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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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.


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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 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,, 2020.

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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,, 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,, 2020.

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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,, 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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.

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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,, 2020.