimental kinetic energy spectra reveal the typical subdivision into two distinct regimes with slopes k-3 for the large scales and k-5/3 for smaller scales. By separating the spectra into a vortex and wave part, it emerges that at the largest scales in the mesoscale range the gravity waves observed in the experiment cause a flattening of the spectra and provide most of the energy. At smaller scales, our data analysis suggests a transition towards a turbulent regime with a forward energy cascade up to where dissipation by diffusive processes occurs.

How to cite: Rodda, C. and Harlander, U.: Transition from geostrophic flows to inertia-gravity waves in the spectrum of a differentially heated rotating annulus experiment., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9374, https://doi.org/10.5194/egusphere-egu2020-9374, 2020.

EGU2020-4290 | Displays | OS4.3

Observed energy cascade from internal solitary waves to turbulence via near N-waves in the ocean

Xiaojiang Zhang, Weimin Zhang, Huizan Wang, and Ren Zhang

High temporal resolution mooring observations reported here revealed that there exist energy cascades from internal solitary wave (ISW) to turbulent mixing via smaller, high-frequency internal waves near the maximum local buoyancy frequency (near N-waves), which are transient, inhomogeneous in space. These near N-waves, riding on the parent ISW, emerged at the trough and gradually extended to the rear face of ISW with their ampltiudes becoming larger and larger. Most of the enlargement occurred in the primary stratifed layer, where the displacements between the density surfaces are largest. The near N-waves riding on a typical ISW held approximately 5 percent of the energy of ISW during its passage. Simulations of the KdV-Burgers equation confirmed the emergence of the near N-waves due to the energy cascade, similar as in the observation. The above results point out a new route of energy cascade from ISWs to turbulence in the ocean, which would be helpful on deepening the understanding of the mechanism of wave-induced mixing and energy cascade in internal waves.

How to cite: Zhang, X., Zhang, W., Wang, H., and Zhang, R.: Observed energy cascade from internal solitary waves to turbulence via near N-waves in the ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4290, https://doi.org/10.5194/egusphere-egu2020-4290, 2020.

EGU2020-8503 | Displays | OS4.3

Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget

Yvan Dossmann and Callum Shakespeare

Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget

Yvan Dossmann,

LEMTA, Université de Lorraine, CNRS, Nancy, France.

Callum Shakespeare,

Climate and Fluid Physics, The Australian National University, Canberra, Australia

 

Usual parameterizations of mixing in global models quantify independently the contribution of internal tides -generated by barotropic flows- and lee waves -generated by quasi-steady flows- relying on a linear approach based on the theory of Bell [1]. However the combined effects of the tidal and quasi-steady flows causes a linear coupling between internal tides and lee waves that has been overlooked in internal wave mixing parameterizations over the last decades [2]. This coupling induces major changes in the internal wave dynamics that has dramatic global impacts on :

  • the energy fluxes to lee waves that is cancelled by 20 % on a global scale and up to 90 % in key areas of the Meridional Overturning Circulation as the Drake passage.

  • the generation of Doppler-shifted internal tides beyond the critical latitudes.

  • the existence of a net wave stress above abyssal hills comparable to the local wind stress.

An accurate description of the cascade from generation to mixing is a necessary step to define relevant parameterizations at the ocean scale and significantly reduce the large uncertainties due to partially represented processes.

The experimental campaign LATMIX led at ANU Canberra in 2019 has confirmed the dynamical effects of this linear coupling on internal wave propagation, energy fluxes and mixing based on high resolution density measurements with the light attenuation technique (LAT). Strong nonlinear processes such as the formation of horizontal vortices have been measured in the bottom boundary layer. The generation of these vortices is only observed when the steady and tidal forcings are combined, while different strong nonlinear structures are present in the case of a pure steady flow [3]. Mixing induced by nonlinear processes overcomes internal wave induced mixing in most relevant parameter regimes for the ocean. These results provide insights to better understand and represent (non-)linear internal wave processes and their impact on mixing at regional and global scales. I will present the main results of this experimental campaign and discuss their implications for the representation of internal wave induced mixing at regional and global scales.

References

[1] Bell, T. H. : Topographically generated internal waves in open ocean », Journal of Geophysical Research, vol. 80, p. 320–327, 1975.

[2] Shakespeare, C. : Interdependence of internal tide and lee wave generation at abyssal hills: global calculations », in Press, Journal of Physical Oceanography, 2020.

[3] Dossmann, Y.; G. Rosevear, M.; Griffiths, R. W.; McC. Hogg, A.; Hughes, G. O. and Copeland, M.: Experiments with mixing in stratified flow over a topographic ridge, Journal of Geophysical Research: Oceans 121 : 6961-6977, 2016.

 

How to cite: Dossmann, Y. and Shakespeare, C.: Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8503, https://doi.org/10.5194/egusphere-egu2020-8503, 2020.

EGU2020-8789 | Displays | OS4.3

Lidar Observations of Seasonal Variability of Gravity-Waves

Irina Strelnikova, Gerd Baumgarten, and Franz-Josef Lübken

In order to understand the generation, propagation and climatology of gravity waves (GWs)observations with high temporal and vertical resolution are required. The observation of gravity waves is important to understand the vertical coupling in the atmosphere.

Recent developments in lidar technology give us new possibilities to study GWs experimentally on a more or less regular basis and resolve spatial sales of 150
meters in the vertical and temporal scales of about 10 minutes. In particular, the capability to operate the lidar during daytime allows for long duration GW observations. The Doppler Rayleigh Iodine Spectrometer (DoRIS) in additionto the established hydrostatic temperature measurement technique yields simultaneous and common volume measurements of winds.

At the ALOMAR observatory in northern Norway (69°N, 16°E)
the gravity wave potential energy density (GWPED) in the stratosphere is shown to have a large seasonal variation with a maximum in winter and a minimum in summer.

In this work we use the phase relation between both zonal and meridional wind components and temperature. We study gravity waves sorted for up- and downward propagating waves under summer and winter conditions to investigate different wave propagation and generation scenarios. We discuss the winter/summer difference not only in terms of total GWPED, but in terms of wave characteristics obtained from our extended analysis technique. We demonstrate, for example, that amount of downward propagating waves is larger in winter than in summer. Also, other wave characteristics like phase speed and mean intrinsic period
will be discussed.

How to cite: Strelnikova, I., Baumgarten, G., and Lübken, F.-J.: Lidar Observations of Seasonal Variability of Gravity-Waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8789, https://doi.org/10.5194/egusphere-egu2020-8789, 2020.

EGU2020-3253 | Displays | OS4.3

Gravity Wave identification using GPS Total Electron Content in South East Asia

Sarthak Srivastava and Amal Chandran

Ionospheric Total Electron Content (TEC) data from ground-based Global Positioning System (GPS) receiver networks have been used previously to detect Travelling Ionospheric Disturbances (TIDs). The TIDs have been shown to arise through coupling of lower atmosphere with the Ionosphere with Gravity Waves as the coupling mechanism. Gravity Waves generated by earthquakes, tsunamis, volcanoes, topography, convection and even solar eclipses have been detected using GPS TEC data. In this study, we identify Gravity Wave signatures in GPS TEC data derived from the Sumatran GPS Array (SuGAr) network. SuGAr is a network of 49 ground-based GPS stations along the convergent plate boundary between Indo-Australian and Asian tectonic plates in western Sumatra, Indonesia. Since initiation in 2002, data from SuGAr has primarily been used to study earthquakes and plate-tectonics in south-east Asia. Due to its location along the seismically-active region, SuGAr can provide valuable data for studying co-seismic Gravity Waves triggered by terrestrial-atmosphere coupling. Frequent occurrence of deep convective clouds in tropical region implies that SuGAr data also provides a unique opportunity to study atmospheric waves generated by convection. 

We have identified Gravity Waves across a wide spectrum corresponding to seismic and tropical convection events in Sumatran region. Upon identifying the wave signatures, we characterized the wave parameters and identified the wave sources through suitable ray tracing calculations. In this paper we show acoustic-gravity waves generated by the 2012 Sumatra great earthquake sequence consisting of 2 largest strike slip earthquakes ever recorded. Spectral analysis indicates the presence of fundamental resonant frequencies for solid Earth-atmosphere coupling. Using a geometric ray tracing method, we also trace the waves very close to the reported epicentres of the double earthquake sequence. We also discuss inertia-gravity waves generated due to convection in South-East Asia using SuGAr TEC data for 2018. Indication of deep convective clouds is confirmed through satellite-based cloud top brightness temperature data.  Ray tracing is performed to further trace the observed waves to the convective system location.

How to cite: Srivastava, S. and Chandran, A.: Gravity Wave identification using GPS Total Electron Content in South East Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3253, https://doi.org/10.5194/egusphere-egu2020-3253, 2020.

EGU2020-13569 | Displays | OS4.3

Gravity wave activity during the southern hemispheric sudden stratospheric warming 2019

Andreas Dörnbrack, Tyler Mixa, Bernd Kaifler, and Markus Rapp

At the end of the austral winter 2019, a sudden stratospheric warming led to an early breakdown of the polar vortex. The meteorological conditions during this event are documented and analysed by means of operational analyses of the Intgrated Forecast System (IFS) of the ECMWF and ERA5 data. Especially, we focus on the decline of stratospheric wave activity over the southern tip of South America. For this region, ground-based and airborne measurements are employed to compare selected diagnostics with fields from the ECMWF's numerical weather prediction model IFS. Furthmore, the meteorological conditions for one selected research flight during the SOUTHTRAC campaign are presented. This part serves as background information for a case study presented by Tyler Mixa.

How to cite: Dörnbrack, A., Mixa, T., Kaifler, B., and Rapp, M.: Gravity wave activity during the southern hemispheric sudden stratospheric warming 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13569, https://doi.org/10.5194/egusphere-egu2020-13569, 2020.

EGU2020-13308 | Displays | OS4.3

First studies of mesospheric momentum flux and wind fields using the SIMONe system over Patagonia

Fede Conte, Jorge Chau, Brian Harding, Ralph Latteck, and Jacobo Salvador

During September of 2019, a state-of-the-art multistatic meteor radar system called SIMONe (Spread Spectrum Interferometric Multistatic meteor radar Observing Network) was installed in southern Patagonia, Argentina. Its main goal being the study of mesospheric waves in one of the (theoretically predicted) most dynamically active regions of the world. SIMONe Patagonia consists of 5 linearly polarized Yagi antennas in a pentagon configuration on transmission, and 5 dual-polarization single Yagi antennas on reception, situated between 30 and 270 km from the transmitters, which locate at 49.6° S. Combining measurements from the 5 links allows for more accurate estimations of mean winds and horizontal momentum flux for altitudes between 75 and 105 km. Furthermore, given the significantly higher amount of meteor detections, one can determine wind fields within the limits of the illuminated volume every 1 hour and 1 km in time and height, respectively. Preliminary results indicate a dominant semidiurnal oscillation in both the zonal and meridional wind components, as well as an enhanced and sustained (in time) gravity wave activity, especially above 90 km of altitude. In addition, momentum flux analysis reveals that the gravity wave activity is stronger than in other parts of the Southern Hemisphere, confirming that Patagonia is indeed a very active region at mesospheric heights.  

How to cite: Conte, F., Chau, J., Harding, B., Latteck, R., and Salvador, J.: First studies of mesospheric momentum flux and wind fields using the SIMONe system over Patagonia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13308, https://doi.org/10.5194/egusphere-egu2020-13308, 2020.

EGU2020-13736 | Displays | OS4.3

Time dependent 3D tomography of a mountain wave over the Andes with GLORIA IR limb imager

Lukas Krasauskas, Markus Geldenhuys, Peter Preusse, jörn Ungermann, Michael Höpfner, Felix Friedl-Vallon, Bernd Kaifler, Markus Rapp, and Riese Martin

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an aircraft-
based Fourier transform spectrometer with a 2D detector array jointly developed by Forschungszentrum
Jülich and Karlsruhe Institute of Technology. Air temperature and volume mixing ratios of various
trace gases are retrieved from the measured IR spectra. GLORIA's viewing direction can be panned between 45 and 135
w. r. t. the flight direction. Combining this capability with flight paths that encircle the observed
atmospheric region, multiple measurements of the same air mass can be performed, allowing for 3D
tomography of the atmosphere with a vertical resolution down to 250 m and horizontal resolution of
around 25 km.
GLORIA flew on the German HALO research aircraft during the SouthTRAC measurement cam-
paign held in southern Argentina in September-November 2019. One of the main goals of the campaign
was gravity wave study using GLORIA, as well as an upward looking ALIMA lidar instrument devel-
oped by Deutsches Zentrum für Luft- und Raumfahrt (DLR), and in situ instruments. During one of
the research flights, a large amplitude mountain wave was observed over the Andes. The air volume
near the mountains was encircled twice, providing a unique opportunity to study the time evolution
of an orographic gravity wave with a help of 3D time dependent temperature retrieval. We present
the initial analysis of this dataset, showing complex temperature structure with several overlapping
gravity wave families at altitudes of 9 to 14 km. GLORIA data is complemented by the ALIMA lidar
temperature retrieval at altitudes between about 20 and 60 km, providing insight into further upward
propagation and breaking of the observed mountain wave. We also compare our results with ECMWF model data.

How to cite: Krasauskas, L., Geldenhuys, M., Preusse, P., Ungermann, J., Höpfner, M., Friedl-Vallon, F., Kaifler, B., Rapp, M., and Martin, R.: Time dependent 3D tomography of a mountain wave over the Andes with GLORIA IR limb imager, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13736, https://doi.org/10.5194/egusphere-egu2020-13736, 2020.

EGU2020-20345 | Displays | OS4.3

Mountain waves penetrating into the mesosphere observed by airborne lidar

Bernd Kaifler, Andreas Dörnbrack, Tyler Mixa, Markus Rapp, Natalie Kaifler, Sonja Gisinger, and Robert Reichert

We present observations by airborne lidar which were obtained over the southern Andes during the SOUTHTRAC campaign in September 2019. Operated onboard the German research aircraft HALO, the Airborne LIdar for Middle Atmosphere research (ALIMA) acquired high resolution temperature profiles in the altitude range 20-80 km. The data show signatures of mountain waves located at the mountain ridges, but often these signatures also extend several hundred kilometer downstream. While during the first days of the campaign mountain waves were able to penetrate into the mesosphere, observations obtained in the following weeks indicate a downward shift of the breaking zone from the mesosphere to the stratosphere which is consistent with the early breakdown of the polar vortex. Our data also indicate evidence for generation of secondary gravity waves within the breaking zone of the mountain waves.

How to cite: Kaifler, B., Dörnbrack, A., Mixa, T., Rapp, M., Kaifler, N., Gisinger, S., and Reichert, R.: Mountain waves penetrating into the mesosphere observed by airborne lidar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20345, https://doi.org/10.5194/egusphere-egu2020-20345, 2020.

EGU2020-18470 | Displays | OS4.3

The Quest for the Gravity Wave Source

Markus Geldenhuys, Isabell Krisch, Peter Preusse, Joern Ungermann, Inna Polichtchouk, Lukas Krasouskas, Felix Friedl-Vallon, and Martin Riese

Apart from orography, no specific sources of parametrized gravity waves are considered in most global circulation models. This is an inadequate generalization in the long run. In 2002, Charron and Manzini stated that no global gravity wave source climatology exist, and in the meantime, little has been done to address this observationally. A single observational case over Greenland is used to illustrate how difficult it is to disentangle the source of a gravity wave. The observation was made during the POLar STRAtosphere in a Changing Climate (POLSTRACC) campaign on 10 March 2016. The campaign was based in Kiruna, Sweden and investigated polar air during the polar vortex breakdown in spring. The gravity wave was observed between 10 and 15 km, with a horizontal wavelength of around 300 km and a vertical wavelength of around 2 km. Several plausible source mechanisms were present at the time of observation, a breaking Rossby wave, jet exit region, cold front, strong wind shear, and topography. POLSTRACC observations, ECMWF high resolution analysis and ERA 5 reanalysis were used to find the gravity wave source. Observations were obtained by the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument based on the High Altitude Long Range (HALO) German research aircraft. GLORIA is an infrared spectrometer that measures many trace gasses in the mid-infrared frequencies. Radiance is changed by inverse-modelling to temperatures. The temperature structure of the gravity wave was tomographically reconstructed. The 3D observations correlate well with the model data. ERA 5 data and the Gravity-wave Regional Or Global Ray Tracer (GROGRAT) was used to determine the exact location of where the gravity wave was emitted. Evidence exists that the gravity wave could have been released by the geostrophic imbalance in the jet. In contrast, dedicated ECMWF model runs suggest that the origin of the gravity wave is in fact orography. However, the Scorer parameter suggests no gravity wave can propagate from the surface upwards. Two ECMWF model runs were used to prove this, one normal operational model run, and the second, with a ‘flat’ orography. This work indicates that care needs to be exercised to diagnose the source of gravity waves, especially without a full informational analysis. Our study illustrates that a better parametrization scheme of gravity wave sources should be included in models for a more realistic representation.

How to cite: Geldenhuys, M., Krisch, I., Preusse, P., Ungermann, J., Polichtchouk, I., Krasouskas, L., Friedl-Vallon, F., and Riese, M.: The Quest for the Gravity Wave Source, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18470, https://doi.org/10.5194/egusphere-egu2020-18470, 2020.

EGU2020-12990 | Displays | OS4.3

Removing spurious inertial instability signals from gravity wave temperature perturbations using spectral filtering methods

Cornelia Strube, Manfred Ern, Peter Preusse, and Martin Riese

Gravity waves are important drivers of dynamic processes in the middle atmosphere, but not the only process that could lead to small-scale perturbations. To analyse atmospheric data for gravity wave signals, gravity wave perturbations have to be separated from atmospheric variability caused by other dynamic processes. Common methods to separate small-scale gravity wave signals from a large-scale background comprise filtering methods in either the horizontal or vertical wavelength domain. Recently, studies showed that vertical wavelengths filtering can mistake other wave-like perturbations, such as inertial instability effects, for gravity wave perturbations.

We use artificial inertial instability perturbations, global model data and satellite observations to assess different spectral background removal approaches on their ability to separate gravity waves and inertial instabilities. Therefore, we investigate a horizontal background removal, applying a zonal wavenumber filter with additional smoothing of the spectral components in meridional and vertical direction, a sophisticated filter based on 2D time-longitude spectral analysis (see Ern et al., 2011) and a vertical wavelength Butterworth filter.

We analyse the results for critical thresholds of the vertical wavelength and zonal wavenumber, respectively. Vertical filtering has to remove a part of the gravity wave spectrum in order to eliminate inertial instability remnants from the perturbations. Horizontal filtering, however, separates the data at scales far beyond the expected gravity wave spectrum for the case we investigated. Furthermore, we show that it is possible to effectively separate inertial instabilities perturbations from gravity waves perturbations for infrared limb-sounding satellite profiles using a cutoff zonal wavenumber of 6.

How to cite: Strube, C., Ern, M., Preusse, P., and Riese, M.: Removing spurious inertial instability signals from gravity wave temperature perturbations using spectral filtering methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12990, https://doi.org/10.5194/egusphere-egu2020-12990, 2020.

The climatology of residual mean circulation – a main component of the Brewer–Dobson circulation – and the potential contribution of gravity waves (GWs) are examined for the annual mean state and each season in the whole stratosphere based on the transformed-Eulerian mean zonal momentum equation using four modern reanalysis datasets. Resolved and unresolved waves in the datasets are respectively designated as Rossby waves and GWs, although resolved waves may contain some GWs. First, the potential contribution of Rossby waves (RWs) to residual mean circulation is estimated from Eliassen–Palm flux divergence. The rest of residual mean circulation, from which the potential RW contribution and zonal mean zonal wind tendency are subtracted, is examined as the potential GW contribution, assuming that the assimilation process assures sufficient accuracy of the three components used for this estimation. The GWs contribute to drive not only the summer hemispheric part of the winter deep branch and low-latitude part of shallow branches, as indicated by previous studies, but they also cause a higher-latitude extension of the deep circulation in all seasons except for summer. This GW contribution is essential to determine the location of the turn-around latitude. The autumn circulation is stronger and wider than that of spring in the equinoctial seasons, regardless of almost symmetric RW and GW contributions around the Equator. This asymmetry is attributable to the existence of the spring-to-autumn pole circulation, corresponding to the angular momentum transport associated with seasonal variation due to the radiative process. The potential GW contribution is larger in September to November than in March-to-May in both hemispheres. The upward mass flux is maximized in the boreal winter in the lower stratosphere, while it exhibits semi-annual variation in the upper stratosphere. The boreal winter maximum in the lower stratosphere is attributable to stronger RW activity in both hemispheres than in the austral winter. Plausible deficiencies of current GW parameterizations are discussed by comparing the potential GW contribution and the parameterized GW forcing.

How to cite: Sato, K. and Hirano, S.: The climatology of the Brewer-Dobson circulation and the contribution of gravity waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1418, https://doi.org/10.5194/egusphere-egu2020-1418, 2020.

EGU2020-20423 | Displays | OS4.3

Sensitivity of middle atmosphere GW forcing to vertical model resolution

Andrea Schneidereit, Hauke Schmidt, and Claudia Stephan

Several current general atmospheric circulation models provide sufficiently high resolutions to resolve important parts of the internal gravity wave spectrum allowing for numerical experiments without GW drag parameterizations. GWs start to be well resolved from horizontal wavelengths of about 7 times the horizontal grid spacing. How much does the resolved wave spectrum and its forcing on the mean circulation depend on the vertical resolution?

−1,The middle atmosphere summer hemisphere provides a suitable background to investigate this question. The mean stratospheric and mesospheric circulation is characterised by prevailing easterlies which prevent planetary wave propagation upwards and represents a mean state driven by IGWs. The sensitivity of the forcing by IGWs is analysed on the basis of the Eliassen-Palm (EP) flux divergence, which describes the forcing on the circulation by resolved eddies.
Model simulations are performed using the upper atmosphere version of the ICON (ICOsahedral Nonhydrostatic) general circulation model, UA-ICON (Borchert et al. 2019, GMD). The simulations start in October and run for an extended austral summer season until March with a horizontal grid spacing of roughly 20 km. The top of the model atmosphere is located at 150 km. Three different model configurations are used with 90, 180, and 360 vertical model layers. The mean vertical grid spacing ranges from roughly 1300 m (90 layers) to 320 m (360 layers) at stratospheric levels, and from roughly 2300 m to 500 m at mesospheric levels. Gravity wave drag parameterizations (orographic and non-orographic) are turned off. The resolved forcing on the mean state due to the EP flux divergence is decomposed into contributions of different scales with respect to horizontal wave numbers. For contributions of IGWs wave numbers above 20 are considered.

The stratospheric and mesospheric easterlies appear stronger in the lower resolution from October to the end of the austral summer season. Westerlies occur above the mesopause. This strong vertical gradient in the zonal mean zonal wind amplifies in the lower resolution. At the beginning of the simulation period, differences between the mean states are weak, of the order of 5 ms−1 , and strengthen during the summer season. The forcing due to internal GWs appears stronger in the lower resolution at higher altitudes and amplifies in the region of the strong vertical gradient of the zonal mean zonal wind. Furthermore, wave spectra are discussed. In accordance with previous studies, an increased vertical resolution results in a reduction of the IGW forcing close to strong zonal mean zonal wind gradients in the upper mesosphere/lower thermosphere.

How to cite: Schneidereit, A., Schmidt, H., and Stephan, C.: Sensitivity of middle atmosphere GW forcing to vertical model resolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20423, https://doi.org/10.5194/egusphere-egu2020-20423, 2020.

EGU2020-7375 | Displays | OS4.3

A multi-wave model for the Quasi-Biennial Oscillation: Plumb’s model extended

Pierre Léard, Daniel Lecoanet, and Michael Le Bars

In the Earth’s stratosphere, equatorial zonal winds reverse from easterlies to westerlies with a period of roughly 28 months. This phenomenon, known as Quasi-Biennial Oscillation (QBO), is driven by internal gravity waves (IGWs) propagating in the stratosphere and interacting with the ambient large-scale flow. Those waves are generated by the turbulent motions of the troposphere. In 1977, an idealised model describing the generation of a reversing large-scale flow by two counter-propagating monochromatic internal gravity waves was developed by Plumb [1]. In 1978, the famous Plumb & McEwan’s experiment [2] validated this model using oscillating membranes to force a standing wave pattern at the boundary of a linearly stratified salty-water layer in a cylindrical shell container.

Recently, the effects of the wave dissipation and wave energy were studied by Renaud et al. [3] using the Plumb model in order to explain the QBO disruption observed in 2016. It was found that as the Reynolds number increases, bifurcations from periodic to non-periodic regimes are seen for the large-scale flow oscillations.

Here, we present the results obtained from an extended version of the Plumb’s model, taking into account the stochastic generation of IGWs in Nature. Our new model includes a wide spectrum of waves as forcing for the large-scale flow. A gaussian distribution of energy is used in order to compare monochromatic forcing results (characterised by a gaussian energy spectrum with a small standard deviation) with multi-wave forcing results (large standard deviation). Unexpectedly, we find that in a large parameter domain, gathering the energy of the forcing into one frequency results in non-periodic oscillations for the QBO while spreading the same amount of energy among many frequencies results in periodic oscillations. We also investigate more realistic distribution of energy for the forcing including classical convective spectra, with or without rotation. We find that different forcings result in very similar reversals. This result is quite relevant for Global Circulation Models (GCMs) where internal gravity waves are parameterised in order to drive a realistic QBO. However, our study suggests that driving a QBO with realistic characteristics (amplitude, period) does not involve that the input forcing (i.e. the wave spectrum characteristics) is realistic as well.

References:

[1] R. A. Plumb, « The interaction of two internal waves with the mean flow: Implications for the theory of the quasi-biennial oscillation », Journal of the Atmospheric Sciences, 1977.

[2] R. A. Plumb and A. D. McEwan, « The instability of a forced standing wave in a viscous stratified fluid: a laboratory analogue of the quasi biennial oscillation », Journal of the Atmospheric Sciences, 1978.

[3] A. Renaud, L.-P. Nadeau, and A. Venaille, « Periodicity Disruption of a Model Quasibiennial Oscillation of Equatorial Winds », Phys. Rev. Lett., vol. 122, no 21, p. 214504, 2019.

How to cite: Léard, P., Lecoanet, D., and Le Bars, M.: A multi-wave model for the Quasi-Biennial Oscillation: Plumb’s model extended, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7375, https://doi.org/10.5194/egusphere-egu2020-7375, 2020.

EGU2020-4997 | Displays | OS4.3

Sensitivity of resolved gravity wave momentum fluxes on different background separation methods in a high resolution simulation.

Zuzana Procházková, Petr Šácha, Aleš Kuchař, Petr Pišoft, and Christopher Kruse

Internal gravity waves (GWs) and their interaction with the atmospheric circulation present a complex problem for global climate models (GCMs) due to a variety of spatial and temporal scales involved. GWs and their effects in GCMs are parameterized by employing various simplifications and restrictions
(propagation, spectrum). Also, our incomplete knowledge of the GW properties in the real atmosphere complicates the situation. Global (satellite) observations of the GW activity are spatiotemporally sparse, making the quantification of the GW interaction with the circulation hardly possible. Recently, atmospheric models capable of resolving most of the GW spectrum have been emerging due to the increasing performance of computing systems. It is increasingly acknowledged that a combination of various types of observations with dedicated high-resolution, GW resolving, simulations has a potential to provide the most precise information about GWs. This combination will allow us to better understand the uncertainty of satellite observations of GW activity, which in turn will be used to develop new GW parameterizations or in development of GW resolving models.
In this study, we will analyze sensitivity of GW momentum flux and its divergence on background separation (and other GW detection) methods and approximations (Boussinesq, anelastic) used in the formulas. We analyze data from high-resolution model simulations produced for an observing system simulation experiment of the ISSI team "New Quantitative Constraints on Orographic GW Stress and Drag" (to be introduced in an invited presentation by Ch. Kruse).

How to cite: Procházková, Z., Šácha, P., Kuchař, A., Pišoft, P., and Kruse, C.: Sensitivity of resolved gravity wave momentum fluxes on different background separation methods in a high resolution simulation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4997, https://doi.org/10.5194/egusphere-egu2020-4997, 2020.

Gravity waves play an important role in the momentum and heat budgets of the middle atmosphere. Global circulation models used for long-term simulations need to parameterize the transport of wave momentum and energy from the lower to the middle atmosphere and the associated wave-mean flow interaction. This gravity wave-mean flow interaction is usually due to dynamical instability triggered by wave refraction and amplitude growth, giving rise to wave dissipation. In addition, gravity waves can interact with the mean flow through the passage of finite wave packets without dissipation. Conventional gravity wave parameterizations cannot describe this effect; nor can they account for wave sources being continuous in space and time, for the finite duration of vertical propagation, or wave acceleration induced by a temporally varying mean flow.
All these effects are accommodated when the wave field is described by the wave-energy density in wave number and physical space, and its evolution is computed by the radiative transfer equation for the wave field. A corresponding parameterization called IDEMIX has successfully been applied in ocean models. Here we present a corresponding parameterization for atmosphere models in single-column approximation. The new scheme is validated in off-line simulations. Results show that the evolution of wave packets forced in the troposphere and propagating upward into stratospheric and mesospheric jets is simulated consistently with theoretical expectations. This includes wave reflection and critical layers. Furthermore, an explicit diffusion scheme was added to account for wave dissipation due to dynamical instability.

How to cite: Mai, M. and Becker, E.: A new parameterization of gravity waves for atmospheric circulation models based on the radiative transfer equation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20262, https://doi.org/10.5194/egusphere-egu2020-20262, 2020.

EGU2020-3157 | Displays | OS4.3

Towards the implementation of a transient gravity wave drag parameterization in atmospheric models

Gergely Bölöni, Young-Ha Kim, Sebastian Borchert, and Ulrich Achatz

The aim of the presented work is to improve the parameterization of subgrid-scale gravity wave (GW) effects on the resolved flow in atmospheric models in a large altitude range from the upper troposphere to the mesopause (~85km). State of the art GW parameterization schemes are using the steady-state approximation for the wave field and therefore assume an instantaneous GW propagation neglecting direct interactions between the GW field and the resolved flow within the whole altitude range mentioned above. As such, these schemes rely on dissipative processes - GW breaking and critical layer filtering - as the only mechanism to accelerate/decelerate the resolved flow. In contrast to this, by dropping the steady-state assumption a contribution to the mean-flow forcing emerges in the form of direct GW-mean-flow interactions. Several idealized studies show that, besides dissipative effects, direct GW-mean-flow interactions contribute to GW dynamics in an important extent (Bölöni et al., 2016, J. Atmos. Sci.}, 73, 4833-4852, Wilhelm et al., 2018, J. Atmos. Sci., 75, 2257-2280, Wei et al., 2019, J. Atmos. Sci., 76, 2715-2738). This motivates the implementation of a transient GW model (MS-GWaM: Multi Scale Gravity Wave Model) to UA-ICON, the upper atmosphere version of ICON (Borchert et al., 2019, Geosci. Model Dev., 12, 3541-3569) which does not rely on the steady-state assumption and thus includes direct GW-mean-flow interactions. As a reference and a representative of currently available GW parameterization schemes a steady-state version of MS-GWaM (ST-MS-GWaM) has been implemented to UA-ICON as well, which shares the treatment of all possible components (wave sources and wave saturation scheme) with the transient MS-GWaM scheme and differs from it "only" in the treatment of propagation, i.e. excluding direct GW-mean-flow interactions and thus transience. Both MS-GWaM and ST-MS-GWaM reproduce the observed wind and temperature climatology (e.g. URAP data: Swinbank, R. and D. A. Ortland, 2003, J. Geophys. Res., 108, D19, 4615) reasonably well, but the transient propagation makes a robust difference in the circulation in perpetual runs. The transient propagation in MS-GWaM substantially contributes to an increase of the GW intermittency in the whole altitude range, giving a better comparison with super-pressure balloon observations (e.g. Hertzog et al., 2012, J. Atmos. Sci., 69, 3433-3448), whereas the lack of transience prevents any occurrence of higher GW momentum flux values than the launch magnitude itself. This is explained by the fact that the direct GW-mean-flow interactions involve a highly transient evolution of the wave action and the vertical group velocity, which often leads to increased pseudo-momentum fluxes as compared to the launch magnitude.

How to cite: Bölöni, G., Kim, Y.-H., Borchert, S., and Achatz, U.: Towards the implementation of a transient gravity wave drag parameterization in atmospheric models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3157, https://doi.org/10.5194/egusphere-egu2020-3157, 2020.

EGU2020-10957 | Displays | OS4.3

Mountain wave parametrization in a transient gravity wave model

Fanni Dora Kelemen, Jan Weinkaemmerer, Gergely Bölöni, and Ulrich Achatz

Orography induced gravity waves are investigated in a Multi Scale Gravity Wave Model (MS-GWaM) over idealized topography. MS-GWaM is a prognostic gravity-wave model, which parametrizes both the propagation and dissipation of subgrid-scale GWs. It is a Lagrangian ray-tracer model, which applies WKB-theory and calculates the propagation of ray volumes in spectral space. Its novelty is that not only the dissipative effect, but also the non-dissipative effects due to direct wave-mean flow interaction are captured. In our conceptual studies we investigate mountain wave generation, which is induced via a time-dependent large-scale wind encountering a prescribed topography. The framework used in our experiments is the PincFloit model, which integrates the pseudo-incompressible equations. We use it both in low resolution with MS-GWaM and in high resolution LES mode as a wave resolving reference. In the reference LES simulations the idealized topography, a mountain chain, is represented with an immersed boundary method. In the MS-GWaM experiments there is no resolved topography, but its effect is modelled as a lower boundary condition. The lower boundary condition is represented by initializing ray volumes with wave number and wave action density depending on the mountain characteristics and the large scale wind speed, based on the assumption that the flow follows the terrain. In the wave resolving reference experiments the flow does not strictly follow the terrain, but other instabilities (rotor formation, boundary layer separation) arise around the mountains. These processes decrease the available momentum transported by GWs, which was initially not accounted for in MS-GWaM. Thus an overestimation of wind deceleration was found in the MS-GWaM parametrization compared to the wave resolving simulation. To correct for this overestimation, an effective mountain height is introduced into Ms-GWaM, which is calculated by a scaling function between mountain height and flow properties using the Froude number.

How to cite: Kelemen, F. D., Weinkaemmerer, J., Bölöni, G., and Achatz, U.: Mountain wave parametrization in a transient gravity wave model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10957, https://doi.org/10.5194/egusphere-egu2020-10957, 2020.

Inertia–gravity waves (IGWs) emitted from jets and fronts are ubiquitous in the atmosphere and have a significant impact on atmospheric processes (Plougonven and Zhang, 2014). Since the mechanism responsible for the spontaneous emission of IGWs during the evolution of an initially balanced flow remain poorly understood, their representation in numerical weather prediction models is challenging (de la Cámara and Lott, 2015). Better understanding of this IGW source mechanism based on idealized numerical simulations is crucial to improve the accuracy of the forecasts. In this study, idealized baroclinic-wave life cycle experiments on the f-plane are performed to investigate spontaneous emission, using a finite-volume solver for the pseudo-incompressible equations (Rieper et al., 2013). In particular, the implementation of a semi-implicit time stepping scheme, along the lines of Smolarkiewicz and Margolin (1997) and Benaccio and Klein (2019), but adjusted to our staggered grid, permits longer simulation runs with much larger domains. A novelty is the implementation of a simple Newtonian heating function based on Held and Suarez (1994), which is used for forcing a baroclinically unstable temperature profile and allows the background state to vary in time (O’Neill and Klein, 2014). The results of the model with semi-implicit time stepping scheme will be documented and compared to an explicit Runge-Kutta scheme. The analysis may serve as a basis for the development and validation of a parameterization scheme for GWs emitted from jets and fronts.

References:

Benaccio, T., and R. Klein, 2019: A semi-implicit compressible model for atmospheric flows with seamless access to soundproof and hydrostatic dynamics. Mon. Wea. Rev., 147, 4221-4240.
de la Cámara, A., and F. Lott, 2015: A parameterization of gravity waves emitted by fronts and jets. Geophys. Res. Lett., 42, 2071-2078.
Held, I.M., and M.J. Suarez, 1994: A Proposal for the Intercomparison of the Dynamical Cores of Atmospheric General Circulation Models. Bull. Amer. Meteor. Soc., 75, 1825-1830.
O’Neill, W.P., and R. Klein, 2014: A moist pseudo-incompressible model. Atmos. Res., 142, 133-141. Plougonven R., and F. Zhang, 2014: Internal gravity waves from atmospheric jets and fronts. Rev. Geophys., 52, 33-76.
Rieper, F., Hickel, S., and U. Achatz, 2013: A conservative integration of the pseudo-incompressible equations with implicit turbulence parameterization. Mon. Wea. Rev., 141, 861-886. Smolarkiewicz, P.K., and L.G. Margolin, 1997: On forward-in-time differencing for fluids: an Eulerian/semi-Langrangian nonhydrostatic model for stratified flows. Atmosphere-Ocean, 35, 127- 152.

How to cite: Schmid, F., Gagarina, E., Klein, R., and Achatz, U.: Generation of inertia-gravity waves in idealized baroclinic-wave life cycles: Explicit vs. semi-implicit time stepping in a finite-volume solver for the pseudo-incompressible equations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2903, https://doi.org/10.5194/egusphere-egu2020-2903, 2020.

EGU2020-1584 | Displays | OS4.3

The long-time spatial and temporal development of Triadic Resonance Instability

Katherine Grayson, Stuart Dalziel, and Andrew Lawrie

How to cite: Grayson, K., Dalziel, S., and Lawrie, A.: The long-time spatial and temporal development of Triadic Resonance Instability , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1584, https://doi.org/10.5194/egusphere-egu2020-1584, 2020.

EGU2020-10630 | Displays | OS4.3

Diapycnal diffusivity induced by the breaking of lee waves
not presented

Thomas Eriksen, Carsten Eden, and Dirk Olbers

A key component in setting the large scale ocean circulation is the process of diapycnal mixing, since this can drive the meridional overturning circulation. Diapycnal mixing in the interior ocean is predominantly associated with the breaking of internal waves. Traditionally, diapycnal mixing has been represented in ocean models by a diapycnal diffusivity either constant or exponentially decreasing with depth. This approach, however, does not take into account the actual physics behind the breaking of internal waves. The energetically consistent internal wave model IDEMIX (Internal wave Dissipation, Energetics and MIXing), on the other hand, computes diffusivities directly on the basis of internal wave energetics. One such type of internal waves are lee waves. These are generated and subsequently dissipated when geostrophic currents interact with bottom topography and are therefore believed to be a source of energy for deep ocean mixing. In this study IDEMIX is coupled to a 1/12th degree regional model of the Atlantic. The lee wave energy flux is calculated and used as a bottom flux at each time step effectively allowing lee waves to propagate, interact with mean flow and waves, and subsequently dissipate. This setup enables not only an estimate of the lee wave energy flux but also a direct investigation of the influence of lee waves on dissipation, stratification and horizontal and overturning circulation.

How to cite: Eriksen, T., Eden, C., and Olbers, D.: Diapycnal diffusivity induced by the breaking of lee waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10630, https://doi.org/10.5194/egusphere-egu2020-10630, 2020.

EGU2020-13808 | Displays | OS4.3

Further developments on estimating inertia-gravity-wave properties: Combining the Riesz transform with machine learning methods

Mozhgan AmirAmjadi, Ali R. Mohebalhojeh, and Mohammad Mirzaei

One of the remaining issues in the parameterization of inertia-gravity waves is the estimation of wave characteristics such as wavenumber and intrinsic frequency. In this survey, we explore a new way to estimate the wave characteristics at the launch level. To this end, we retrieve the wavenumber using the Riesz Transform which is the generalized form of the Hilbert Transform applicable in the multi-dimensional analysis. For this purpose, the high-resolution horizontal divergence field has been employed since it filters the background flow and thus provides a reasonable representation of the inertia-gravity wave signal. This is followed by the application of machine learning to reconstruct the retrieved wavenumber using the coarse-grained resolvable variables including the horizontal wind speed, the large scale vertical velocity, the cross-stream ageostrophic wind speed, the frontogenesis function and the latent heat released during condensation as explanatory variables at the launch level.
We have employed the ERA5 dataset in this survey, having observed that the dataset can directly resolve the inertia-gravity waves at its full resolution. In order to avoid mountain waves and focus on non-orographic inertia-gravity waves, two areas far from the significant obstacles over the midlatitude of the Atlantic Ocean and Northern Pacific Ocean are considered from December 2018 to February 2019. The results show a reasonable correlation between the reconstructed wavenumber using low-resolution explanatory variables and the retrieved one using the Riesz Transform so that this method can be utilized to estimate the inertia-gravity wave number at the launch level.

How to cite: AmirAmjadi, M., Mohebalhojeh, A. R., and Mirzaei, M.: Further developments on estimating inertia-gravity-wave properties: Combining the Riesz transform with machine learning methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13808, https://doi.org/10.5194/egusphere-egu2020-13808, 2020.

EGU2020-12268 | Displays | OS4.3

The Arrested Ekman Layer Escapes! Ventilation of the BBL by Internal Swash.

Kurt Polzin, Zhankun Wang, Binbin Wang, and Angel Ruiz Angulo

Moored data from the northern Deepwater Gulf of Mexico in the vicinity of DeepWater Horizon are presented.  Subinertial flows of O(0.1-0.2 m/s) are in the sense of Kelvin wave propagation and support a downwelling Ekman layer with reduced near boundary stratification.  The moored data document cross-slope and vertical buoyancy fluxes dominated by a frequency band that includes diurnal and inertial frequencies and extend to about an order of magnitude larger than inertial.  We refer to this frequency band as internal swash and the region of reduced stratification at the bottom boundary exhibiting these fluxes as the internal swash zone.  Vertical fluxes of cross-slope momentum associated with internal swash band frequencies are large, of similar order of magnitude as the drag associated with the viscous no-flow bottom boundary condition on the cross-slope subcentral current.  Typical mixing efficiencies of (Γ ~ 0.2) are found in association with elevated mixing O(100 times background) one-to-two hundred meters above the bottom. This enhanced turbulence appears in conjunction with near-inertial frequency motions that may be dynamically coupled to the mean flow.  

 

How to cite: Polzin, K., Wang, Z., Wang, B., and Ruiz Angulo, A.: The Arrested Ekman Layer Escapes! Ventilation of the BBL by Internal Swash. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12268, https://doi.org/10.5194/egusphere-egu2020-12268, 2020.

OS4.4 – Tides of the past, present and future

EGU2020-2835 | Displays | OS4.4 | Highlight

Sea-level rise impacts on the tides of the European Shelf: mechanisms analysis

Deborah Idier, François Paris, Goneri Le Cozannet, Faiza Boulahya, and Franck Dumas

Sea-level rise (SLR) can modify not only total water levels, but also tidal dynamics. Several studies have investigated the effects of SLR on the tides of the western European continental shelf (mainly the M2 component). Idier et al. (2017) further investigate this issue using a modelling-based approach, considering uniform SLR scenarios from −0.25 m to +10 m above present-day sea level. Assuming that coastal defences are constructed along present-day shorelines, the patterns of change in high tide levels (annual maximum water level) are spatially similar, regardless of the magnitude of sea-level rise (i.e., the sign of the change remains the same, regardless of the SLR scenario) over most of the area (70%). These changes are generally proportional to SLR, as long as SLR remains smaller than 2 m. Depending on the location, they can account for +/−15% of regional SLR. Changes in high tide levels are much less proportional to SLR when flooding is allowed, in particular in the German Bight. However, some areas (e.g., the English Channel) are not very sensitive to this option, meaning that the effects of SLR would be predictable in these areas, even if future coastal defence strategies are ignored.

In the present work, we focus on the mechanisms driving these tide changes, especially the bed friction damping, the resonance properties and the reflection at the coast, i.e., local and non-local processes. Additional simulations are done to quantify the effect of these mechanisms on tide changes.

 

Reference: Idier D., Paris F., Le Cozannet G., Boulahya F., Dumas F. (2017) Sea-level rise impacts on the tides of the European Shelf. Continental Shelf Research, 137, 56-71.

How to cite: Idier, D., Paris, F., Le Cozannet, G., Boulahya, F., and Dumas, F.: Sea-level rise impacts on the tides of the European Shelf: mechanisms analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2835, https://doi.org/10.5194/egusphere-egu2020-2835, 2020.

EGU2020-10007 | Displays | OS4.4

Tidal and near-inertial energy density and energy fluxes over the Reykjanes Ridge

Clément Vic, Bruno Ferron, Ivane Salaün, Virginie Thierry, and Herlé Mercier

The Reykjanes Ridge is a key topographic structure stretching south of Iceland and located at the crossroad of the Atlantic Meridional Overturning Circulation upper and lower limbs. It has been inferred to host significant mixing and water mass transformation, yet the mechanisms at play remain obscure. Using data from an array of 7 moorings deployed for two years over the Reykjanes Ridge in a cross-ridge direction, we computed internal waves’ energy density and energy fluxes in the dominating wavebands, i.e., near-inertial and semi-diurnal (tidal) bands to assess the contribution of several mechanisms at fuelling a route to energy dissipation and mixing. Internal tide fluxes are dominating the energy fluxes by an order of magnitude right on top of the ridge and follow a clear spring-neap cycle; but rapidly fade away and become decoherent O(100) km away from the ridge, suggesting a strong scattering by mesoscale turbulence and seafloor topography. Near-inertial energy fluxes and density are surface-intensified and follow a seasonal cycle, with a winter intensification due to storms and intense low-pressure weather systems. The level of near-inertial energy density is roughly explained by the local wind power input, and rapid decay of energy with depth suggests that most of the dissipation occurs in the surface layers, thus is not important for deep water mass transformation. 

How to cite: Vic, C., Ferron, B., Salaün, I., Thierry, V., and Mercier, H.: Tidal and near-inertial energy density and energy fluxes over the Reykjanes Ridge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10007, https://doi.org/10.5194/egusphere-egu2020-10007, 2020.

EGU2020-16421 | Displays | OS4.4 | Highlight

The impact of Arctic sea ice cover on seasonal modulation of the M2 tide

Inger Bij de Vaate, Amey Vasulkar, Cornelis Slobbe, and Martin Verlaan

The impact of Arctic sea ice decline on future global tidal and storm surge extreme water levels is unknown. Regional studies show that the impact can be substantial; causing increased erosion and posing higher risks to fragile Arctic ecosystems in low-lying areas. Since Arctic tides and surges influence global water levels, consequences of Arctic sea ice decline will be noticed across the globe. In the ongoing FAST4Nl project, an Arctic Total Water Level model will be used to quantify this impact. The model will be developed as an extension of the operational Global Tide and Surge Model (GTSM) and includes the effect of sea ice on tides.

Here we present the results of a study on the seasonal variability of the M2 tide with respect to differences in sea ice cover. The effect of sea ice on the M2 amplitude was modelled for minimal and maximal sea ice configurations. In addition, tidal harmonic analysis was performed on a global tide gauge data set, supplemented by SAR altimeter derived water levels from the Arctic region. The high along-track resolution of SAR altimeters (300 m) enables to derive water levels from leads in the sea ice. Here, the retrieved sea surface heights within a given region were stacked, in order to obtain a sufficiently large data set for analysis of the predominantly ice-covered areas. This allowed to gain insight in the seasonal modulation of both local and global tides and directly relate these processes to variations in sea ice.

How to cite: Bij de Vaate, I., Vasulkar, A., Slobbe, C., and Verlaan, M.: The impact of Arctic sea ice cover on seasonal modulation of the M2 tide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16421, https://doi.org/10.5194/egusphere-egu2020-16421, 2020.

EGU2020-12500 | Displays | OS4.4

The dissipation of the internal tide inferred from a global ocean model, altimetry, and in-situ observations

Maarten Buijsman, Harpreet Kaur, Zhongxiang Zhao, Amy Waterhouse, and Caitlin Whalen

In this presentation we combine several model and observational data sets to better understand the dissipation of the diurnal and semidiurnal internal tide in the global ocean, which is relevant for maintaining the global overturning circulation. We compute depth-integrated internal tide dissipation rates from a realistically-forced global HYbrid Coordinate Ocean Model (HYCOM) simulation with a horizontal resolution of 4 km (1/25 degrees) and 41 layers. We also compute dissipation rates from altimetry in two ways: 1) from the low-mode flux divergence away from topography and 2) by fitting exponential decay curves along low-mode internal tide beams. The internal-tide sea-surface height amplitude is computed with a least-squares harmonic analysis over a 20+ year altimetry data set. Hence, the altimetry-inferred dissipation rates both reflect the tidal dissipation and the energy scattered from the stationary to the nonstationary internal tide. To account for the dissipation of the nonstationary tide, we apply a spatially-varying correction factor to the stationary dissipation inferred from altimetry.  This correction factor is computed from a global 8-km HYCOM simulation with a duration of 6 years, from which the stationary and nonstationary internal tides can be easily isolated. We compare the simulated and the corrected altimetry-inferred dissipation rates with dissipation rates from finescale and microstructure observations. Preliminary results show that the simulated dissipation is up to a factor of two larger than the depth-integrated dissipation rates inferred from finescale methods, but smaller than the dissipation rates from microstructure.

How to cite: Buijsman, M., Kaur, H., Zhao, Z., Waterhouse, A., and Whalen, C.: The dissipation of the internal tide inferred from a global ocean model, altimetry, and in-situ observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12500, https://doi.org/10.5194/egusphere-egu2020-12500, 2020.

The Free Core Nutation (FCN) is a retrograde mode related to the slight misalignment of the rotation axis of the fluid outer core and the elastic mantle, with a period of about 430 sidereal days in the celestial frame. In the Earth-fixed reference frame, the (complex) frequency of the Free Core Nutation (FCN) is inside the diurnal tidal band and causes a resonant response (Free Core Resonance, FCR) of some diurnal tidal waves to the tide-generating forces.
The FCN is usually investigated through its effects on gravity tides and Earth nutations. Here we analyse about 7 years of discontinuous strain records from two 90-m long laser interferometers (strainmeters) operating under the Gran Sasso (Italy) massif and about 4.6 years of discontinuous strain records from two 70-m-long laser interferometers operating the Central Pyrenees (Spain).
Starting from the expressions for the vector displacements due to diurnal and semi-diurnal solid tides, we express  extension along any azimuthal direction in terms of three complex parameters (related to areal strain and the two shear strain components), which are functions of the latitude-dependent Love and Shida numbers. Those three complex parameters are affected by the FCR through three complex resonance strengths.
We find that we can infer 4 model parameters from the inversion of our data, i. e. from the comparison between amplitudes and phases of the measured and theoretical diurnal tides close to the resonance: the FCN period, the FCR quality factor, the imaginary part of one of the three resonance strengths, and the real part of another resonance strength. However, local deformation is distorted with respect to regional deformation because of siting effects. Coupling between local extension (measured by the interferometers) and regional deformation can be described by three coupling coefficients per interferometer, thus introducing 12 additional unknown in the inversions.
We minimize misfit between amplitudes and phases of the measured and theoretical tidal strain jointly for all the interferometers by sampling the 4D model parameter space, while optimal coupling coefficients for each interferometer are computed through a simple matrix inversion at each sampled point.
Theoretical strain tides is corrected for the effects of the water load oscillations caused by ocean tides. We use FES2014 and TPXO9 ocean models, while the appropriate Earth model for different ocean load areas is chosen basing on the widths of the continental shelves nearby the stations and the inversion misfits.
Although we analyse records from two stations only and the amount of data is relatively small, our results for the FCN period and (to some extent) the FCR quality factor are robust and comparable to those obtained from gravity tides and  nutations. Moreover, we obtain reliable values of the resonance strengths and robust estimates of the coupling coefficients for all the interferometers.

How to cite: Amoruso, A. and Crescentini, L.: Free Core Resonance parameters from diurnal strain tides recorded by the Gran Sasso (Italy) and Canfranc (Spain) underground geodetic interferometers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10392, https://doi.org/10.5194/egusphere-egu2020-10392, 2020.

EGU2020-4387 | Displays | OS4.4

The time-varying characteristics in tidal duration asymmetry

Guo Wenyun, Song Dehai, Guo Leicheng, Ge Jianzhong, Ding Pingxing, and Wang Xiaohua

Tides always behaves different rising and falling durations, which can mostly attribute to the shallow-water effect and interactions among tidal constituents. The duration asymmetry may lead to an inequality in flood/ebb tidal current magnitudes, affecting the net sediment transport. Tidal duration asymmetry has time-dependent characteristics. We deducted a general framework for identifying the time-variability in tidal duration asymmetry. The application to the global tides showed that the fortnightly variability in tidal asymmetry is universal and that duration asymmetry can be stronger during neap tide than during spring tide. Then the framework is applied to the tides in the Changjiang Estuary. Prominent seasonal variation in tidal asymmetry is revealed, mainly relate to the river-tide interaction. Application to the tides in the Yangshan Harbor sea area revealed that the local-scale tidal asymmetry can be changed strongly by a large coastal engineering.

How to cite: Wenyun, G., Dehai, S., Leicheng, G., Jianzhong, G., Pingxing, D., and Xiaohua, W.: The time-varying characteristics in tidal duration asymmetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4387, https://doi.org/10.5194/egusphere-egu2020-4387, 2020.

EGU2020-12115 | Displays | OS4.4 | Highlight

The effects of tidal changes on the frequency of nuisance flooding events in the United States

Sida Li, Thomas Wahl, David Jay, Stefan Talke, and Lintao Liu

Nuisance flooding (NF) or high tide flooding describes minor nondestructive flooding which can nonetheless cause substantial negative socio-economic impacts to coastal communities. The frequency of NF events has increased and accelerated over the past decades along the U.S. coast, leading to changes ranging from 300% to 900%. This is mainly a result of sea level rise reducing the gap between high tidal datum and flood thresholds. While long-term relative sea level rise is the main driver for the increased number of NF events, other factors such as variability in the Gulf stream, the storm climate, and infragravity waves can also contribute. Another important driver that is often overlooked is related to changes in coastal and estuary tides, through secular trends in the amplitudes of major tidal constituents. In this presentation we assess the role of tidal changes in modulating the frequency of NF events along the U.S. coastline. We analyze hourly records from 49 U.S. tide gauges for which the National Weather Service has defined NF thresholds. We find that (1) overall across all tide gauges the number of NF days has increased since 1950 due to changes in coastal tides, adding up to 100 NF days in recent years (on top of the increase due to relative sea level rise), (2) more tide gauges experience an increase in NF events than a decrease due to changes in tides, (3) tide gauges in major estuaries which have undergone major anthropogenic alterations experience the strongest changes; in Wilmington (Cape Fear estuary), for example, 10-40% of NF events in recent years can be attributed to tidal changes. 

How to cite: Li, S., Wahl, T., Jay, D., Talke, S., and Liu, L.: The effects of tidal changes on the frequency of nuisance flooding events in the United States, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12115, https://doi.org/10.5194/egusphere-egu2020-12115, 2020.

EGU2020-10207 | Displays | OS4.4

New unconstrained global ocean tide solutions for satellite gravimetry including minor tides

Roman Sulzbach, Henryk Dobslaw, and Maik Thomas

The quality of global ocean tide models has increased drastically over the last decades due to the availability of dense open-ocean observations from satellite altimetry. In regions of poor altimetry coverage (e.g., polar seas and coastal areas) and for minor tides with a small signal-to-noise ratio, however, reliable estimates from unconstrained global numerical models are still (and will remain) critically important. We will present in this contribution recent results from the purely-hydrodynamic, barotropic tidal model TiME (Weis et al., 2008) that benefit from a newly introduced rotated grid avoiding the singularity at the North Pole; a revised scheme for dynamic feedbacks of self-attraction and loading; and revised bathymetry data-sets that also include water column height modifications in cavities underneath the Antarctic ice-shelves.

By focussing exemplarily on the M2 tide, we will demonstrate the individual impact of all those changes on the simulated water height variations. It will be shown that the effects of ice-shelf cavities extend well beyond the Southern Ocean and affect even amphidromic systems in the Northern Hemisphere. We will also emphasize the ability of unconstrained numerical models as TiME to explicitly simulate minor tidal lines, thereby allowing to thoroughly test (and subsequently improve) admittance-based methods currently employed for the processing of satellite gravimetry data from the GRACE and GRACE-FO missions.

How to cite: Sulzbach, R., Dobslaw, H., and Thomas, M.: New unconstrained global ocean tide solutions for satellite gravimetry including minor tides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10207, https://doi.org/10.5194/egusphere-egu2020-10207, 2020.

We present the results of our studies of singular value decomposition (SVD) of the forward operator in tidal analysis. Using the resolution matrix and the ratio between singular values, we distinguish significant contributions that compose the tidal signal and we study cross-talk within and between tidal groups. Using all harmonics from the tidal catalogue we investigate the resolution matrix properties with decreasing amplitude of harmonics. We demonstrate the loss of resolution even for harmonics of large amplitude with decreasing time-series length. Our further investigation shows the cross-talk from atmospherically induced gravity variation into a tidal signal (expected and unexpected, e.g. S1, Fi1, Sig1). We investigate the ability to determine the ratio of gravimetric factors of degree 2 and degree 3 tides from the specific tidal gravity signal recordings.

The main interest of tidal analysis is the accurate and precise determination of tidal parameters, which are amplitude (gravimetric) factor and phase lag, the quantities describing the Earth response to the tidal forcing. Tidal catalogues define the tide generating potential in terms of harmonics. Widely used software, like ETERNA or Baytap-G, uses a-priori grouping of harmonics which is based on reasonable considerations like the Rayleigh criterion of spectral resolution. Wave grouping is a model parameterisation used to make the analysis problem overdetermined by using assumptions regarding the model parameters (e.g. credo of smoothness, known free-core resonance parameters, known ratio between response to degree 2 and degree 3 forcing). If those assumptions are incorrect, this can lead to artefacts which might go unnoticed. This presents a limitation for example in the search for causes of temporal variation of tidal parameters, as reported recently. SVD of the unparameterised problem allows us to investigate these limitations.

In our analysis, SVD is a factorisation of a linear regression matrix. The regression matrix consists of tidal harmonics in-phase and quadrature signal for rigid Earth tide (tidal forcing to Earth surface). We compute time series for each harmonic present in Tamura tidal catalogue by using a modified version of "Predict" (ETERNA package). Resulting values can be, but do not need to be, grouped prior to SVD analysis. Other than with conventional programs, wave groups can not only be defined along the frequency axis. They can as well be used to separate harmonics of degree 2 and degree 3. SVD allows us to study the significance of tidal harmonics, cross-talk between harmonics or groups and matrix null space. Thus, we can discriminate the parameters with small singular value, which do not significantly contribute to the predicted tidal data or are noise-sensitive.

How to cite: Ciesielski, A. and Forbriger, T.: Resolution and significant contributions of tidal forcing in flexible harmonic grouping computed using Singular Value Decomposition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17758, https://doi.org/10.5194/egusphere-egu2020-17758, 2020.

EGU2020-5446 | Displays | OS4.4

Efficient Calibration of a Global Tide and Surge Model

xiaohui wang, Martin Verlaan, and Hai Xiang Lin

Combined tide and surge models are very useful tools to issue warnings for storm surges as well as for assessment of the potential impacts of sealevel rise. Over the past decade, a Global Tide and Surge Model (GTSM) has been developed by Deltares with improvements in physics, grid resolution and skill in the each new version. The uncertainties in bathymetry and friction are currently a major part of the remaining model uncertainty. Improved estimates of these parameters would be desirable, but the required computing speed and memory storage are limiting the possibilities at the moment. Here, we propose an efficient coarse grid parameter estimation scheme for the high resolution GTSM to estimate the bathymetry. OpenDA software is combined with GTSM using DUD algorithm (Does not Use Derivative) making use of the FES2014 dataset as observations in deep water. Even though parallel computing is implemented for model simulation, calibration of the fine grid model directly would still require too much computer time, for instance, e.g. it takes 9 hours on 20 cores to simulate 45 days and calibration typically requires many of these simulations. Therefore, a coarse-to-fine strategy is developed by replacing the fine grid with coarse grid in parameter estimation iterations to reduce the computing cost by 67%. Moreover, via a sensitivity analysis we are able to reduce the parameter dimension from O(106) to O(102) which leads to a  further reduction of the required computation and memory. The results of the estimation and model validation demonstrate the parameter estimation scheme to improve the accuracy of the model by approximately 30% with affordable computational and storage demands.

 

How to cite: wang, X., Verlaan, M., and Lin, H. X.: Efficient Calibration of a Global Tide and Surge Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5446, https://doi.org/10.5194/egusphere-egu2020-5446, 2020.

EGU2020-22142 | Displays | OS4.4

Ensemble Water Level Prediction System: Improving the Representation of Model Uncertainty

Natacha Bernier, Oleksandr Huziy, Keith Thompson, Pengcheng Wang, Benoit Pouliot, and Syd Pell

Concern over increased flooding and the need for earlier and more reliable risk forecasts motivate the continued development of operational forecasts of coastal water level. We report here on results from a year long ensemble of total water level forecasts calculated using a dynamical ocean model forced with ensemble atmospheric forcing and tidal boundary conditions. We focus on the east coast of Canada. The domain includes the Gulf of St. Lawrence, the Labrador Shelf, the Scotian Shelf, and the Gulf of Maine. The water level ensemble is made of a control and 20 perturbed members. Individual forecasts are produced twice daily for 16 days.

 

The novelty of the present study is in the exploration of perturbations of the ocean contributions. In addition to examining how uncertainty in atmospheric forcing maps into flood risk, we also explore the feasibility, and impact, of perturbing the ocean tides. We use a recent case study to demonstrate our findings.

 

How to cite: Bernier, N., Huziy, O., Thompson, K., Wang, P., Pouliot, B., and Pell, S.: Ensemble Water Level Prediction System: Improving the Representation of Model Uncertainty , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22142, https://doi.org/10.5194/egusphere-egu2020-22142, 2020.

EGU2020-1359 | Displays | OS4.4

Tidal asymmetry in the Sylt-Romo Bight, south-eastern North Sea

Vera Fofonova, Alexey Androsov, Lasse Sander, Ivan Kuznetsov, Felipe Amorim, H. Christian Hass, Finn Mielck, and Karen Helen Wiltshire

 

The study is dedicated to the tidal dynamics in the Sylt-Rømø Bight with a focus on the non-linear processes. The FESOM-C model was used as the numerical tool, which works with triangular, rectangular or mixed grids and is equipped with a wetting/drying option. As the model’s success at resolving currents largely depends on the quality of the bathymetric data, we have created a new bathymetric map for an area based on recent studies of Lister Deep, Lister Ley, and the Højer and Rømø Deep areas. This new bathymetric product made it feasible to work with high resolution grids (up to 2 m in the wetting/drying zone). As a result, we were able to study the tidal energy transformation and the role of higher harmonics in the domain in detail. The tidal ellipses, maximum tidally-induced velocities, energy fluxes and residual circulation maps were constructed and analysed for the entire bight. Additionally, tidal asymmetry maps were introduced and constructed. The full analysis was performed on two grids with different structures and showed a convergence of the results as well as fulfillment of the energy balance. The tidal residual circulation and asymmetric tidal cycles largely define the circulation pattern, transport and accumulation of sediment and the distribution of bedforms in the bight, therefore the results are necessary and useful benchmarks for further studies in the area, including baroclinic and sediment dynamics investigations.

How to cite: Fofonova, V., Androsov, A., Sander, L., Kuznetsov, I., Amorim, F., Hass, H. C., Mielck, F., and Wiltshire, K. H.: Tidal asymmetry in the Sylt-Romo Bight, south-eastern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1359, https://doi.org/10.5194/egusphere-egu2020-1359, 2020.

EGU2020-1640 | Displays | OS4.4

A comparison study of tidal prediction techniques for applications in the German Bight

Andreas Boesch and Simon Jandt-Scheelke

The harmonic representation of inequalities (HRoI) is a technique for tidal analysis and prediction. The HRoI has been used at BSH for over six decades to calculate heights and times of high and low waters for German tide tables. In its original form, it is tailored to predict the vertices of semi-diurnal tides. A more generalized version of the HRoI allows analysing the full tidal curve at equal fractions of the mean lunar day. We compare results from the HRoI with other tidal analysis techniques, e.g. the common harmonic method, for locations in the German Bight. The study includes several tide gauges in the rivers Ems, Weser and Elbe. Short durations of rise and rapid water level changes after low water often characterize the tide curves in these rivers and pose challenges to tidal predictions.

How to cite: Boesch, A. and Jandt-Scheelke, S.: A comparison study of tidal prediction techniques for applications in the German Bight, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1640, https://doi.org/10.5194/egusphere-egu2020-1640, 2020.

EGU2020-8382 | Displays | OS4.4

Interannual stratification changes affect tides in the Gulf of Maine

Daniel Kotzian, Michael Schindelegger, Mattias Green, and Sophie Stolzenberger

Analyses of water level measurements in the Gulf of Maine and Bay of Fundy suggest interannual changes in the tidal M2 amplitude of significant size (∼1–5 cm) and remarkable spatial coherence. These signals are of unknown origin and cannot be caused by the observed sea-level fluctuations of less than 10 cm. Here we use a regional ocean model setup to link interannual M2 variations to changes in stratification, which may alter the surface tide through processes of turbulent mixing and baroclinic wave scattering as the tide propagates into and across the gulf. We run short (20-day) simulations at 1/30° horizontal resolution, omit atmospheric forcing, and prescribe background temperature and salinity fields at annual intervals starting in 1993. Tidal velocities are introduced along with geostrophic currents at open boundaries and are not varied in between runs. M2 amplitudes inferred from these experiments exhibit a year-to-year variability of 1–2 cm throughout the Gulf of Maine, mostly reflecting the sign (but not always the magnitude) of measured amplitude changes at tide gauges Boston, Portland, and Eastport. In particular, we are able to reproduce the 3-cm drop in the M2 time series from 1993 to 1998 and the subsequent increase by 1.5 cm until the year 2000. Over the period 1993–2013 and at all three stations, our simulations explain 22–29% of the locally observed M2 variance, with linear correlations ranging from 50 to 56%. Although model sensitivities and the exact mechanisms underlying these signals are yet to be worked out, our study provides appreciable evidence that varying stratification may indeed be a significant driver for the gulf's tidal changes on interannual and perhaps secular time scales.

How to cite: Kotzian, D., Schindelegger, M., Green, M., and Stolzenberger, S.: Interannual stratification changes affect tides in the Gulf of Maine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8382, https://doi.org/10.5194/egusphere-egu2020-8382, 2020.

EGU2020-13544 | Displays | OS4.4

Impact of sea level variations on hydrographic survey around Taiwan

Wen-Hau Lan, Chung-Yen Kuo, Sheng-Fong Lin, and Chien-Hsing Lu

Taiwan is an island entirely surrounded by oceans, so living and economics are significantly influenced by the oceans. The electronic navigational chart system is extremely important for improving the safety of marine navigation and ocean depth is the essential data for electronic charts. Sea surface variations affected by ocean tide and sea level change are the main error sources in hydrographic surveys since the traditional tidal correction only using tide gauge stations, ignoring geographically non-uniform ocean tides and sea level anomalies around Taiwan. In this research, we evaluate two factors impacting the accuracy of hydrographic surveys, including ocean tides and seasonal sea level variations, using tide gauge records, satellite altimeter data and ocean tide models around Taiwan, and also analyze the accuracy of the ocean tide models around Taiwan. In addition, sea level anomalies are strongly influenced by climate changes in recent years. An understanding of seasonal sea level cycle and its spatial and temporal changes are importance because its temporal changes can result in the variation of the frequency and magnitude of coastal hazards. Therefore, we will apply the Ensemble Empirical Mode Decomposition to sea level data to assess the stability of the long-term seasonal sea level fluctuations with time.

How to cite: Lan, W.-H., Kuo, C.-Y., Lin, S.-F., and Lu, C.-H.: Impact of sea level variations on hydrographic survey around Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13544, https://doi.org/10.5194/egusphere-egu2020-13544, 2020.

EGU2020-1046 | Displays | OS4.4 | Highlight

Sea level rise in the Venetian lagoon inferred from the 150-year-long tidal record

Sara Rubinetti, Carla Taricco, Davide Zanchettin, Enrico Arnone, and Angelo Rubino

The city of Venice (Northern Italy), together with its lagoon, is a historic, cultural and artistic heritage of inestimable value. One of its peculiarities consists in the recurrent storm surge phenomena, referred to as acqua alta. Sea level rise and local subsidence made their frequency to increase dramatically with respect to the past, causing severe damages to the lagoon and in particular to the city centre, as during the exceptional high tide verified on November 12, 2019.
Here we show the analysis of the historical time series of tidal maxima and minima recorded in the Venetian lagoon, covering the period 1872-2018. It is the longest and most complete historical series of the Venetian area and one of the longest records of the entire Mediterranean region. During this period, the relative sea level height has increased of about 30 cm with respect to the reference level, while the average number of acqua alta events – evaluated over a 40-year time interval - has passed from about 4 to 70 per year. These events usually occur during the fall season (from October to December), even if a not negligible number has been also recorded during winter. Therefore, we analyse the October-March average annual time series with advanced spectral analysis methods, like Monte Carlo Singular Spectrum Analysis (MC-SSA), to extract and reconstruct the significant variability modes characterizing the record. They are the increasing long-term trend and components with multidecadal, decadal and interannual periods. The trend results from the superposition on the global eustacy of the local subsidence affecting the Venetian lagoon, which is due to both natural causes and human activities. We also discuss the possible linkage of the other significant spectral components to large scale climatic patterns. In particular, the decadal-scale oscillation is one of the most important variability modes affecting Northern Italian hydrology.
Finally, we apply simple statistical methods (autoregressive models and feed-forward neural networks) to forecast the long-term evolution of sea level over the next ten years. In this contribution, we illustrate results from this state of the art two-fold statistical prediction system that provides robust predictions of sea level in the Venetian lagoon for the next decade and discuss them in the light of current longer-term projections of future sea level rise. Finally, we will test the predictive skill of the applied methods using tidal measurements recorded during 2019, to verify if our predictions are able to describe tidal variability characterizing the current year.       

How to cite: Rubinetti, S., Taricco, C., Zanchettin, D., Arnone, E., and Rubino, A.: Sea level rise in the Venetian lagoon inferred from the 150-year-long tidal record, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1046, https://doi.org/10.5194/egusphere-egu2020-1046, 2020.

Information on extremes of the sea-level is obtained from tide-gauge
records.  Such records may have gaps.

Estimates of potential changes in the size and/or frequency of sea-level
extremes are hampered by long gaps, or when just the high extremes are
missing due, e.g. to equipment failure.

Methods used for filling such gaps can be based on having multiple
records from gauges near each other; but what to do if there is
only one record? This problem can typically occur when old tide-gauge
records are used -- the use of multiple recorders at the same place is
more wide-spread today. However, especially older and therefore longer
records hold the key to obtaining long-baseline insights into the temporal
evolution of extreme tides and thus impacts of e.g. climate change.

In this work, we review and assess methods for gap filling. We asses using
the 'known truth' method, i.e. by applying realistic gaps to complete
gauge records and reconstructing and then comparing errors calculated as
the diffrence between modelled and actual values.  We compare a simple
harmonic model fit method to various spline methods as well as Neural
network and deep learning approches.  We also test a hybrid method
which uses not just tide-gauge data but also air pressure readings
from a meteorological station near the tide-gauge.

We then attempt to fill in the missing maxima of the Esbjerg, Denmark
hourly tide-gauge record since 1889. Particularly, before 1910 the maxima
above 300 cm are missing (Bijl, et al., 1999), and we try to fill these in.

How to cite: Thejll, P.: Review and Assessment of gap-filling methods from tide-gauges: Maxima missing at the Esbjerg, Denmark station, before 1910., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4487, https://doi.org/10.5194/egusphere-egu2020-4487, 2020.

EGU2020-1561 | Displays | OS4.4

Automating Tide Gauge Quality Control

Joanne Williams and Andrew Matthews

Tide gauges provide a vital component in coastal flooding alert systems, and as a record of past events. They are used to record short duration extremes such as tsunamis, storm surges lasting a few hours, regular tides, and long term changes in relative sea-level. Globally, there is far more tide gauge data in existence than is available in the public domain for research. A significant factor obstructing the release of data is that quality control of tide-gauge records is still carried out with a great deal of manual inspection, and is therefore labour-intensive. Automated systems must carefully distinguish between spikes due to instrumental error and genuine rare extreme events; and between damaged instruments and still water. The National Oceanography Centre automatic quality control software aims to enable analysis of any high-frequency tide-gauge record around the world with minimal manual intervention or parameter selection. We demonstrate the implementation in Matlab and discuss the successes and challenges of the software.

 

How to cite: Williams, J. and Matthews, A.: Automating Tide Gauge Quality Control, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1561, https://doi.org/10.5194/egusphere-egu2020-1561, 2020.

EGU2020-596 | Displays | OS4.4 | Highlight

Back to the future 3: Analysing tectonically induced tidal resonance with conceptual models

Hannah Davies, J.A. Mattias Green, and Joao C. Duarte

Recent research of coupled tidal and tectonic modelling has found that during periods in an ocean’s Wilson cycle, (i.e. during dispersal, and subsequent convergence of oceans due to plate tectonic movement), oceans occasionally become resonant for the semi-diurnal component of the tide (M2). This results in an approximately 20-Million-year long period of enhanced tidal dissipation in the resonant ocean (assuming continental plate drift rates of ~5 cm yr-1). This resonant “Super-tide” has been simulated in numerical tidal models for both past and future tectonic scenarios, and they show that the current tides are among the most energetic found.

Here we use an established tidal model to analyse the conditions required for open ocean tidal resonance. Our conceptual “Earths” consist of two or more simplified oceans, which are shaped to represent conceptual versions of oceans of the past, present, and future: triangular (Tethys ocean), circular (Pacific and Arctic oceans), rectangular (Southern and Indian oceans), and rhomboid shaped (North, and South Atlantic Ocean). Each scenario was conducted using ocean bathymetry ranging from a “bathtub” ocean (a uniformly deep flat abyssal plane from coast to coast), to a continental shelf with no abyssal bathymetry, to a “realistic” ocean with ocean shelves, ridges, and subduction zones. The global ocean land ratio and ocean volume was conserved to present-day in most conceptual scenarios however, to investigate the maximum tidal dissipation possible on Earth, some scenarios deviated from the ocean volume and global coverage. In every scenario, ocean width is progressively increased relative to the predominant ocean boundaries, simulating plate tectonic opening of each ocean.

The aim of the work was to assess the frequency of the occurrence of resonance in the open ocean, and the upper limit for tidal dissipation of the semi-diurnal tide on Earth. We found that super-tides are common in the results with their dissipative strength varying from weaker than present day to five times present day.

The occurrence of tidal resonances in modelled conceptual oceans further confirms the link between tectonics and tidal evolution. These super-tidal periods of markedly increased tidal dissipation alter the ocean’s energy budget, nutrient dispersal and the carrying capacity of coastal and oceanic ecosystems.

How to cite: Davies, H., Green, J. A. M., and Duarte, J. C.: Back to the future 3: Analysing tectonically induced tidal resonance with conceptual models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-596, https://doi.org/10.5194/egusphere-egu2020-596, 2020.

EGU2020-9930 | Displays | OS4.4

The impact of tidal dissipation changes on the Last Glacial Maximum AMOC

Sophie-Berenice Wilmes, Mattias Green, and Andreas Schmittner

The global mean sea-level decrease of 120 – 130 m during the Last Glacial Maximum (LGM; 26 – 19 kyr BP) is thought to have substantially altered semidiurnal tidal dynamics in the glacial North Atlantic. This more than doubled global open ocean tidal dissipation in comparison to present day and increased the amount of energy available for diapycnal mixing which is important for driving the global meridional overturning circulation. Reconstructions of the glacial ocean have generally suggested a more sluggish Atlantic meridional overturning circulation (AMOC) during the LGM together with weaker mixing. Here, we investigate the impact of tidal dissipation changes on the LGM AMOC and the carbon cycle using the intermediate complexity ocean model UVic coupled to the biogeochemistry model MOBI forced with three different LGM dissipation estimates. The simulations are constrained with LGM δ13C and radiocarbon data from sediments. Our results suggest that our simulations, as previously inferred, most closely agree with a weakened LGM AMOC (8 – 9 Sv), and importantly, that the agreement is consistent with increased LGM tidal mixing. These results firstly imply that a weakened AMOC state can occur with stronger tidal mixing without hampering the agreement with the sediment isotope data. Secondly, this work highlights the importance of considering tidal dissipation changes when modelling the paleo-ocean.

How to cite: Wilmes, S.-B., Green, M., and Schmittner, A.: The impact of tidal dissipation changes on the Last Glacial Maximum AMOC, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9930, https://doi.org/10.5194/egusphere-egu2020-9930, 2020.

EGU2020-11923 | Displays | OS4.4 | Highlight

Tidal circulation in an Early Permian epicontinental sea: insights from a mathematical modeling approach

Mariane Candido, Joice Cagliari, and Ernesto Luiz Lavina

The intracratonic Paraná Basin and its western extension - Chaco-Paraná Basin - are located in the south-central portion of South America and cover an area of about  ~1.8 million km2, including portions in Brazil, Argentina, Uruguay, and Paraguay. The Early Permian epicontinental sea was shallow and likely connected with the Panthalassa in the southern portion of Uruguay (Lavina, 1992). The transgressive sedimentary succession of the Guatá Group is composed of coastal plain and shallow marine deposits (Rio Bonito Formation), in complex associations due to base level fluctuations and irregular deglaciation paleotopography, and offshore-transitional deposits (Palermo Formation) (Lavina and Lopes, 1987). The Rio Bonito Formation is mostly preserved within paleovalleys carved by glaciers and tectonic. The tidal-influence in this formation occurs throughout the succession and are mainly characterized by medium- to coarse-grained arkosic and quartz sandstones with uni- and bidirectional cross-bedding, herring-bone cross-bedding, tidal bundles, reactivation surfaces, mud drapes, and double mud drapes (Fritzen et al., 2019; Lopes and Lavina, 2001). Besides the tidal sedimentological aspects, the conditions that governed tide in this epicontinental sea are poorly understood. In this work, we present a theoretical perspective on the behavior of tides in the Paraná Basin epicontinental sea during the Early Permian. Mathematical models were applied to test the existence of amphidromic points in the basin, to verify the possibility of resonance, as well as to test the tidal amplification inside two paleovalleys. The obtained results were compared to Hudson Bay, considered here a modern analog. According to the paleogeography, paleolatitude (Southern Hemisphere), depositional records and insights from the modern analog, the studied Early Permian epicontinental sea likely had bear more than one clockwise-rotation amphidromic system. Resonant effects may also have affected circulation, especially at sea depth below 100 m. In the simulated scenarios, tidal amplification in both valleys was variable but concentrated between micro to mesotidal amplitudes. This is the first contribution to the understanding of the tidal behavior of the Early Permian epicontinental sea.

 

References:

Fritzen, M.R., Cagliari, J., Candido, M., Lavina, E.L., 2019. Tidal bar cyclicity record in the Lower Permian (Rio Bonito Formation of the Paraná Basin). Sedimentary Geology 381, 76-83.

Lavina, E.L., 1992. Geologia sedimentar e paleogeografia do Neopermiano e Eotriassico (intervalo Kanzaniano - Scythiano) da Bacia do Parana. Ph.D. Thesis, UFRGS University.

Lavina, E.L., Lopes, R.C., 1987. A Transgressão Marinha do Permiano Inferior e a Evolução Paleogeográfica do Supergrupo Tubarão no Estado do Rio Grande do Sul. Paula-Coutiana 1, 51-103.

Lopes, R.C., Lavina, E.L., 2001. Estratigrafia de sequências nas Formações Rio Bonito e Palermo (Bacia do Paraná), na região carbonífera do Jacuí, Rio Grande do Sul, in: Severiano Ribeiro, H.J.P. (Ed.), Estratigrafia de sequências: fundamentos e aplicações. Edunisinos, São Leopoldo, pp. 391-419.

How to cite: Candido, M., Cagliari, J., and Lavina, E. L.: Tidal circulation in an Early Permian epicontinental sea: insights from a mathematical modeling approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11923, https://doi.org/10.5194/egusphere-egu2020-11923, 2020.

OS4.5 – Ocean Remote Sensing

EGU2020-19069 | Displays | OS4.5

Harmony's ocean elevation measurements: potential and performance

Andreas Theodosiou, Paco Lopez Dekker, Marcel Kleinherenbrink, and Gert Mulder

Harmony, an Earth Explorer 10 candidate mission, consists of two receive-only Synthetic Aperture Radar (SAR) satellites using Sentinel-1D as the illuminator. The mission will switch between close formation phases and StereoSAR phases, dedicated to relative surface elevation and relative surface motion respectively. Interferometric observations of the ocean have, in the past, been hindered by the quick temporal decorrelation of the sea surface; a result of the along-track baseline that often comes with the cross-track baseline necessary for interferometry. Specialised SAR systems aiming to observe the oceans need to account for the decorrelation of the surface. SWOT overcomes the issue by fixing the two SAR antennas to physically eliminate their along-track separation. Due to the squinted, bistatic nature of the formation, Harmony can act as an altimeter, observing relative sea-surface heights (SSH) over unprecedented wide swaths. Hence, the mission promises to have highly coherent observations of the sea surface, leading to accurate surface elevation measurements. The wide swath will enable the recovery of mesoscale features of the ocean surface in a single pass. We will present the first results of the performance analysis of the mission's observations of elevation over the oceans. The effect of errors, namely the residual Doppler, baseline errors and sea-state bias, on the observations will also be discussed.

How to cite: Theodosiou, A., Lopez Dekker, P., Kleinherenbrink, M., and Mulder, G.: Harmony's ocean elevation measurements: potential and performance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19069, https://doi.org/10.5194/egusphere-egu2020-19069, 2020.

EGU2020-7356 | Displays | OS4.5

Combining remote sensing and in situ observations to study the physical-biological coupling at fine scale: recent Mediterranean campaigns and outlook.

Andrea M. Doglioli, Gérald Grégori, Francesco d'Ovidio, Anne A. Petrenko, Stéphanie Barrillon, Jean-Luc Fuda, Melilotus Thyssen, Roxane Tzortzis, Lloyd Izard, Franck Dumas, Pierre Garreau, Ananda Pascual, Pierre Marrec, Louise Rousselet, Nagib Bhairy, Frédéric Cyr, Marc Tedetti, Léo Berline, and François Carlotti

The oceanic fine scales are highly energetic features (eddies, fronts, meanders, filaments) with relatively short lifetimes (days/weeks to months). Due to their associated strong gradients in physical and biogeochemical properties, they crucially affect ocean physics and ecology with potential impacts at the climate scale. The temporal scale associated with these horizontal and vertical fine scales is the same as many important ecological processes including phytoplankton growth and competition. This temporal resonance is one of the reasons behind the fine-scale variability appearing in the marine ecosystems structure and related domains, including biogeochemical cycles, trophic food-webs up to resources and biodiversity.

Over the past few decades, great progresses have been made in characterizing fine scales through modeling. Remote sensing is also improving rapidly in terms of resolution, with landmark missions like SWOT expected to be operational very soon (2022). However, in situ sampling remains challenging due to the difficulties of mapping a large domain covering the length of a filament or the diameter of an eddy (~100km) at high spatio-temporal frequency (~km and ~daily).

Here we present some sampling strategies we are developing for addressing this issue by combining remote sensing and in situ multi-platform high-resolution sampling of physical, biogeochemical and biological variables. 

In a series of campaigns in the Mediterranean Sea (OSCAHR 2015, PROTEVSMED-SWOT 2018, FUMSECK 2019), satellite-based adaptive and Lagrangian strategies proved to be successful to target and follow fine scale structures in situ. When paired with in situ biological measurements, like automated cytometry, these strategies highlight the important role of the fine scales in structuring the phytoplankton community by acting as fluid dynamical barriers and biodiversity hot-spots.

To extend these observations to other regions, we support an international coordinated experimental effort of the fine scale community at several sites all around the world to fully exploit the great opportunities offered by the launch of the satellite SWOT.

How to cite: Doglioli, A. M., Grégori, G., d'Ovidio, F., Petrenko, A. A., Barrillon, S., Fuda, J.-L., Thyssen, M., Tzortzis, R., Izard, L., Dumas, F., Garreau, P., Pascual, A., Marrec, P., Rousselet, L., Bhairy, N., Cyr, F., Tedetti, M., Berline, L., and Carlotti, F.: Combining remote sensing and in situ observations to study the physical-biological coupling at fine scale: recent Mediterranean campaigns and outlook., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7356, https://doi.org/10.5194/egusphere-egu2020-7356, 2020.

EGU2020-15821 | Displays | OS4.5

New evidences of seasonal deep ocean current variability in the north-eastern tropical Pacific Ocean impacted by remote gap winds

Kaveh Purkiani, André Paul, Annemiek Vink, Maren Walter, and Michael Schulz

There has been a steady increase of interest in mining of deep-sea mineral in the Clarion-Clipperton Zone in the eastern Pacific Ocean during the last decade. This region is known as one of the most eddy-rich regions, typically at the mesoscale, which are mainly generated by the intense wind burst channelled through gaps in the Sierra Madre mountains in Central America. Here we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability at the region of potential future mining and Eddy Kinetic Energy (EKE) at the vicinity of gap winds.

A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years from 1993 to 2016 in order to study the main characteristic parameters and the spatio-temporal variability of mesoscale eddies in the north-eastern tropical Pacific Ocean. Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anti-cyclonic) were found. For eddies with lifetimes longer than 7 days, the total number of cyclonic eddies exceeds that of anticyclonic eddies by about 16%. However, anticyclonic eddies are larger in size and greater in vorticity, and survive longer in the ocean than cyclonic eddies (often 90 days or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration for investigating the variability of current velocity at deep ocean region as we found eddies originated by Tehuantepec (TT) gap wind lasting longer in the ocean and travel farther distances in different direction compare to eddies emanated from Papagayo gap wind. Long-lived anticyclonic eddies generated at the vicinity of the TT gap wind are observed to travel long distances up to 4500 km far offshore west of 110° W.

EKE anomalies observed in the surface of the interior ocean at a distance of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5-6 months. This is consistent with the required time for an anticyclone eddy with the average translation speed of 12 cm/s to reach the ocean interior. Significant seasonal variability of deep ocean current velocity recorded by ocean-bottom moorings at depth of 4100 m likely reflecting the energy transfer of surface EKE generated by the gap winds to the deep ocean is also found. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El-Niño Southern Oscillation was found, whereas no robust correlation was detected for anticyclonic eddies.

How to cite: Purkiani, K., Paul, A., Vink, A., Walter, M., and Schulz, M.: New evidences of seasonal deep ocean current variability in the north-eastern tropical Pacific Ocean impacted by remote gap winds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15821, https://doi.org/10.5194/egusphere-egu2020-15821, 2020.

EGU2020-9454 | Displays | OS4.5

Ocean surface dynamics derived from TerraSAR-X/TanDEM-X and hydrodynamic modeling

Andreas Lehmann and Steffen Suchandt

Dynamical processes at the ocean surface are of high interest because they control the exchange processes between ocean and atmosphere. Furthermore, ocean surface drift determines the dispersion of heat, salt and material such as harmful substances or plastic litter. Still the measurement of ocean surface currents is a challenge because of wave and wave-breaking processes. Here we demonstrate the usefulness of TerraSAR-X/TanDEM-X data to determine ocean surface currents, wave and wind fields. Up to now there are no spatially resolved ocean surface currents measurements available, so that for the validation of surface currents a combined SAR and hydrodynamic modeling methodology is applied. Ocean surface currents are derived from SAR Along-track Interferometry, and the hydrodynamic model is a coupled wave sea ice-ocean model of the Baltic Sea. The model is driven by ERA-Interim atmospheric reanalysis data. Hydrodynamic model data are also used to support the geophysical interpretation of the multiparametrical information of the ocean surface provided by SAR.

How to cite: Lehmann, A. and Suchandt, S.: Ocean surface dynamics derived from TerraSAR-X/TanDEM-X and hydrodynamic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9454, https://doi.org/10.5194/egusphere-egu2020-9454, 2020.

EGU2020-12198 | Displays | OS4.5

Scatterometer, Radiometer and GNSS-R observations of Hurricane Dorian

Zorana Jelenak, Zorana Jelenak, Paul Chang, Joe Sapp, Suleiman Alsweiss, Seubson Soisuvarn, Faozi Said, Joenghwan Park, and Zorana Jelenak

Hurricane Dorian was the first major hurricane of the 2019 Atlantic season. Dorian formed on August 24, from a tropical wave in the Central Atlantic. Over next several days it gradually strengthened and become a hurricane on August 28th. Dorian rapidly intensified and reached Category 4 status on August 31st. Next day, Dorian reached Category 5 intensity, with maximum sustained winds of 185 mph while making landfall in Elbow Cay, Bahamas first and then another one on Grand Bahamas several hours later. Dorian stalled just north of Grand Bahamas for about a day. It was the strongest known tropical system to impact the Bahamas. A combination of cold water upwelling and an eyewall replacement cycle weakened Dorian to a Category 2 hurricane and it began to move slowly towards the north-northwest. In the early hours of September 6, Dorian weakened to Category 1 intensity as it picked up speed and turned northeast.

 

Hurricane Dorian was exceptionally well sampled by NOAA and Airforce hurricane hunter aircrafts. Large dataset of SFMR surface winds collected during the flights as well as dropsondes documented changes in Dorian’s wind field throughout its duration. AMSR-2 and SMAP radiometer, ASCAT and ScatSat-1 scatterometers as well as CYGNSS GNSS-R wind observations over Dorian were collected at NOAA for analysis. Understanding the differences between remote sending technologies utilized for wind observations is crucial for NOAA operational users. Large and rapid changes in Dorian’s wind field represent an interesting case study where strengths and weaknesses of different wind measurement technologies can be assessed and compared. SFMR wind measurements represent common surface ground truth that can be utilized to bring all these different measurements together.

 

While SMAP L-band radiometer sensitivity to high winds is not affected by rain its low measurement resolution smears wind field variations in the inner core depending on the storm size. AMSR-2 wind retrievals obtained from C-band measurements have somewhat larger measurement resolution and ability to provide more insight of the inner core wind variations although they can be enhanced by rain signature. These measurements are being utilized for 50 and 64kts wind radii. C-band scatterometer measurements such as ASCAT, even though attenuated by heaviest rain, provide both wind speed and direction information making then invaluable for early detection of tropical depression formation. As a matter of fact ASCAT measurements were used by NHC to initiate warnings for Dorian on August 24th, 2019: “Two ASCAT passes between 1200-1300Z this morning indicated that the system had a closed circulation and surface winds of at least 30 kt, and that is the intensity set for this advisory.” Ku-band scatterometer ScatSat-1 is most impacted by rain and its winds can measurably differ from ASCAT in the areas of heavy rain. Understanding the thresholds when this occurs is imperative for successful utilization of ScatSat-1 winds in NOAA operations. Finally winds from CYGNSS GNSS-R measurements are assessed utilizing NOAA CYGNSS wind product. Examples of all measurements during Dorian duration will be presented and compared with aircraft observations.

How to cite: Jelenak, Z., Jelenak, Z., Chang, P., Sapp, J., Alsweiss, S., Soisuvarn, S., Said, F., Park, J., and Jelenak, Z.: Scatterometer, Radiometer and GNSS-R observations of Hurricane Dorian , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12198, https://doi.org/10.5194/egusphere-egu2020-12198, 2020.

EGU2020-18916 | Displays | OS4.5

NOC GNSS-R Global Ocean Wind Speed and Sea-Ice Products

Matthew Hammond, Giuseppe Foti, Christine Gommenginger, Meric Srokosz, Martin Unwin, and Josep Rosello

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative and rapidly developing approach to Earth Observation that makes use of signals of opportunity from Global Navigation Satellite Systems, which have been reflected off the Earth’s surface. Using GNSS-R data collected by the UK TechDemoSat-1 (TDS-1) between 2014 and 2018, the National Oceanography Centre (NOC) has developed a GNSS-R wind speed retrieval algorithm called the Calibrated Bistatic Radar Equation (C-BRE), which now features updated data quality control mechanisms including flagging of radio frequency interference (RFI) and sea-ice detection based on the GNSS-R waveform. Here we present an assessment of the performance of the latest NOC GNSS-R ocean wind speed and sea-ice products (NOC C-BRE v1.0) using validation data from the ECMWF ERA-5 re-analysis model output. Results show the capability of spaceborne GNSS-R sensors for accurate wind speed retrieval and sea-ice detection. Additionally, ground-processed Galileo returns collected by TDS-1 are examined and the geophysical sensitivity of reflected Galileo data to surface parameters is investigated. Preliminary results demonstrate the feasibility of spaceborne GNSS-R instruments receiving a combination of GNSS signals transmitted by multiple navigation systems, which offers the opportunity for frequent, high-quality ocean wind and sea-ice retrievals at low relative cost. Other advancements in GNSS-R technology are represented by future mission concepts such as HydroGNSS, a proposed ESA Scout mission opportunity by SSTL offering support for enhanced retrieval capabilities exploiting dual polarisation, dual frequency, and coherent reflected signal reception.

How to cite: Hammond, M., Foti, G., Gommenginger, C., Srokosz, M., Unwin, M., and Rosello, J.: NOC GNSS-R Global Ocean Wind Speed and Sea-Ice Products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18916, https://doi.org/10.5194/egusphere-egu2020-18916, 2020.

EGU2020-1537 | Displays | OS4.5

The European HF Radar Node: focal point to promote land-based remote sensing of coastal surface currents and its applications

Lorenzo Corgnati, Carlo Mantovani, Anna Rubio, Julien Mader, Emma Reyes, Jose Luis Asensio Igoa, Antonio Novellino, Patrick Gorringe, and Annalisa Griffa

How to cite: Corgnati, L., Mantovani, C., Rubio, A., Mader, J., Reyes, E., Asensio Igoa, J. L., Novellino, A., Gorringe, P., and Griffa, A.: The European HF Radar Node: focal point to promote land-based remote sensing of coastal surface currents and its applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1537, https://doi.org/10.5194/egusphere-egu2020-1537, 2020.

EGU2020-7571 | Displays | OS4.5 | Highlight

Innovation in Satellite Sensors for Ocean Observations : An OceanObs19 overview

Rosemary Morrow and the OceanObs19 Satellite Innovation team

Over the next decade, new satellite sensors are being developed or proposed to enhance our global observations of ocean surface parameters, many aiming at finer scale processes. These sensors include the new generation of satellite altimeters with finer resolution in alongtrack SAR mode or with swath SAR-interferometry; missions to observe total surface currents and wind-wave interactions; high resolution sea surface temperature and salinity; and ocean color, polarimetry and lidar missions for biogeochemistry, among others. Key observational challenges are to have finer-resolution across open ocean fronts, to observe the surface dynamical interactions over multiple scales, and to extend our satellite observing systems into the coastal and polar regions. Understanding smaller-scale variability will have benefits for climate, ocean operations and ocean health.

This presentation will give an overview of the OceanObs19 discussions on the opportunities and priorities for new satellite ocean sensors for the upcoming decade.

How to cite: Morrow, R. and the OceanObs19 Satellite Innovation team: Innovation in Satellite Sensors for Ocean Observations : An OceanObs19 overview, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7571, https://doi.org/10.5194/egusphere-egu2020-7571, 2020.

EGU2020-7513 | Displays | OS4.5

Overview of the CCI+SSS project

Jacqueline Boutin, Nicolas Reul, Julia Koehler, Adrien Martin, Rafael Catany, and Climate Change Initiative Salinity Consortium

Sea Surface Salinity (SSS) is an Essential Climate Variable (ECV) that plays a fundamental role in the density-driven global ocean circulation, the water cycle, and climate. The satellite SSS observation from the Soil Moisture and Ocean Salinity (SMOS), Aquarius, and Soil Moisture Active Passive (SMAP) missions have provided an unprecedented opportunity to map SSS over the global ocean since 2010 at 40-150km scale with a revisit every 2 to 3 days. This observation capability has no historic precedent and has brought new findings concerning the monitoring of SSS variations related with climate variability such as El Niño-Southern Oscillation, Indian Ocean Dipole, and Madden-Julian Oscillation, and the linkages of the ocean with different elements of the water cycle such as evaporation and precipitation and continental runoff. It has enhanced the understanding of various ocean processes such as tropical instability waves, Rossby waves, mesoscale eddies and related salt transport, salinity fronts, hurricane haline wake, river plume variability, cross-shelf exchanges. There are also emerging use of satellite SSS to study ocean biogeochemistry, e.g. linked to air-sea CO2 fluxes.

Following the success of the initial oceanographic studies implementing this new variable, the European Space Agency (ESA) Climate Change Initiative CCI+SSS project (2018-2020) aims at generating improved calibrated global SSS fields over 10 years period (2010-2019) from all available satellite L-band radiometer measurements, extended at regional scale to 2002-2019 from C-band radiometer measurements. It fully exploits the ESA/Earth explorer SMOS mission complemented with SMAP and AQUARIUS satellite missions. The project gathers teams involved in earth observation remote sensing, in the validation of satellite data and in climate variability study. In this presentation, we will present the first CCI+SSS product released to the scientific community (https://catalogue.ceda.ac.uk/uuid/9ef0ebf847564c2eabe62cac4899ec41). The comparisons with in situ ground truth indicate much better performances than the ones obtained with a single satellite data product, with global rmsd against in situ references of 0.16 pss. Large scale interannual variability is successfully reproduced and SSS variability in very variable regions like the Bay of Bengale and in river plumes in the Atlantic Ocean is very satisfactory, confirming the usefulness of these products for scientific studies. Nevertheless we also identify some caveats that will be discussed as well as the ways envisaged to resolve part of them in the next version of the product to be delivered publicly in Summer 2020.

The ESA CCI+SSS consortium gathers scientists and engineers from various European research institutes and companies (LOCEAN/IPSL, LOPS, University of Hamburg, NOC, ICM, ARGANS, ACRI-st, ODL) and is conducted in collaboration with US colleagues from NASA and Remote Sensing System.

How to cite: Boutin, J., Reul, N., Koehler, J., Martin, A., Catany, R., and Salinity Consortium, C. C. I.: Overview of the CCI+SSS project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7513, https://doi.org/10.5194/egusphere-egu2020-7513, 2020.

EGU2020-16939 | Displays | OS4.5

First regional SMOS Sea Surface Salinity products over the Baltic Sea and its oceanographic added-value

Veronica Gonzalez-Gambau, Estrella Olmedo, Cristina Gonzalez-Haro, Antono Turiel, Justino Martinez, Carolina Gabarro, Pekka Alenius, Laura Tuomi, Petra Roiha, Manuel Arias, Rafael Catany, Diego Fernandez, and Roberto Sabia

Accurate satellite-based sea surface salinity (SSS) fields would address some gaps of knowledge and benefit the understanding of Baltic Sea salinity dynamics.  In particular, these fields can contribute to the monitoring of long-term salinity changes and to the detection of periods with anomalous salinity. These products can also be very useful as initial fields and validation data for improving the existing numerical models.


The Baltic Sea is one of the most challenging regions for the retrieval of SSS from L-band satellite measurements. Nowadays, available EO-based SSS products are quite limited over this region both in terms of spatio-temporal coverage and quality. This is mainly due to several technical limitations that strongly affect the SMOS TB particularly over semi-enclosed seas, such as the high contamination by Radio-Frequency Interference (RFI) sources and the contamination close to land and ice edges. Besides, the sensitivity of TB to SSS changes is very low in cold waters and much larger errors are expected compared to temperate oceans. Salinity and temperature values are very low in this basin, which implies that dielectric constant models are not fully tested in such conditions. In the recent years, the Barcelona Expert Center team has been working on the development of innovative algorithms for improving the quality of SMOS TB and SSS retrievals dealing with the main processing issues. 


In the context of the ESA Baltic+ Salinity Dynamics project (https://balticsalinity.argans.co.uk/), these methodologies have been adapted and consolidated towards the generation of the first  regional SMOS SSS product (2011-2020) that would suit to the needs of the Baltic research community. Very recently, the first version of the Baltic+ SSS product has been produced (3-year series) and is currently under validation against in-situ measurements. The quality assessment of the SSS product in the Baltic Sea is also an issue and its representativeness must be carefully assessed. The basin is strongly stratified and then, the differences between SMOS measurements (first centimeters) and in-situ observations (few meters depth) can be noticeable. Differences are more probable during ice melting and high runoff events in spring where there might be a freshwater layer at the top shallow surface. Feedback from the users will help identifying the limitations of the product. Additional technical developments will be addressed to meet the requirements of the communities working in the study of Baltic processes. 


We will present at the conference the Baltic+ SSS v1 product and its added-value with respect to other existing EO-based datasets. The potential scientific impact of this satellite SSS product in advancing on-going regional research initiatives like the Baltic Earth Working Group on Salinity dynamics will be discussed.

How to cite: Gonzalez-Gambau, V., Olmedo, E., Gonzalez-Haro, C., Turiel, A., Martinez, J., Gabarro, C., Alenius, P., Tuomi, L., Roiha, P., Arias, M., Catany, R., Fernandez, D., and Sabia, R.: First regional SMOS Sea Surface Salinity products over the Baltic Sea and its oceanographic added-value, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16939, https://doi.org/10.5194/egusphere-egu2020-16939, 2020.

EGU2020-9414 | Displays | OS4.5

A convolutional neural network with error estimates to reconstruct sea surface temperature satellite observations (DINCAE)

Alexander Barth, Aida Alvera Azcárate, Matjaz Licer, and Jean-Marie Beckers

A method to reconstruct missing data in satellite data using a neural network is presented. Satellite observations working in the optical and infrared bands are affected by clouds, which obscure part of the ocean underneath. In this paper, a neural network with the structure of a convolutional auto-encoder is developed to reconstruct the missing data based on the available cloud-free pixels in satellite images. However, it is unclear how to handle missing data (or data with variable accuracy) in a neural network when using incomplete satellite data in the training phase. The present work shows a consistent approach which uses essentially the satellite data and its expected error variance as input and provides the reconstructed field along with its expected error variance as output. The approach is motivated by the way models and observations are combined in the frame of data assimilation. The neural network is trained by maximizing the likelihood of the observed value. The corresponding error variances are estimated during training and do not need to be known a priori. The approach, called DINCAE (Data-Interpolating Convolutional Auto-Encoder) is applied to a relatively long time-series of Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature data and compared to DINEOF (Data Interpolating Empirical Orthogonal Functions), a method to reconstruct missing data based on an EOF decomposition. The reconstruction error of both approaches is computed using cross-validation and in situ observations from the World Ocean Database. DINCAE results have lower error, while showing higher variability than the DINEOF reconstruction. The resulting error estimates are also validated using the cross-validation data and they follow closely the expected Gaussian distribution.

How to cite: Barth, A., Alvera Azcárate, A., Licer, M., and Beckers, J.-M.: A convolutional neural network with error estimates to reconstruct sea surface temperature satellite observations (DINCAE), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9414, https://doi.org/10.5194/egusphere-egu2020-9414, 2020.

EGU2020-17404 | Displays | OS4.5

Filtering tide-generated internal waves using Convolutional Neural Networks

Redouane Lguensat, Ronan Fablet, Julien Le Sommer, Sammy Metref, and Emmanuel Cosme

Starting from 2021, Surface  Water Ocean Topography (SWOT) satellite altimetry mission will provide an unprecedented amount of Sea Surface Height (SSH) measurements. In addition to allowing for a higher spatial resolution, SWOT will deliver two-dimensional horizontal SSH data thanks to its wide swath capacities, which is a remarkable leap compared to conventional current altimeters.

With the aim of extracting a clean SSH signal from the SWOT measurements, several challenges are expected to be encountered. In this work, we focus on filtering the footprints of Internal Gravity Waves (IGWs), this is of high interest for physical oceanographers who seek to better understand mesoscale and submesoscale ocean physics.

Thanks to recent developments in ocean numerical simulation, we can now have access to a considerable amount of simulation data with exceptional high spatial resolutions up to 1/60° and hourly temporal resolution. Here, we benefit from an advanced North Atlantic simulation of the ocean circulation (eNATL60) that models tidal motions, and design a supervised machine learning experiment that aims to test several techniques for filtering IGWs.

In particular, we show that deep convolutional neural networks are a relevant candidates for this task and presents promising results with regard to conventional linear filtering techniques. We also show how our method can be adapted to the context of the fast-sampling phase of SWOT, and can also take advantage from the presence of additional data such as Sea Surface Temperature.

How to cite: Lguensat, R., Fablet, R., Le Sommer, J., Metref, S., and Cosme, E.: Filtering tide-generated internal waves using Convolutional Neural Networks , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17404, https://doi.org/10.5194/egusphere-egu2020-17404, 2020.

EGU2020-2415 | Displays | OS4.5

The global extent of artificial light pollution in the marine environment

Tim Smyth, Thomas Davies, and David McKee

Coastlines globally are increasingly being illuminated with Artificial Light At Night (ALAN) from various urban infrastructures such as houses, offices, piers, roads, ports and dockyards. Artificial sky glow can now be detected above 22% of the world’s coasts nightly and will dramatically increase as coastal human populations more than double by the year 2060. One of the clearest demonstrations that we have entered another epoch, the urbanocene, is the prevalence of ALAN visible from space.

Photobiological life history adaptations to the moon and sun are near ubiquitous in the surface ocean (0-200m), such that cycles and gradients of light intensity and spectra are major structuring factors in marine ecosystems. The potential for ALAN to reshape the ecology of coastal habitats by interfering with natural light cycles and the biological processes they inform is increasingly recognized and is an emergent focus for research.

In this paper we derive a methodology to quantify and map the depths to which biologically relevant ALAN penetrates in the marine environment.  We use two satellite derived global datasets to achieve this: an artificial night sky brightness world atlas (Falchi et al., 2016) and an in-water Inherent Optical Property (Lee et al., 2002) dataset derived from ESA’s Ocean Colour Climate Change Initiative (OC-CCI https://www.oceancolour.org/).  These primary datasets are both used in conjunction with in-situ derived measurements and radiative transfer modelling in order to quantify the critical depth (Zc) to which biologically relevant ALAN penetrates throughout the global ocean’s estuarine, coastal and near shore regions, in particular the area defined by an individual country’s Exclusive Economic Zone. 

How to cite: Smyth, T., Davies, T., and McKee, D.: The global extent of artificial light pollution in the marine environment , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2415, https://doi.org/10.5194/egusphere-egu2020-2415, 2020.

EGU2020-4412 | Displays | OS4.5

Ocean Surface Currents in the northern Nordic Seas from a combination of multi-mission satellite altimetry and numerical modeling

Felix L. Müller, Denise Dettmering, Claudia Wekerle, Christian Schwatke, Marcello Passaro, Wolfgang Bosch, and Florian Seitz

Satellite altimetry is an important part of the Global Geodetic Observing System providing precise information on sea level on different spatial and temporal scales. Moreover, satellite altimetry-derived dynamic ocean topography heights enable the computation of ocean surface currents by applying the well-known geostrophic equations. However, in polar regions, altimetry observations are affected by seasonally changing sea-ice cover leading to a fragmentary data sampling.

In order to overcome this problem, an ocean model is used to fill in data gaps. The aim is to obtain a homogeneous ocean topography representation that enables consistent investigations of ocean surface current changes. For that purpose, the global Finite Element Sea-ice Ocean Model (FESOM) is used. It is based on an unstructured grid and provides daily water elevations with high spatial resolution.

The combination is done based on a Principal Component Analysis (PCA) after reducing both quantities by their constant and seasonal signals. In the main step, the most dominant spatial patterns of the modeled water heights as provided by the PCA are linked with the temporal variability of the estimated dynamic ocean topography elevations from altimetry. At the end, the seasonal signal as well as the absolute reference from altimetry is added back to the data set.

This contribution describes the combination process as well as the generated final product: a daily, more than 17 years covering dataset of geostrophic ocean currents. The combination is done for the marine regions Greenland Sea, Barents Sea and the Fram Strait and includes sea surface height observations of the ESA altimeter satellites ERS-2 and Envisat. In order to evaluate the combination results, independent surface drifter observations, corrected for a-geostrophic velocity components, are used.

How to cite: Müller, F. L., Dettmering, D., Wekerle, C., Schwatke, C., Passaro, M., Bosch, W., and Seitz, F.: Ocean Surface Currents in the northern Nordic Seas from a combination of multi-mission satellite altimetry and numerical modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4412, https://doi.org/10.5194/egusphere-egu2020-4412, 2020.

Temperature conditions of development juvenile NEA cod in the Barents sea for 1998-2015 on the basis of satellite data

Vanyushin G. P., Bulatova T. V.

Russian Federal Research Institute of Fisheries and Oceanography (VNIRO)

107140 17, V. Krasnoselskaya str., Moscow

tel: 8(499)264-01-33, fax: 8(499)264-91-87,

e-mail: ladimon@mail.ru

 

Abstract

The paper considers the real temperature conditions in the main spawning area of North-East Arctic cod in the Norwegian sea and the development of its juveniles in the Barents sea in the periods from March to October 1998-2015. Here was taken as a principle the analysis of materials Bank mean weekly maps of sea surface temperature (SST) built on complex process: infrared digital data from metrological satellites of the series "NOAA" and quasisynchronous temperature data "in situ" from ships, buoys and coastal stations. A continuous series of indicators on temperature variability in the surface layer of sea water in coastal zone of the Norwegian sea during spawning periods and later on during the early ontogenesis of juvenile cod in the Barents sea  allowed to establish the dynamics of interannual seasonal temperature trends on a mesoscale period of time (1998-2015). This made it possible to assess the indirect impact of temperature conditions on the prospect of survival and, accordingly, the number of juvenile cod in the first year of its life after spawning – the most important stage in the life cycle of a new generation of cod. The paper presents calculations of monthly and seasonal average values of SST and SST anomalies in the Norwegian and Barents seas, shows the interannual seasonal dynamics of these characteristics. Given for these years, the results of the comparative analysis between: seasonal values of temperature in the water surrounding the Lofoten Islands (March-April – time of the main spawning) and in the water of the Barents sea (May-October - time of the early onthogenesis of juvenile cod) and professional expert estimates the number of yearlings cod. The relationship between these statistical data was positive and about equal to R= + 0,67. Information on the number of generations of cod at different stages of its life cycle was taken from the annual reports of the Arctic Fisheries Working Group ICES.

Keywords: satellite monitoring, sea surface temperature (SST), the  Northeast Arctic cod, main spawning and habitat waters, yearlings of the cod.

How to cite: Vanyushin, G. and Bulatova, T.: Temperature conditions of development juvenile NEA cod in the Barents sea for 1998-2015 on the basis of satellite data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1350, https://doi.org/10.5194/egusphere-egu2020-1350, 2020.

In this study, Sea Surface Salinity (SSS) Level 3 (L3) daily product derived from Soil Moisture Active Passive (SMAP) during the year 2016, was validated and compared with SSS daily products derived from Soil Moisture and Ocean Salinity (SMOS) and in-situ measurements. Generally, the Root Mean Square Error (RMSE) of the daily SSS products is larger along the coastal areas and at high latitudes and is smaller in the tropical regions and open oceans. Comparisons between the two types of daily satellite SSS product revealed that the RMSE was higher in the daily SMOS product than in the SMAP, whereas the bias of the daily SMOS was observed to be less than that of the SMAP when compared with Argo floats data. In addition, the latitude-dependent bias and RMSE of the SMAP SSS were found to be primarily influenced by the precipitation and the Sea Surface Temperature (SST).Then, aregression analysis method which has adopted the precipitation and SST data was used to correct the larger bias of the daily SMAP product. It was confirmed that the corrected daily SMAP product could be used for assimilation in high-resolution forecast models, due to the fact that it was demonstrated to be unbiased and much closer to the in-situ measurements than the original uncorrected SMAP product.

How to cite: Qin, S.: Validation and correction of sea surface salinity retrieval from SMAP, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4389, https://doi.org/10.5194/egusphere-egu2020-4389, 2020.

EGU2020-7472 | Displays | OS4.5

New insights into SMOS Sea Surface Salinity retrievals in the Arctic Ocean.

Alexandre Supply, Jacqueline Boutin, Jean-Luc Vergely, Nicolas Kolodziejczyk, Gilles Reverdin, Nicolas Reul, and Anastasiia Tarasenko

Since 2010, the Soil Moisture and Ocean Salinity (SMOS) satellite mission monitors the earth emission at L-Band, providing the longest time series of Sea Surface Salinity (SSS) from space over the global ocean. However, retrieving SSS at high latitudes with a reasonable accuracy remains challenging, in particular due to the low sensitivity of L-Band radiometric measurements to SSS in cold waters and to the contamination of SMOS measurements by the vicinity of continents and sea ice as well as the presence of Radio Frequency Interferences. In this paper, we assess the quality of weekly SSS fields derived from swath-ordered instantaneous SMOS SSS (so called Level 2) distributed by the European Space Agency. These products are filtered according to new criteria. We use the pseudo-dielectric constant retrieved from SMOS brightness temperatures to filter SSS pixels polluted by sea ice. We identify that the dielectric constant model and the sea surface temperature auxiliary parameter used as prior information in the SMOS SSS retrieval are significant sources of uncertainty. We develop a novel correction methodology accordingly.

SSS Standard deviation of differences (STDD) between weekly SMOS SSS and in-situ near surface salinity significantly decrease after applying the SSS correction, from 1.46 pss to 1.26 pss. The correlation between new SMOS SSS and in-situ near surface salinity reaches 0.94. SMOS estimates better capture SSS variability in the Arctic Ocean in comparison to TOPAZ reanalysis (STDD = 1.86 pss), particularly in river plumes fresher by about 10 pss than surrounding waters. Furthermore, comparisons with in-situ measurements ranging from 1 to 11 m depths identify huge vertical stratification in fresh regions. This emphasizes the need to consider in-situ salinity as close as possible to the sea surface when validating L-band radiometric SSS which are representative of the first top centimeter.

How to cite: Supply, A., Boutin, J., Vergely, J.-L., Kolodziejczyk, N., Reverdin, G., Reul, N., and Tarasenko, A.: New insights into SMOS Sea Surface Salinity retrievals in the Arctic Ocean., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7472, https://doi.org/10.5194/egusphere-egu2020-7472, 2020.

EGU2020-11683 | Displays | OS4.5

Global validation of the ESA CCI+SSS products

Adrien Martin, Sébastien Guimbard, Jacqueline Boutin, Nicolas Reul, and Rafael Catany

The European Space Agency (ESA) Climate Change Initiative for Sea Surface Salinity (CCI+SSS) project aims at generating long-term, improved, calibrated global SSS fields from space. The project started in mid-2018 and in its first year has produced a 9-year dataset (2010-2018) from the three available L-band radiometer satellites (SMOS: Soil Moisture and Ocean Salinity; Aquarius; SMAP: Soil Moisture Active Passive) and validated it against in situ references (Argo and ISAS: In Situ Analysis System). The dataset is available at https://catalogue.ceda.ac.uk/uuid/9ef0ebf847564c2eabe62cac4899ec41.

The comparisons with in situ ground truth indicate much better performances than the ones obtained with a single satellite data product, with global precision against in situ references of 0.16 pss and 0.10 pss in areas with low variability. There is a very good agreement between the CCI dataset and references, including long-term stability, with differences within +-0.05 pss for global ocean within [40°S-20°N]. At higher latitude, we observe seasonal oscillation of the CCI SSS difference against references. The CCI SSS products uncertainty have been validated against references and show good agreement as long as the spatial representativeness is considered in presence of strong spatial gradients in salinity.

How to cite: Martin, A., Guimbard, S., Boutin, J., Reul, N., and Catany, R.: Global validation of the ESA CCI+SSS products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11683, https://doi.org/10.5194/egusphere-egu2020-11683, 2020.

EGU2020-20753 | Displays | OS4.5

SMOS-based estimation and validation of Total Alkalinity in the Mediterranean basin

Roberto Sabia, Estrella Olmedo, Giampiero Cossarini, Aida Alvera-Azcárate, Veronica Gonzalez-Gambau, and Diego Fernández-Prieto

ESA SMOS satellite [1] has been providing first-ever Sea Surface Salinity (SSS) measurements from space for over a decade now. Until recently, inherent algorithm limitations or external interferences hampered a reliable provision of satellite SSS data in semi-enclosed basin such as the Mediterranean Sea. This has been however overcome through different strategies in the retrieval scheme and data filtering approach [2, 3]. This recent capability has been in turn used to infer the spatial and temporal distribution of Total Alkalinity (TA - a crucial parameter of the marine carbonate system) in the Mediterranean, exploiting basin-specific direct relationships existing between salinity and TA.

Preliminary results [4] focused on the differences existing in several parameterizations [e.g, 5] relating these two variables, and how they vary over a seasonal to interannual timescale.

Currently, to verify the consistency and accuracy of the derived products, these data are being validated against a proper ensemble of in-situ, climatology and model outputs within the Mediterranean basin. An error propagation exercise is also being planned to assess how uncertainties in the satellite data would translate into the final products accuracy.

The resulting preliminary estimates of Alkalinity in the Mediterranean Sea will be linked to the overall carbonate system in the broader context of Ocean Acidification assessment and marine carbon cycle.

References:

[1] J. Font et al., "SMOS: The Challenging Sea Surface Salinity Measurement From Space," in Proceedings of the IEEE, vol. 98, no. 5, pp. 649-665, May 2010. doi: 10.1109/JPROC.2009.2033096

[2] Olmedo, E., J. Martinez, A. Turiel, J. Ballabrera-Poy, and M. Portabella,  “Debiased non-Bayesian retrieval: A novel approach to SMOS Sea Surface Salinity”. Remote Sensing of Environment 193, 103-126 (2017).

[3] Alvera-Azcárate, A., A. Barth, G. Parard, J.-M. Beckers, Analysis of SMOS sea surface salinity data using DINEOF, In Remote Sensing of Environment, Volume 180, 2016, Pages 137-145, ISSN 0034-4257, https://doi.org/10.1016/j.rse.2016.02.044.

[4] Sabia, R., E. Olmedo, G. Cossarini, A. Turiel, A. Alvera-Azcárate, J. Martinez, D. Fernández-Prieto, Satellite-driven preliminary estimates of Total Alkalinity in the Mediterranean basin, Geophysical Research Abstracts, Vol. 21, EGU2019-17605, EGU General Assembly 2019, Vienna, Austria, April 7-12, 2019.

[5] Cossarini, G., Lazzari, P., and Solidoro, C.: Spatiotemporal variability of alkalinity in the Mediterranean Sea, Biogeosciences, 12, 1647-1658, https://doi.org/10.5194/bg-12-1647-2015, 2015.

 

 

How to cite: Sabia, R., Olmedo, E., Cossarini, G., Alvera-Azcárate, A., Gonzalez-Gambau, V., and Fernández-Prieto, D.: SMOS-based estimation and validation of Total Alkalinity in the Mediterranean basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20753, https://doi.org/10.5194/egusphere-egu2020-20753, 2020.

EGU2020-21449 | Displays | OS4.5

Colder and smaller : 10 years of observations of surface salinity by SMOS, Aquarius and SMAP to study mesoscale eddies in the Southern Ocean

Audrey Hasson, Cori Pegliasco, Jacqueline Boutin, and Rosemary Morrow

Since 2010, space missions dedicated to Sea Surface Salinity (SSS) have been providing observations with almost complete coverage of the global ocean and a resolution of about 45 km every 3 days. The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission was the first orbiting radiometer to collect regular SSS observations from space. The Aquarius and SMAP (Soil Moisture Active-Passive) missions of the National Aeronautics and Space Administration (NASA) then reinforced the SSS observing system between mid-2011 and mid-2015 and since mid-2015, respectively.

Using the most recent SSS Climate Change Initiative project dataset merging data from the 3 missions, this study investigates the SSS signal associated with mesoscale eddies in the Southern Ocean. Eddies location and characteristics are obtained from the daily v3 mesoscale eddy trajectory atlas produced by CLS. SSS anomalies along the eddies journey are computed and compared to Sea Surface Temperature (SST) anomalies (v4 Remote Sensing Systems) as well as the SubAntarctic Front (SAF) position (CTOH, LEGOS). The vertical structure of the eddies is further investigated using profiles from colocated Argo autonomous floats. 

This study highlights a robust signal in SSS depending on both the eddies rotation (cyclone/anticyclone) and latitudinal position with respect to the SAF. Moreover, this dependence is not found in SST. These observations reveal oceanic the interaction of eddies with the larger scale ocean water masses. SSS and SST anomalies composites indeed show different patterns either bi-poles linked with horizontal stirring of fronts, mono-poles from trapping water or vertical mixing changes, or a mix of the two.

This analysis gives strong hints for the erosion of subsurface waters, such as mode waters, induced by enhanced mixing caused by the deep-reaching eddies of the southern ocean.

How to cite: Hasson, A., Pegliasco, C., Boutin, J., and Morrow, R.: Colder and smaller : 10 years of observations of surface salinity by SMOS, Aquarius and SMAP to study mesoscale eddies in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21449, https://doi.org/10.5194/egusphere-egu2020-21449, 2020.

EGU2020-7545 | Displays | OS4.5

CATDS CEC-LOCEAN debiased version 4 Sea Surface Salinity

Clovis Thouvenin-Masson, Jacqueline Boutin, Jean-Luc Vergely, Dimitry Khvorostyanov, and Stéphane Tarot

The Centre Aval de Traitement des Données SMOS (CATDS), developped by the CNES in collaboration with the CESBIO and IFREMER, produces and continuously improves SMOS sea surface salinity (SSS) products.

The aim of this poster is to present the last version of CATDS L3 products developed by the LOCEAN CATDS Expertise Center (CEC-LOCEAN debiased v4, https://www.catds.fr/Products/Available-products-from-CEC-OS/CEC-Locean-L3-Debiased-v4), and to highlight its main improvements with respect to previous version 3.

The L3 products are available for 9-day and 18-day Gaussian averaging. Both versions 3 and 4 contain a bias correction based on internal consistency of SMOS SSS retrieved in various locations across swath, and on seasonal variability of salinity. The main evolutions of version 4 consist in refining the absolute correction methodology, limiting wind speed to 16m/s, add a refined filtering for sea ice and radio frequency contamination based on SMOS retrieved pseudo dielectric constant, the so-called ACARD (Waldteufel et al. 2004) and an improved sea surface temperature (SST) correction in cold waters based on Dinnat et al. (2019) observed dependency.

Improvements with respect to version 3 are assessed through systematic validation that consists in two main stages: (1) Comparison with respect to in-situ measurements (repetitive ship transects across Atlantic and Arctic regions, and Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) moorings); (2) Comparison with the In-Situ Analysis System (ISAS) monthly fields (Kolodziejczyk, 2017), in terms of both mean spatial maps and time series of key statistics parameters. The key statistics parameters are computed both over the global ocean and for individual areas of interest. Thus, both the mean spatial patterns and temporal variability in various regions are evaluated.

Comparisons between the two last versions exposed in this poster are based on relevant examples from this systematic validation: main improvements are observed in high latitudes (over 45° latitude).In the Southern Ocean modification of wind speed filtering and SST correction lead to a decrease in the mean difference between SMOS  and ISAS SSS south of 45S from 0.16+/-0.07 to 0.02+/-0.05pss. Std of the differences and r2 are also improved over global ocean. Statistics obtained with this new version are close to the ones obtained with SMAP RemSS v4 SSS.

 

Dinnat, E.P.; Le Vine, D.M.; Boutin, J.; Meissner, T.; Lagerloef, G. Remote Sensing of Sea Surface Salinity: Comparison of Satellite and In Situ Observations and Impact of Retrieval Parameters. Remote Sens. 2019, 11, 750.

Kolodziejczyk Nicolas, Prigent-Mazella Annaig, Gaillard Fabienne (2017). ISAS-15 temperature and salinity gridded fields. SEANOE. https://doi.org/10.17882/52367

Waldteufel, P., J. L. Vergely, and C. Cot, A modified cardioid model for Processing multiangular radiometric observations, IEEE Transactions on Geoscience and Remote Sensing, vol.42, issue.5, pp.1059-1063, 2004. DOI : 10.1109/TGRS.2003.821698.

How to cite: Thouvenin-Masson, C., Boutin, J., Vergely, J.-L., Khvorostyanov, D., and Tarot, S.: CATDS CEC-LOCEAN debiased version 4 Sea Surface Salinity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7545, https://doi.org/10.5194/egusphere-egu2020-7545, 2020.

Poorly treated or completely untreated sewage water discharges are common problem which might have major consequences in coastal water regions, smaller water basins and semi-enclosed seas. Although satellite remote sensing has a great potential for coastal water quality monitoring such outfalls are difficult for detection due to the small scale of the events and the complex effects on the physical and biogeochemical parameters. In search for an appropriate technique for detection of  sewage discharges through satellite remote sensing, we examine areas with similar optical water properties, such as small river plumes flowing into the sea. They are expected to be visible in a similar manner as they have high turbidity levels, higher nutrients concentration and are fresh compared to the salty sea water.

In the current study we examine small river inflows in the Black Sea as they have optical and radar properties comparable with poorly or completely untreated sewage discharges in the region. Additionally, the Black Sea is an intriguing study area because of the unique ecosystem with challenging optical properties and water characteristics.

The temporal and spatial variability of the inherent optical properties and sea surface roughness are studied in the area of river plumes and are compared with open sea values. The impact of atmospheric conditions given by wind speed, wind direction and precipitation on the river plume detectability is observed in the regions of interest. Long time series of images for three years are analysed in order to reveal the seasonal and annual variability of the events. The satellite data is taken from the Sentinel missions and the atmospheric variables are from the ERA5 reanalysis.

The outcome of the study gives a solid base for estimation of the potential of satellite remote sensing for monitoring of poorly treated or completely untreated sewage outfalls or other land sources flowing into the sea.

How to cite: Gancheva, I., Campbell, G., and Peneva, E.: Satellite remote sensing techniques for detection of riverine and other sources flows in the coastal part of the Black Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8336, https://doi.org/10.5194/egusphere-egu2020-8336, 2020.

EGU2020-11812 | Displays | OS4.5

Evaluation and scientific exploitation of CryoSat ocean products for oceanographic studies

Francisco Mir Calafat, Chris Banks, Helen Snaith, Christine Gommenginger, Andrew Shaw, Paolo Cipollini, Nadim Dayoub, Jérôme Bouffard, and Marco Meloni

CryoSat’s ability to operate in different operating modes over water surfaces led to the first in-orbit evidence of the value of SAR-mode altimetry for oceanography, with the mission continuing to provide high-quality data and information not just over ice but also over the open ocean, polar waters and coastal regions. Approaching ten years in orbit, CryoSat routinely delivers a number of oceanographic products for global ocean applications. A dedicated operational CryoSat ocean processor (COP) has existed since April 2014 generating data products available in near real time (FDM/NOP), within ~3 days (IOP) and a geophysical ocean product (GOP) available within a month. An improved processing baseline was introduced in late 2017 and the same processing chain has now been applied to provide consistent ocean data products from the start of the mission. 
Within the ESA funded CryOcean-QCV project, the National Oceanography Centre (NOC) in the UK is responsible for routine quality control and validation of CryoSat Ocean Products. Activities include the production of daily and monthly reports containing global assessments and quality control of sea surface height anomaly (SSHA), significant wave height (SWH), backscatter coefficient (Sigma0) and wind speed, as well as a suite of validation protocols involving in situ data, model output and data from other satellite altimetry missions. This presentation will review some of the metrics and results obtained for CryoSat Ocean Products for SSHA, SWH and wind speed when assessed against data from tide gauges, wind and wave buoys, WaveWatch III wave model output, HF radar surface current data and comparisons with Jason-2 and Jason-3. Example metrics include SSHA along-track power spectra and the characterisation of offsets and variability regionally and in different sea states. 
In this presentation, we demonstrate the quality and scientific value of the CryoSat data in the open ocean where the altimeter operates mainly in conventional low-resolution-mode (LRM) but also over selected ocean regions where CryoSat operates in SAR-mode. 
Finally, scientific exploitation of the CryoSat data for oceanographic studies will be illustrated, focusing on CryoSat sea surface height anomalies. We will present examples of the benefits of CryoSat ocean products for oceanographic studies based on a dedicated Level 3 gridded product, featuring investigations of propagating ocean features (e.g. Rossby-type wave propagation) and their signatures in CryoSat in comparisons with data from other sources including SMOS, Sentinel 3A and 3B. 

How to cite: Mir Calafat, F., Banks, C., Snaith, H., Gommenginger, C., Shaw, A., Cipollini, P., Dayoub, N., Bouffard, J., and Meloni, M.: Evaluation and scientific exploitation of CryoSat ocean products for oceanographic studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11812, https://doi.org/10.5194/egusphere-egu2020-11812, 2020.

EGU2020-22600 | Displays | OS4.5

Taking advantage of Multisensor Synergy: New discovery and analysis tools

Lucile Gaultier, Fabrice Collard, Ziad El Khoury Hanna, Gilles Guitton, Sylvain Herlédan, and Guillaume Le Séach

Numerous new satellites and sensors have arised during the past decade. This satellite constellation has never been so dense and diverse. It provides a wide range of view angles to the ocean surface from the coast to the open ocean, at various scales and from physical to biological processes. Sentinel 1-2-3 program covers various sensors such as SAR, Optical, radiometer or altimeter with a repeat subcycle of only a few days, yet the repeat frequency for each sensor alone is not enough to monitor meso to submeso scales.

In the other hand, in-situ data are sparse in space but offers a high sample frequency and therefore complementary to remote sensing
observations. Handling consistently these huge heterogeneous datasets in a simple, fast and convenient way is now possible using the free and open Ocean Virtual Laboratory online portal or its standalone version. These tools are starting to be widely used by the scientific community to better discover, understand and monitor oceanic processes. We will demonstrate the potential and functionalities of these tools using various test cases:

Collocating Sentinel 1-2-3 for wave current interaction analysis
Creating synoptic charts of fronts and eddies, highlighting strong and energetic ocean currents
Campaign at sea planning and real time analysis of in-situ / remote sensing data. 
Validation and comparison of currents (derived from satellite and models) with a Lagrangian approach using SEAScope stand alone interactive tool. 


Online tool is available at https://ovl.oceandatalab.com and standalone version at https://seascope.oceandatalab.com. A splinter-meeting will
be organised at the conference to provide hands-on demonstration. 

How to cite: Gaultier, L., Collard, F., El Khoury Hanna, Z., Guitton, G., Herlédan, S., and Le Séach, G.: Taking advantage of Multisensor Synergy: New discovery and analysis tools, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22600, https://doi.org/10.5194/egusphere-egu2020-22600, 2020.

EGU2020-17583 | Displays | OS4.5

EUMETSAT Copernicus Marine Training and User Support

Hayley Evers-King, Christine Traeger-Chatterjee, Sally Wannop, Lauren Biermann, and Oliver Clements

EUMETSAT provides user support and training for all users of the Copernicus Marine Data Stream (CMDS). The CMDS refers to all the level 1 and level 2 marine data from sensors on the Sentinel-3 and Jason-3 satellites, including ocean colour, sea surface temperature, and surface topography data. Details on the products and processing methodologies are available through handbooks, product notices, and a number of services including a help desk, and online forum. The training service aims to support all users wishing to explore potential applications of the CMDS. The service is primarily based around the delivery of two week, blended courses with both an online and classroom component.  The online component is hosted on a Moodle platform and uses a variety of prepared resources including short articles, videos, software installation, and basic software tutorials; supported by discussion forums, to prepare participants for the classroom phase. The classroom phase is focused on practical work, with no lectures given. Participants are led through examples of workflows using SNAP and Jupyter Notebooks/Python, and are then given one-on-one/small group trainer support to work for 3 days on personal projects that they defined during the online phase. These projects yield a diverse range of synergistic use cases of ocean remote sensing data for societally relevant applications The training service has also run a variety of collaborative courses with community led initiatives, and proposes to develop online courses and resources in response to community needs. Feedback and requests from the ocean remote sensing community are welcomed.

 

How to cite: Evers-King, H., Traeger-Chatterjee, C., Wannop, S., Biermann, L., and Clements, O.: EUMETSAT Copernicus Marine Training and User Support , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17583, https://doi.org/10.5194/egusphere-egu2020-17583, 2020.

EGU2020-21206 | Displays | OS4.5

Preliminary Evaluation of the Atmospheric Correction Look-Up-Tables (LUTs) of the COCTS-HY1C

Keping Du, Shuguo Chen, Jing Ding, and Zhongping Lee

The Chinese Ocean Colour and Temperature Scanner (COCTS), Coastal Zone Imager (CZI) and the novel Ultra-Violet Imager (UVI) which on-board the Chinese Ocean Satellite  HY-1C was launched in September 2018. The atmospheric correction of ocean color sensors was a critical step for accurate retrieval of the remote sensing reflectance, and the look-up-tables (LUTs), for the Rayleigh scattering, the aerosol scattering, and the diffuse transmittance, which were built bases on a Successive Order Scattering Vector Radiative Transfer Solver, played an important role in the processing step. Preliminary evaluation has been performed using the SeaWiFS LUTs and the MODIS data, it showed that COCTS can provide accurate ocean color products.

How to cite: Du, K., Chen, S., Ding, J., and Lee, Z.: Preliminary Evaluation of the Atmospheric Correction Look-Up-Tables (LUTs) of the COCTS-HY1C, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21206, https://doi.org/10.5194/egusphere-egu2020-21206, 2020.

EGU2020-10760 | Displays | OS4.5

Spectral reflectance of suspended solid sediments: a laboratory experiment.

Martina Carlino and Silvia Di Francesco

Ocean color remote sensing proved to be a good alternative to traditional methods for total suspended solids concentration (TSS) monitoring purposes: numerous sensors have been developed for ocean color applications and different algorithms to retrieve TSS from remotely sensed data already exist.

Nevertheless, their application is generally limited by site-specific factors, and presently there is no uniform remote sensing model to estimate TSS.

The present study is focused in the development, evaluation and validation of different algorithms to estimate total suspended solids concentration based on laboratory reflectance data.

At this aim, a laboratory experiment was designed to collect the spectral reflectance of water containing fixed suspended particulate matter in terms of its concentration.

During the experiment, a total of 10 silty clay loam sediment samples were introduced into a tank filled with clear water up to a depth of 22 cm, illuminated by two 45 W lamps focused on center of water surface. After sieving, sediments were weighed so that TSS concentration ranging from 150 up to 2000 mg/L were obtained in the tank, being soil sediments suspension guaranteed by means of a mechanical pump-driven device.

Optical data were collected few minutes after each sediment introduction, using an Ocean Optics Jaz spectroradiometer mounted on a platform above the tank.

In accordance with previous studies, collected reflectance spectra of water containing sediments showed that, whatever is sediment concentration in water, reflectance in the red region is larger than that in the NIR region. Furthermore, reflectance spectra generally present two peaks: one between 550 nm and 750 nm, and the other between 750 nm and 850 nm, being the second peak insignificant for samples with very small TSS (e.g., SSC=150 mg/L), due to strong absorption of water.

After collection, laboratory reflectance spectra were integrated over the bandpass of different sensors’ selected bands, modulated by their relative response functions (RSR).

The basic principle of using a specific band, or band ratios to estimate a water parameter is to select spectral bands representative of its scattering/absorption features.

Band selection was achieved testing some previously formulated ocean color algorithms for the estimation of water quality parameters.

After band selection, linear regression model was applied to estimate the relationship between sensors’ reflectance at these bands and suspended solids concentration.

Results showed high correlation between selected sensors’ spectral red band and total suspended solids concentration higher than 500 mg/L up to 1360 mg/L, while less accuracy was observed for TSS concentrations higher than 1360 mg/L. Furthermore, the ratio between spectral red and green bands better estimates TSS in waters where total suspended concentration is not higher than 500 mg/L.

 

How to cite: Carlino, M. and Di Francesco, S.: Spectral reflectance of suspended solid sediments: a laboratory experiment., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10760, https://doi.org/10.5194/egusphere-egu2020-10760, 2020.

EGU2020-8655 | Displays | OS4.5

Spatial and temporal scales of variability in the Meghna estuary from ocean remote sensing

Masuma Chowdhury, Irene Laiz Alonso, and Isabel Caballero de Frutos

Abstract

Meghna Estuary is a very complex and dynamic estuarine system because of its irregular shape, wide seasonal variation and the changing role of tides. Every year, a major portion of the flow laden with high amount of sediments from the Ganges-Brahmaputra-Meghna river system are transferred through this estuary to the Bay of Bengal (BoB). The purpose of this analysis is to extract the dominant patterns of satellite Chlorophyll-a (CHL), Total Suspended Matter (TSM) and Sea Surface Temperature (SST) variability off the Meghna Estuary. CHL and TSM data were downloaded from GlobColour Project (http://globcolour.info/) and SST data were downloaded from Ocean Colour Project (https://oceancolor.gsfc.nasa.gov) using the monthly, 4km resolution of the global earth domain over eleven years (April 2002-April 2012). A temporal Empirical Orthogonal Function (EOF) analysis was performed to all the variables. The results revealed that the mean spatial distribution of the selected environmental variables in the Meghna estuary and its adjacent area during the study period showed clear latitudinal patterns in case of CHL and TSM, with higher values around the northern coast and a gradual offshore decreasing gradient. The SST map showed the same spatial pattern but with lower temperature values around the northern coastal fringe and higher values offshore. Only the first EOFs mode was found to be relevant, representing 25.55%, 15% and 89.43% of the CHL, TSM and SST variance, respectively, and depicted that the seasonal variability is the dominant pattern in this complex estuarine environment.

How to cite: Chowdhury, M., Laiz Alonso, I., and Caballero de Frutos, I.: Spatial and temporal scales of variability in the Meghna estuary from ocean remote sensing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8655, https://doi.org/10.5194/egusphere-egu2020-8655, 2020.

EGU2020-19123 | Displays | OS4.5

Multi-scale ocean colour synergy products for coastal water quality monitoring

Dimitry Van der Zande, Aida Alvera-Azcárate, Charles Troupin, João Cardoso Dos Santos, and Dries Van den Eynde

High-quality satellite-based ocean colour products can provide valuable support and insights in the management and monitoring of coastal ecosystems. Today’s availability of Earth Observation (EO) data is unprecedented including medium resolution ocean colour systems (e.g. Sentinel-3/OLCI), high resolution land sensors (e.g. Sentinel-2/MSI) and geostationary satellites (e.g. MSG/SEVIRI). Each of these sensors offers specific advantages in terms of spatial, temporal or radiometric characteristics. In the Multi-Sync project, we developed advanced ocean colour products (i.e. remote sensing reflectance, turbidity, and chlorophyll a concentration) through the synergetic use of these multi-scale EO data taking advantage of spectral characteristics of traditional medium resolution sensors, the high spatial resolution of some land sensors and the high temporal resolution of geostationary sensors.

To achieve this goal a multi-scale DINEOF (Data Interpolating Empirical Orthogonal Functions) approach was developed to reconstruct missing data using empirical orthogonal functions (EOF), reduce noise and exploit spatio-temporal coherency by joining several spatial and temporal resolutions. Here we present the capacity of DINEOF to extract multi-scale information through the integration of Sentinel-3, Sentinel-2 and SEVIRI datasets.

The functionality of the advanced multi-scale products will be demonstrated in a case study for the Belgian Coastal Zone (BCZ) highly relevant to the user community: sediment transport modelling near the harbour of Zeebrugge in support of dredging operations. As stated in the OSPAR treaty (1992), Belgium is obliged to monitor and evaluate the effects of all human activities on the marine ecosystem. Dredging activities in and near Belgian harbors fall under this treaty and are performed daily to ensure accessibility of the port by ships. Optimization of these dredging activities requires monitoring data which is typically acquired through in situ observations or modelling data. In this case study we take advantage of Sentinel-3, Sentinel-2 and SEVIRI data characteristics to provide a satellite product that meets the end user requirements in terms of product quality and temporal/spatial resolution.

 

How to cite: Van der Zande, D., Alvera-Azcárate, A., Troupin, C., Cardoso Dos Santos, J., and Van den Eynde, D.: Multi-scale ocean colour synergy products for coastal water quality monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19123, https://doi.org/10.5194/egusphere-egu2020-19123, 2020.

Hourly sea surface height (SSH) from a coastal ocean model off eastern Newfoundland is used to generate simulated surface water and ocean topography (SWOT) data by a SWOT simulator. The simulated SWOT data are then used to reconstruct SSH by applying optimal interpolation (OI) in time and space. The reconstructed SSH is further used to calculate geostrophic currents associated with the inshore Labrador Current. The simulated SWOT data are also assimilated into the coastal ocean model by applying a deterministic ensemble Kalman filter (DEnKF). It is found that the inshore Labrador Current is fairly well reconstructed at the weekly scale, while the DEnKF assimilation reproduces well the  inshore Labrador Current at not only weekly but also daily scales.

How to cite: Han, G. and Ma, Z.: Reconstruction of the Inshore Labrador Current using SWOT: from OI to DENKF, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3626, https://doi.org/10.5194/egusphere-egu2020-3626, 2020.

EGU2020-19273 | Displays | OS4.5

Mapping altimetry in the forthcoming SWOT era by back-and-forth nudging the quasi-geostrophic dynamics

Florian Le Guillou, Sammy Metref, Maxime Ballarotta, Clément Ubelmann, Emmanuel Cosme, Julien Le Sommer, and Jacques Verron

For 20 years, ocean surface topography maps, essential for understanding the ocean circulation, have been built by statistically interpolating sea surface height (SSH) data provided by along-track nadir altimeters.  The space-time distribution of observed data limits the resolution of the maps to approximately 150 km and 20 days in wavelength. The launch of the next-generation SWOT altimetry mission in 2021 opens the way to high resolution maps thanks to an unprecedented kilometric resolution over a swath wide of 120 km. However, new advanced mapping techniques that involve information on ocean dynamics should be explored to take advantage of the high spatial resolution of SWOT. In this study, a data assimilation algorithm, the back and forth nudging (BFN), is implemented with a one and half layer quasi geostrophic model (QG) to dynamically interpolate the altimetric data.  We test the QG/BFN system to dynamically map SSH with synthetic but realistically distributed altimetric observations in the framework of Observing System Simulation Experiments (OSSE). The study focuses on two regions of the North-Atlantic ocean, presenting different dynamics and characterized by different temporal samplings of SWOT.

A systematic comparison with the traditional objective mapping technique demonstrates that the QG/BFN brings a significant improvement of the quality of the maps using only conventional nadir altimeter data. This outperformance of the QG/BFN is further increased when SWOT is added in the altimetric dataset. Our method is particularly effective for reconstructing non-linear surface dynamics at small mesoscales (40-100 km) that are smoothed out by conventional methods based on optimal mapping.

How to cite: Le Guillou, F., Metref, S., Ballarotta, M., Ubelmann, C., Cosme, E., Le Sommer, J., and Verron, J.: Mapping altimetry in the forthcoming SWOT era by back-and-forth nudging the quasi-geostrophic dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19273, https://doi.org/10.5194/egusphere-egu2020-19273, 2020.

EGU2020-5909 | Displays | OS4.5

Composite distribution of North Atlantic extreme wind waves inferred from satellite altimetry data

Sonia Ponce de León and Joao Bettencourt

The north Atlantic Ocean is regularly traversed by extratropical cyclones and winter low pressure systems originated in the Western part of the basin that can potentially generate dangerous extreme sea states. The region where these extreme sea states occur is linked to the tracks of the low-pressure systems in the north Atlantic basin.

Extreme sea states are usually generated by storms that can traverse whole ocean basins and generate high-energy swells that can propagate for thousands of kilometers. Additionally, rogue waves are a recognized source of extreme waves that needs to be considered when designing for operation at sea.

This study aims at the spatial distribution of the mean and extreme wave significant wave height inside the extratropical cyclones. We studied the significant wave height distribution of extratropical cyclones using merged satellite altimetry data to produce composite maps of this sea state variable. Although there are large variations among individual cyclones, the compositing method allows obtaining general features. We find that the higher waves are in the south-eastern quadrant of the cyclone, due to the extended fetch mechanism. The highest wave heights are found during the 48h period when the cyclone’s strength is maximum.

 

How to cite: Ponce de León, S. and Bettencourt, J.: Composite distribution of North Atlantic extreme wind waves inferred from satellite altimetry data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5909, https://doi.org/10.5194/egusphere-egu2020-5909, 2020.

Altimeters can provide global long-duration observations of oceanic wind speed and wave height. However, altimeters face the undersampling problem in estimating wind and wave climate because of their sparse sampling pattern and the changing number of in-orbit satellites. In this study, the undersampling error of altimeters was studied by sampling the ERA5 oceanic wind speed and wave height data using the track information of multiplatform altimeters. Comparisons were made between the statistics (mean, 90th and 99th percentiles, and long-term trends of them) of the original ERA5 data and the gridded along-track sampling of the ERA5 data. The results show a large discrepancy with respect to the extreme values (90th and 99th percentiles). The undersampling of altimeters can lead to significant underestimations of monthly extreme values of oceanic wind speed and wave height. Meanwhile, this underestimation is alleviated with the increase of the number of in-orbit altimeters, leading to very large overestimations of long-term trends of these extreme values over the period 1985-2018. In contrast, the annual extreme values of oceanic wind speed and wave height and their long-term trends are more reliable, although slight aforementioned biases of extreme values still exist and the data from GEOSAT are not suitable for computing annual statistics. For altimeter data, the annual values are a better option to compute long-term trends than the monthly data. This study also presents a correction scheme of using model data to compensate for the wind and wave events missed by altimeter tracks. After the correction, the global trends in oceanic wind speed and wave height over 1992-2017 are recomputed using annual statistics. The results show a clear discrepancy between the trends of wind speed and wave height during this period: the wind speed increased, while the wave height decreased. However, the reason for this discrepancy is unknown at this stage.

How to cite: Jiang, H.: Evaluation of altimeter undersampling in estimating global wind and wave climate using virtual observation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2191, https://doi.org/10.5194/egusphere-egu2020-2191, 2020.

EGU2020-4870 | Displays | OS4.5

Estimation of wave-induced contribution to Sentinel-1 Doppler shift observations

Artem Moiseev, Harald Johnsen, and Johnny Johannessen

The Doppler Centroid Anomaly (DCA) registered by microwave Synthetic Aperture Radar (SAR) contains information about ocean surface motion in the radar line-of-sight direction. The recorded signal is associated with the motion induced by the total wavefield (i.e., both wind waves and swell) and underlying ocean surface currents. Hence, accurate estimates of the wave-induced contribution to the observed DCA is required in order to obtain reliable information about underlying ocean surface current. In this study, we develop an empirical geophysical model function for the estimation of the wave-induced DCA. The study is based on two months of Sentinel-1 SAR Wave mode (WV) DCA observations collocated with wind field at 10m height from the ECMWF model and sea state information from the WAVEWATCH III model.

Analysis of two months of observations acquired over land showed that thanks to the novel Sentinel-1 DCA calibration, the uncertainty in the data does not exceed 3Hz (corresponding to a radial velocity of 0.21/014 m/s in the near/far range. The relationship between the DCA and the near-surface wind is in agreement with previously reported findings under the assumption of fully developed seas; the DCA is about 24% of the range wind speed at 23° incidence angle and decreasing (up to 50%) with increasing incidence angle from 23° to 36°. However, the difference between upwind (i.e., the wind blows towards antenna) and downwind (i.e., wind blows away from the antenna) configurations is inconsistent from study to study. Reliable information about the wave field indeed helps to describe the spread in the DCA, especially at low and moderate wind speeds, and when the ocean surface is dominated by the remotely generated swell.

The CDOP model is used as a baseline for estimating the wind-wave-induced Doppler shift. Retraining of the CDOP model for the Sentinel-1 SAR observations (CDOP-S) yielded a significantly better fit. Then, we extended the GMF with parameters of the wavefield (significant wave height, mean wave period and direction) in the moment of SAR acquisition. Combining information about near-surface wind and ocean surface wave fields also considerably improves the accuracy of the wave-induced Doppler shift estimates. In turn,  the accuracy of the ocean surface current retrievals are improved as demonstrated by the promising agreement with the near-surface ocean surface current climatology based on multiyear drifter observations.

How to cite: Moiseev, A., Johnsen, H., and Johannessen, J.: Estimation of wave-induced contribution to Sentinel-1 Doppler shift observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4870, https://doi.org/10.5194/egusphere-egu2020-4870, 2020.

EGU2020-19669 | Displays | OS4.5

Aviso+: what’s new on the reference portal in satellite altimetry?

Laurent Soudarin, Françoise Mertz, Vinca Rosmorduc, Catherine Schgounn, Thierry Guinle, Florence Birol, and Fernando Niño

Aviso (Archiving, Validation and Interpretation of Satellite Oceanographic data) is a service set up by CNES to process, archive and distribute data and products from altimetry satellite missions. Its portal AVISO+ (www.aviso.altimetry.fr) is the entry point to freely access more than 40 products from CNES (Centre National d'Etudes Spatiales) and CTOH (Center for Topographic studies of the Ocean and Hydrosphere) not only for ocean-oriented applications but also for hydrology, coastal, ice applications. In addition, the website proposes information (handbooks, use case, outreach material, …) to discover the products and their use. New products are regularly added to the catalogue, whether they are operational or demonstration products. For example, in 2019, the catalogue has been enriched with L2P level products from the Copernicus Sentinel-3A & 3B missions, the new version of the Mean Dynamic Topography CNES/CLS 2018, the mesoscale Eddy Trajectory Atlas in Near Real Time, several datasets of along-track and gridded SSALTO/DUACS experimental products, and some others. In 2020, some products of the CFOSAT (China-France Oceanography SATellite) mission devoted to the monitoring of ocean surface winds and waves, as well as CTOH ice and snow products will be available via Aviso+.

This presentation gives an overview of available products and services. We also present here all the recent novelties, and those to come in 2020.

How to cite: Soudarin, L., Mertz, F., Rosmorduc, V., Schgounn, C., Guinle, T., Birol, F., and Niño, F.: Aviso+: what’s new on the reference portal in satellite altimetry?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19669, https://doi.org/10.5194/egusphere-egu2020-19669, 2020.

EGU2020-10051 | Displays | OS4.5

On the use of deep learning for the improvement of swh and wave spectra from CFOSAT

Jiuke Wang, Lotfi Aouf, Alice Dalphinet, and Benxia Li

The SWIM (Surface Waves Investigation and Monitoring) carried by the CFOSAT (Chinese-French Oceanography Satellite) is designed to obtain the nadir wave height and directional spectrum. This work first introduces the efficiency of deep learning technique to improve wave height and spectrum observation from CFOSAT. A set of deep learning neural networks (DNN) are established and trained to improve the accuracy of SWIM nadir wave height. According to the assessment based on independent buoy observations, the DNN reduces the root mean square error (RMSE) of significant wave height by 32.2% (from 0.26 m to 0.17 m), and the scatter index by 25.7% (from 14 % to 10 %). The result shows that the bias is significantly decreased from 0.11 m to -0.02 m. To correct the SWIM wave spectra, 6 months of NDBC frequency spectra have been used to obtain 19 sets of DNN, each of them is corresponding to one effective frequency of SWIM accordingly. Each set of DNN contains 14 hidden layers with the input as the energy from the different beams 6°, 8° and 10°. Then, the DNN forms a new combined spectrum based on wave spectra of all beams of the SWIM instrument. The independent assessment shows that the wave spectra which come from the 19 sets of DNN significantly reduced the relative error (RE) by 10% to 46% in comparison with beam 10°, which has the best accuracy performance among all the beams. The deep learning technique is also used as a quality control procedure before the assimilation of SWIM wave data. The Siamese convolution neural network (CNN) connected with a deep learning neural network (DNN) is applied to perform such comparison and verification for the SWIM directional spectra. The SWIM directional wave spectra are considered as the 2-dimensional “energy-pictures” with a matrix dimension of 17 frequencies and 9 directions. The Siamese CNN network is made up of 2 pairs of convolution layer and pooling layer, in which the 32 groups of convolution kernels are used to generate one-dimensional features from the directional wave spectra. Both wave spectra of SWIM instrument and wave model are inputted into the same Siamese CNN network, being transformed into 2 sets of features accordingly. Then the features would go to the DNN to generate the index of the similarity. This gives a quantitative description of how different between the SWIM directional spectra and the ones from wave model. Through the training of 26014 pairs of directional wave spectra, the Siamese CNN and DNN have showed a consistency of 78% to 86% with the “partition distance” method in the independent testing data, but with a much faster computation speed. We revealed that the Siamese technique increases the number of wave spectra passing through the verification than the “partition distance” method. This will ensure larger impact of the assimilation of SWIM data in the analysis and forecast period. The case study by running the wave model MFWAM using Siamese network and “partition distance” in assimilation is investigated.

How to cite: Wang, J., Aouf, L., Dalphinet, A., and Li, B.: On the use of deep learning for the improvement of swh and wave spectra from CFOSAT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10051, https://doi.org/10.5194/egusphere-egu2020-10051, 2020.

EGU2020-20388 | Displays | OS4.5

Remote measurement of near-surface currents via wave spectra: currents varying vertically and horizontally

Benjamin K Smeltzer, Ida Seip Gundersen, and Simen Ådnøy Ellingsen

Remote sensing of ocean near-surface currents based on measurements of the wave spectrum is an attractive means of mapping currents over a large area simultaneously. The most common wave measurement method involves marine X-band radar (Lund et al. 2015), with optical video measurements using drones more recently being used as an alternative (Streßer, Carrasco & Horstmann, 2017). In both cases, analysis of the wave dispersion within a subset window of the spatial domain is performed to determine the spatially varying near-surface current. An improved method for determining the depth-dependence of sub-surface currents from measured wave spectra was recently developed by our group (Smeltzer et al 2019).

Our long-term goal is to develop methods whereby the best possible representation of the three-dimensional sub-surface current can be obtained from remote measurement of waves. Methods based on current retrieval from wave spectra must assume that horizontal current variations are slow compared to a typical wavelength, but this is not always so. To resolve horizontal space, retrieved images must be subdivided into windows and the velocity vector at the midpoint is determined from the 3D spectrum of the waves within the window only.

In this work we examine the dependence of the spatial window size on the results of the current reconstruction. When the window size is decreased, greater spatial resolution is achieved being able to capture currents that vary on a faster horizontal length scale, at the expense of lower resolution in wavevector spectral space which may decrease the accuracy of the reconstructed currents, especially when information as the depth-dependence of the flow is desired. When the window size is larger, the reconstructed current may not be representative of the average current within the window. We present experiments conducted in a laboratory where spatially varying currents and waves of can be well-controlled and measured in situ, a valuable test-bed setup compared to field measurements. We investigate the factors involved which determine the optimal choice of window size.

References

Lund, B., et al. A new technique for the retrieval of near-surface vertical current shear from marine X-band radar images. J. Geophys. Res.: Oceans (2015) 120 8466-8496.

Smeltzer, B.K., Æsøy, E., Ådnøy, A. and Ellingsen S.Å., An improved method for determining near-surface currents from wave dispersion measurements. J. Geophys. Res.: Oceans. (2019) 124, https://doi.org/10.1029/2019JC015202.

Streßer, M., Carrasco, R. and Horstmann, J., Video-based estimation of surface currents using a low-cost quadcopter, IEEE Geosci. Remote Sens. Lett. (2017) 14 2027-2031.

How to cite: Smeltzer, B. K., Gundersen, I. S., and Ellingsen, S. Å.: Remote measurement of near-surface currents via wave spectra: currents varying vertically and horizontally, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20388, https://doi.org/10.5194/egusphere-egu2020-20388, 2020.

EGU2020-5286 | Displays | OS4.5

Retrieving the Ocean Interior Structure from Surface Data:FOAGRNN and SQG-mEOF-R

Huizan Wang, Ren Zhang, Senliang Bao, Henqian Yan, Weimin Zhang, and Xiaojiang Zhang

As many oceanic observations can only reflect the sea surface, such as satellite data, the retrieval of the ocean interior structure from sea surface information is of great importance for us to understand the three-dimensional ocean. In order to retrieve the three-dimensional salinity and density structure from surface data, two new method are proposed as follows. One proposed method is called generalized regression neural network with the fruit fly optimization algorithm (FOAGRNN), which is a nonlinear method and used to estimate subsurface salinity profiles from sea surface parameters. Compared with linear methodology, the estimated salinity profiles from the FOAGRNN method are in better agreement with the measured profiles at the halocline.The results suggest a potential new approach for salinity profile estimation derived from sea surface data. The other proposed method is called SQG-mEOF-R, which estimate the interior density from the sea surface density (SSD) and sea surface height (SSH) by combining the dynamical surface trapped mode derived from the Surface Quasi-Geostrophic (SQG) function with the statistical mode calculated from multivariate EOF reconstruction (mEOF-R) method and. This method is applied to the eddy-resolving OFES (Ocean General Circulation Model For the Earth simulator) simulation and compared with the conventional SQG or isQG (interior plus SQG) and mEOF-R methods. The results manifest that, no matter in the NorthWest Pacific (NWP) region dominated by surface-intensified eddies or the SouthEast Pacific (SEP) region characterized by subsurface-intensified eddies, SQG-mEOF-R perform a robust work in mesoscale density reconstruction.

How to cite: Wang, H., Zhang, R., Bao, S., Yan, H., Zhang, W., and Zhang, X.: Retrieving the Ocean Interior Structure from Surface Data:FOAGRNN and SQG-mEOF-R, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5286, https://doi.org/10.5194/egusphere-egu2020-5286, 2020.

EGU2020-10011 | Displays | OS4.5

Spatio-temporal variability of internal waves in the Caspian Sea

Olga Lavrova and Andrey Kostianoy

Internal waves (IWs) are an intrinsic feature of all density stratified water bodies: oceans, seas, lakes and reservoirs. IWs occur due to various causes. Among them are tides and inertial motions, variations in atmospheric pressure and wind, underwater earthquakes, water flows over bottom topography, anthropogenic factors, etc. In coastal areas of oceans and tidal seas,  IWs induced by tidal currents over shelf edge predominate. Such IWs are well-studied in multiple field, laboratory and numerical experiments. However, the data on IWs in non-tidal seas, such as the Black, Baltic and Caspian Seas, are scarce. Meanwhile, our multi-year satellite observations prove IWs to be quite a characteristic hydrophysical phenomenon of the Caspian Sea. The sea is considered non-tidal because tide height does not exceed 12 cm at the coastline. And yet surface manifestations of IWs are regularly observed in satellite data, both radar and visible. The goal of our study was to reveal spatial, seasonal and interannual variability of IW surface manifestations in the Caspian Sea in the periods of 1999-2012 and 2018-2019 from the analysis of satellite data. All available satellite radar and visible data were used, that is data from ERS1/2 SAR; Envisat ASAR; Sentinel-1A,1B SAR-C; Landsat-4,5 TM; Landsat-7 ETM+; Landsat-8 OLI; Sentinel-2A,2B MSI sensors. During the year, IWs were observed from the beginning of May to mid-September. In certain years, depending on hydrometeorological conditions, such as water heating, wind field, etc., no IWs could be seen in May or September. IWs regularly occur in the east of Middle Caspian and in the northeast of South Caspian. In North Caspian, due to its shallowness and absence of pronounced stratification, IWs are not generated, at least their surface signatures cannot be found in satellite data. In the west of the sea, IWs are scarcely observed, primarily at the beginning of the summer season. IW trains propagate toward the coast, their generation sites are mainly over the depths of 50-200 m.

According to the available data for the studied periods, the time of the first appearance of IW signatures differs significantly from year to year. For example, in 1999 and 2000 it happened only in July.

Since no in situ measurements were conducted in the sites of regular IW manifestations, an attempt  was made to establish the dependence of IW occurrence frequency  on seasonal and interannual variations of sea surface temperature, an indirect indicator of the depth of the diurnal or seasonal thermocline, that is where IW were generated. Sea surface temperature was also estimated from satellite data.

Another issue addressed in the work was the differentiation between the sea surface signatures of IWs in the atmosphere and the sea. The Caspian Sea is known for their close similarity in spatial characteristics.

The work was carried out with financial support of the Russian Science Foundation grant #19-77-20060.  Processing of satellite data was carried out by Center for Collective Use “IKI-Monitoring” with the use of “See The Sea” system, that was implemented in frame of Theme “Monitoring”, State register No. 01.20.0.2.00164.

How to cite: Lavrova, O. and Kostianoy, A.: Spatio-temporal variability of internal waves in the Caspian Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10011, https://doi.org/10.5194/egusphere-egu2020-10011, 2020.

EGU2020-21497 | Displays | OS4.5

Korteweg-deVriès limitations for the interpretation of SAR images in the Strait of Gibraltar: impact of different stratifications on ISW surface signature.

Morgane Dessert, Xavier Carton, Jean-Marc Le Caillec, Christophe Messager, Lucie Bordois, Marc Honnorat, and Tran Vu La

Internal Solitary Waves (ISW) are particularly large amplitude internal waves which may propagate in the ocean over tens of kilometres while preserving their shape via a balance between non-linearity and non-hydrostatics effects. These waves may have wide impacts on the ocean dynamics (mixing or inducing vertical currents) and on human activities (fisheries, underwater acoustic or offshore activities).

ISW can be detected on satellite scenes. For instance, they may induce surface currents and thus enhance or damp the capillary waves at the sea surface which signed on the Synthetic Aperture Radar (SAR) scenes. On SAR images, ISW appear as successions of bright and dark bands over a grey background. From these images, the amplitude of the ISW and the depth of the pycnocline may be inferred using the Korteweg-DeVries (KdV) theoretical framework. Several SAR images interpretation methods have been developed based on curve fitting or Peak-to-Peak methods (Zheng et al., 2001) or parametric autoregressive techniques (Le Caillec, 2006). The KdV theory relies on the weakly nonlinear approximation and a Two-Layers Ocean Model (TLOM).

In Gibraltar Strait, the tidal dynamic leads to strong periodic currents. The exchanges between the Mediterranean sea and the Atlantic ocean occurred according a two layer scheme that maintains large density gradient located at the interface between Atlantic and Mediterranean Waters.  At some tidal outflow, an internal hydraulic jump is formed above Camarinal sill, when the tidal ouflow slackens, it is released and leads to the formation of eastward propagating internal solitary waves. The site is thus considered as an ISW “hot-spot”. Part of the energy carried by these waves propagates eastward into the Alborean Sea, although the stratification may differ from the TLOM.

If the stratification differs from TLOM, a given surface signature of ISW could match to several configurations of the pycnocline geometry and ISW amplitude, depending on the associated stratification.

In order to assess the impact of the stratification on the surface signature of the ISW, we implemented an idealized 2DV (one vertical and one longitudinal directions) configuration with the Coastal and Regional Ocean modelling COmmunity model (CROCO) using its non-Boussinesq (pseudo compressible) capability. The bathymetry and the density profile are inspired from oceanic observations. The tidal forcing is simplified to a pure monochromatic M2 tide.

First, simulations are initialized with a two-layer density profile and different pycnocline depths. Then, we added continuous stratification in each of the two (surface/bottom) layers. We tested also several tidal regimes in order to represent the various strengths between the neap and spring tide. SAR images interpretation techniques are then tested in each configurations. Pycnocline depths and ISW amplitudes computed from SAR methods are then compared with the ones initially simulated by the CROCO model.

 

Le Caillec, J.-M., 2006. Study of the SAR signature of internal waves by nonlinear parametric autoregressive models. IEEE Trans. Geosci. Remote Sens. 44, 148–158. https://doi.org/10.1109/TGRS.2005.859954

Zheng, Q., Yuan, Y., Klemas, V., Yan, X.-H., 2001. Theoretical expression for an ocean internal soliton synthetic aperture radar image and determination of the soliton characteristic half width. J. Geophys. Res. Oceans 106, 31415–31423. https://doi.org/10.1029/2000JC000726

How to cite: Dessert, M., Carton, X., Le Caillec, J.-M., Messager, C., Bordois, L., Honnorat, M., and La, T. V.: Korteweg-deVriès limitations for the interpretation of SAR images in the Strait of Gibraltar: impact of different stratifications on ISW surface signature. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21497, https://doi.org/10.5194/egusphere-egu2020-21497, 2020.

EGU2020-8634 | Displays | OS4.5

Semi-Empirical Model for the Ka-band Sea Surface Doppler Centroid

Yury Yurovsky, Vladimir Kudryavtsev, Semyon Grodsky, and Bertrand Chapron

The sea surface Doppler spectrum centroid is a principal parameter for the sea surface current retrieval from Doppler radar measurements. Satellite Doppler scatterometers are proposed to operate in the Ka-band (SKIM, DopplerScatt/WaCM, SEASTAR) in order to achieve sufficient measurement accuracy. Todays documentation of the Ka-band sea surface backscattering parameters is poor, thus this work is aimed at presenting a model for the sea surface Doppler spectrum centroid (DC) deducted from field data collected from the Black Sea research platform. The model relies on the well-known two-scale surface separation approach. Within this framework, the small-scale waves are the scatterers moving at their inherent speed (Bragg wave phase velocity or specular point velocity), which, in turn, are advected by the large-scale wave orbital velocities. These modulations lead to correlated variations of local scatterer cross-section and speed. The inherent scatterer velocity is computed theoretically, while the modulation term is described by the empirical modulation transfer function (MTF) which naturally involves both tilt and hydrodynamics components as a function of look geometry and sea state. The proposed semi-empirical DC model is in good agreement with measurements if in situ wave gauge directional spectrum is used as a wave input. Based on this finding, we extrapolate the semi-empirical DC model on the arbitrary surface described by the physical model of the wind wave spectrum. The resulting DC model is compared to the published empirical models and measurements (SAXON-FPN, DopplerScatt, AirSWOT, Wavemill field campaigns, and CDOP Envisat ASAR model). The model DC dependencies on incidence angle and wind speed are consistent with Ku-band
SAXON-FPN, Ka-band AirSWOT, and DopplerScatt datasets, but differs from C-band CDOP model and X-band Wavemill dataset, which generally have higher DC magnitude (besides longer operating radar wavelength, the difference can be attributed to swell dominated sea observed in the CDOP and Wavemill cases). The model predicts that the DC rises with wind speed at small incidence angles, 20–30o, but the DC level is almost independent of wind at larger incidence angles, 50–55o. Such behavior is explained by the balance between opposing wind dependencies of the MTF magnitude and the magnitude of modulating wave orbital velocities.

The work is supported by the Russian Science Foundation under grant No. 17-77-30019.

How to cite: Yurovsky, Y., Kudryavtsev, V., Grodsky, S., and Chapron, B.: Semi-Empirical Model for the Ka-band Sea Surface Doppler Centroid, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8634, https://doi.org/10.5194/egusphere-egu2020-8634, 2020.

Seawater temperature and salinity are the two key parameters related to the regional sea level variability. In this study, the spatial-temporal variabilities of the thermal and halo steric height over the South China Sea (SCS) are investigated using multi-senor satellite remote sensing products, in-situ measurements and reanalysis. The sea surface temperature and salinity products are used to reconstruct the upper layer sea level components, and the relative contribution of these two components are quantified. It is revealed that the thermal and halo components vary in an out-of-phase pattern, and dominant different regions within SCS. Variabilities of the sea level components on different timescale are further analyzed, and the linkage with large scale processes, such as the indo-pacific warm pool, will be presented.

How to cite: Sha, J. and Li, X.: Variabilities of sea level components over the South China Sea based on multi-sensor satellite observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6198, https://doi.org/10.5194/egusphere-egu2020-6198, 2020.

 

  • Sargassum horneri is one of the major components of the floating sargassum that is distributed widely along the coast, including Korea, China and Japan. Sargassum horneri has an air pocket called an ‘air sec’ on its body which leads to a floating life, and drifts by ocean currents and winds. Due to these characteristics, blooms of Sargassum horneri have occurred April-June in the East China Sea. If these blooms flow into Jeju Island in South Korea, the blooms can cause enormous damage to fishing activities and the marine tourism industry. In order to minimize the damage caused by these blooms, we have been studied using remote sensing and field measurements.

    This study investigates environmental factors associated to the inflow of Sargassum horneri into Korean Peninsula. We used floating algae detection algorithm developed by a Geostationary Ocean Color Imager (GOCI). Since GOCI provides data of the seas surrounding the Korea eight times a day (00 to 07 UTC), it is suitable to monitor the blooms. The algorithm was made using the Red-edge effect which has a sharply rising reflectivity at around 700 nm but a low reflectivity in the red area (660-680 nm). And it was considered that the reflectivity of background seawater which varies from place to place is eliminated. Based on the results of the algorithm for detecting floating algae, Sargassum horneri’s inflow into the Korean Peninsula was analyzed January to June for six years (2014 to 2019). Also, the environment factors affecting to the inflow path were investigated each months and years.

 

How to cite: Kim, J. and Park, Y.: Investigation of the environmental factors associated with the inflow of Sargassum horneri into the Korean Peninsula using GOCI, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22278, https://doi.org/10.5194/egusphere-egu2020-22278, 2020.

EGU2020-22318 | Displays | OS4.5

Beach Litter Detection and Monitoring Using UAV Image and Deep Neural Network under Imbalanced Data

Suho Bak, Minji Jeong, Nakyeong Kim, and Hongjoo Yoon

Beach litters such as plastic bottles and styrofoam destroys coast ecosystems and creates aesthetic discomfort that lowers the value of the beach. Also, these beach litters are consumed by marine creatures, causing secondary ecosystem destruction. In order to solve this beach litters problem, it is necessary to study the generation and distribution pattern of waste and the cause of the inflow. However, the data for the study were only sample data collected in some areas of the beach. Also, most of the data covers only the total amount of beach litters. The total amount obtained from the sampling method was difficult to represent the total amount of beach litters.

UAV(Unmanned Aerial Vehicle) and Deep Neural Network can be effectively used to detect and monitor beach litter. Using UAV, it is possible to easily survey the entire beach. Recently, Object Detection technologies based on the Convolutional Neural Network have produced remarkable results in the general object recognition field. The Deep Neural Network can also identify the type of coastal litters. Therefore, using UAV and Deep Neural Network, it is possible to acquire spatial information by type of beach litters.

This paper proposes a Beach litter detection algorithm based on UAV and Deep Neural Network and a Beach litter monitoring process using it. It also offers optimal shooting altitude and image duplication to detect small beach litter such as plastic bottles and styrofoam pieces found on the beach. We are also suggest to effectively methods for training on imbalanced data.

In this study, DJI Mavic 2 Pro was used. The camera on the UAV is a 1-inch CMOS with a resolution of 20MP. The images obtained through UAV are produced as orthoimages and input into a pre-trained neural network algorithm.

How to cite: Bak, S., Jeong, M., Kim, N., and Yoon, H.: Beach Litter Detection and Monitoring Using UAV Image and Deep Neural Network under Imbalanced Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22318, https://doi.org/10.5194/egusphere-egu2020-22318, 2020.

EGU2020-22321 | Displays | OS4.5

Characteristics of Marine Debris Movement Pattern Using Location Tracking Buoys

Minji Jeong, Suho Bak, Nakyeong Kim, and Hongjoo Yoon

Recently, marine debris, which is emerging most in the marine environment, becomes a global problem and becomes a factor of marine environmental pollution. In particular, the United Nation General Assembly adopted the Sustainable Development Goals(SDGs), the largest joint goal of the United Nations and the international community, which is to be implemented from 2016 to 2030, including the reduction of marine waste by 2025 in the detailed marine sector. Marine debris, which is emerging as an international environmental issue, is emphasized mainly by coastal countries and can be seen in domestic coasts. Accordingly, coastal debris monitoring is carried out to investigate debris on designated coasts in Korea. However, the current monitoring methods have limitations in identifying the amount and type of coastal debris by region. And when comparing the amount of inflow and collection of marine debris in Korea, the collection amount is less than half compared to the inflow amount. Therefore, it is necessary to understand the movement pattern characteristics by tracking the behavior of marine debris. In this study uses the location tracking buoys which can identify the movement characteristics of marine debris in accordance with the floating and direct influence in real-time. Therefore, GPS terminals built into objects that are shaped like marine debris, suggesting that they can track what movement patterns are characteristic of current and wind effects.

How to cite: Jeong, M., Bak, S., Kim, N., and Yoon, H.: Characteristics of Marine Debris Movement Pattern Using Location Tracking Buoys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22321, https://doi.org/10.5194/egusphere-egu2020-22321, 2020.

EGU2020-9671 | Displays | OS4.5

Remote sensing mixed layer depth using ocean ambient noise

Najeem Shajahan and David Barclay

Ambient noise measurements have been widely used to estimate environmental information such as water column sound speed, pH, seabed properties, and wind speed. In this study, 30 days of ambient noise data recorded on two vertically oriented hydrophones deployed near Alvin canyon on the New England shelf break were used to estimate the ocean mixed layer depth (MLD). The vertical noise coherence was computed and compared to a wave-number integral noise model comprised of a two-segment piecewise linear summer sound speed profile in a shallow water waveguide. Measurements of noise and sound speed profiles, together with a wavenumber integral ambient noise model were used to calculate the mixed layer thickness. Noise model results showed variations in the first zero-crossing frequency, which was in accordance with the semi-diurnal variability of the MLD. MLD was determined by matching the zero-crossing frequency of the real part of measured coherence with the model results for the entire one-month period. The comparison of the estimated MLD using ambient noise showed good agreement with the measured MLD from the temperature sensors.

How to cite: Shajahan, N. and Barclay, D.: Remote sensing mixed layer depth using ocean ambient noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9671, https://doi.org/10.5194/egusphere-egu2020-9671, 2020.

EGU2020-22319 | Displays | OS4.5

Evaluation of Sea fog Detection Accuracy Based on Geostationary Satellite Image Using Machine Learning

NaKyeong Kim, Suho Bak, Minji Jeong, and Hongjoo Yoon

A sea fog is a fog caused by the cooling of the air near the ocean-atmosphere boundary layer when the warm sea surface air moves to a cold sea level. Sea fog affects a variety of aspects, including maritime and coastal transportation, military activities and fishing activities. In particular, it is important to detect sea fog as they can lead to ship accidents due to reduced visibility. Due to the wide range of sea fog events and the lack of constant occurrence, it is generally detected through satellite remote sensing. Because sea fog travels in a short period of time, it uses geostationary satellites with higher time resolution than polar satellites to detect fog. A method for detecting fog by using the difference between 11 μm channel and 3.7 μm channel was widely used when detecting fog by satellite remote sensing, but this is difficult to distinguish between lower clouds and fog. Traditional algorithms are difficult to find accurate thresholds for fog and cloud. However, machine learning algorithms can be used as a useful tool to determine this. In this study, based on geostationary satellite imaging data, a comparative analysis of sea fog detection accuracy was conducted through various methods of machine learning, such as Random Forest, Multi-Layer Perceptron, and Convolutional Neural Networks.

How to cite: Kim, N., Bak, S., Jeong, M., and Yoon, H.: Evaluation of Sea fog Detection Accuracy Based on Geostationary Satellite Image Using Machine Learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22319, https://doi.org/10.5194/egusphere-egu2020-22319, 2020.

EGU2020-22412 | Displays | OS4.5

Effect of the Ushant SST front on regional weather forecasting in the event of anticyclonic events

Francoise Orain, Marie-Noelle Bouin, Jean-Luc Redelsperger, and Valérie Garnier

The representation of the Ushant front in Meteo-France numerical models is not accurate. The aim of this study is to evaluate the impact of a better representation of this front derived from SST satellite observation data on the weather forecast in Brittany.

 

The study consisted in finding and selecting cases from 2016 to 2018 where the Ushant front was present in satellite SST analysis (high spatial and temporal resolution ) with differences in weather pattern between North and South Brittany. Then compare this to the operational Arome model (Meteo France non hydrostatic model).

Situations of disagreement between the model and the observations were selected. Some weather forecast simulations with Mesonh model (very close to Arome) were performed on these cases with a better definition of the Ushant front. We present some results.

 

 

How to cite: Orain, F., Bouin, M.-N., Redelsperger, J.-L., and Garnier, V.: Effect of the Ushant SST front on regional weather forecasting in the event of anticyclonic events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22412, https://doi.org/10.5194/egusphere-egu2020-22412, 2020.

OS4.7 – The Copernicus Marine Environment Monitoring Service (CMEMS)

EGU2020-10322 | Displays | OS4.7 | Highlight

Copernicus Marine Service: achievements, future challenges and long-term evolution

Pierre-Yves Le Traon

The Copernicus Marine Environment Monitoring Service (CMEMS) provides regular and systematic reference information on the physical state, variability and dynamics of the ocean, ice and marine ecosystems for the global ocean and the European regional seas.  The Copernicus Marine Service has run a successful initial phase over the past five years.  Operational capabilities have been demonstrated, user uptake and user base have been steadily increasing and service evolution activities have allowed regular improvements of the products and services provided to users.  CMEMS now serves a wide range of users (more than 21,000 subscribers are registered to the service) and applications (maritime safety, marine resources, coastal and marine environment, weather, seasonal forecast and climate).  An overview of CMEMS achievements will be given and the presentation will highlight the essential role of R&D activities.  CMEMS priorities and scientific challenges for Copernicus 2 (2021-2027) will then be discussed.   

How to cite: Le Traon, P.-Y.: Copernicus Marine Service: achievements, future challenges and long-term evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10322, https://doi.org/10.5194/egusphere-egu2020-10322, 2020.

EGU2020-7062 | Displays | OS4.7

WAVERYS : A CMEMS global wave reanalysis during the altimetry period

Stephane Law Chune, Lotfi Aouf, Alice Dalphinet, Bruno Levier, and Yann Drillet

As part of the Copernicus Marine Core service, WAVERYS is the multi-year wave reanalysis that aims to provide global wave data with a grid resolution of 1/5°. The wave reanalysis covers the period of 1993-2018 and provides 3-hourly classical integrated wave parameters describing the sea state at the ocean surface. The wave model used is the V4 version of the model MFWAM, which is driven by atmospheric forcing (winds and ice fraction) from ECMWF ERA5 reanalysis. This latter has showed a significant improvement regarding to the previous reanalysis ERA-Interim. WAVERYS includes the assimilation of altimeters wave data available during the period starting from Topex-Poseidon until Sentinel-3A missions. Directional wave spectra from Synthetic Aperture Radar (SAR) of Sentinel-1A and 1B missions are also assimilated. This is the first time that such directional wave spectra are used in a global wave reanalysis.

Further, WAVERYS uses a 3 hour surface current forcing provided by ocean reanalysis GLORYS12 implemented by Mercator-Ocean in the frame of Copernicus Marine Service with a grid resolution of 1/12°. The wave reanalysis is high skilled for ocean regions with dominant wave-currents interactions. Preliminary validation tests have shown improvement by 15% in scatter index for large scale high currents areas. This paper will give detailed characteristics of the wave system and will insist on the benefits of taking into account ocean currents and a physics calibrated for realistic swell propagation.

 

How to cite: Law Chune, S., Aouf, L., Dalphinet, A., Levier, B., and Drillet, Y.: WAVERYS : A CMEMS global wave reanalysis during the altimetry period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7062, https://doi.org/10.5194/egusphere-egu2020-7062, 2020.

EGU2020-19468 | Displays | OS4.7 | Highlight

Monitoring marine heatwaves in CMEMS ocean analysis systems

Eric de Boisseson

Since 2015, CMEMS has been providing near-real time and multi-year ocean analyses that describe both past and current ocean states. In the recent years, the increased frequency of marine heatwave events has raised the attention of the community. Strong and long-lasting events have been shown to have a significant impact on the marine ecosystems and industries. In the work presented here, recent cases of marine heatwaves have been analysed in the ECMWF ORAS5 reanalysis that is part of the CMEMS catalogue of multi-year products. Marine heatwaves are detected from time series of Sea Surface Temperature using a tool developed at CSIRO. Various aspect of the heatwaves are investigated in ORAS5 fields such as the strength and duration of the events and their propagation into the subsurface. The characteristics of recent heatwave events in the North Pacific and off New Zealand as captured in ORAS5 will also be discussed. Particular attention will be brought onto the Pacific Ocean 'Blob', the longest marine heatwave on record that lasted from 2014 to 2016. ORAS5 captured the ‘Blob’ and its propagation down the vertical column in coastal regions where the fishing industry usually strives. The extent of the ecological and economical impact of such an event is still felt to this day. The evolution of future marine heatwave events will be monitored in ORAS5. The predictability of these heat waves at monthly and seasonal range is under investigation.

How to cite: de Boisseson, E.: Monitoring marine heatwaves in CMEMS ocean analysis systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19468, https://doi.org/10.5194/egusphere-egu2020-19468, 2020.

EGU2020-5243 | Displays | OS4.7

Modelling the marine ecosystem of IBI European waters for CMEMS operational applications

Elodie Gutknecht, Guillaume Reffray, Alexandre Mignot, Tomasz Dabrowski, and Marcos Sotillo

As part of the Copernicus Marine Environment Monitoring Service (CMEMS), a physical-biogeochemical coupled model system has been developed to monitor and forecast the ocean dynamics and marine ecosystem of the European waters. The model domain, called Iberia-Biscay-Ireland (IBI), covers the North-East Atlantic Ocean from the Canary Islands to Iceland, including the North Sea and the Western Mediterranean. Based on NEMO-PISCES with a horizontal resolution of 1/36°, the CMEMS IBI coupled model has provided 7-day weekly ocean forecasts for CMEMS since April 2018. But prior to its operational launch, a pre-operational qualification simulation (2010-2016) has allowed assessing the model's capacity to reproduce the main biogeochemical and ecosystem features of the IBI area. 
The objective is to evaluate the capacity of the coupled model to reproduce the spatial distribution and seasonal dynamics of the main biogeochemical variables, using this 7-year qualification simulation. The model results are compared with available satellite estimates as well as in situ observations, with a focus on BGC-Argo floats. 
The evaluation confirms that this model system can be a useful tool to better understand the current state and changes in the marine biogeochemistry of European waters and can also provide key variables for developing indicators to monitor the health of marine ecosystems. 

How to cite: Gutknecht, E., Reffray, G., Mignot, A., Dabrowski, T., and Sotillo, M.: Modelling the marine ecosystem of IBI European waters for CMEMS operational applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5243, https://doi.org/10.5194/egusphere-egu2020-5243, 2020.

EGU2020-21903 | Displays | OS4.7

CMEMS Primary production from satellite remote sensing: spatial and temporal evolution and comparison with other products

Marine Bretagnon, Philippe Garnesson, and Antoine Mangin

Half of the global primary production is produced in the ocean by phytoplankton and the reaction of photosynthesis. For the marine environment, primary production is at the basis for the food web, by the supply of energy for higher trophic levels. Monitor primary production appears therefore to be a guideline to reach sustainable fisheries. In addition to its role on the trophic web, primary production is also important for its role on CO2 fluxes. Indeed, while phytoplankton creates matter from nutrients and CO2. The produced matter can be grazed by higher trophic levels or sink towards sediment. Amount of carbon sequestrated and exported out of the productive layer give some clues efficiencies of the oceanic biological carbon pump. Primary production is therefore important not only for economic resources, but also for climatic studies, to investigate if the ocean is a carbon sink or sources.

A strategy of algorithm validation / inter-comparison was used as part as the CMEMS project to identify most accurate primary production algorithm among the most used in the literature.

Primary production validation is based on the commonly used comparison with in situ data, as well as the frequency and the intensity of the annual bloom in different basin. Inter-comparison with model were performed at the basin scale of the Mediterranean Sea to assess the robustness and the consistency of different type of estimates.

Satellite estimate of primary production, as proposed by CMEMS, give now access to an archive of 21 years for user community, to investigate evolution of primary production at the global scale or in specific basin.

 

How to cite: Bretagnon, M., Garnesson, P., and Mangin, A.: CMEMS Primary production from satellite remote sensing: spatial and temporal evolution and comparison with other products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21903, https://doi.org/10.5194/egusphere-egu2020-21903, 2020.

EGU2020-8564 | Displays | OS4.7

Benefits of dynamically modelled river discharge input for ocean and coupled system.

Hao Zuo, Eric de Boisseson, Ervin Zsoter, Shaun Harrigan, Patricia de Rosnay, Fredrik Wetterhall, and Christel Prudhomme

River freshwater input is crucial in modelling global ocean. Most ocean models used in CMEMS services rely on climatological river discharge data with various deficiencies, which can lead to biased simulated ocean states. The Copernicus Emergency Management Service (CEMS) Global Flood Awareness System (GloFAS) provides state-of-the-art global flood forecasts and downstream river discharge. A GloFAS-ERA5 global river discharge reanalysis dataset has been produced using the same system, modelled by routing runoff from ECMWF’s (European Centre for Medium- Range Weather Forecasts) atmospheric reanalysis ERA5 via a river network. As a global gridded data set that covers from 1979 to near-real-time, GloFAS-ERA5 reanalysis can provide an improved and standardized input of land freshwater input for global, regional and coastal ocean models. Evaluation results suggest that the overall performance of this new river discharge reanalysis is reasonably good in general when verified against a global network of 1801 discharge observation stations. A new method has been developed for conversion of the GloFAS-ERA5 reanalysis data into land freshwater input for the NEMO ocean model. This method has been tested with a climatology of GloFAS-ERA5 river discharge. Compared to the DRAKKAR climatology of land freshwater input (BT06, hereafter) used by most CMEMS services, this new data set has an increased global mean value of ~1.33 Sv, but with reduced seasonal variations. Assessment of this GloFAS-ERA5 land freshwater input has been carried out with the operational ECMWF ocean analysis system-OCEAN5, driven by the same ERA5 atmospheric forcing. Evaluation of simulated ocean state against in-situ observations show improvements in regions affected by Amazon freshwater input when use GloFAS-ERA5 instead of BT06, by reducing a negative sea surface salinity bias in these regions. However, negative impact from switching to GloFAS-ERA5 land freshwater input is also visible in several regions, e.g. in the Maritime Continent and west coast of central America, which is associated with a large positive bias in the GloFAS-ERA5 river discharge at these regions. This issue can be mitigated by applying bias-correction to the GloFAS-ERA5 land freshwater input, and by adding extra vertical mixing in several affected regions that are close to the river mouth. Assessments of module simulated ocean Essential Climate Variables (ECVs) have been carried out to quantify the benefit of this realistic freshwater time series input. Improvements in climate signals like the Atlantic Meridional Overturning transports is also recorded.

How to cite: Zuo, H., de Boisseson, E., Zsoter, E., Harrigan, S., de Rosnay, P., Wetterhall, F., and Prudhomme, C.: Benefits of dynamically modelled river discharge input for ocean and coupled system., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8564, https://doi.org/10.5194/egusphere-egu2020-8564, 2020.

EGU2020-22004 | Displays | OS4.7

An adaptive optimal interpolation based on analog forecasting: application to SSH in the Gulf of Mexico

Yicun Zhen, Pierre Tandeo, Stephanie Leroux, Sammy Metref, Thierry Penduff, and Julien Le Sommer

Because of the irregular sampling pattern of raw altimeter data, many oceanographic applications rely on information from sea surface height (SSH) products gridded on regular grids where gaps have been filled with interpolation. Today, the operational SSH products are created using the simple, but robust, optimal interpolation (OI) method. If well tuned, the OI becomes computationally cheap and provides accurate results at low resolution. However, OI is not adapted to produce high resolution and high frequency maps of SSH. To improve the interpolation of SSH satellite observations, a data-driven approach was recently proposed: analog data assimilation (AnDA). AnDA adaptively chooses analog situations from a catalog of SSH scenes -- originating from numerical simulations or a large database of observations -- which allow the temporal propagation of physical features at different scales, while each observation is assimilated. In this article, we review the AnDA and OI algorithms and compare their skills in numerical experiments. The experiments are observing system simulation experiments (OSSE) on the Lorenz-63 system and on an SSH reconstruction problem in the Gulf of Mexico. The results show that AnDA, with no necessary tuning, produces comparable reconstructions as does OI with tuned parameters. Moreover, AnDA manages to reconstruct the signals at higher frequencies than OI. Finally, an important additional feature for any interpolation method is to be able to assess the quality of its reconstruction. This study shows that the standard deviation estimated by AnDA is flow-dependent, hence more informative on the reconstruction quality, than the one estimated by OI.

How to cite: Zhen, Y., Tandeo, P., Leroux, S., Metref, S., Penduff, T., and Le Sommer, J.: An adaptive optimal interpolation based on analog forecasting: application to SSH in the Gulf of Mexico, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22004, https://doi.org/10.5194/egusphere-egu2020-22004, 2020.

The ambition of CMEMS is to be the primary source of information for monitoring and prediction of the physical and bio-geochemical properties of the marine environment, both in the global deep ocean and in European coastal waters. This requires that the teams developing CMEMS identify and address several key scientific and technical challenges. Some challenges relate to the ocean model used to generate CMEMS products. For example to ensure that it simulates the most important aspects of the flow as faithfully as other ocean models and that it is able to perform efficiently on relevant High Performance Computer (HPC) systems. Some challenges relate to the data assimilation. For example to ensure that the ocean re-analyses are not compromised by problems caused by biases in the ocean model and to develop more effective collaboration within the team of data assimilation developers. Other challenges relate to biogeochemistry (BGC). For example to develop monitoring systems sufficient to support the validation and development of the BGC models, and suitable systems for joint assimilation of physical and BGC measurements. The presentation will focus on illustrating and discussing these challenges.

How to cite: Bell, M.: Challenges in ocean modelling and data assimilation for CMEMS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5680, https://doi.org/10.5194/egusphere-egu2020-5680, 2020.

EGU2020-20070 | Displays | OS4.7

Sources of uncertainty of Baltic Sea future projections

Markus Meier, Christian Dieterich, and Matthias Gröger

In an ensemble of regional scenarios for the Baltic Sea we analyzed the sources of uncertainty in climate indices and environmental quality indicators. The ensemble is based on 32 regionalized scenarios where four different external drivers have been varied. Climate is represented by four different Earth System Models (ESMs). Uncertain future greenhouse gas emissions are represented by two different Representative Concentration Pathways (RCPs). Two nutrient load scenarios, broadly equivalent to two Shared Socio-economic Pathways (SSPs), describe two distinct evolutions of the regional population development, agricultural practices and food demand and two scenarios for global mean sea level rise (GMSL) measure the impact of the water level on the biogeochemical cycle in the Baltic Sea. The volume averaged temperature increase at the end of the century relative to the reference period 1976-2005 is 1.3 to 2.2 K (RCP 4.5) and 2.9 to 4.2 K (RCP 8.5). Averaged salinity changes by -2.1 and +0.2 g/kg (RCP 4.5) and -3.2 and -0.2 g/kg (RCP 8.5). For temperature, uncertainties before 2080 are dominated by natural variability and ESM biases. After 2080 the largest source of uncertainty is related to the unknown greenhouse gas concentrations. As expected, uncertainties related to either SLR or nutrient loads are negligible. For salinity, the dominating source of uncertainty during the entire 21st century is explained by the biases of the ESMs. However, natural variability and, in particular by the end of the century, uncertainties due to unknown greenhouse gas concentrations and sea level rises are important as well. For hypoxic area, uncertainties before 2040 are dominated by ESM biases. After 2040 the largest source of uncertainty is related to the unknown nutrient loads (SSPs). However, ESM biases, natural variability, unknown greenhouse gas concentrations and unknown sea level rises play an important role as well. Hence, the predictability of hypoxic area on long time scales requires accurate knowledge of various drivers and accurate quality of ESMs.

 

How to cite: Meier, M., Dieterich, C., and Gröger, M.: Sources of uncertainty of Baltic Sea future projections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20070, https://doi.org/10.5194/egusphere-egu2020-20070, 2020.

EGU2020-8657 | Displays | OS4.7

Impact of wave-induced sea ice fragmentation on sea ice dynamics in the MIZ

Guillaume Boutin, Timothy Williams, Pierre Rampal, Einar Olason, and Camille Lique

The decrease in Arctic sea ice extent is associated with an increase of the area where sea ice and open ocean interact, commonly referred to as the Marginal Ice Zone (MIZ). In this area, sea ice is particularly exposed to waves that can penetrate over tens to hundreds of kilometres into the ice cover. Waves are known to play a major role in the fragmentation of sea ice in the MIZ, and the interactions between wave-induced sea ice fragmentation and lateral melting have received particular attention in recent years. The impact of this fragmentation on sea ice dynamics, however, remains mostly unknown, although it is thought that fragmented sea ice experiences less resistance to deformation than pack ice. In this presentation, we will introduce a new coupled framework involving the spectral wave model WAVEWATCH III and the sea ice model neXtSIM, which includes a Maxwell-Elasto Brittle rheology. We use this coupled modelling system to investigate the potential impact of wave-induced sea ice fragmentation on sea ice dynamics. Focusing on the Barents Sea, we find that the decrease of the internal stress of sea ice resulting from its fragmentation by waves results in a more dynamical MIZ, in particular in areas where sea ice is compact. Sea ice drift is enhanced for both on-ice and off-ice wind conditions. Our results stress the importance of considering wave–sea-ice interactions for forecast applications. They also suggest that waves likely modulate the area of sea ice that is advected away from the pack by ocean (sub-)mesoscale eddies near the ice edge, potentially contributing to the observed past, current and future sea ice cover decline in the Arctic. 

How to cite: Boutin, G., Williams, T., Rampal, P., Olason, E., and Lique, C.: Impact of wave-induced sea ice fragmentation on sea ice dynamics in the MIZ, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8657, https://doi.org/10.5194/egusphere-egu2020-8657, 2020.

EGU2020-10281 | Displays | OS4.7

Simulating bio-optical properties in the Mediterranean Sea.

Paolo Lazzari, Eva Alvarez, Elena Terzic, Stefano Salon, Emanuele Organelli, Fabrizio D'Ortenzio, and Vincenzo Vellucci

We present the results of a series of simulations performed by a multi-spectral bio-optical model developed in the framework of the BIOPTIMOD Service Evolution project for the Copernicus Marine Environment Monitoring System (CMEMS-SE). In this research, we integrate the CMEMS Mediterranean Sea biogeochemical model MedBFM (multi-stoichiometric, phytoplankton functional types -PFT- based) with a bio-optical model able to resolve light propagation along the water column with 25 nm resolution in the visible range. Recent optical data streams provided by novel observations platforms, such as the Biogeochemical Argo floats (BGC-Argo) and the multi-spectral satellite sensors (ESA-CCI and Sentinel-OLCI), are used for the model validation. Our approach aims to improve the quality and reduce the uncertainty of paramount CMEMS biogeochemical products such as phytoplankton biomass and primary production.  

We present: 1) the optical dataset gathered from BGC-Argo floats and satellite sensors; 2) the multi-spectral upgrade of the bio-optical model including specific PFT optical properties  and CDOM formulation; 3) the assessment of the new bio-optical model within the CMEMS quality framework with particular reference to Remote Sensing Reflectance (Rrs) data and to light attenuation as inferred from BGC-Argo floats.

In particular, the analysis of Rrs spatial and temporal variability allows to evaluate the skill of different parameterizations for PFT (e.g. photochemical efficiency) and CDOM dynamics (e.g. photobleaching rate), using an ensemble of multi-annual 3D simulations.

The role of the CDOM deep inventory and river discharge in modulating light attenuation is also evaluated. Results indicates that the novel bio-optical model allows to reconstruct the West-East DCM gradient as a self-emergent property.

Major impacts of the project, potentially strategic for CMEMS users, involve: the improvement of CMEMS biogeochemical products quality, the development of new optics-related biogeochemical products for CMEMS, and the strengthening of the scientific and technological links with the new generation of marine bio-optical sensors, which may also support a stronger collaboration between modelling, remote sensing and experimental communities.



How to cite: Lazzari, P., Alvarez, E., Terzic, E., Salon, S., Organelli, E., D'Ortenzio, F., and Vellucci, V.: Simulating bio-optical properties in the Mediterranean Sea., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10281, https://doi.org/10.5194/egusphere-egu2020-10281, 2020.

EGU2020-5741 | Displays | OS4.7

Gathering knowledge on mesopelagic ecosystems: insights from a parsimonious modelling approach

Anna Conchon, Olivier Titaud, Inna Senina, Beatriz Calmettes, Audrey Delpech, and Patrick Lehodey

SEAPODYM-LMTL is the Lower (zooplankton) and Mid (micronekton) Trophic levels model of the Spatial Ecosystem And POpulation DYnamic Modeling framework. Currently, there is one zooplankton and 6 micronekton functional groups defined according to their vertical behavior and development times. The model is global and spatially explicit with transport described through a system of advection-diffusion equations. The vertical dimension is simplified into three layers -- epipelagic, upper and lower mesopelagic -- defined relatively to the euphotic depth. There are three vertically migrant and three non-migrant functional groups. The model is parsimonious with only a few parameters (6 for the zooplankton and 11 for the micronekton) that control the energy transfer efficiency from the primary production and the mortality and time of development that are linked to the water temperature. A data assimilation framework has been implemented to estimate those parameters.  We present briefly the latest results and future challenges of this model. They include the validation of vertical layer boundaries, the first zooplankton and micronekton parameters estimation using existing biomass observations, and the developments needed to use large global datasets of acoustic data. 

How to cite: Conchon, A., Titaud, O., Senina, I., Calmettes, B., Delpech, A., and Lehodey, P.: Gathering knowledge on mesopelagic ecosystems: insights from a parsimonious modelling approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5741, https://doi.org/10.5194/egusphere-egu2020-5741, 2020.

EGU2020-2714 | Displays | OS4.7

Earth Observation services for Wild Fisheries, Oystergrounds Restoration and Bivalve Mariculture along European Coasts*

Luis Rodriguez Galvez, Ghada El Serafy, Sonja Wanke, Daniel Twigt, and Nicky Villars

Sea related activities are set to increase and the growth in food production from sea enhancing global food security is already a reality. However, this growth must be aligned with increasing environmental constraints as well as complying and restoring regulations and frameworks. This requires the adoption of improved and efficient behaviors based on wider incorporation of available information and knowledge from the industry and citizens alike. Marine and coastal managers must make decisions to maintain the social, economic, and ecological health of marine and coastal areas in coastal and nearshore areas and to operate, plan and manage their activities at sea. The European funded FORCAST project represents a step forward in this direction by bringing the coastal water quality and met-ocean information closer to the target sectors: wild fisheries, oystergrounds restoration, and bivalve mariculture. FORCOAST will develop, test and demonstrate, in operational mode, novel Copernicus-based downstream information services that will incorporate and combine Copernicus Marine Environment Monitoring Service (CMEMS), Copernicus Land Monitoring Service (CLMS) and Climate Change Monitoring Service (CMS), local monitoring data and advanced modelling in the service. FORCOAST will provide consistent high resolution data products for coastal applications, based on a standardized data processing scheme. Furthermore, FORCOAST will make use DIAS which will help to develop the data access and cloud processing service. FORCOAST will provide those services in eight pilot service uptake sites covering five different regional waters (North Sea, Baltic Sea, Mediterranean Sea, Black Sea and the coastal Atlantic Ocean). The outcome of FORCAST is a novel commercial service that will provide Copernicus-based downstream information coastal services to a variety of stakeholders, which will result in an operation, planning and management improvement of different marine activities in the sectors of wild fisheries and aquaculture, having an economic and societal positive effect on the involved parties.

*This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870465

How to cite: Rodriguez Galvez, L., El Serafy, G., Wanke, S., Twigt, D., and Villars, N.: Earth Observation services for Wild Fisheries, Oystergrounds Restoration and Bivalve Mariculture along European Coasts*, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2714, https://doi.org/10.5194/egusphere-egu2020-2714, 2020.

EGU2020-13029 | Displays | OS4.7 | Highlight

Marine Litter Drift Monitoring (Forecast and Hindcast) in the Channel and the North Atlantic

Anne Vallette, Fatimatou Coulibaly, and Stephen Emsley

Meteorological events, such as storms and/or gale force winds, act as triggers to influx of macro litter into the hydrological cycle via run off from land into rivers. These rivers discharge into the sea and the marine litter is then transported through the region by currents and wind either becoming entrained in the sea, possibly sinking and/or disintegrating into micro marine litter or ending up being stranded at the coast then washed back ashore or flown on to the land. Thanks to a Copernicus Marine Environment Monitoring Service (CMEMS) grant, ARGANS Ltd has developed a web-based service, called Litter-TEP, that aims to track marine litter from the source. It uses a parametric model of riverine macro litter discharge, to seed drift models of the NE Atlantic Shelf Region (OSPAR II/III), providing to end-users a 5-day running forecast of macro-litter density in the sea, potential beach stranding at the coast and, inversely, where a beach litter event is identified to provide the likelihood of where the litter entered the sea. In order to determine drift trajectories, we use ocean current, wave and wind forecasts from Copernicus Marine Service high quality analysis and forecast products for the European North West Shelf seas. The main issues which have been identified, and for which we perform additional R&D, are the following: a) source’s modelling and estimation of volume of litter introduced to the sea, b) litter’s types for which the drift model should be adapted, and c) the spatial resolution of models in the littoral area (nearshore) vs. offshore. In fact, for the beaching & refloating models, we need of a bathymetry at the scale of 1/3000 and a coastal cartography at 1/1000 to obtain the beach profile, then calculate the runoff on the beach, the rip currents, etc. The next enhancement, driven by users’ requirements, is to improve the land discharge model vide collection of litter seeings with citizen crowdsourcing apps, and records of beach litter surveys, and beach cleaning campaigns. Another improvement, in the mid-term, targets the discharge models, using refined hydrologic schemes for the watersheds, and better estimates of habitats (rural, urban, industrial, …).  ARGANS Ltd service is the next-generation tool for planning beach cleaning and helping local authorities to track back the trash to their sources, leading the fight against litter pollution and for improvement of the river water quality. 

How to cite: Vallette, A., Coulibaly, F., and Emsley, S.: Marine Litter Drift Monitoring (Forecast and Hindcast) in the Channel and the North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13029, https://doi.org/10.5194/egusphere-egu2020-13029, 2020.

Oceanic fronts, as important mesoscale phenomena, are transition zones between two water mass with different hydrological features, which has important significances in various fields, such as fishery, environment protection and marine military. In order to effectively obtain the ocean front information, a gradient-based Canny edge detection algorithm is applied to detect the ocean front, and post-processing procedure specifically designed for high resolution hindcast and reanalysis is utilized to extract primary ocean fronts, ensuring the balance of frontal continuity and positioning accuracy. Based on CMEMS reanlaysis and satellite SST data and regional model hindcast from NMEFC in China, a long-term intercomparison and validation is performed to evaluate the frontal detection method and model behavior in aspect of mesoscale forecasting/hindcasting. Frontal statistical product include mean status, seasonal and interannual analysis are utilized to intercompare. We also develop ‘CLASS 4’ validation metric to validate daily results in different datasets. Further more, forecast error, bias and skill of operational ocean front product are also shown for potential users.

How to cite: Ren, S., Zhu, X., and Marie, D.: Intercomparison and validation of detected SST fronts based on CMEMS high-resolution reanalysis data and satellite observations in the South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2521, https://doi.org/10.5194/egusphere-egu2020-2521, 2020.

The aim of this work is to obtain high-resolution values of sea surface salinity (SSS) and temperature (SST) in the global ocean by using raw satellite data (i.e., without any band data pre-processing or atmospheric correction). Sentinel-2 Level 1-C Top of Atmosphere (TOA) reflectance data is used to obtain accurate SSS and SST information. A deep neural network is built to link the band information with in situ data from different buoys, vessels, drifters, and other platforms around the world. The neural network used in this paper includes shortcuts, providing an improved performance compared with the equivalent feed-forward architecture. The in situ information used as input for the network has been obtained from the Copernicus Marine In situ Service. Sentinel-2 platform-centred band data has been processed using Google Earth Engine in areas of 100 m x 100 m. Accurate salinity values are estimated for the first time independently of temperature. Salinity results rely only on direct satellite observations, although it presented a clear dependency on temperature ranges. Results show the neural network has good interpolation and extrapolation capabilities. Test results present correlation coefficients of 82% and 84% for salinity and temperature, respectively. The most common error for both SST and SSS is 0.4 C and 0.4 PSU. The sensitivity analysis shows that outliers are present in areas where the number of observations is very low. The network is finally applied over a complete Sentinel-2 tile, presenting sensible patterns for river-sea interaction, as well as seasonal variations. The methodology presented here is relevant for detailed coastal and oceanographic applications, reducing the time for data pre-processing, and it is applicable to a wide range of satellites, as the information is directly obtained from TOA data.

How to cite: Medina-Lopez, E.: High-resolution sea surface salinity and temperature in coastal areas from Sentinel-2 and Copernicus Marine in situ data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3465, https://doi.org/10.5194/egusphere-egu2020-3465, 2020.

EGU2020-7234 | Displays | OS4.7

CURAE – bridging the gap between regional CMEMS forecasts and coastal high-resolution applications

Marc Mestres, Johannes Pein, Manuel Espino, Johannes Schulz-Stellenfleth, Lars Boye Hansen, Joanna Staneva, and Agustín Sánchez-Arcilla

The sustainable management of coastal areas with limited natural resources and subject to high anthropogenic pressures, based on complete forecast and analysis systems, is one of the main concerns of coastal authorities. Paradoxically, current oceanographic operational predictions are often insufficient to provide a good description of these regions due to low resolution/accuracy, the omission of relevant land-sea interactions, and the inadequate inclusion in the models of physical processes specific to the coastal fringe.

The CURAE project bridges this gap between the regional scales and the local coastal needs by establishing a 2-way connection between the existing CMEMS products and an innovative coastal information system (required by coastal water managers and littoral users alike) that considers hydro-morphodynamic interactions in an operational manner. The paper will present a new set of robust downscaling and coupling tools that have been developed to enhance the coastal dimension of present CMEMS products. The main advances refer to coastal processes not commonly incorporated in oceanographic predictions and that interact with the shelf sea, including the continental discharge in terms of water, sediment and nutrient fluxes.

The project is focused on two hydrodynamically contrasting sites with different uses (aquaculture and dredging) that demonstrate the added value and potential of CMEMS for coastal applications. The first case is the semi-enclosed microtidal Fangar Bay in the Ebro Delta (Spanish Mediterranean), in which bivalve farming coexists with tourism and nutrient- and sediment-laden freshwater discharges from rice fields in a hydrodynamically weak environment interacting with active domain geometries. The second study case is the macrotidal Wadden Sea/German Bight estuaries area, with concentrated plus distributed land discharge and significant dredging activities required for port operations. To ensure that the tools and conclusions derived from the project are as generic as possible, different numerical approaches (structured vs. unstructured grids) have been applied at each site and will be compared and discussed in the paper. The obtained results illustrate the benefits of quantitative forecasts for the healthy functioning of coastal systems, while proving the potential of CMEMS for supporting sustainable interventions in the coastal zone. The obtained advances are encouraging from scientific and application standpoints and underpin the need for a high-resolution coastal extension of CMEMS.

How to cite: Mestres, M., Pein, J., Espino, M., Schulz-Stellenfleth, J., Hansen, L. B., Staneva, J., and Sánchez-Arcilla, A.: CURAE – bridging the gap between regional CMEMS forecasts and coastal high-resolution applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7234, https://doi.org/10.5194/egusphere-egu2020-7234, 2020.

EGU2020-7799 | Displays | OS4.7

High-resolution model Verification Evaluation (HiVE). Part 2: Using object-based methods for the evaluation of algal blooms

Marion Mittermaier, Rachel North, Christine Pequignet, and Jan Maksymczuk

HiVE is a CMEMS funded collaboration between the atmospheric Numerical Weather Prediction (NWP) verification and the ocean community within the Met Office, aimed at demonstrating the use of spatial verification methods originally developed for the evaluation of high-resolution NWP forecasts, to CMEMS ocean model forecast products. Spatial verification methods provide more scale appropriate ways to better assess forecast characteristics and accuracy of km-scale forecasts, where the detail looks realistic but may not be in the right place at the right time. As a result, it can be the case that coarser resolution forecasts verify better (e.g. lower root-mean-square-error) than the higher resolution forecast. In this instance the smoothness of the coarser resolution forecast is rewarded, though the higher-resolution forecast may be better. The project utilised open source code library known as Model Evaluation Tools (MET) developed at the US National Center for Atmospheric Research (NCAR).

 

This project saw, for the first time, the application of spatial verification methods to sub-10 km resolution ocean model forecasts. The project consisted of two parts. Part 1 is described in the companion poster to this one. Part 2 describes the skill of CMEMS products for forecasting events or features of interest such as algal blooms.  

 

The Method for Object-based Diagnostic Evaluation (MODE) and the time dimension version MODE Time Domain (MTD) were applied to daily mean chlorophyll forecasts for the European North West Shelf from the FOAM-ERSEM model on the AMM7 grid. The forecasts are produced from a “cold start”, i.e. no data assimilation of biological variables. Here the entire 2019 algal bloom season was analysed to understand: intensity and spatial (area) biases; location and timing errors. Forecasts were compared to the CMEMS daily cloud free (L4) multi-sensor chlorophyll-a product. 

 

It has been found that there are large differences between forecast and observed concentrations of chlorophyll. This has meant that a quantile mapping approach for removing the bias was necessary before analysing the spatial properties of the forecast. Despite this the model still produces areas of chlorophyll which are too large compared to the observed. The model often produces areas of enhanced chlorophyll in approximately the right locations but the forecast and observed areas are rarely collocated and/or overlapping. Finally, the temporal analysis shows that the model struggled to get the onset of the season (being close to a month too late), but once the model picked up the signal there was better correspondence between the observed and forecast chlorophyll peaks for the remainder of the season. There was very little variation in forecast performance with lead time, suggesting that chlorophyll is a very slowly varying quantity.  

 

Comparing an analysis which included the assimilation of observed chlorophyll shows that it is much closer to the observed L4 product than the non-biological assimilative analysis. It must be concluded that if the forecast were started from a DA analysis that included chlorophyll, it would lead to forecasts with less bias, and possibly a better detection of the onset of the bloom.  

 

How to cite: Mittermaier, M., North, R., Pequignet, C., and Maksymczuk, J.: High-resolution model Verification Evaluation (HiVE). Part 2: Using object-based methods for the evaluation of algal blooms , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7799, https://doi.org/10.5194/egusphere-egu2020-7799, 2020.

EGU2020-8083 | Displays | OS4.7

The Copernicus Marine Environment Monitoring Service as a platform to map marine ecosystem services: a Lithuanian case study

Miguel Inácio, Marius Kalinauskas, Katarzyna Miksa, Eduardo Gomes, and Paulo Pereira

Oceans and seas have always played an important role in supporting human wellbeing through the deliverance marine ecosystem services (MES). Nevertheless, the anthropogenic driven environmental degradation coupled with changes in socio-economic dynamics affected the capacity to deliver MES in quantity and quality. While it is of the utmost importance and need to map and assess MES, data deficiencies, data standardization, lack of knowledge on the functioning of multiple MES and poor spatial and temporal coverage, hinder its operationalization. The objective of this work was to test the applicability of databases and platforms like the Copernicus Marine Environment Monitoring Service (CMEMS) to be used as a unified point to map and assess MES in Lithuanian marine area. To map different indicators of MES such as wave weight and direction as well as chlorophyll-a concentration, data was extracted from physical and biogeochemical model outputs within CMEMS, covering the whole extent of the Lithuanian Exclusive Economic Zone. From a user perspective, the use of CMEMS to map and assess MES, allows: (1) to overcome complex challenges such as ecological modelling, utilizing outputs directly; (2) to cover spatial and temporal extent in areas where information is scarce and (3) to have vertically resolved data, important for the understanding and mapping of MES. In this perspective, CMEMS plays and will potentially play a higher role towards the operationalization of MES, contributing to better and more informed decision in the sphere of marine environmental management.

This work has received funding from the European Social Fund project Lithuanian National Ecosystem Services Assessment and Mapping (LINESAM) No. 09.3.3-LMTK-712-01-0104 under grant agreement with the Research Council of Lithuania (LMTLT).

How to cite: Inácio, M., Kalinauskas, M., Miksa, K., Gomes, E., and Pereira, P.: The Copernicus Marine Environment Monitoring Service as a platform to map marine ecosystem services: a Lithuanian case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8083, https://doi.org/10.5194/egusphere-egu2020-8083, 2020.

EGU2020-8626 | Displays | OS4.7 | Highlight

Ocean reporting of the Copernicus Marine Environment Monitoring Service

Karina von Schuckmann and Pierre-Yves Le Traon

The Copernicus Marine Environment Monitoring Service (CMEMS) ocean state-of-the-art ocean reporting for the global ocean and European seas is part of the production center service elements in order to establish a unique reference of value-added expert information at a regular frequency. This is achieved through two principal activities:

  1. Annual release of the peer-reviewed CMEMS Ocean State Report containing a state-of-the-art value-added synthesis of the ocean state, variability and change from the past to present
  2. Ocean Monitoring Indicators and related operational framework on the CMEMS web portal. In particular, CMEMS has developed several indicators based on global or regional ocean reanalyses. For a series of indicators, consistency estimates are available, based on a multiproduct approach inherited from CLIVAR/GODAEIV-TT ORA IP.

This activity is aiming to reach a wide audience from the scientific community, over climate and environmental service and agencies, environmental reporting bodies, decision maker to the general public. Currently, the ocean state report activity is in its 5th cycle, and a huge number of indicators have been made freely available via the CMEMS web portal, including numerical data, scientific and quality context and product documentation. We will give here an overview on the CMEMS ocean reporting activity, highlight main outcomes, and introduce future plans and developments.

How to cite: von Schuckmann, K. and Le Traon, P.-Y.: Ocean reporting of the Copernicus Marine Environment Monitoring Service, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8626, https://doi.org/10.5194/egusphere-egu2020-8626, 2020.

EGU2020-8681 | Displays | OS4.7

High-resolution model Verification Evaluation (HiVE). Part 1: Using neighbourhood techniques for the assessment of ocean model forecast skill

Jan Maksymczuk, Ric Crocker, Marion Mittermaier, and Christine Pequignet

HiVE is a CMEMS funded collaboration between the atmospheric Numerical Weather Prediction (NWP) verification and the ocean community within the Met Office, aimed at demonstrating the use of spatial verification methods originally developed for the evaluation of high-resolution NWP forecasts, with CMEMS ocean model forecast products. Spatial verification methods provide more scale appropriate ways to better assess forecast characteristics and accuracy of km-scale forecasts, where the detail looks realistic but may not be in the right place at the right time. As a result, it can be the case that coarser resolution forecasts verify better (e.g. lower root-mean-square-error) than the higher resolution forecast. In this instance the smoothness of the coarser resolution forecast is rewarded, though the higher-resolution forecast may be better. The project utilised open source code library known as Model Evaluation Toolkit (MET) developed at the US National Center for Atmospheric Research. 

 

This project saw, for the first time, the application of spatial verification methods to sub-10 km resolution ocean model forecasts. The project consisted of two parts. Part 1 describes an assessment of the forecast skill for SST of CMEMS model configurations at observing locations using an approach called HiRA (High Resolution Assessment). Part 2 is described in the companion poster to this one.  

 

HiRA is a single-observation-forecast-neighbourhood-type method which makes use of commonly used ensemble verification metrics such as the Brier Score (BS) and the Continuous Ranked Probability Score (CRPS). In this instance all model grid points within a predefined neighbourhood of the observing location are considered equi-probable outcomes (or pseudo-ensemble members) at the observing location. The technique allows for an inter-comparison of models with different grid resolutions as well as between deterministic and probabilistic forecasts in an equitable and consistent way. In this work it has been applied to the CMEMS products delivered from the AMM7 (~7km) and AMM15 (~1.5km) model configurations for the European North West Shelf that are provided by the Met Office. 

 

It has been found that when neighbourhoods of equivalent extent are compared for both configurations it is possible to show improved forecast skill for SST for the higher resolution AMM15 both on- and off-shelf, which has been difficult to demonstrate previously using traditional metrics. Forecast skill generally degrades with increasing lead time for both configurations, with the off-shelf results for the higher resolution model showing increasing benefits over the coarser configuration. 

How to cite: Maksymczuk, J., Crocker, R., Mittermaier, M., and Pequignet, C.: High-resolution model Verification Evaluation (HiVE). Part 1: Using neighbourhood techniques for the assessment of ocean model forecast skill , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8681, https://doi.org/10.5194/egusphere-egu2020-8681, 2020.

EGU2020-8804 | Displays | OS4.7

A new version of the IBI near real time system for November 2020: what will be changed?

Guillaume Reffray, Mounir Benkiran, Bruno Levier, Elodie Gutknecht, Roland Aznar, Karen Guihou, and Marcos Garcia-Sotillo

The IBI (Iberian-Biscay-Ireland) MFC (Monitoring and Forecasting Centre) is in charge of providing ocean forecasts for the IBI region in the framework of the Copernicus Marine Environment Monitoring Service (CMEMS). To do so, a coupled model system is operated daily to monitor and forecast the ocean dynamics and marine ecosystem. The domain of the model is a subset of the ORCA grid at the resolution of 1/36° and covers the North-East Atlantic Ocean from the Canary Islands to Iceland, including the North Sea and the Western Mediterranean.

The IBI system is operated since 2010 and regularly released. The coupled model is based on NEMO-PISCES. A data assimilation method based on SEEK Kalman filter is applied to the physical component. This coupled system with data assimilation has been operational since April 2018, providing 5-day physical forecasts on a daily basis and 7-day ecosystem and carbon forecasts on a weekly basis.

A new release of this system is planned for November 2020, and includes a lot of changes about the physical NEMO model especially on its vertical component (turbulence, solar penetration, bulk formulaes, ...), a new tuning for the data assimilation method and a new CMEMS global biogeochemical system to initialise and force at the open boundaries of the IBI system. The objective is to summarize these changes and to illustrate their impacts by confronting the model results to in-situ data profiles, SST satellites products, BGC-Argo floats ...

How to cite: Reffray, G., Benkiran, M., Levier, B., Gutknecht, E., Aznar, R., Guihou, K., and Garcia-Sotillo, M.: A new version of the IBI near real time system for November 2020: what will be changed?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8804, https://doi.org/10.5194/egusphere-egu2020-8804, 2020.

EGU2020-9016 | Displays | OS4.7

Zooplankton and Micronekton products from the CMEMS Catalogue for better monitoring of Marine Resources and Protected Species

Patrick Lehodey, Olivier Titaud, Anna Conchon, inna Senina, and Jacques Stum

Since 2019, the Copernicus Marine portfolio is providing a new model-based product on Zooplankton and Micronekton, representing the low and mid-trophic levels (LMTL) of the ocean food chain (Global Ocean low and mid trophic levels biomass multi year product: ). Zooplankton are organisms in the size range of less than 1 to a few millimeters (e.g., copepods) constituting prey of all fish larvae and small pelagic fish species (e.g herring, sardines and anchovies). Micronekton are also relatively small but actively swimming organisms such as crustaceans, fish, and cephalopods that are typically about 1 to 10 centimeters in size. Micronekton are prey of large fish and other oceanic predators. They are also predators of fish larvae. Therefore, zooplankton and micronekton distributions are key explanatory variables to understand fish recruitment mechanisms, individual behaviour of satellite tracked individuals of protected species (e.g., marine turtles, seabirds, sharks and marine mammals), and dynamics of large populations of fish species either targeted by fisheries (mackerel, tuna, swordfish, etc.) or strictly controlled in by-catch (e.g., bluefin tuna or sharks). The product includes 2-dimensional (latitude-longitude) maps of biomass for zooplankton and groups of micronekton on a weekly basis from 1998 to 2018 at a 25 km horizontal resolution. It is a model-based estimation of micronekton biomass, relying on the most possible realistic forcings (temperature, currents and primary production) assimilating satellite and in situ data. Continuous progress in the development and validation of these new models and products will help to reduce our knowledge gaps and will also support the Science & Climate sector as zooplankton and micronekton are key to better quantifying the carbon uptake and storage in the ocean, known as the “biological carbon pump”.

How to cite: Lehodey, P., Titaud, O., Conchon, A., Senina, I., and Stum, J.: Zooplankton and Micronekton products from the CMEMS Catalogue for better monitoring of Marine Resources and Protected Species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9016, https://doi.org/10.5194/egusphere-egu2020-9016, 2020.

EGU2020-9136 | Displays | OS4.7

The neXtSIM-F sea ice forecasting platform

Timothy Williams, Anton Korosov, Pierre Rampal, and Einar Olason

The neXtSIM-F forecast system consists of a stand-alone sea ice model, neXtSIM, forced by the TOPAZ ocean forecast and the ECMWF atmospheric forecast, combined with daily data assimilation.

neXtSIM is a novel sea ice model which is able to reproduce sea ice deformation properties and statistics, such as spatial localisation and temporal intermittency,
even at relatively low resolutions. For our forecast we run it at 10km resolution, over a pan-Arctic domain. We assimilate OSISAF SSMI and AMSR2 sea ice concentration products and the SMOS sea ice thickness product by modifying the initial conditions daily and adding a compensating heat flux to prevent removed ice growing back too quickly. 

We present an evaluation of the platform over the period from November 2018 to present, looking at sea ice drift and concentration and extent, and thin ice thickness.

neXtSIM-F is scheduled to become part of the CMEMS Arctic Marine Forecast Center in June 2020.

How to cite: Williams, T., Korosov, A., Rampal, P., and Olason, E.: The neXtSIM-F sea ice forecasting platform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9136, https://doi.org/10.5194/egusphere-egu2020-9136, 2020.

EGU2020-9400 | Displays | OS4.7

A baroclinic tidal forecasting model for the Mediterranean Sea - First validation results

Anna Chiara Goglio, Emanuela Clementi, Massimiliano Drudi, Alessandro Grandi, Rita Lecci, Valentina Agresti, Simona Masina, Giovanni Coppini, and Nadia Pinardi

In the framework of the Copernicus Marine Environment Monitoring Service (CMEMS) Mediterranean

Analysis and Forecasting Physical System (MedFS), a specific modeling upgrade has been carried out

by including the main lunisolar tides.

Mediterranean tides, even if characterized by small amplitudes, play an important role on the dynamics

of the Mediterranean sea and the introduction of tides in the hydrodynamic numerical model simulations

represent the first step in the development of a numerical forecasting model that considers explicitly the

tidal dynamics and the mesoscales.

MedFS is an operational system that produces weekly analysis and daily 10-days forecasts of the main

physical fields with a resolution of around 4.5km over the whole Mediterranean basin including the

Atlantic Ocean adjacent area (Clementi et al., 2018).

Baroclinic high resolution numerical experiments have been performed including a tidal potential and

forcing the model at the Atlantic boundaries with tidal elevation downscaled from a global model

FES2014 and tidal velocity derived from the TUGOm (http://sirocco.omp.obsmip.

fr/ocean_models/tugo) ocean hydrodynamic model. The experiments have been carried out

including the 8 most relevant tidal constituents in the Mediterranean Sea, namely M2, S2, K1, O1, K2,

N2, P1 and Q1.

In this work, first results of baroclinic tidal model experiments are presented together with their

validation with respect to insitu and satellite data as well as comparing with available literature studies.

In particular the harmonic analysis of tidal amplitudes and phases highlight the model ability to

correctly represent the tide gauges observations in the whole basin and in the areas of large tidal signal.

How to cite: Goglio, A. C., Clementi, E., Drudi, M., Grandi, A., Lecci, R., Agresti, V., Masina, S., Coppini, G., and Pinardi, N.: A baroclinic tidal forecasting model for the Mediterranean Sea - First validation results , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9400, https://doi.org/10.5194/egusphere-egu2020-9400, 2020.

EGU2020-9873 | Displays | OS4.7

Validation of the CMEMS-IBI wave model with data assimilation in a high resolution regional configuration.

Malek Ghantous, Lotfi Aouf, Alice Dalphinet, Cristina Toledano, Lorea García San Martín, Ernesto Barrera, Pablo Lorente, and Marcos García Sotillo

One of the challenges of the Iberia-Biscay-Ireland (IBI) Monitoring Forecasting Centre in CMEMS phase 2 is the implementation of the assimilation of altimeter wave data in the wave forecast system.  In this work we explored the impact of the assimilation of altimeter wave data in the IBI domain.  We ran the Météo France version of the WAM wave model (MFWAM) in the IBI domain for 2018 and 2019, with data assimilated from the Jason 2 and 3, Saral, Cryosat 2 and Sentinel 3 altimeters.  This high-resolution (0.05 degree) configuration was forced by 0.05 degree ECMWF winds, and boundary conditions were provided by a 0.1 degree global model run.  We also included refraction from currents generated with the NEMO-IBI ocean circulation model.  We present results with and without wave–current interactions.  Validation against both buoy data and the HaiYang 2 altimeter shows that the assimilation of data leads to a marked reduction in scatter index and model bias compared to the run without data assimilation; the gains from including currents meanwhile are modest.  

The data assimilation scheme presently implemented in MFWAM uses an optimal interpolation algorithm where constant model and observational errors are assumed.  To add some sophistication, we experimented with non-constant background errors derived from a model ensemble.  Though the effect was small, the method suggests a way to improve the data assimilation performance without substantially altering the algorithm.

How to cite: Ghantous, M., Aouf, L., Dalphinet, A., Toledano, C., García San Martín, L., Barrera, E., Lorente, P., and García Sotillo, M.: Validation of the CMEMS-IBI wave model with data assimilation in a high resolution regional configuration., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9873, https://doi.org/10.5194/egusphere-egu2020-9873, 2020.

EGU2020-13293 | Displays | OS4.7

Assessment of the CMEMS global biogeochemical forecasting operational system, with assimilation of Ocean Colour data

Julien Lamouroux, Alexandre Mignot, Coralie Perruche, Giovanni Ruggiero, Julien Paul, Charles-Emmanuel Testut, Jean-Michel Lellouche, and Yann Drillet

The operational production of data-assimilated biogeochemical state of the ocean is one of the challenging core projects of the Copernicus Marine Environment Monitoring Service (hereafter CMEMS). In that framework, Mercator Ocean International is in charge of improving the realism of its global 1⁄4° coupled physical-biogeochemical simulations, analyses and re‐analyses, and to develop an effective capacity to routinely estimate the biogeochemical state of the ocean, including, amongst others, the implementation of biogeochemical data assimilation. Primary objectives are to enhance the time representation of the seasonal cycle in the real time and reanalysis systems, and to provide a better control of the production in the equatorial regions.

In that framework, Mercator Ocean International has successfully updated its global biogeochemical analysis and forecasting system with an Ocean Color data assimilation capability. In this system, successfully commissioned in September 2019, the biogeochemical model (NEMO/PISCES) is offline coupled with the dynamical ocean (1/12° coarsened to 1/4° resolution) from the CMEMS global physical analysis and forecasting operational system (PSY4), at a daily frequency, and benefits from the assimilation of satellite (SSH-SST-SIC) and in situ physical data. Nevertheless, a biogeochemical climatological damping is activated in the biogeochemical model in order to mitigate the impact of some misconstrained processes (vertical velocities) from this physical data-assimilated forcing (under investigation). The dedicated assimilation of biogeochemical data relies on a simplified version of the SEEK filter, where the forecast error covariances are built from a fixed-basis - but seasonally variable - ensemble of anomalies computed from a multi-year numerical experiment (without biogeochemical data assimilation) with respect to a running mean. Regarding Ocean Colour observations, the system relies, as a first step, on the CMEMS Global Ocean surface chlorophyll concentration products, delivered in NRT.

The objective of this presentation is thus to provide (1) a short description of the implementation of the aforementioned data assimilation methodology in the forecasting system; (2) a synthesis of the assessment of this global biogeochemical forecasting system, by cross-comparing the assimilated solution with various datasets, both spatial (Ocean Colour) and in situ (BGC-Argo, GLODAP), and (3) a synthetic overview of the impact/benefit of the assimilation of the Ocean Colour data.

How to cite: Lamouroux, J., Mignot, A., Perruche, C., Ruggiero, G., Paul, J., Testut, C.-E., Lellouche, J.-M., and Drillet, Y.: Assessment of the CMEMS global biogeochemical forecasting operational system, with assimilation of Ocean Colour data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13293, https://doi.org/10.5194/egusphere-egu2020-13293, 2020.

EGU2020-13432 | Displays | OS4.7

How CMEMS INSTAC contributes to the monitoring of the ocean?

Mélanie Juza, Tanguy Szekely, Jérôme Gourrion, Inmaculada Ruiz-Parrado, Sylvie Pouliquen, and Joaquin Tintoré

CMEMS IN SITU TAC (INSTAC) collects in situ observations from various platforms (e.g. Argo floats, gliders, drifters, ships, fixed stations, marine mammals, high-frequency radar). It provides free, open and quality-controlled physical and biogeochemical ocean data in both delayed mode and near-real time, in support to the operational oceanography, the ocean health and the climate change. Monitoring the 4-dimensional ocean at various spatial and temporal scales, the INSTAC multi-year products provide an essential information on the ocean state, variability and changes. Hence, the INSTAC group contributes substantially to the elaboration of the annual CMEMS Ocean State Report (Von Schuckmann et al., 2016, 2018, 2019, 2020), in collaboration with internal and external scientific experts, as well as with other CMEMS TACs and MFCs.

A general overview of the INSTAC contributions to the CMEMS Ocean State Report is presented, highlighting its capacity to describe, analyze and understand the ocean state and variability of both physical and biogeochemical components from the sea surface to the deep ocean, from the coastal to open sea waters at both short-term (event) and long-term temporal scales. The INSTAC team contributes to the CMEMS Ocean Monitoring Indicators (e.g. temperature, salinity, ocean heat content, water mass and heat exchange, extreme event detection), investigates the ocean circulation variability (e.g. cold and fresh blob in North Atlantic, mesoscale eddy anomaly), analyses the impacts of climate change on marine ecosystem and ocean circulation (e.g. water mass responses to climate warming, cyclones), and develops operational applications and services (pollution risk, search-and-rescue, storm and wave alerts, river discharges).

How to cite: Juza, M., Szekely, T., Gourrion, J., Ruiz-Parrado, I., Pouliquen, S., and Tintoré, J.: How CMEMS INSTAC contributes to the monitoring of the ocean?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13432, https://doi.org/10.5194/egusphere-egu2020-13432, 2020.

EGU2020-13683 | Displays | OS4.7

Towards a unified framework for maximum wave computation from numerical models: outcomes from the LATEMAR project.

Alvise Benetazzo, Francesco Barbariol, Paolo Pezzutto, Luciana Bertotti, Luigi Cavaleri, Silvio Davison, and Mauro Sclavo

Reliable prediction of oceanic waves during severe marine storms has always been foremost for offshore platform design, coastal activities, and navigation safety. Indeed, many damaging accidents and casualties during storms were ascribed to the impact with abnormal and unexpected waves. However, predicting extreme wave occurrence is a challenging task, at first, because of their inherent randomness, and because the observation of large ocean waves, of primary importance to assess theoretical and numerical models, is limited by the costs and risks of deployment during severe open-ocean sea-state conditions.

In the context of the EU-based Copernicus Marine Environment Monitoring Service (CMEMS) evolution, the LATEMAR project (https://www.mercator-ocean.fr/en/portfolio/latemar/) aimed at improving the modelling of large wave events during marine storms. Indeed, at present, operational systems only provide average and peak wave parameters, with no information on individual waves whatsoever. However, developments of the state-of-the-art third-generation wave models demonstrated that using the directional wave spectrum moments into theoretical statistical models for wave extremes, forecasters are able to accurately infer the expected shape and likelihood of the maximum waves during storms.

The main purpose of the activity is therefore to provide the wave models WAM and WAVEWATCH III with common procedures to explicitly estimate the maximum wave heights for each sea state. LATEMAR achieved this goal by: performing an extensive assessment of the model maximum waves using field observations collected from an oceanographic tower; comparing WAM and WAVEWATCH III maximum wave estimates in the Mediterranean Sea; investigating the sensitivity of the maximum waves on the main sea state parameters. All model developments and evaluations resulting from this research project will be directly applicable to the wave model forecasting systems to expand their catalogue.

How to cite: Benetazzo, A., Barbariol, F., Pezzutto, P., Bertotti, L., Cavaleri, L., Davison, S., and Sclavo, M.: Towards a unified framework for maximum wave computation from numerical models: outcomes from the LATEMAR project., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13683, https://doi.org/10.5194/egusphere-egu2020-13683, 2020.

EGU2020-15559 | Displays | OS4.7

Improved ocean wind forcing products

Rianne Giesen, Ana Trindade, Marcos Portabella, and Ad Stoffelen

The ocean surface wind plays an essential role in the exchange of heat, gases and momentum at the atmosphere-ocean interface. It is therefore crucial to accurately represent this wind forcing in physical ocean model simulations. Scatterometers provide high-resolution ocean surface wind observations, but have limited spatial and temporal coverage. On the other hand, numerical weather prediction (NWP) model wind fields have better coverage in time and space, but do not resolve the small-scale variability in the air-sea fluxes. In addition, Belmonte Rivas and Stoffelen (2019) documented substantial systematic error in global NWP fields on both small and large scales, using scatterometer observations as a reference.

Trindade et al. (2019) combined the strong points of scatterometer observations and atmospheric model wind fields into ERA*, a new ocean wind forcing product. ERA* uses temporally-averaged differences between geolocated scatterometer wind data and European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis fields to correct for persistent local NWP wind vector biases. Verified against independent observations, ERA* reduced the variance of differences by 20% with respect to the uncorrected NWP fields. As ERA* has a high potential for improving ocean model forcing in the CMEMS Model Forecasting Centre (MFC) products, it is a candidate for a future CMEMS Level 4 (L4) wind product. We present the ongoing work to further improve the ERA* product and invite potential users to discuss their L4 product requirements.

References:

Belmonte Rivas, M. and A. Stoffelen (2019): Characterizing ERA-Interim and ERA5 surface wind biases using ASCAT, Ocean Sci., 15, 831–852, doi: 10.5194/os-15-831-2019.

Trindade, A., M. Portabella, A. Stoffelen, W. Lin and A. Verhoef (2019), ERAstar: A High-Resolution Ocean Forcing Product, IEEE Trans. Geosci. Remote Sens., 1-11, doi: 10.1109/TGRS.2019.2946019.

How to cite: Giesen, R., Trindade, A., Portabella, M., and Stoffelen, A.: Improved ocean wind forcing products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15559, https://doi.org/10.5194/egusphere-egu2020-15559, 2020.

EGU2020-15573 | Displays | OS4.7

A high resolution reanalysis for the Mediterranean Sea

Romain Escudier, Emanuela Clementi, Massimiliano Drudi, Jenny Pistoia, Alessandro Grandi, Andrea Cipollone, Rita Lecci, Mohamed Omar, Ali Aydogdu, Simona Masina, Giovanni Coppini, and Nadia Pinardi

In order to be able to predict the future ocean climate and weather, we need to understand what happened in the past and the mechanisms responsible for the ocean variability. This is particularly true in a complex area such as the Mediterranean Sea with diverse dynamics such as deep convection and thermohaline circulation or coastal hydrodynamics. To this end, effective tools are reanalyses or reconstructions of the past ocean state. 

Here we present a new physical reanalysis of the Mediterranean Sea at high resolution, developed in the CMEMS Med-MFC framework. The hydrodynamic model is based on the Nucleus for European Modelling of the Ocean (NEMO) combined with a variational data assimilation scheme (OceanVAR). A series of system developments have been carried out to upgrade the current Med-MFC reanalysis to the new one with high resolution, including new NEMO version and configuration, the new version of atmospheric forcing (ERA-5) datasets and revised OceanVAR scheme.

The model has a horizontal resolution of 1/24° and 141 vertical z* levels and provides daily and monthly 3D values of temperature, salinity, sea level and currents. Hourly ERA-5 atmospheric fields force the model and daily boundary conditions in the Atlantic are taken from the global CMCC C-GLORS reanalysis. 39 rivers model the freshwater input to the basin plus the Dardanelles. The reanalysis covers 30-years, initialized from World Ocean Atlas climatology in January 1985, getting to a nominal state after a two years spin-up and ending in 2018. In-situ data from CTD, ARGO floats, XBT are assimilated into the model in combination with satellite altimetry data.

This reanalysis has been validated and assessed through comparison to in-situ and satellite observations as well as literature climatologies. The results show good agreement with observations and a better representation of the main dynamics of the region compared to the previous, lower resolution (1/16°) reanalysis. The new reanalysis will allow the study of physical processes at multi-scales, from the large scale to the transient small mesoscale structures.

How to cite: Escudier, R., Clementi, E., Drudi, M., Pistoia, J., Grandi, A., Cipollone, A., Lecci, R., Omar, M., Aydogdu, A., Masina, S., Coppini, G., and Pinardi, N.: A high resolution reanalysis for the Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15573, https://doi.org/10.5194/egusphere-egu2020-15573, 2020.

EGU2020-17659 | Displays | OS4.7

The COASTAL CRETE downscaled forecasting system

Nikolaos Kampanis, Katerina Spanoudaki, George Zodiatis, Maria Luisa Quarta, Marco Folegani, and George Galanis

The island of Crete is known to be at the crossroads of historic sea routes that served as conveyors of trade, knowledge and culture throughout history, linking some of the world's earliest sophisticated civilizations and currently attracts millions of tourists and cruise passengers. At the same time, the coastal area of Crete is an area of increasing interest due to the recent hydrocarbon exploration and exploitation activities in the Eastern Mediterranean sea and the increase of the maritime transport after the enlargement of the Suez Canal. National and local authorities, like ports and the coast guard, who are involved in maritime safety, such as oil spill prevention, safety of ships, the tourism industry and policy makers involved in coastal zone management, are only few of end users groups seeking high spatial and temporal resolution forecasting products and information to support their maritime activities in the coastal sea area of the island. To this end, a high-resolution, operational forecasting system, namely COASTAL CRETE, has been development for the coastal area of Crete to assist local end users and response agencies to strengthen their capacities in maritime safety and marine conservation. COASTAL CRETE implements advanced numerical hydrodynamic and sea state models, nested in the Copernicus Marine Environmental Monitoring Service of the Mediterranean Sea –CMEMS Med MFC. COASTAL CRETE produces, on a daily basis, 5-days hourly and 6-hourly averaged high-resolution forecasts of important marine parameters, such as sea currents, temperature, salinity, as well as waves. The COASTAL CRETE high-resolution (~1km) hydrodynamic model is based on a modified POM novel parallel code previously implemented by the CYCOFOS  in the Eastern Mediterranean and the Levantine Basin, while for wave forecasts, the last ECMWF CY46R1 parallel version  including  a number of new features,  a state-of-the-art wave analysis and prediction model with high accuracy in both shallow and deep waters has been implemented with a resolution of 1km. The harvesting of the CMEMS Med MFC products has been set in an automatic way and managed through the EODATASERVICE technology developed by MEEO, i.e. ADAM (Advanced geospatial Data Management platform - https://adamplatform.eu/). This application provides automatic data exchange management capabilities between the CMEMS Med MFC and the COASTAL CRETE models, enabling data visualization, combination, processing and download through the implementation of the Digital Earth concept. The downscaled high-resolution COASTAL CRETE forecasts will be used to deliver on demand information and services in the broader objectives of the maritime safety, particularly for oil spill and floating objects predictions.

Acknowledgements: Copernicus Marine Environment Monitoring Service (CMEMS) DEMONSTRATION COASTAL-MED SEA. COASTAL-CRETE, Contract: 110-DEM5-L3.

How to cite: Kampanis, N., Spanoudaki, K., Zodiatis, G., Quarta, M. L., Folegani, M., and Galanis, G.: The COASTAL CRETE downscaled forecasting system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17659, https://doi.org/10.5194/egusphere-egu2020-17659, 2020.

EGU2020-17977 | Displays | OS4.7

Framework for improving land boundary conditions in regional ocean products

Francisco Campuzano, Flávio Santos, Ana Isabel Ramos de Oliveira, Lucian Simionesei, Rodrigo Fernandes, David Brito, Estrella Olmedo, Antonio Turiel, Marco Alba, Antonio Novellino, Marina Tonani, Huw Lewis, Marcos García Sotillo, Arancha Amo Baladrón, Tomasz Dabrowski, Benjamin Jacob, Joanna Staneva, and Ramiro Neves

Currently hydrological models are not generally coupled to coastal and regional ocean models because, even if regarded as a powerful and useful tool, they do not fully accomplish to estimate accurately the right volume of water reaching the coastal zone for many reasons including water management activities such as human consumption, irrigation, etc. For this reason, many coastal and ocean models continue to use river climatologies as boundary conditions for representing such an active boundary. Furthermore, continuous salinity observations in the coastal area are scarce and sensors are highly unreliable while current Earth Observation (EO) products for salinity poorly represents the coastal gradients.

In this presentation, the current state-of-the-art and the results of the LAMBDA Project (λ) (LAnd-Marine Boundary Development and Analysis) will be shown. The main aim of the project was to demonstrate an improvement in the thermohaline circulation in coastal areas by a better characterisation of the land-marine boundary conditions, with special regard to the salinity fields. The LAMBDA project analysed the opportunity of improving the land-marine boundary conditions by exploring the capacities of state-of-the-art hydrologic models. In order to achieve those objectives, the project strategy used an integrated approach that went from watershed models to validation in the coastal area by fit-for-purpose EO products, developed by SMOS, and passing through methods and proxies for integrating the freshwater flows into regional mesoscale grids. The watershed and estuarine proxies were modelled using the MOHID Water modelling system (http://www.mohid.com/) an open source model capable of simulating a wide range of processes, i.e. hydrodynamics, transport, water quality, oil spills, in surface water bodies (oceans, coastal areas, estuaries and reservoirs).

The project products were evaluated in Portugal Continental waters, tested in CMEMS regional products and evaluated by local experts in Germany, Ireland, Portugal, UK and Spain.

How to cite: Campuzano, F., Santos, F., Ramos de Oliveira, A. I., Simionesei, L., Fernandes, R., Brito, D., Olmedo, E., Turiel, A., Alba, M., Novellino, A., Tonani, M., Lewis, H., García Sotillo, M., Amo Baladrón, A., Dabrowski, T., Jacob, B., Staneva, J., and Neves, R.: Framework for improving land boundary conditions in regional ocean products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17977, https://doi.org/10.5194/egusphere-egu2020-17977, 2020.

EGU2020-18033 | Displays | OS4.7

SOSeas: An assessment tool for predicting the dynamic risk of drowning on beaches

Javier F. Bárcena, Javier García-Alba, Felipe Fernández, Javier Costas-Veiga, Marcos Mecías, María Luisa Sámano, David Szpilman, and Andrés García

This study focuses on the development of an operational service to prevent one of the major public health problems worldwide, drowning (https://soseas.ihcantabria.com/). Approximately, there are 360,000 annual deaths from drowning all around the world, although, global estimates may significantly underestimate the real values. In order to reduce the third leading cause of unintentional injury death worldwide, there is an urgent need to increase the understanding of drowning causes in highly, nonlinear and complex dynamic systems, such as beaches.

Usually, process-based models have been directly used to provide information for risk analysis or by means of hybrid downscaled systems in local areas. Nevertheless, this type of modelling is not operative to generate a worldwide system capable to offer a forecasting risk of drowning as a function of hydrodynamic information, suggesting that computational requirements will be an impediment to applications where a quick answer is required, e.g., managing temporary closures of bathing sites. Accordingly, different techniques have been proposed to overcome the large computational burden associated with process-based models, called dynamic emulation modelling. From a technical perspective, Artificial Neural Networks (ANN) models have a strong predictive ability for nonlinear systems allowing it to be synchronized with another system, and can enhance the overall reliability and applicability of process-based models simplifying the mathematical descriptions of the physical structure and mechanism of chaotic systems. From the operational perspective, the implementation of ANN models is highly efficient at a very low cost compared to the implementation of process-based models.

The catalogue of events included the information of metocean conditions provided by the global reanalysis and forecasts of Copernicus Marine Service (CMEMS) and the information provided by the Brazilian Life Saving Society (SOBRASA) about drowning in Santa Catarina beaches (Brazil). During the last 18 years, lifeguards have collected more than 132,000 observations from 139 coastal beaches, which have 346 lifeguard posts. This information has been provided in two databases: (1) a database of events (drownings or similar) and (2) a database of status flag at each lifeguard post. Events database contained 52,712 records from January 2001 to May 2019. The Flags database contained 79,487 records from November 2016 to July 2019.

From these databases, the chosen key-variables to predict the dynamic risk of drowning on beaches using electronic flags were geomorphological variables: morphological modal state, beach orientation, presence of estuary/river mouth, and metocean variables: Waves (significant total height, mean wave period, direction of waves), Wind (magnitude of wind velocity, direction of winds), Water Level (water level variation), and Currents (magnitude of marine currents).

The application to the Santa Catarina beaches demonstrated ANN models are viable surrogates of highly nonlinear process-based models and highly variable forcings to understand the synchronization between metocean conditions and drowning risks at beaches. The results showed that the neural networks conveniently reproduced the status flag of beaches. Finally, it is worthy to mention this service has created the availability of non-existent tools that enhance safety in these aquatic spaces, generating economic assets as a sign of high quality tourism.

How to cite: Bárcena, J. F., García-Alba, J., Fernández, F., Costas-Veiga, J., Mecías, M., Sámano, M. L., Szpilman, D., and García, A.: SOSeas: An assessment tool for predicting the dynamic risk of drowning on beaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18033, https://doi.org/10.5194/egusphere-egu2020-18033, 2020.

EGU2020-18332 | Displays | OS4.7

Assimilation of chlorophyll data into a stochastic ensemble simulation for the North Atlantic ocean

Yeray Santana-Falcón, Pierre Brasseur, Jean Michel Brankart, and Florent Garnier

Satellite-derived surface chlorophyll data are daily assimilated into a three-dimensional 24 member ensemble configuration of an online-coupled NEMO-PISCES model for the North Atlantic ocean. A one-year multivariate assimilation experiment is performed to evaluate the impacts on analyses and forecast ensembles. Our results demonstrate that the integration of data improves surface analysis and forecast chlorophyll representation in a major part of the model domain, where the assimilated simulation outperforms the probabilistic skills of a non-assimilated analogous simulation. However, improvements are dependent on the reliability of the prior free ensemble. A regional diagnosis shows that surface chlorophyll is overestimated in the northern limit of the subtropical North Atlantic, where the prior ensemble spread does not cover the observation's variability. There, the system cannot deal with corrections that alter the equilibrium between the observed and unobserved state variables producing instabilities that propagate into the forecast. To alleviate these inconsistencies, a one-month sensitivity experiment in which the assimilation process is only applied to model fluctuations is performed. Results suggest the use of this methodology may decrease the effect of corrections on the correlations between state vectors. Overall, the experiments presented here evidence the need of refining the description of model's uncertainties according to the biogeochemical characteristics of each oceanic region.

How to cite: Santana-Falcón, Y., Brasseur, P., Brankart, J. M., and Garnier, F.: Assimilation of chlorophyll data into a stochastic ensemble simulation for the North Atlantic ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18332, https://doi.org/10.5194/egusphere-egu2020-18332, 2020.

EGU2020-22241 | Displays | OS4.7

A novel statistical approach for Near-Real Time Quality Control of hydrographic observations

Jérôme Gourrion, Delphine Dobler, and Tanguy Szekely

This study concerns a quality check (QC) test in which temperature and salinity observations are compared to the range of variability as known from a reference dataset.

For delayed-time QC, Gourrion et al. (2019) have shown that a validity range built from local minimum and maximum values performs better than the standard one estimated from the local mean +/- N standard deviations. It allows to account for the assymetry and peakedness of the local parameter distribution. The performance of the test is significantly improved, reducing strongly the number of hydrographic stations to be checked by a delayed-time operator.

Nevertheless, for near-real-time applications, the available operator time is severely reduced ; the method needs to be automatized and the number of erroneous detections further reduced.

Here, we first present the impact on the performance brought by the increase of the temporal extent of the reference dataset. Then, an adhoc widening of the validity interval is proposed and shown to further improve the performance, reaching a point of applicability for NRT processing. Finally, performance differences between temperature and salinity are highlighted. 

 

How to cite: Gourrion, J., Dobler, D., and Szekely, T.: A novel statistical approach for Near-Real Time Quality Control of hydrographic observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22241, https://doi.org/10.5194/egusphere-egu2020-22241, 2020.

EGU2020-22304 | Displays | OS4.7

Progresses in the CMEMS BS-MFC for improving forecasting capabilities and monitoring the Black Sea region through high quality modelling systems

Stefania Angela Ciliberti, Atanas Palazov, Marilaure Gregoire, Joanna Staneva, Elisaveta Peneva, Simona Masina, Rita Lecci, Veselka Marinova, Marius Matreata, Sergio Creti, Luc Vandenbulcke, Arno Behrens, Francesco Palermo, Eric Jansen, Leonardo Lima, Laura Stefanizzi, Farshid Daryabor, Diana Azevedo, Nadezdha Valcheva, and Paola Agostini and the CMEMS Black Sea Monitoring and Forecasting Center Team

The BS-MFC (Black Sea Monitoring and Forecasting Centre) delivers near real time and multi-year products for the Black Sea region with the scope to describe its physical, biogeochemical and wave conditions in the frame of CMEMS. This is done through 3 Production Units – Physics, Biogeochemistry and Waves – that implement state-of-the-art and accurate modelling approaches for forecasting and monitoring purposes. In 2019, the BS-MFC offer has been updated to include i) updated versions of the BS-PHY and BS-WAV NRT products and new BS-BIO product, ii) update of the MY products timeseries up to Dec 2018 and iii) inclusion of Ocean Monitoring Indicators (OMI).

Considering NRT systems, the systems are performing assimilation of in-situ and satellite products provided by CMEMS TACs with PHY and BIO products centered to 12:00Z and WAV product instantaneous fields covering 10-days forecast. BS-BIO offers new product since Jul 2019, including CHL, PHYC, O2, NO3, PO4, Primary Production and carbonate system components (pH, DIC, Alkalinity, air-sea flux of CO2). To support ocean monitoring purposes, describing the current state of the Black Sea physical dynamics, environmental and extreme events, the BS-MFC implements a set of OMI: a) vertically integrated oxygen content, b) oxygen penetration density and depth, c) sea surface temperature and salinity anomalies, d) significant wave height extremes.

To improve forecasting capabilities and prepare the next generation of BS products, the BS-MFC is working on several scientific topics, the most challenged are the increased resolution in vertical of the physical system, the problem of the Bosporus Strait as boundary condition, improved data assimilation capabilities, coupling strategies among PHY, BIO and WAV and improvement of upstream data ingestion in NRT and MY systems, including the usage of hourly forcing in WAV production system and forecast data of the Danube River discharge and nutrients in the PHY and BIO systems. Furthermore, the BS-MFC is working on enforce operational capacities and define pre-operational evaluation to estimate accuracy of operational and new products.

 

How to cite: Ciliberti, S. A., Palazov, A., Gregoire, M., Staneva, J., Peneva, E., Masina, S., Lecci, R., Marinova, V., Matreata, M., Creti, S., Vandenbulcke, L., Behrens, A., Palermo, F., Jansen, E., Lima, L., Stefanizzi, L., Daryabor, F., Azevedo, D., Valcheva, N., and Agostini, P. and the CMEMS Black Sea Monitoring and Forecasting Center Team: Progresses in the CMEMS BS-MFC for improving forecasting capabilities and monitoring the Black Sea region through high quality modelling systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22304, https://doi.org/10.5194/egusphere-egu2020-22304, 2020.

OS4.8 – Numerical modelling of the ocean: new scientific advances in ocean models to foster exchanges within NEMO community and contribute to future developments

EGU2020-3390 | Displays | OS4.8

A parameterization of local and remote tidal mixing

Casimir de Lavergne, Clément Vic, Gurvan Madec, Fabien Roquet, Amy Waterhouse, Caitlin Whalen, Yannis Cuypers, Pascale Bouruet-Aubertot, and Toshiyuki Hibiya

Vertical mixing is often regarded as the Achilles’ heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. Implemented in the NEMO global ocean model, the scheme improves the representation of deep water-mass transformation and obviates the need for a constant background diffusivity.

How to cite: de Lavergne, C., Vic, C., Madec, G., Roquet, F., Waterhouse, A., Whalen, C., Cuypers, Y., Bouruet-Aubertot, P., and Hibiya, T.: A parameterization of local and remote tidal mixing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3390, https://doi.org/10.5194/egusphere-egu2020-3390, 2020.

EGU2020-5618 | Displays | OS4.8

VENM: An Algorithm to Accurately Calculate Neutral Slopes and Gradients

Sjoerd Groeskamp, Paul Barker, Trevor McDougall, Ryan Abernathey, and Stephen Griffies

Mesoscale eddies stir along the neutral plane, and the resulting neutral diffusion is a fundamental aspect of subgrid-scale tracer transport in ocean models. Calculating neutral diffusion traditionally involves calculating neutral slopes and three-dimensional tracer gradients. The calculation of the neutral slope traditionally occurs by computing the ratio of the horizontal to vertical locally referenced potential density derivative. However, this approach is problematic in regions of weak vertical stratification, prompting the use of a variety of ad hoc regularization methods that can lead to rather nonphysical dependencies for the resulting neutral tracer gradients.

Here we introduce VENM; a search algorithm that requires no ad hoc regularization and significantly improves the numerical accuracy of calculating neutral slopes, neutral tracer gradients, and associated neutral diffusive fluxes. We compare and contrast VENM against a more traditional method, using an independent objective neutrality condition combined with estimates of spurious diffusion, heat transport, and water mass transformation rates. VENM is more accurate, both physically and numerically, and should form the basis for future efforts involving neutral diffusion calculations from observations and possibly numerical model simulations.

How to cite: Groeskamp, S., Barker, P., McDougall, T., Abernathey, R., and Griffies, S.: VENM: An Algorithm to Accurately Calculate Neutral Slopes and Gradients, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5618, https://doi.org/10.5194/egusphere-egu2020-5618, 2020.

Organic particle populations in the mesopelagic layer span a wide range of sizes and sinking speeds. A long-standing paradigm in ocean biogeochemistry posits that large, fast-sinking detrital particles contribute to most of the vertical export flux of particulate organic carbon (POC), whereas small, slow-sinking or suspended particles comprise most (>90%) of the stock. Over the last decades, most studies have placed emphasis on understanding and predicting the vertical fluxes driven by large particles owing to their influence on ocean carbon storage. Yet, there are compelling reasons to study the dynamics of suspended and slow-sinking small POC (sPOC) in greater detail. First, sPOC likely supports most of the mesopelagic respiration. Second, recent studies have shown that a number of mechanisms can inject large amounts of sPOC into the mesopelagic layer, to the point that the sPOC fraction may seasonally dominate total vertical POC fluxes. Thus, better accounting for sPOC fluxes might allow us overcome historical difficulties in balancing mesopelagic carbon budgets.

In the last decade, hundreds of bio-optical sensors deployed on autonomous profiling robots (bgc-Argo floats) have enabled observation of small particle stocks between the sea surface and 1000 m depth with a profiling frequency of 1-10 days. These observations are showing that mesopelagic sPOC follows distinct seasonal cycles in different oceanic areas, and allow identification of sPOC supply events (e.g., caused by vertical mixing or by disaggregation or fragmentation of larger particles) and of net sPOC removal. Regarding ocean biogeochemistry models, they have traditionally been tuned to estimate sinking POC fluxes but failed to capture POC stocks. Recently, the formulation of POC degradation rates in the model PISCESv2 was changed and a POC reactivity continuum approach was adopted, which greatly improved the representation of small and big POC stocks. These parallel developments now enable the quantitative assessment of mesopelagic POC dynamics.

Here we analyze the annual cycles of mesopelagic sPOC in the subpolar North Atlantic, as seen by biogeochemical Argo floats, and compare them to their PISCESv2-simulated counterparts. We then discuss the processes that drive mesopelagic sPOC seasonality in the observations and in 1D model simulations. Finally, we present a genetic algorithm approach that uses biogeochemical Argo float observations to optimize the PISCESv2 parameters that influence sPOC, focusing on the interplay between particle degradation rate and sinking speed.

How to cite: Galí, M., Falls, M., and Bernardello, R.: Seasonal dynamics of mesopelagic organic particles in the subpolar North Atlantic. Learning from the crosstalk between biogeochemical Argo float measurements and PISCESv2 simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16602, https://doi.org/10.5194/egusphere-egu2020-16602, 2020.

EGU2020-18680 | Displays | OS4.8

An atmospheric boundary layer model to improve air-sea interactions in eddying ocean models

Guillaume Samson, Florian Lemarié, Théo Brivoal, Romain Bourdallé-Badie, Hervé Giordani, Jean-Luc Redelsperger, and Gurvan Madec

High-resolution ocean-atmosphere coupled models are able to simulate realistically air-sea interactions taking place at mesoscale between ocean eddies and fronts, and the lower atmosphere. These coupled processes have the potential to improve oceanic simulations by modulating wind work input and turbulent heat fluxes. However, the computational cost and the complexity of such coupled models appear prohibitive and inadequate in the context of global eddying oceanic simulations.

We propose here an alternative approach based on a one-dimensional vertical atmospheric boundary layer (ABL) model driven by large-scale atmospheric data (forecasts or reanalysis). Its intermediate complexity between a bulk parameterization and a full atmospheric model associated with a limited computational cost makes this approach well suited for applications ranging from process studies to global operational oceanography.

First, the ABL model is validated against a set of classic atmospheric testcases such as a SST front. The comparison with analytical and LES solutions indicates a good agreement with the ABL model results.

Then, two realistic configurations based on NEMO ocean model are presented to assess air-sea interactions: a global 1/4° configuration including sea-ice and a regional 1/36° configuration covering western Europe.

We show that the ocean-ABL coupled model produces negative correlations between surface current and wind stress mesoscale curl anomalies (oceanic eddy damping effect), and positive correlations between surface current and wind speed mesoscale curl anomalies (wind adjustment and ocean re-energization effects) in good agreement with literature. We also show that the simulated wind speed positively correlates with SST mesoscale anomalies, as observed with satellite data and full coupled models.

To summarize, the ocean-ABL coupled model is able to realistically represent mesoscale dynamical and thermal feedbacks while keeping a good consistency with the atmospheric forcing, and with a very limited computational cost (10% of the ocean model). The ABL model will be released with the next NEMO version.

1. Choisir un champ pour le nom d'utilisateur
Vous pouvez également utiliser les nombres pour choisir un champ depuis le clavier.
Raccourcis-clavier:
Ignorer
Ignorer
Continuer
Confirmer
Afficher plus
Supprimer la sélection
 

How to cite: Samson, G., Lemarié, F., Brivoal, T., Bourdallé-Badie, R., Giordani, H., Redelsperger, J.-L., and Madec, G.: An atmospheric boundary layer model to improve air-sea interactions in eddying ocean models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18680, https://doi.org/10.5194/egusphere-egu2020-18680, 2020.

Kinetic energy backscatter (KEB) parameterizations of subgrid 2d turbulence have shown their efficiency in ocean models as they restore activity of mesoscale eddies. Modern KEBs utilize only two properties of badly resolved inverse energy cascade: KEB tendency should be larger than turbulent viscosity in spatial scale and amount of returning energy should compensate energy loss due to eddy viscosity. Typical operators for KEB tendency are Laplace operator with negative viscosity coefficient and stochastic process. Application of artificial neural networks (ANN) to approximate subgrid forces may give rise to new KEB models. The main challenge in this direction is to preprocess subgrid forces in such a way to reveal a part corresponding to returning of energy from subgrid scales. In this work, we propose to define subgrid forces as a term nudging a coarse-resolution model toward high-resolution model. This force is energy-generating and may be approximated with ANN. Conventional KEBs and ANN model are compared in Double-Gyre configuration of NEMO ocean model.

How to cite: Perezhogin, P.: A new data-driven subgrid 2d turbulence parameterization and comparison with conventional kinetic energy backscatter parameterizations in NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19890, https://doi.org/10.5194/egusphere-egu2020-19890, 2020.

EGU2020-21486 | Displays | OS4.8

Implementation of NEMO v4.0.1 as ocean component for the regional climate modeling framework

Janna Abalichin, Birte-Marie Ehlers, and Frank Janssen

The ‘German Strategy for Adaptation to Climate Change’ (DAS) provides the political framework to climate change mitigation and adaptation in Germany. The associated ‘Adaption Action Plan’ envisages the establishment of an operational forecasting and projection service for climate, extreme weather and coastal and inland waterbodies. This service is intended to make use of a regional climate modeling framework, with NEMO v4.0.(1) as the ocean component. The atmospheric component will be provided by the German Weather Service (either the current weather forecasting model ICON or COSMO will be used) and will be coupled to NEMO after testing and calibration of NEMO on the regional scale.

The area of interest includes besides the North Sea and the Baltic Sea the entire North-West-Shelf to take into account cross-shelf transport, the water exchange between North Sea and Baltic Sea and the impact of North Atlantic weather systems on the internal dynamics of the seas. One focus area will be German Bight, well known for its large tidal flats, which make wetting & drying a desirable model feature, which will be tested in future. The used/implemented bathymetry includes the up to date measurements of the sea floor from the EMODNET network.

To achieve a proper description of the dynamics in this region the model has to be calibrated with regard to the timing and amplitude of the water levels in the coastal waters, the water inflow through the Danish straits, the thermal stratification as well as the seasonality and thickness of the sea ice in the Northern Baltic Sea.

These efforts are carried out in the pilot project ‘Projection Service for Waterways and Shipping’ (ProWaS).

How to cite: Abalichin, J., Ehlers, B.-M., and Janssen, F.: Implementation of NEMO v4.0.1 as ocean component for the regional climate modeling framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21486, https://doi.org/10.5194/egusphere-egu2020-21486, 2020.

EGU2020-16204 | Displays | OS4.8

An evaluation of the mixed precision version of NEMO 4.0.1

Oriol Tintó, Stella Valentina Paronuzzi Ticco, Mario C. Acosta, Miguel Castrillo, Kim Serradell, and Francisco J. Doblas-Reyes

One of the requirements to keep improving the science produced using NEMO is to enhance its computational performance. The interest in improving its capability to efficiently use the computational infrastructure its two-fold: on one side there are experiments that would only be possible if a certain threshold of throughput is achieved, on the other side any development that achieves an increase in efficiency would help saving resources while reducing the environmental impact of our experiments. One of the opportunities that raised interest in the last few years is the optimization of the numerical precision. Historical reasons brought many computational models to over-engineer the numerical precision: solving this miss-adjustment can payback in terms of efficiency and throughput. In this direction, a research was carried out in order to safely reduce the numerical precision in NEMO which led to a mixed-precision version of the model. The implementation has been developed following the approach proposed by Tintó et al. 2019, in which the variables that require double precision are identified automatically and the remaining ones are switched to use single-precision. The implementation will be released in 2020 and this work presents its evaluation in terms of both performance and scientific results.

How to cite: Tintó, O., Paronuzzi Ticco, S. V., Acosta, M. C., Castrillo, M., Serradell, K., and Doblas-Reyes, F. J.: An evaluation of the mixed precision version of NEMO 4.0.1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16204, https://doi.org/10.5194/egusphere-egu2020-16204, 2020.

EGU2020-7225 | Displays | OS4.8

Implementation and assessment of a flux-limiter based wetting and drying scheme

Enda O'Dea, Mike Bell, Andrew Coward, and Jason Holt

Wetting and drying processes in shallow water systems by surges, tides and seiches have important societal, physical and biological impacts. Operational regional models are now of sufficient resolution, O(1 km), that the processes of wetting and drying need to be included. Here we describe a flux limiter based approach that allows a numerical ocean model with a flux formulation of tracer advection to wet and dry. Following Warner et al. (2013), the flux limiter approach limits the outflow from a cell whose depth is below a critical value defined by the user. The limiter can be a step function or a smooth function of the water depth flux limiter, the latter increases model stability and avoids rapid alternation between dry and wet states on long slopes as the critical depth is approached. Furthermore, the user may proportionally limit the baroclinic fluxes as a cell transitions from wet to dry over the course of the large baroclinic time step. The simplicity of the flux limiter approach lends itself to its application within existing numerical models without significant intrusion into the code base. Here we explore the scheme's effectiveness, sensitivities and limitations within the 3D NEMO ocean model by assessing it using test cases of increasing complexity. It is shown to perform well in classic channel test cases and 2D parabolic test cases with analytic solutions. Its performance against analytical 1D dam break experiments is explored and used to interpret its performance against laboratory measurements of a 2D dam break. The scheme is also shown to run stably for a realistic 3D regional domain of the North West European shelf and to improve some aspects of the model's performance against tide gauges.

How to cite: O'Dea, E., Bell, M., Coward, A., and Holt, J.: Implementation and assessment of a flux-limiter based wetting and drying scheme, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7225, https://doi.org/10.5194/egusphere-egu2020-7225, 2020.

EGU2020-22157 | Displays | OS4.8

Explicit modelling of external and internal tidal waves in the global 1/12° NEMO ocean model

Jérôme Chanut, Romain Bourdallé-Badie, Benoît Tranchant, Damien Allain, Loren Carrère, Ariane Koch-Larrouy, Florent Lyard, and Yves Morel

Ocean tides play a major role in ocean mixing, and setting up water properties, both in the deep and the shallow ocean. Whether parameterized or explicitly simulated, tides can not be ignored in modern ocean prediction models. Representing them explicitly, the approach followed here to prepare the upcoming CMEMS (Copernicus Marine Environment Monitoring Service) global prediction system, allows for the generation of a large quantity of internal waves propagating at great distances. This is a useful information for future high resolution wide swath altimetry missions but also for forcing regional systems, enabling remotely generated internal waves to enter the user domain, providing in some places an important part of the high frequency
energy.

In the present work, we review numerical aspects in the NEMO ocean model influencing the explicit representation of both external and internal tidal waves in a global 1/12° configuration. The numerical core of NEMO has indeed largely evolved recently to simulate tides, by now including the lunisolar tidal potential, Self Attraction and Loading effects, a split-explicit barotropic solver but also Lagrangian vertical coordinates to limit the spurious numerical diffusion. We report here the effect of various parameters, using data assimilative tidal models and altimetry data as references. Semi-diurnal energy budgets are also computed. Throughout this work, a systematic comparison to HYCOM results (Ansong et al. 2015 and Buijsman et al. 2016) is performed.

How to cite: Chanut, J., Bourdallé-Badie, R., Tranchant, B., Allain, D., Carrère, L., Koch-Larrouy, A., Lyard, F., and Morel, Y.: Explicit modelling of external and internal tidal waves in the global 1/12° NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22157, https://doi.org/10.5194/egusphere-egu2020-22157, 2020.

EGU2020-22177 | Displays | OS4.8

The impact of a parameterisation of submesoscale mixed layer eddies on mixed layer depths in the NEMO ocean model

Daley Calvert, George Nurser, Mike Bell, and Baylor Fox-Kemper

A parameterisation scheme for restratification of the mixed layer by submesoscale mixed layer eddies is implemented in the NEMO ocean model. Its impact on the mixed layer depth (MLD) is examined in 30-year integrations of "uncoupled" ocean-ice and "coupled" atmosphere-ocean-ice-land global climate configurations used by the Met Office Hadley Centre. The specification of the mixed-layer Rossby radius in the scheme is shown to affect its impact on the MLD in the 1/4 degree uncoupled configuration by up to a factor of 2 in subtropical and mid-latitudes. This factor has been limited in the extent to which small mixed-layer Rossby radii are utilised to guard against CFL-type instabilities in the scheme, but such a limit was not found to be necessary for this implementation. An alternative form of the scheme is described that approximates the mixed-layer Rossby radius as a function only of latitude. This form is shown to yield similar results to the original formulation for an appropriate choice of parameters. The global mean impact of the scheme on the MLD is found to be almost twice as large in the 1 degree and 2 degree uncoupled configurations as it is in the 1/4 degree configuration, although the parameterised vertical buoyancy fluxes have closer agreement. This is shown to be the result of the scheme overcompensating for the decay in strength of resolved mixed layer density fronts in this model with decreasing horizontal grid resolution. The MLD criterion defining the depth scale of the scheme is shown to affect its global mean impact on the MLD by nearly a factor of 3 in the 1/4 degree uncoupled and coupled configurations, depending on whether the criterion is chosen to capture the actively mixing layer or well-mixed layer. Climatological MLD biases are improved overall in both cases, substantively reducing deep winter biases whilst slightly increasing shallow summer biases.

How to cite: Calvert, D., Nurser, G., Bell, M., and Fox-Kemper, B.: The impact of a parameterisation of submesoscale mixed layer eddies on mixed layer depths in the NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22177, https://doi.org/10.5194/egusphere-egu2020-22177, 2020.

EGU2020-9731 | Displays | OS4.8

Evaluation of a multilayer snow scheme in NEMO-LIM3

Gaëlle Gilson, Thierry Fichefet, Olivier Lecomte, Pierre-Yves Barriat, Jean Sterlin, and François Massonnet

EGU2020-9732 | Displays | OS4.8

Refactoring the Memory Access Pattern to Improve Computational Performance in NEMO

Italo Epicoco, Francesca Mele, Silvia Mocavero, Marco Chiarelli, Alessandro D'Anca, and Giovanni Aloisio

In the roadmap of modern parallel architectures development, the computing power of a node grows much more quickly than main memory performance (capacity, bandwidth). This leads to an even much higher gap between computing and memory resources. An efficient use of the cache memory is becoming ever more essential as optimization technique.
The NEMO model uses a finite difference integration method and a regular cartesian grid for space discretization. The NEMO code reflects this choice: a generic field is represented in memory as a 3D array; and the code is mainly composed of three-level nested loops. These loops often include only a few operations in the body; the results are stored in a temporary 3D array and then used in subsequent loops until the final calculation.
The aim of this work is to make better use of the cache memory by fusing DO loops together. The loop fusion is a transformation which takes two or more adjacent loops that have the same iteration space traversal and combines their bodies into a single loop.
The fusion of the loops is not trivial, and it could require introducing additional redundant operations to solve data dependencies. Unfortunately, this leads to a drawback of the overall performance. To avoid the redundant operation, we can adopt pointers to arrays and implement a pointer rotation at each loop iteration.
We have developed the loop fusion transformation in an advection kernel extracted from the NEMO oceanic model. We have compared 3 different versions of the optimized advection kernel, with 3 different levels of loop fusion.
The first prototype refers to the implementation where the extreme fusion is applied, and all loops in the routine have been fused. In this version, the operations are replicated up to 3 times. In the second prototype the buffer rotation has been applied only in the outermost loop. In the third prototype, the buffer rotation has also been implemented for the second dimension, and this version introduces only a limited amount of redundant operations.

The tests have been performed on the Athena cluster located at the CMCC supercomputing center. The supercomputing infrastructure is based on the Intel Xeon E5-2670 processors. The memory hierarchy is composed of 32KB of L1 cache, 256KB of L2 and 20MB L3 cache shared among the cores. The results clearly proved the effectiveness of the loop fusion approach that reaches a speedup of 2x with a high number of cores. The third prototype has proven to be the most promising solution. Prototypes 1 and 2 provide a good improvement up to 256 cores then the redundant operations lead to a loss of performance.
A deeper analysis measuring the Last Level Cache misses also showed how the loop transformation significantly reduced the number of cache misses.
Despite the good results achieved with the loop fusion optimization, we can remark that this optimization is strictly linked to the computing architecture. A fully portable performance improvement can be ensured by the adoption of a DSL (Domain Specific Language).

How to cite: Epicoco, I., Mele, F., Mocavero, S., Chiarelli, M., D'Anca, A., and Aloisio, G.: Refactoring the Memory Access Pattern to Improve Computational Performance in NEMO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9732, https://doi.org/10.5194/egusphere-egu2020-9732, 2020.

EGU2020-15049 | Displays | OS4.8

Verification of the Storm surge Forecasting System for the Korean Coastal Area

Nary La, Byoung Woong An, KiRyong Kang, Sang Myeong Oh, and YoonJae Kim

In recent years, coastal disasters have been frequently caused by typhoons and storm surges accompanied by high waves due to global warming and the changing marine environment. In addition, the development of coastal areas in Korea has also led to suffering great damage to society every year. 

To cope with this issue, we have developed a new storm-surge prediction system based on the NEMO model for improving the predictability both the tide and the surge. This new regional tide-surge prediction system (RTSM) is constructed with a two-dimensional barotropic sigma coordinates and has a 1/12 degrees horizontal resolution. To find optimal coefficients of this model, several sensitivity experiments were conducted and verified with tide gauge measurements from the KHOA (Korea Hydrographic and Oceanographic Agency). Finally, we selected a bathymetry from SRTM (Shuttle Radar Topography Mission), Charnock coefficient as a constant value of 0.275 and the reference pressure for the inverse barometric effect as the domain mean. As the result of comparing surge-height predictions with the currently operating model (OPER-RTSM), the new system (RTSM) showed roughly 30% higher in forecast accuracy than the previous OPER-RTSM.

How to cite: La, N., An, B. W., Kang, K., Oh, S. M., and Kim, Y.: Verification of the Storm surge Forecasting System for the Korean Coastal Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15049, https://doi.org/10.5194/egusphere-egu2020-15049, 2020.

EGU2020-20134 | Displays | OS4.8

Comparison of Dynamic Topography Bias in HIROMB and NEMO-Nordic Model by Utilizing Marine Geoid

Vahidreza Jahanmard, Nicole Delpeche-Ellmann, and Artu Ellmann

Hydrodynamic models (HDM) provide a reasonable estimate of the sea conditions. Thus making them a vital tool for climate change, engineering, and marine ecosystems. One of the parameters often derived from HDM is the Sea Surface Height (SSH). There exists however a very important hidden characteristic with respect to SSH derived from HDM. For instance, the modelled sea level may have a bias relative to a geodetic reference system datum. In many cases, this bias can change both spatially and temporally. This study now examines this bias by comparison of HDM modelled SSH with tide gauges derived SSH that are geodetically referenced to a more stable vertical reference frame such as the marine geoid (equipotential surface of the earth i.e. is the shape of the ocean surface under the influence of the gravity and rotation of Earth alone).

In this study, the performance of two HDM is analysed for the period 2014‒2015: the Nemo-Nordic (utilised for the Baltic and the North Sea) and the HIROMB-BOOS (used for operational sea forecast in Estonia). In these models, the derived SSH is compared to the fourteen tide gauges (TG) located along the Estonian coastal zone of the Baltic Sea. The vertical reference frame for these tide gauges is fitted to that of a regional high-resolution geoid model, thus deriving the Dynamic Topography. The methodology consisted of: (i) determining the offshore points that are closest to the tide gauge location, (ii) filtering and averaging of the data sets to remove outliers and high-frequency fluctuations (iii) calculation of the SSH bias between TG and HDM (iv) calculation of the standard deviation and root mean square error (RMSE).

In general, results show that both models conform to a similar trend as tide gauge. The bias however between tide gauge and models varied randomly in magnitude (both spatially and temporally) between both models. The maximum bias for the HIROMB was calculated to be an overestimation of 57 cm and for the Nemo an underestimation of 64 cm. These results hint of possible improvement that can be made in HDM by utilizing a high resolution geoid model that can assist in accurate engineering and scientific studies.

How to cite: Jahanmard, V., Delpeche-Ellmann, N., and Ellmann, A.: Comparison of Dynamic Topography Bias in HIROMB and NEMO-Nordic Model by Utilizing Marine Geoid, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20134, https://doi.org/10.5194/egusphere-egu2020-20134, 2020.

EGU2020-22097 | Displays | OS4.8

Performance of ECCC surge forecasting systems near Canada's East Coast during the passage of hurricane Dorian

Oleksandr Huziy, Natacha Bernier, Benoit Pouliot, Patrick Timko, Pengcheng Wang, and Devon Telford

Accurate forecasts of storm surges caused by winds and atmospheric pressure are important for the protection of life and property in coastal regions and also for safe navigation. Therefore, Environment and Climate Change Canada (ECCC) maintains and develops surge prediction systems. This study focuses on the assessment of the timing and amplitude of predicted surges at selected locations during the passage of hurricane Dorian. The systems use barotropic ocean models to simulate water levels and currents at 1/30 and 1/12 degree horizontal resolutions.

The relatively low tidal range at the time of Dorian’s landfall helped prevent catastrophic flooding. However, with a closer superposition of the peak surge and high tide, the damage could have been more significant. Reasonably well predicted timing of the highest surge by the system helped prevent the over-issuance of warnings. Sensitivity of the forecast quality to the lead time, resolution and atmospheric forcing for the event will be presented.

How to cite: Huziy, O., Bernier, N., Pouliot, B., Timko, P., Wang, P., and Telford, D.: Performance of ECCC surge forecasting systems near Canada's East Coast during the passage of hurricane Dorian, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22097, https://doi.org/10.5194/egusphere-egu2020-22097, 2020.

EGU2020-22147 | Displays | OS4.8

Overview of the first year of the NEMO global 1/36° configuration (ORCA36) development

Clement Bricaud and Miguel Castrillo
Mercator Ocean International operates global high-resolution forecasting systems in the framework of the Copernicus Marine Environment Monitoring Service. The current system has a 1/12° resolution. In order to prepare the increase of its resolution, the development of a new global configuration has started in 2019, with a higher resolution (1/36°). This configuration is also expected in the H2020 IMMERSE project as a demonstrator for the HPC improvements developed in NEMO OGCM and in the H2020 ESIWACE2 project as a demonstrator for production runs at unprecedented resolution on pre-exascale supercomputers. We present here the first 0RCA36 configuration and the first results of a simulation performed on several months forced
by ERAinterim with NEMO 4. We compare it with its twin global ¼° and 1/12° configurations. We also present some results of NEMO 4/ORCA36 performances and scalability, performed by BSC on Mare Nostrum supercomputer.

How to cite: Bricaud, C. and Castrillo, M.: Overview of the first year of the NEMO global 1/36° configuration (ORCA36) development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22147, https://doi.org/10.5194/egusphere-egu2020-22147, 2020.

The tuning phase of IPSL-CM6A-LR, a climate model of CMIP6 using NEMO as ocean component, lasted for 4 years, during which we explored different numerical recipes controlling ocean vertical mixing, among others. Analysis of all simulations is still ongoing, but two lessons can be learned so far. [1] After more than 2,000 yr of integration (using pre-industrial external forcings), the deep ocean has not reached an equilibrium, yet. [2] Sensitivity experiments exploring structural and parametric uncertainties indicate that some intrinsic climatic features of this model are quite robust. Overall, this suggests that we currently have little control on the backbone of numerical oceans.

How to cite: Deshayes, J.: Challenges raised by global ocean configurations in the context of climate modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22151, https://doi.org/10.5194/egusphere-egu2020-22151, 2020.

EGU2020-22213 | Displays | OS4.8

The NEMO (Nucleus for European Modelling of the Ocean) numerical ocean platform

Guillaume Samson, Claire Levy, and Nemo System Team

The Nucleus for European Modelling of the Ocean (NEMO) is a state-of-the art modelling platform for oceanographic research, operational oceanography, sesonnal forecasts and climate studies. NEMO includes three major components; the blue ocean (dynamics), the white ocean (sea-ice), the green ocean (ocean biogeochemistry). It also allows coupling through interfaces with atmosphere (through OASIS software), waves, ice-shelves, so as nesting through the adaptive mesh refinement software AGRIF. Some reference configurations and test cases allowing to explore, to set-up and to validate the applications, and a set of tools to use the platform are also available to the community. The whole platform and its documentation are available under free licence.

The evolution and reliability of NEMO are organised and controlled by a European Consortium between CMCC (Italy), CNRS (France), MOI France), NOC (UK), UKMO (UK).

Consortium members agree on long term strategy and yearly plans, sharing expertise and efforts within the NEMO System Team: the core team of NEMO developers in order to ensure the successful and sustainable development of the NEMO System as a well-organised, state-of-the-art ocean model code system suitable for both research and operational work

How to cite: Samson, G., Levy, C., and System Team, N.: The NEMO (Nucleus for European Modelling of the Ocean) numerical ocean platform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22213, https://doi.org/10.5194/egusphere-egu2020-22213, 2020.

EGU2020-1478 | Displays | OS4.8

Assessment of relationships between Arctic sea ice and stratification of water column modelled by NEMO version 4.0

Byoung Woong An and Pil-Hun Chang

EGU2020-9591 | Displays | OS4.8

Ocean sensitivity to bulk formulae parameterization: a NEMO-ORCA025 model study

Giulia Bonino, Doroteaciro Iovino, and Simona Masina

Surface wind stress and heat fluxes (e.g. sensible and latent) are the major driving forces that modify the ocean dynamics and thermodynamics. Therefore, in a modelling framework, realistic momentum and heat fluxes are essential for simulating the global ocean. In the NEMO ocean general circulation model, these air-sea fluxes, which are difficult to be measured directly, are derived using bulk formulas. A large set of bulk formulas exist and this work tries to quantify the ocean response to the different bulk formulations implemented in the newest version of NEMO global ocean model (version 4.0.1). A set of experiments based on the CMCC ORCA025 configuration (1/4° of horizontal resolution) is run and examined. The numerical experiments differ for the bulk formula used  to estimate air-sea fluxes (ECMWF, NCAR, COARE and COARE3.5) and they consist of 2-years simulations forced by the recent JRA55-do-v1.3 (∼ 55km resolution). Preliminary results on the ocean sensitivity in terms of sea surface temperature, sea surface salinity and ocean currents, also in comparison with available observations, will be shown.

How to cite: Bonino, G., Iovino, D., and Masina, S.: Ocean sensitivity to bulk formulae parameterization: a NEMO-ORCA025 model study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9591, https://doi.org/10.5194/egusphere-egu2020-9591, 2020.

EGU2020-11444 | Displays | OS4.8

Antarctic Circumpolar Current Biases in a hierarchy of HadGEM3-GC3.1 Models

Tomas Jonathan, Helen Johnson, David Marshall, Mike Bell, and Patrick Hyder

The Southern Ocean is a crucial part of the global ocean circulation. The unique bathymetry and lack of meridional boundary in conjunction with an equator to pole temperature gradient and strong westerly winds results in an eastward flowing Antarctic Circumpolar Current (ACC). The ACC is the strongest ocean current in the world (173.3 ± 10.7Sv), vital in transporting heat, carbon and nutrients between the major ocean basins. 

Using prototype UK CMIP6 (HadGEM3-GC3.1) simulations at 1°, 1/4° and 1/12° spatial resolutions we illustrate the strong resolution dependence of the strength of the ACC through the Drake Passage. All three model resolutions exhibit a weak ACC compared to observations. The 1/4° and 1/12° models show a significant weakening over the first 50 years, stabilizing at 60Sv and 120Sv respectively.

We analyse the source of the weaker volume transport by decomposing the ACC transport into components arising due to northern and southern boundary density profiles (relative to the bottom density), Ekman transport and depth-independent flow. We attribute the weaker ACC in the 1/4° model to a lightening of the southern density profile and the formation of a reverse flow along the coast of Antarctica.

Our decomposition highlights the significant contribution to the ACC’s volume transport and variability made by both northern and southern density profiles, as well as the depth-independent component of the flow.

How to cite: Jonathan, T., Johnson, H., Marshall, D., Bell, M., and Hyder, P.: Antarctic Circumpolar Current Biases in a hierarchy of HadGEM3-GC3.1 Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11444, https://doi.org/10.5194/egusphere-egu2020-11444, 2020.

EGU2020-18908 | Displays | OS4.8

Climate scenarios of the Gulf of Bothnia using a high-resolution regional ocean model (NEMO-SCOBI)

Sam Fredriksson, Simo Siiriä, Annu Oikkonen, Petra Roiha, Jani Särkkä, Robinson Hordoir, Anders Höglund, Jenny Hieronymus, Kari Eilola, Itzel Ruvalcaba Baroni, and Lars Arneborg

How will the Gulf of Bothnia be impacted by future climate change? A changing climate will, in addition to warming, reduce the ice-season, change the salinity, sea level, and wave and current conditions. This will in turn have implications for eco systems, habitats, biodiversity, as well as human activities such as fishing, aquaculture, and wind parks. The SmartSea project aims to estimate the climate change impacts in this area. This study, which is part of the project SmartSea, assesses the changing physical and biogeochemical properties up to year 2059 using numerical experiments with forcing from two different Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and four different global climate models. Here NEMO3.6 with LIM3 sea ice model is coupled to the biogeochemical model SCOBI. The model comprises the Gulf of Bothnia with a horizontal resolution of approximately one nautical mile.

The preliminary results comparing periods 1975-2005 and 2030-2059 and the pathway RCP4.5 and RCP8.5 show significant changes in sea ice conditions including a decrease of the ice season length, annual maximum ice volume, and extent of ice cover. In addition, the annual maximum ice volume is seen to arise earlier in the season. The temperature increases consistently, although the actual increase between the different simulations varies considerably. A general trend of decreasing salinity can also be seen. This is, however, less systematic than for ice conditions and temperature. The simulations indicate that the changes in both temperature and salinity are not spread evenly, but some areas will be affected more than others. The flow speed trends have been studied by comparing simulations for the period 1980-2005 and the pathway RCP4.5 and RCP8.5 for 2040-2059. The simulations indicate a rise in both local maximum flow speeds, as in average flow speeds, both in surface currents and depth averaged currents (barotropic currents).

How to cite: Fredriksson, S., Siiriä, S., Oikkonen, A., Roiha, P., Särkkä, J., Hordoir, R., Höglund, A., Hieronymus, J., Eilola, K., Ruvalcaba Baroni, I., and Arneborg, L.: Climate scenarios of the Gulf of Bothnia using a high-resolution regional ocean model (NEMO-SCOBI), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18908, https://doi.org/10.5194/egusphere-egu2020-18908, 2020.

EGU2020-22192 | Displays | OS4.8

Eddy-Diffusivity Mass-Flux Parameterization: A Approach to unify Diffusion and Convection in Ocean Models

Romain Bourdallé-Badie, Hervé Giordani, and Gurvan Madec

A new one-dimensional parameterization of penetrative deep convection
has been developed for ocean general circulation models (OGCMs). This work is
motivated by the necessity for OGCMs to better represent the vertical mixing,
the production and properties of deep and intermediate waters, the overflows
and the transport of biogeochemical materials. Our approach is inspired from
atmospheric parameterizations of shallow convection which assumes that in the
convective boundary layer, the subgrid-scale fluxes result from two different
mixing scales : small eddies which are represented by an eddy-diffusivity
contribution, and large eddies associated with thermals which are represented
by a mass-flux contribution. The local (small eddies) and non-local
(large eddies) contributions are unified into an Eddy-Diffusivity-Mass-Flux
(EDMF) parameterization which treats simultaneously the whole vertical mixing.
EDMF is implemented and tested into the communautary ocean model NEMO. Deepening of dense water in 1D analytic cases, sucessfully reproduced in LES simulations, is significantly better captured with EDMF than with standard diffusion parameterizations. Also the convective events observed during winter 2013 in the western Mediterranean at the Lion station are more realistic in terms of sequencing and intensity with EDMF. We show that the best performances of EDMF are due to a best representation of the non-local entrainment fluxes which are counter-gradient in the stratified zone of the thermocline.

How to cite: Bourdallé-Badie, R., Giordani, H., and Madec, G.: Eddy-Diffusivity Mass-Flux Parameterization: A Approach to unify Diffusion and Convection in Ocean Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22192, https://doi.org/10.5194/egusphere-egu2020-22192, 2020.

OS4.9 – Marine Pollution Monitoring, Predictions and Risk Mapping

EGU2020-6032 | Displays | OS4.9

A general methodology for beached oil spill hazard mapping and its application to the Atlantic basin coasts

Nadia Pinardi, Augusto Sepp-Neves, Francesco Trotta, and Antonio Navarra

The current lack of a standardized approach to compute the coastal oil spill hazard due to maritime traffic accidental releases has hindered an accurate estimate of its global impact, which is paramount to manage and intercompare the associated risks. We propose here a hazard estimation approach that is based on ensemble simulations and the extraction of the relevant frequency distributions. We demonstrate that both open ocean and beached oil concentration distributions fit a Weibull curve, a two-parameter fat-tail probability distribution function. The simulation experiments are carried out in all the coastal areas of the Atlantic ocean basin. An indicator that quantify the coastal oil spill hazard is proposed and applied to the study areas.

How to cite: Pinardi, N., Sepp-Neves, A., Trotta, F., and Navarra, A.: A general methodology for beached oil spill hazard mapping and its application to the Atlantic basin coasts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6032, https://doi.org/10.5194/egusphere-egu2020-6032, 2020.

EGU2020-13905 | Displays | OS4.9

Relevant CMEMS products to predict oil slick drift in the Grande America accident

Pierre Daniel, Marie Drevillon, Bruno Levier, and Vincent Gouriou

The Copernicus Marine Environmental Monitoring Service (CMEMS) is a unique capability to provide daily state-of-the-art ocean analyses and forecasts. In the event of accidental marine pollution incidents, those products can help predict where slicks of pollutant and other substances spilled at sea will drift. For a given area, several products are generally available and it is sometimes difficult to know which is the most suitable for this type of use. The use of several CMEMS products during a major accident in the Bay of Biscay is presented here.

On March 12, 2019, the merchant ship Grande America sank at a depth of 4600 m, 350 km off the French coast, in the Bay of Biscay. It caused a spill of bunker oil and loss of containers. The MOTHY drift model was used daily during the aerial surveillance and recovery at sea period. It provided drift forecasts for oil slicks and containers up to 3 days in deterministic mode and up to 10 days in probabilistic mode. Long-term modelling of residual diffused pollution was also carried out, in particular to manage continuous leakage from the wreck. A technical committee of experts met daily to evaluate drift observations and forecasts. It focused on the best choices of available ocean models.

The operational ocean analysis and forecasting systems IBI (Iberian Biscay Irish) at 1/36 degree and Global Mercator at 1/12 degree were used. They led to significant differences in drift predictions, and only one of the two systems was retained after a few days of use. These differences are analysed in the light of available observations.

Drift forecasts did not indicate any oil arrival to the coast. This allowed the authorities to organize the response at sea without mobilizing resources ashore. No pollution was indeed observed on the coasts.

How to cite: Daniel, P., Drevillon, M., Levier, B., and Gouriou, V.: Relevant CMEMS products to predict oil slick drift in the Grande America accident, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13905, https://doi.org/10.5194/egusphere-egu2020-13905, 2020.

EGU2020-2982 | Displays | OS4.9 | Highlight

Oil Pollution in the Mediterranean Sea: An Overview

Angela Carpenter

Oil pollution can enter the marine environment from many sources including land, shipping, and oil installations. It can have significant impacts on the marine environment that, depending on the type of oil, can last for prolonged periods of time. Monitoring oil pollution in the Mediterranean Sea region has been conducted using both aerial and satellite surveillance. This presentation will provide an overview of the sources and volumes of oil entering the Mediterranean, identify impacts on the marine ecosystem in general terms, and will review surveillance activities in the region, including cooperative activities undertaken by regional and EU agencies, for example.

 

How to cite: Carpenter, A.: Oil Pollution in the Mediterranean Sea: An Overview, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2982, https://doi.org/10.5194/egusphere-egu2020-2982, 2020.

EGU2020-21297 | Displays | OS4.9 | Highlight

Fate and dynamic of marine micro plastics in the Baltic Sea

Jens Murawski and Jun She

The fate of micro plastic litter (<5mm) in the marine environment is still largely unknown. H2020 project CLAIM (Cleaning Litter by developing and Applying Innovative Methods) uses model based assessments to improve our knowledge on the micro plastic sources, sinks and pathways. A systematic approach for assessing the sources and pathways of land emitted microplastics was developed and applied for two types of micro plastic from car tyres and cosmetic products. After entering the sea, micro plastic pollutants are affected by transport, biofouling, sinking and sedimentation. A 3D modelling tool has been developed by including these processes. Multi-years-studies (2013-2018) were performed to evaluate seasonal drift pattern and accumulation zones. DMI’s high resolution ocean circulation model HBM, in 900m resolution for the entire Baltic Sea serves as the modelling platform for the assessments. It was found that the fate of micro plastics at sea depends largely on transport and the sinking processes, including biofouling. Tyre wear is heavier than sea water and is accumulating in the deeper parts of the Baltic Sea, whereas micro plastics from cosmetic products are lighter than sea water and are floating near the surface, until biofouling has increased their density sufficiently enough. Key processes that determine the fate micro plastics at sea are introduced and the results of the multi-year study: drift pattern and accumulation zones are presented.

How to cite: Murawski, J. and She, J.: Fate and dynamic of marine micro plastics in the Baltic Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21297, https://doi.org/10.5194/egusphere-egu2020-21297, 2020.

EGU2020-22636 | Displays | OS4.9 | Highlight

Cleaning marine Litter by developing and Applying Innovative Methods (the CLAIM H2020 project)

George Triantafyllou, Jun She, Joydeep Dutta, Marco Faimali, Michael St. John, Roy Brouwer, Pavel Stoev, and George Triantaphyllidis

EGU2020-5977 | Displays | OS4.9

Backtracing of marine litter and microplastic from OSPAR beaches in the North Atlantic

Johannes Röhrs, Knut-Frode Dagestad, Cecilie Mauritzen, Kjersti Opstad Strand, Bjørn Einar Grøsvik, and Leticia Antunes Nogueira

Marine litter has been systematically registered at selected beaching sites within a framework of the OSPAR commission. We select a number of sites in the North Sea, Norwegian Sea and the Barents Sea to investigate where marine litter at these site could come from. Using results from hydrodynamic ocean models, wave models and atmospheric forecasts we backtrace litter from the beaching sites to possible origins within a limited time frame at the order of years. While the identified sources are hypothetical at first, we compare the types of registered plastic litter with reasonable sources in the regions identified as possible origins from the model. Thereby we distinguish between fishery related litter, industrial litter and litter from personal consumption, as the composition between these types differ between the OSPAR sites. Our modeling experiments are designed in co-production with  stakeholders for planning strategies to address and reduce marine litter.

How to cite: Röhrs, J., Dagestad, K.-F., Mauritzen, C., Opstad Strand, K., Grøsvik, B. E., and Antunes Nogueira, L.: Backtracing of marine litter and microplastic from OSPAR beaches in the North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5977, https://doi.org/10.5194/egusphere-egu2020-5977, 2020.

EGU2020-8659 | Displays | OS4.9

Drift of buoyant objects in the Baltic sea – model data analysis

Liga Bethere, Aigars Valainis, Juris Sennikovs, and Uldis Bethers

Marine debris and pollution of the sea is well recognized problem. Knowledge of the potential destination and time of arrival for buoyant or nearly buoyant contaminants as, for example, microplastics, is necessary for effective policy planning.

This work analyzes characteristics of buoyant objects in the Baltic Sea using simulations of Lagrangian particle movement. Simulations are based on current and wind model data. Initially particles are regularly distributed (spaced 5 km) over the Baltic Sea and a new simulation and particle release is started every day over a period of 10 years – years 2008-2017. It is assumed that upon reaching the coast particle gets washed out on the coast.

The aim of this work was to acquire following 3 drift characteristics for possible buoyant object movement in the Baltic Sea:

  1. How many days does it take from different regions of the Sea to reach the coast, what regions (clusters) can be identified that share similar behavior for different seasons;
  2. Which coastal regions are most at risk – which regions get particles washed out the most;
  3. What are main pathways for the particles – which sea regions affect which coastal regions the most.

As the distributions of floating time and location are non-normal then the methods of Symbolic Data Analysis (SDA) were used. To be more exact, statistics from each sea point or coastal segment was described by empirical distribution function (histogram) and differences/similarities were calculated using squared Wasserstein distance. The simulations cover multiple seasons – therefore the difference between seasons is also examined for each of 3 drift characteristics.

Part of the research is supported from the Latvian Academy of Sciences, project lzp-2018/1-0162 DRIMO - DRIft MOdelling for pollution reduction and safety in the Baltic Sea, 2018 - 2021.

How to cite: Bethere, L., Valainis, A., Sennikovs, J., and Bethers, U.: Drift of buoyant objects in the Baltic sea – model data analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8659, https://doi.org/10.5194/egusphere-egu2020-8659, 2020.

EGU2020-2946 | Displays | OS4.9

Oil spill modeling for the Port of Taranto (SE Italy)

Svitlana Liubartseva, Ivan Federico, Giovanni Coppini, and Rita Lecci

The Taranto Sea is a Mediterranean lagoon where alarming pressure is expected to further increase, due to industrialization, heavy ship traffic, and densely populated coasts. The area hosts the Trading Port, Industrial Port, and Container Terminal. There is an important refinery, owned by ENI's Refining&Marketing, with a potential of 6 million tons per year (Autorità di Sistema Portuale del Mar Ionio – Porto di Taranto, 2017). A buoyed area in the Mar Grande is used by tankers of up to 300,000 GRT carrying petroleum for the refinery. Being at risk of oil pollution, the Taranto Sea became a pilot site for the development of a universal relocatable platform aimed at the real time management of marine pollution events in the harbors and ports in the framework of the IMPRESSIVE Project.

According to a Project paradigm, marine pollution forecasting system in harbors includes (1) EO observation technologies (satellite, ASV, UAV); (2) high-resolution hydrodynamic models based on downscaling of CMEMS products, and (3) pollution transport models.

To implement the system components for the Taranto Sea the Lagrangian oil spill model MEDSIK-II has been coupled to Southern Adriatic Northern Ionian coastal Forecasting System (SANIFS http://sanifs.cmcc.itFederico et al., 2017) and ECMWF atmospheric forecast. To this end, the SANIFS output discretized on the unstructured horizontal grid at a variable resolution of 3–4 km for the open sea and of 50–500 m for the coastal area is interpolated to a regular grid with a resolution of 150 m. For the first time, MEDSLIK-II can use currents and sea surface temperature of such the resolution, which is almost 15 times less than previously exploited horizontal resolution for the Pilot sites in the framework of coupling to the Adriatic Forecasting System (AFS) (Guarnieri et al., 2010).

The new coupling is planned to run the MEDSLIK-II simulations in stochastic mode in order to evaluate the environmental consequences of possible accidents and malfunctions in the ENI petroleum transport system.

This work is performed in the framework of the IMPRESSIVE project (#821922) co-funded by the European Commission under the H2020 Programme.

References:

Autorità di Sistema Portuale del Mar Ionio – Porto di Taranto, 2017. Three-year operational plan 2017–2019 and Port vision 2030 of the Port of Taranto. http://www.port.taranto.it/index.php/en/

Federico, I., Pinardi, N., Coppini, G., Oddo, P., Lecci, R., Mossa, M. 2017. Coastal ocean forecasting with an unstructured grid model in the southern Adriatic and northern Ionian seas. Nat. Hazards Earth Syst. Sci., 17, 45–59, doi: 10.5194/nhess-17-45-2017.

Guarnieri, A., Oddo, P., Pastore, M., Pinardi, N., 2010. The Adriatic Basin Forecasting System new model and system development. Coastal to Global Operational Oceanography: Achievements and Challenges, pp. 184–190.

How to cite: Liubartseva, S., Federico, I., Coppini, G., and Lecci, R.: Oil spill modeling for the Port of Taranto (SE Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2946, https://doi.org/10.5194/egusphere-egu2020-2946, 2020.

EGU2020-9407 | Displays | OS4.9

Development of a system for monitoring and assessment of underwater sound

Veselka Marinova and Asen Stefanov

The Bulgarian Black Sea coast is an area with intense human activity, but also there is a complex ecosystem. Several anthropogenic sources generate loud sound levels in this area. The two most wide spread are maritime transport and hydrocarbon exploration and exploitation in the Bulgarian offshore sector. Underwater sound may have negative impact on animals in the Bulgarian waters that are sensitive to sound, such as marine mammals and certain fish species. 

The knowledge of ambient noise levels is very important for the characterization of the environmental status with regard to the European Marine Strategy Framework Directive (MSFD). The directive is aiming at a more effective protection of the marine environment including the protection of marine life exposed to noise, and the improvement of the health of the marine environment as a whole.

To estimate this impact the ocean technologies department of Bulgarian Institute of Oceanology developed a system to monitor the sound generated by marine activities following the TSG Noise guidance. The aim was to provide an integrated solution to monitor and asses the noise impact of ship traffic or other marine activities. The development was funded by program BG02 "Integrated management of marine and inland waters" financed by the financial mechanism of the European economic Area (EEA FM) 2009-2014.

The system consists of monitoring and simulation components. The combination of numerical modelling and noise measurements at selected locations offers a credible solution to the problem of underwater noise monitoring. The monitoring component comprises an array of passive sound recorders, equipped with hydrophones, self-contained power supplies, data acquisition and storage electronics. The simulation tool is in development. Suitable modelling approaches, modelling scenarios, and acoustic model input values are being selected and applied for the most important sources of sound and for underwater sound propagation in the Bulgarian waters. The tool computes sound maps produced by multiple noise point sources, as input for assessment of the environmental status. For optimum results, the simulation tool will be validated using acoustic measurements provided by the monitoring tool.

Future work includes development of a post-processing tool of the sound maps to obtain indicators relevant to both the noise anthropogenic pressure and biological effect of underwater noise impact on marine life.

How to cite: Marinova, V. and Stefanov, A.: Development of a system for monitoring and assessment of underwater sound, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9407, https://doi.org/10.5194/egusphere-egu2020-9407, 2020.

Marine environmental security are facing increasingly prominent problems in recent years. Under the current context of big data, most of the emergency event participants are in a dual dilemma of "knowledge ocean" and "knowledge deficiency" in their responses to emergencies, leading to a result that they fail to quickly acquire and use relevant knowledge. For the need of risk prevention and control over marine environmental security events, it is necessary to make research on the knowledge base of marine environmental security risks’ prevention and control and organize the knowledge base system based on the knowledge related to storm surges, enteromorpha, marine oil spills, tsunamis, sea ice and other marine environmental security events, including such contents as basic knowledge, laws and regulations, plans, cases, prevention and preparation, monitoring and early warning, disposal and rescue, and so on. Through such methods as knowledge maps, multi-type knowledge coupling, digitization of risk prevention and control knowledge and the like, the ontology and detailed element sets are constructed for the knowledge base on prevention and control over comprehensive risks against marine environmental safety, thus forming the entity of the risk prevention and control knowledge base.

How to cite: Wu, L., Xu, F., and Sun, L.: Research on the construction of knowledge base system of Marine disasters and emergencies risk prevention and control, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12815, https://doi.org/10.5194/egusphere-egu2020-12815, 2020.

EGU2020-20228 | Displays | OS4.9

Oil spill forecasting systems uncertainty assessment in the Aegean Sea

Konstantinos Kampouris, Vassilios Vervatis, John Karagiorgos, and Sarantis Sofianos

The Aegean Sea is one of the world’s busiest waterways, combined with complex and intense weather and sea current patterns with strong seasonality, complicated coastline and bathymetry. Therefore, the uncertainty assessment of the oil spill forecasting systems in this region is of great interest. The purpose of this study is to evaluate the impact of the uncertainty of the atmospheric forcing on the performance of the oil spill modelling and the dispersion of the pollutants in the marine environment. Ensemble simulations were carried out using the ECMWF Ensemble Prediction System and the oil spill model MEDSLIK II. The Aegean Sea was chosen as the study area performing ensembles of 50 members with seven days forecast lead time, during different seasons. Three types of oil were chosen representing lighter, medium and heavier oil spills, covering also a wide range of oil densities. The oil spill duration and the spill rate were chosen taking into account significant accidents of the past like for instance the Prestige case. Preliminary results suggest that the model errors in the oil spill trajectories are sensitive to the atmospheric forcing uncertainties.

Keywords: Aegean Sea, MEDSLIK II, Oil spill, ensemble simulation, uncertainty

How to cite: Kampouris, K., Vervatis, V., Karagiorgos, J., and Sofianos, S.: Oil spill forecasting systems uncertainty assessment in the Aegean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20228, https://doi.org/10.5194/egusphere-egu2020-20228, 2020.

EGU2020-20673 | Displays | OS4.9

Estimate hydrodynamic connectivity and probability of contamination through Lagrangian experiments in a high resolution shelf sea model

Michele Bendoni, Carlo Brandini, Maria Fattorini, Chiara Lapucci, and Carlo Pretti

Coastal areas are experiencing an increasing anthropic pressure worldwide, especially due to port activities. In addition, valuable ecosystems such as Marine Protected Areas (MPA) might be located close to ports and be potentially subject to pollutant driven by the local current pattern. It is then fundamental to develop tools to analyze and quantify the tendency of a MPA to be affected by generic pollutant released from a port. Present work is based on a series of Lagrangian experiments carried out on a domain containing the port of Livorno and the Meloria Sholas MPA, located in the Tuscany Archipelago (Italy). The flow field employed to force the experiments is obtained from a downscaling modelling chain implemented with the 3D ROMS software. The top level is a 1.2 km low-resolution model covering the North-West portion of the Mediterranean basin which feeds with a one-way nesting algorithm a 400 m mid-resolution model for the Tuscany Archipelago, extending West of Corsica Island and up to the Gulf of Genova. The inner level of the modelling chain is a 50 m high-resolution coastal model (offline nesting) which covers the area of Meloria Shoals, the port, and their surroundings. Hydrodynamic simulations are carried out for one year. Initial conditions are provided by the CMEMS (1/24° res) model Analysis, as well as boundary conditions for the low-resolution model. Atmospheric forcing comes from the downscaling of the ERA-5 reanalysis dataset, consisting on the BOLAM model implemented on a 7 km grid of the Med-CORDEX domain, in which the MOLOCH model is nested on a 2.5 km spaced grid. Lagrangian numerical experiments are carried out considering the consecutive release of passive particles in the port area, at finite intervals for one year, following the trajectories for ten days. To estimate the degree of hydrodynamic connectivity between the port and the MPA and give a measure of the probability of contamination, the “oceanographic distance” is computed in several ways from the calculated trajectories. Preliminary results show the main transport pattern is mostly distributed alongshore, making the MPA less connected to the port compared to areas placed at the same distance.

How to cite: Bendoni, M., Brandini, C., Fattorini, M., Lapucci, C., and Pretti, C.: Estimate hydrodynamic connectivity and probability of contamination through Lagrangian experiments in a high resolution shelf sea model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20673, https://doi.org/10.5194/egusphere-egu2020-20673, 2020.

EGU2020-8914 | Displays | OS4.9

Particle fluxes and trace metal dispersal in the Gulf of Vera, SW Mediterranean Sea

Marta Tarrés, Marc Cerdà-Domènech, Anna Sanchez-Vidal, Rut Pedrosa-Pàmies, Aitor Rumín-Caparrós, Antoni Calafat, and Miquel Canals

Quantifying fluxes of particulate trace metals to the deep sea is a necessary step to understand their role in many oceanographic processes. The Gulf of Vera, in the SW Mediterranean Sea, is an ideal place to quantify these fluxes given the presence of a metal-rich mine tailings deposit in Portmán Bay, in its northern shore. Portmán Bay is one of the most extreme cases in Europe of mine waste impacts on coastal ecosystems, both inshore and offshore. About 57 million tons of tailings enriched in Fe, Pb, Mn, Zn, As, Ti and other metals were dumped there from 1957 to 1990 [1].

In the frame of the NUREIEV project, five mooring lines equipped with sediment traps and current meters were deployed in the wider Gulf of Vera. Three of the moorings were located in the middle course of three submarine canyons, one in the open continental slope and one in the deep basin. The monitoring period lasted for an annual cycle (March 2015 - March 2016). Our research focused on (1) measuring the temporal variability of particle fluxes in the study area, and (2) determining the concentration of particulate trace metals and metalloids (Zn, Pb, As, Ni, Cd, Cu, Co) as well as other metals (Fe, Mn, Ti) in settling particles.

Preliminary results within the following project NUREIEVA indicate that marine storms are the main processes triggering the transfer of particulate matter to the deep margin, mainly through hundred meters thick nepheloid layers moving not only along the submarine canyons but also across the open slope. High metal fluxes after storms reached up to 120.4 mg Pb m-2d-1, 282.3 mg Zn m-2d-1 and 40.4 mg As m-2d-1 in the canyon stations, four orders of magnitude higher than calculated in open Ligurian Sea [2]. This suggests that submarine canyons are efficient pathways for the transfer of pollutants from the shelf to the deep margin and basin [3].

Peak metal concentrations found so far in the wider Gulf of Vera exceed those found in other canyons in the Mediterranean Sea [2, 4]. The hypothesis that this could be related to the release of metals from the mine tailings deposit in Portmán is plausible but would require further work to be confirmed, including determination of enrichment factors in each station, and discerning between Portmán’s and other potential metal sources.

 

[1] Oyarzun, R. et al., 2013. Sci. Total Environ. 454-455, 245-249.

[2] Heimbürger, L.E. et al., 2012. Chem. Geol. 291, 141-151.

[3] Canals, M. et al., 2013. Prog. Oceanogr. 118, 1-27.

[4] Palanques, A. et al., 2008. Mar. Geol. 248, 213-227.

How to cite: Tarrés, M., Cerdà-Domènech, M., Sanchez-Vidal, A., Pedrosa-Pàmies, R., Rumín-Caparrós, A., Calafat, A., and Canals, M.: Particle fluxes and trace metal dispersal in the Gulf of Vera, SW Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8914, https://doi.org/10.5194/egusphere-egu2020-8914, 2020.

EGU2020-12653 | Displays | OS4.9

NOOS-Drift, an innovative operational transnational multi-model ensemble system to assess ocean drift forecast accuracy.

Sebastien Legrand, Knut-Frode Dagestad, Pierre Daniel, Michel Kapel, and Samuël Orsi

In case of maritime pollution, man-overboard, or objects adrift at sea, national maritime authorities of the 9 countries bordering the European North West Continental Shelf (NWS) rely on drift model simulations in order to better understand the situation at stake and plan the best response strategy. So far, the drift forecast services are mainly managed at national levels with almost no integration at the transnational level. Designed as a support service to the national drift forecasting services, NOOS-Drift has the ambition to change this paradigm.   

NOOS-Drift is a distributed transnational multi-model ensemble system to assess and improve drift forecast accuracy in the European North West Continental Shelf. Developed as a one-stop-shop web service, the service allows registered users (national drift model operators or trained maritime authorities) to submit on-demand drift simulation requests to be run by all the national drift forecasting services connected to NOOS-Drift. Within 15 minutes after activation, the NOOS-Drift users shall get access to the drift simulation results of the individual ensemble members, as well as the results of a multi-models joint analysis assessing the ensemble spread and delineating risk areas to locate possible maritime pollution. This operation of such a distributed multi-models service is to our knowledge a world premiere. 

In this communication, we will present the technical and scientific developments that had to be done to make this service possible, including: 

  1. a robust, secure and latency-free communication system that coordinates the execution of the different national models
  2. a strategy to build the multi-model ensemble
  3. a definition of drift forecast accuracy
  4. the joint multi-model analysis tools
  5. the standard file formats and visualisation means 

Finally we will illustrate on an example how the NOOS-Drift service could change the decision making process.

How to cite: Legrand, S., Dagestad, K.-F., Daniel, P., Kapel, M., and Orsi, S.: NOOS-Drift, an innovative operational transnational multi-model ensemble system to assess ocean drift forecast accuracy., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12653, https://doi.org/10.5194/egusphere-egu2020-12653, 2020.

EGU2020-15191 | Displays | OS4.9

Monitoring and forecasting of marine pollution in the Mediterranean Sea: the ODYSSEA project approach

Katerina Spanoudaki, Nikolaos Kokkos, Konstantinos Zachopoulos, Georgios Sylaios, Nikolaos Kampanis, Dave de Koning, Lorinc Meszaros, Sonja Wanke, and Ghada El Serafy

The H2020 funded project ODYSSEA (http://odysseaplatform.eu/) aims to make Mediterranean marine data easily accessible and operational to a broad range of users of the marine space. ODYSSEA develops an interoperable and cost-effective platform, fully integrating networks of observing and forecasting systems across the Mediterranean basin, addressing both the open sea and the coastal zone. The platform integrates marine data from existing Earth Observing Systems, such as Copernicus and EMODnet, receives and processes novel, newly produced datasets (through high-resolution models and on-line sensors such as a novel microplastics sensor) from nine prototype Observatories established across the Mediterranean basin, and applies advanced algorithms to organise, homogenise and fuse the large quantities of data in order to provide to various end-user groups and stakeholders both primary data and on-demand derived data services.

The nine ODYSSEA Observatories are established across the whole Mediterranean basin, covering also areas of marine data gaps along the North African and Middle East coastline. The Observatories comprise observing and forecasting systems and cover coastal and shelf zone environments, Marine Protected Areas and areas with increased human pressure. The operational forecasting system of the Observatories consists of a ‘chain’ of dynamically coupled, high-resolution numerical models comprised of a) the hydrodynamic model Delft3D-FLOW, b) the wave model Delft3D-WAVE (SWAN), c) the water quality model DELWAQ, d) the oil spill fate and transport model MEDSLIK-II, e) the ecosystem model ECOPATH, and f) the in-house mussel farm model developed by the Democritus University of Thrace. This operational system provides forecasts, early warnings and alerts for currents, waves, water quality parameters, oil spill pollution and ecosystem status. In this work, the ODYSSEA forecasting system (developed with the Delft-FEWS software) is implemented for simulating oil spill pollution for the Thracian Sea Observatory.  The area is biodiversity rich and an important spawning and nursery ground for small pelagic species, while in Kavala Gulf, oil exploitation takes place. The Lagrangian oil spill model MEDSLIK-II has been coupled to high-resolution oceanographic fields (currents, temperature, Stokes drift velocity), produced by Delft3D-FLOW and SWAN, and NOAA GFS atmospheric forcing. The hydrodynamic and wave models have been configured for the Thracian Sea based on dynamic downscaling of CMEMS products to a grid resolution of 1/120°. Seasonal hazard maps (surface oil slick, beached oil) are produced employing multiple oil spill scenarios using multi-year hydrodynamics. The results highlight the hazard faced by Thracian Sea Observatory coasts. 

Acknowledgements: This research has received funding from the European Union’s Horizon 2020 research and innovation programme ODYSSEA: OPERATING A NETWORK OF INTEGRATED OBSERVATORY SYSTEMS IN THE MEDITERRANEAN SEA, GA No 72727.

How to cite: Spanoudaki, K., Kokkos, N., Zachopoulos, K., Sylaios, G., Kampanis, N., de Koning, D., Meszaros, L., Wanke, S., and El Serafy, G.: Monitoring and forecasting of marine pollution in the Mediterranean Sea: the ODYSSEA project approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15191, https://doi.org/10.5194/egusphere-egu2020-15191, 2020.

EGU2020-5199 | Displays | OS4.9

Identification of marine litter accumulation areas in estuaries

Paula Núñez, Andrés García, Ana J. Abascal, Inés Mazarrasa, Javier F. Bárcena, Helios Chiri, Tamara Rodriguez-Castillo, José A. Juanes, and Raúl Medina

Marine litter is one of the main threats for the marine environment, causing significant damage at ecological, economic and social levels. Approximately, 80% of the marine litter comes from land-based sources, mainly from rivers. A significant percentage of this litter reaches the open oceans and the rest are retained inside the estuaries. The mechanisms that favor the marine litter accumulation inside the estuaries are determined by the interactions between the tide, the river flow, the waves, and the wind, as well as by the physical features of the estuary. In addition, tides and waves can contribute to the introduction of litter from marine sources. Therefore, estuaries frequently act as sinks for marine litter. In this study, a methodology to identify the most probable areas of marine litter accumulation inside estuaries is developed. The methodology is based on numerical modeling and statistical analysis and consists of 4 fundamental steps. First, a series of metocean scenarios (tidal conditions, river flow, waves, and wind), statistically representative of the studied estuary, are identified using the clustering algorithm K-means. Second, these conditions are used as forcing and boundary conditions of a hydrodynamic model to obtain the high spatial resolution currents and waves that determine the transport of litter. Third, with these high-resolution drivers, a Lagrangian transport model is fed to generate a database of potential marine litter trajectories from different litter-sources, where each litter-source is carefully selected according to the activity developed in the area. Finally, the statistical analysis of these trajectories allows identifying the most probable areas of marine litter accumulation. The efficacy of this methodology is demonstrated by its application to the Pas estuary (northern coast of Spain) by comparing the numerical results with field data. Results show that the greatest accumulations of marine litter take place in the curves that imply important changes in the direction of the flow, where the probabilities range between 20 and 30%, and that the distance to the litter-sources also plays a fundamental role.

How to cite: Núñez, P., García, A., Abascal, A. J., Mazarrasa, I., Bárcena, J. F., Chiri, H., Rodriguez-Castillo, T., Juanes, J. A., and Medina, R.: Identification of marine litter accumulation areas in estuaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5199, https://doi.org/10.5194/egusphere-egu2020-5199, 2020.

EGU2020-5386 | Displays | OS4.9

Evaluation of the Leviathan offshore platform environmental studies in the Eastern Mediterranean Sea

George Zodiatis, Svitlana Liubartseva, Loizos Loizides, Marco Pellegatta, Giovanni Coppini, Robin Lardner, George Kallos, Christina Kalogeri, Roberto Bonarelli, Antonio Augusto Sepp Neves, Panicos Nikolaides, and Avigdor Brillant

One of the largest last decade discoveries of hydrocarbons in the Eastern Mediterranean Sea is the Leviathan field, which constitutes a large-scale energy program of the State of Israel. Gas and condensate from the Leviathan well are transferred via pipeline to an offshore platform located ~10 km from the Israeli shoreline, and from there via a pipeline to the coastal Leviathan energy installation. The local communities are concerned from the pollution implications that might occur in case of spillage and/or any malfunction in regular operation.

The present work includes review of previous environmental studies regarding the Leviathan energy project 2007-2011, new extended simulations 2015-2018 for condensate, diesel and grey water leaks and resultant evaporation simulations caused by possible condensate spillage from the offshore platform and the pipe rupture.

In the framework of the current study concerning the Leviathan offshore platform, a robust statistics  is obtained by 5844 spill simulation runs for condensate and diesel against12 runs as mentioned in the previous studies, while for the pipe rupture a robust statistics was made with 104 runs for condensate and diesel compared to12 runs as performed previously.

The previous spillage scenarios from the offshore platform had underestimated by almost order of magnitude the content per design itself (1000bbls vs. ~6000bbls) and documentation of permits. Similarly, the pipe rupture spillage scenarios underestimated by almost half order of magnitude (1200bbls vs. ~3000bbls). Therefore, the current simulations predicted larger spillage quantities, compared to the aforementioned previous simulations.

The main conclusions driven from the 10km counter simulations for the offshore platform spillage show the following: First oil arrival at the Israeli coast from the offshore platform is predicted to be within 8 hours after start of spillage event in winter, and within 11 hours in summer.The first impacted area is predicted to be the coastline between Zichron-Ya'akov/Dor and Atlit. In winter on average, it is predicted that 17% of the spillage is beached, while in summer, twice as higher, i.e. up to 35%. Deposition of spilled condensate in the Hadera desalination plant is estimated to be the highest among the 5 desalination plants examined.

Similarly, the main conclusions driven from the 1km pipe rupture spillage counter show that the first impact on the Israel is predicted to be within 5-6 hours after start of spillage in winter, and within 3-4 hours in summer, with the worst case scenario occurring within half an hour after start of the spillage. The coastline of Zichron-Ya'akov is found to be an epicenter of the highest condensate deposition up to 15 tons/km, regardless the season. Due to the proximity of the pipe rupture to the shore, it is predicted that 38-40% of the condensate washed up the shore nearby, without any significant seasonal or monthly variability. The condensate spillage from the pipe rupture located 1 km from the shoreline will affect mostly the Atlit, Ma'agan-Michael and Caesarea National parks’ and the Hadera desalination plant coastlines.

How to cite: Zodiatis, G., Liubartseva, S., Loizides, L., Pellegatta, M., Coppini, G., Lardner, R., Kallos, G., Kalogeri, C., Bonarelli, R., Sepp Neves, A. A., Nikolaides, P., and Brillant, A.: Evaluation of the Leviathan offshore platform environmental studies in the Eastern Mediterranean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5386, https://doi.org/10.5194/egusphere-egu2020-5386, 2020.

CC BY 4.0