OS1.2

EDI
The North Atlantic: natural variability and global change

The North Atlantic exhibits a high level of natural variability from interannual to centennial time scales, making it difficult to extract trends from observational time series. Climate models, however, predict major changes in this region, which in turn will influence sea level and climate, especially in western Europe and North America. In the last years, several projects have been focused on the Atlantic circulation changes, for instance OVIDE, RACE, OSNAP, and ACSIS. Another important issue is the interaction between the atmosphere and the ocean as well as the cryosphere with the ocean, and how this affects the climate.

Please note that while we hope to hold a session in the traditional format, we anticipate that some part or all of the session may be held online.

We welcome contributions from observers and modelers on the following topics:

-- climate relevant processes in the North Atlantic region in the atmosphere, ocean, and cryosphere
-- response of the atmosphere to changes in the North Atlantic
-- atmosphere - ocean coupling in the North Atlantic realm on time scales from years to centuries (observations, theory and coupled GCMs)
-- interpretation of observed variability in the atmosphere and the ocean in the North Atlantic sector
-- Comparison of observed and simulated climate variability in the North Atlantic sector and Europe
-- Dynamics of the Atlantic meridional overturning circulation
-- variability in the ocean and the atmosphere in the North Atlantic sector on a broad range of time scales
-- changes in adjacent seas related to changes in the North Atlantic
-- role of water mass transformation and circulation changes on anthropogenic carbon and other parameters
-- linkage between the observational records and proxies from the recent past

Co-organized by AS1/CL4
Convener: Richard Greatbatch | Co-conveners: Caroline Katsman, Monika Rhein, Bablu Sinha
vPICO presentations
| Fri, 30 Apr, 11:00–17:00 (CEST)

vPICO presentations: Fri, 30 Apr

Chairperson: Richard Greatbatch
11:00–11:05
11:05–11:15
|
EGU21-2928
|
solicited
Antje Weisheimer, Damien Decremer, David MacLeod, Chris O'Reilly, Tim Stockdale, Stephanie Johnson, and Tim Palmer

Predictions of the winter NAO and its small signal-to-noise ratio have been a matter of much discussion recently. Here we look at the problem from the perspective of 110-year-long historical hindcasts over the period 1901-2010 performed with ECMWF’s coupled model. Seasonal forecast skill of the NAO can undergo pronounced multidecadal variations: while skill drops in the middle of the century, the performance of the reforecasts recovers in the early twentieth century, suggesting that the mid-century drop in skill is not due to a lack of good observational data. We hypothesize instead that these changes in model predictability are linked to intrinsic changes of the coupled climate system. 

The confidence of these predictions, and thus the signal-to-noise behaviour, also strongly depends on the specific hindcast period. Correlation-based measures like the Ratio of Predictable Components are shown to be highly sensitive to the strength of the predictable signal, implying that disentangling of physical deficiencies in the models on the one hand, and the effects of sampling uncertainty on the other hand, is difficult. These findings demonstrate that relatively short hindcasts are not sufficiently representative for longer-term behaviour and can lead to skill estimates that may not be robust in the future.

See also: Weisheimer, A., D. Decremer, D. MacLeod, C. O'Reilly, T. Stockdale, S. Johnson and T.N. Palmer (2019). How confident are predictability estimates of the winter North Atlantic Oscillation? Q. J. R. Meteorol. Soc., 145, 140-159, doi:10.1002/qj.3446.

How to cite: Weisheimer, A., Decremer, D., MacLeod, D., O'Reilly, C., Stockdale, T., Johnson, S., and Palmer, T.: How confident are predictability estimates of the winter North Atlantic Oscillation?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2928, https://doi.org/10.5194/egusphere-egu21-2928, 2021.

11:15–11:17
|
EGU21-12580
André Düsterhus, Leonard Borchert, Vimal Koul, Holger Pohlmann, and Sebastian Brune

The North Atlantic Oscillation (NAO) has over the year a major influence on European weather. In many applications, being it in modern or paleo climate science, the NAO is assumed to varying in strength, but otherwise often understood as being a constant feature of the pressure system over the North Atlantic. In recent years investigations on the seasonal-predictability of the winter NAO has shown that the prediction skill is varying over time. This opens the question, why this is the case and how well models are able to represent the NAO in all its variability over the 20th century.

To investigate this further we take a look at a seasonal prediction of the NAO with the Max Planck Institute Earth System Model (MPI-ESM) seasonal prediction system, with 30 members over the 20th century. We analyse its dependence of prediction skill on various features of the NAO and the North Atlantic system, like the Atlantic Multidecadal Variability (AMV). As such we will demonstrate, that the NAO is a much less stable system over time as currently assumed and that models may not be in the position to predict its full variability appropriately.

How to cite: Düsterhus, A., Borchert, L., Koul, V., Pohlmann, H., and Brune, S.: Variability of the North Atlantic Oscillation in the 20th century, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12580, https://doi.org/10.5194/egusphere-egu21-12580, 2021.

11:17–11:19
|
EGU21-9501
|
ECS
Florian Börgel, Claudia Frauen, Thomas Neumann, and H. E. Markus Meier

European climate is heavily influenced by the North Atlantic Oscillation (NAO). However, the spatial structure of the NAO is varying with time, affecting its regional importance. By analyzing an 850-year global climate model simulation of the last millennium it is shown that the variations in the spatial structure of the NAO can be linked to the Atlantic Multidecadal Oscillation (AMO). The AMO changes the zonal position of the NAO centers of action, moving them closer to Europe or North America. During AMO+ states, the Icelandic Low moves further towards North America while the Azores High moves further towards Europe and vice versa for AMO- states. The results of a regional downscaling for the East Atlantic/European domain show that AMO-induced changes in the spatial structure of the NAO reduce or enhance its influence on regional climate variables of the Baltic Sea such as sea surface temperature, ice extent, or river runoff.

How to cite: Börgel, F., Frauen, C., Neumann, T., and Meier, H. E. M.: The Atlantic Multidecadal Oscillation controls the impact of the North Atlantic Oscillation on North European climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9501, https://doi.org/10.5194/egusphere-egu21-9501, 2021.

11:19–11:21
|
EGU21-2671
Hai Bui and Thomas Spengler

Sea surface temperatures (SSTs) can influence the development of extratropical cyclones by providing latent and sensible heat through surface fluxes as well as by modifying the environmental low-level baroclinicity. As surface fluxes as well as low-level baroclinicity maximize along the prominent SST fronts associated with the Gulf Stream and Kuroshio, the influence of these mechanisms on cyclone development is anticipated to be strongest along SST fronts. To map the sensitivity to the structure and position of SST fronts during the development of extratropical cyclones, we examine the response of cyclones when they cross an SST front at different directions and speeds. The results are based on idealized numerical simulations with the WRF model, where we prescribe moving SST fronts and a baroclinically unstable environment with an incipient cyclone. Cyclones moving towards the warmer side of the SST front deepen faster and have a faster crossing speed. The diabatic production of eddy available potential energy through latent heating, mainly associated with convection, plays a dominant role in the deepening. Cyclones that move to the colder side of the SST front weaken due to a reduction of available moisture for diabatic processes. However, before these cyclones weaken, they experience a brief period of faster deepening attributable to the enhanced environmental low-level baroclinicity associated with the SST gradient.

How to cite: Bui, H. and Spengler, T.: Response of Extratropical Cyclones when Crossing a Sea Surface Temperature Front, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2671, https://doi.org/10.5194/egusphere-egu21-2671, 2021.

11:21–11:23
|
EGU21-10219
|
ECS
Luca Famooss Paolini, Alessio Bellucci, Paolo Ruggieri, Panos Athanasiadis, and Silvio Gualdi

Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient, which anchors zones of intense upward motion extending up to the upper-troposphere and shapes zones of intense baroclinic eddy activity (storm tracks). For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a potentially relevant source of atmospheric predictability. 

 

General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, the number of studies on this subject is still limited. Furthermore, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. 

 

The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). This project was designed with the specific objective of investigating the impact of increased model horizontal resolution on the representation of the observed climate. Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. 


Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.

How to cite: Famooss Paolini, L., Bellucci, A., Ruggieri, P., Athanasiadis, P., and Gualdi, S.: Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10219, https://doi.org/10.5194/egusphere-egu21-10219, 2021.

11:23–11:25
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EGU21-7685
Thomas Bracegirdle, Hua Lu, and Jon Robson

Decadal variability in indices of North Atlantic (NA) atmospheric circulation plays a major role in changing climate over western Europe. However, reproducing characteristics of this variability in climate models presents a major challenge. Climate models broadly exhibit weaker-than-observed multi-decadal variability in atmospheric circulation indices. A prominent explanation for this is that model-simulated links between anomalous sea-surface temperatures (SSTs) and atmospheric variability are too weak. The dominant mode of basin-wide NA SST variability is Atlantic multi-decadal variability (AMV), which on multi-decadal timescales is expressed more strongly over the NA sub-polar gyre (SPG). SSTs over the SPG region (SSTSPG) are therefore the main focus here.

Studies to date have shown that variability in the North Atlantic Oscillation (NAO) exhibits strongest correlations with AMV indices in late winter, but the reasons for this are not clear. Here we show that this stronger late-winter correlation is particularly clear for SSTSPG and coincides with a climatological equatorward shift of the eddy-driven NA westerly jet from early-to-late winter. To help gain dynamical insight, indices of eddy-driven jet latitude (JLI) and speed (JSI) were correlated with SSTSPG and it was found that they exhibit more pronounced early-to-late winter shifts in correlations than for the NAO; In particular,  correlations strengthen from early-to-late winter for JLI while weaken for JSI. Our results suggest that the jet-SSTSPG linkages progress through winter from JSI dominant in early winter to JLI dominant in late winter.

CMIP5 and CMIP6 models were then evaluated for representation of these observed characteristics in ocean-atmosphere linkages. Consistent with the observed sub-seasonal links between climatological jet latitude and atmosphere-ocean correlation strength, CMIP models with larger equatorward jet biases exhibit weaker JSI-SSTSPG correlations and stronger JLI-SSTSPG correlations. A pronounced early-winter equatorward bias in jet latitude in CMIP models could partially explain the weaker-than observed linkage between SSTs and atmospheric variability.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Observed and CMIP-simulated links between North Atlantic climatological winter jet latitude and inter-annual to decadal ocean-atmosphere coupling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7685, https://doi.org/10.5194/egusphere-egu21-7685, 2021.

11:25–11:27
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EGU21-1310
|
ECS
Shenjie Zhou, Xiaoming Zhai, and Ian Renfrew

The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems – to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.

How to cite: Zhou, S., Zhai, X., and Renfrew, I.: North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1310, https://doi.org/10.5194/egusphere-egu21-1310, 2021.

11:27–11:29
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EGU21-2403
Tobias Schulzki, Klaus Getzlaff, and Arne Biastoch

The southward flow of North Atlantic Deep Water makes up the major component of the AMOC's deepwater limb. In the subtropical North Atlantic, it's flow is concentrated along the continental slope, forming a coherent Deep Western Boundary Current (DWBC). Both, observations and models show a high variability of the flow in this region.
We use an eddy-rich ocean model to show that this variability is mainly caused by eddies and meanders that are generated by barotropic instability. They occur along the entire DWBC pathway and introduce several reciruculation gyres that result in a decorrelation of DWBC transport at 26.5°N and 16°N, despite the fact that a considerable mean transport of 20 Sv connects the two latitudes. Water in the DWBC at 26.5°N is partly returned northward. Because the amount of water returned depends on the DWBC transport itself, a stronger DWBC does not necessarily lead to an increased amount of water that reaches 16°N. 
Along the pathway to 16°N, the transport signal is altered by a broad and temporally variable transit time distribution. Thus, advection in the DWBC cannot account for coherent AMOC changes on interannual timescales seen in the model.

How to cite: Schulzki, T., Getzlaff, K., and Biastoch, A.: On the variability of the DWBC transport between 26.5°N and 16°N in an eddy-rich ocean model and its implications for meridionally coherent changes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2403, https://doi.org/10.5194/egusphere-egu21-2403, 2021.

11:29–11:31
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EGU21-5071
|
Simon Wett, Monika Rhein, Arne Biastoch, Claus Böning, and Klaus Getzlaff

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role for the climate system of Europe and the Arctic. It is responsible for the northward transport of warm and saline water in the upper water column and the southward transport of cold and fresh water in the deep.

Since the early 2000s, observations from ship-based measurements and moorings are available which allow estimates of the individual components of the AMOC. However, the spatial resolution of mooring measurements is coarse and ship-based surveys are mostly done only once a year, adding to the uncertainty of these measurements. Earlier observational studies in the subpolar North Atlantic have found decadal trends of individual AMOC components. However, whether the entirety of the AMOC exhibits a trend remains unclear. Due to the observational limitations, most knowledge about the recent AMOC development is based on model simulations. Comparing these model simulations with observations remains an important task to understand the changes in the AMOC strength in the last decades and improve model representations of the AMOC.

We analyze a realization of the high-resolution VIKING20X ocean model from 1980 to 2019 offering a large overlap with the available observations. We compare it to measurements of the NOAC array at 47°N and sections obtained from repeated ship surveys. We aim to merge observations and model simulation to better estimate recent AMOC changes and increase our understanding of the underlying processes.

How to cite: Wett, S., Rhein, M., Biastoch, A., Böning, C., and Getzlaff, K.: AMOC Evolution at 47°N in the Last Decades in Observations and a High-Resolution Ocean Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5071, https://doi.org/10.5194/egusphere-egu21-5071, 2021.

11:31–11:33
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EGU21-11010
Oliver Gutjahr, Johann H. Jungclaus, Nils Brüggemann, Helmuth Haak, and Jochem Marotzke

Recent observations suggest that deep convection and water mass transformation in the Irminger Sea southeast of Greenland, together with overflows from the Nordic Seas, may be more important for the variability of the Atlantic meridional overturning circulation (AMOC) than the Labrador Sea. The preconditioning for and triggering of deep convection in the Irminger Sea is strongly associated with topography-induced mesoscale wind phenomena, such as Greenland tip jets, katabatic winds and marine cold air outbreaks. However, the resolution of current coupled climate models is too coarse to capture all the properties of these wind systems or to capture them at all. Here we explore the air-ice-ocean interactions induced by mesoscale wind phenomena in the Irminger Sea in a 1-year global coupled 5km simulation with ICON-ESM. The model is able to capture the complex interactions of the wind field and the ocean. We find that strong downward katabatic winds cause substantial heat loss from the Irminger Sea in addition to Greenland tip jets. The outflowing katabatic winds form narrow streaks of cold air that extend across the entire Irminger basin from southeast Greenland to Iceland. In addition, cold air outbreaks from the sea ice lead to the genesis of mesoscale cyclones, which in turn can cause Greenland tip jets before moving off to the east. All these wind phenomena cause substantial heat loss that preconditions the ocean for deep convection. If these wind systems are not resolved, the water mass transformation in the Irminger Sea could be too weak, contributing to why the Labrador Sea dominates AMOC variability in models. We conclude that resolving these mesoscale wind systems in an Earth system model could have significant implications for deep convection and water mass transformation in the Irminger Sea, and thus for AMOC variability.

How to cite: Gutjahr, O., Jungclaus, J. H., Brüggemann, N., Haak, H., and Marotzke, J.: The Irminger Sea – air-ice-ocean interactions in a 5 km coupled simulation with ICON-ESM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11010, https://doi.org/10.5194/egusphere-egu21-11010, 2021.

11:33–11:35
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EGU21-4608
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ECS
Jing Sun, Mojib Latif, and Wonsun Park

There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.

How to cite: Sun, J., Latif, M., and Park, W.: Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4608, https://doi.org/10.5194/egusphere-egu21-4608, 2021.

11:35–11:37
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EGU21-2725
Laura Jackson

The Atlantic Meridional Overturning Circulation (AMOC) influences our climate by transporting heat northwards in the Atlantic ocean. The subpolar North Atlantic plays an important role in this circulation, with transformation of water to higher densities, deep convection and formation of deep water. Recent OSNAP observations have shown that the overturning is stronger to the east of Greenland than the west.

Here we analyse a CMIP6 climate model at two resolutions (HadGEM3 GC3.1 LL and MM) and show both compare well with the OSNAP observations. We explore the source of low frequency variability of the AMOC and how it is related to the surface water mass transformation in different regions. We also investigate time-mean and low frequency water mass transformations in other CMIP6 climate models.

How to cite: Jackson, L.: OSNAP and water mass transport in CMIP6 models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2725, https://doi.org/10.5194/egusphere-egu21-2725, 2021.

11:37–11:39
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EGU21-8434
Adrian New, David Smeed, Arnaud Czaja, Adam Blaker, Jenny Mecking, Jamie Mathews, and Alejandra Sanchez-Franks

Labrador Slope Water (LSLW) is found in the Slope Sea on the US-Canadian eastern shelf-slope as a relatively fresh and cool water mass, lying between the upper layer water masses and those carried by the Deep Western Boundary Current. It originates from the Labrador Current and has previously only been reported in the Eastern Slope Sea (east of 66°W). We here use the EN4 gridded database and the Line W hydrographic observations to show for the first time that the LSLW also penetrates into the Western Slope Sea, bringing it into close contact with the Gulf Stream. We also show that the LSLW spreads across the entire Slope Sea north of the Gulf Stream, and is both fresher and thicker when the Atlantic Meridional Overturning Circulation (AMOC) is high at the RAPID array at 26°N. The fresher, thicker LSLW is likely to contribute an additional 1.5 Sv of Gulf Stream transport. The spreading of the LSLW is also investigated in a high-resolution ocean general circulation model (NEMO), and is found to occur both as a western boundary current and through the extrusion of filaments following interaction with Gulf Stream meanders and eddies. The mechanism results in downward vertical motion as the filaments are entrained into the Gulf Stream. We conclude that the LSLW (rather than the deeper Labrador Sea Water) provides the intermediate depth water masses which maintain the density contrast here which partly drives the Gulf Stream, and that the transport of the LSLW from the Labrador shelf-slope offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting high latitude changes in the Subarctic with subsequent variability in the Gulf Stream and AMOC.

How to cite: New, A., Smeed, D., Czaja, A., Blaker, A., Mecking, J., Mathews, J., and Sanchez-Franks, A.: Labrador Slope Water Connects the Subarctic with the Gulf Stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8434, https://doi.org/10.5194/egusphere-egu21-8434, 2021.

11:39–11:41
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EGU21-7770
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ECS
Damien Couespel, Marina Lévy, and Laurent Bopp

The decline in ocean primary production is one of the most alarming consequences of anthropogenic climate change. This decline could indeed lead to a decrease in marine biomass and fish catch, as highlighted by recent policy-relevant reports. Because of computational constraints, current Earth System Models used to project ocean primary production under global warming scenarios have to parameterize flows occurring below the resolution of their computational grid (typically 1°). To overcome these computational constraints, we use an ocean biogeochemical model in an idealized configuration representing a mid-latitude double-gyre circulation, and perform global warming simulations under increasing horizontal resolution  (from 1° to 1/27°) and under a large range of parameter values for the eddy parameterization employed in the coarse resolution configuration. In line with projections from Earth System Models, all our simulations project a marked decline in net primary production in response to the global warming forcing. Whereas this decline is only weakly sensitive to the eddy parameters in the eddy-parametrized coarse resolution, the simulated decline in primary production is halved at the finest eddy-resolving resolution (-12% at 1/27° vs -26 at 1°). This difference stems from the high sensitivity of the subsurface nutrient transport to model resolution. Our results call for improved representation of the role of eddies on nutrient transport below the seasonal mixed-layer to better constrain the future evolution of marine biomass and fish catch potential for decision-making.

How to cite: Couespel, D., Lévy, M., and Bopp, L.: Oceanic Primary production decline halved in eddy-resolving simulations of global warming, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7770, https://doi.org/10.5194/egusphere-egu21-7770, 2021.

11:41–11:43
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EGU21-12539
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ECS
Charlotte O'Brien, Peter Spooner, David Thornalley, Jack Wharton, Eirini Papachristopoulou, Francesco Pallottino, Svetlana Radionovskaya, Nicolas Dutton, Tianying Li, Rebecca Garratt, and Delia Oppo

Traditionally, deep-sea ecosystems have been considered to be insulated from the effects of modern climate change. Yet, with the recognition of the importance of food supply from the surface ocean and deep-sea currents to sustaining these systems, the potential for rapid response of benthic systems to climate change is gaining increasing attention. North Atlantic benthic responses to past climate change have been well-documented using marine sediment cores on glacial-interglacial timescales, and ocean sediments have also begun to reveal that planktic species assemblages are already being influenced by global warming. However, very few ecological time-series exist for the deep ocean covering the Holocene-through-industrial era. Here, we use benthic and planktic foraminifera found in Northeast Atlantic (EN539-MC16-A/B and RAPID-17-5P), Northwest Atlantic (KNR158-4-10MC and KNR158-4-9GGC) and Labrador Sea (RAPID-35-25B and RAPID-35-14P) sediments to show that, in locations beneath areas of major North Atlantic surface water change, benthic ecosystems have also changed significantly over the industrial era relative to the Holocene. We find that the response of the benthos is dependent on changes in the surface ocean near to the study sites. Our work highlights the spatial heterogeneity of these benthic ecosystem changes and therefore the need for local-regional scale modelling and observations to better understand responses to deep-sea circulation changes and modern surface climate change. 

How to cite: O'Brien, C., Spooner, P., Thornalley, D., Wharton, J., Papachristopoulou, E., Pallottino, F., Radionovskaya, S., Dutton, N., Li, T., Garratt, R., and Oppo, D.: Variable response of North Atlantic deep-sea benthic ecosystems to industrial-era climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12539, https://doi.org/10.5194/egusphere-egu21-12539, 2021.

11:43–12:30
Break
Chairperson: Caroline Katsman
13:30–13:40
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EGU21-940
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solicited
Damien Desbruyères, Léon Chafik, and Guillaume Maze

The Subpolar North Atlantic (SPNA) is known for rapid reversals of decadal temperature trends, with ramifications encompassing the large-scale meridional overturning and gyre circulations, Arctic heat and mass balances, or extreme continental weather. Here, we combine datasets derived from sustained ocean observing systems (satellite and in situ), and idealized observation-based modelling (advection-diffusion of a passive tracer) and machine learning technique (ocean profile clustering) to document and explain the most-recent and ongoing cooling-to-warming transition of the SPNA. Following a gradual cooling of the region that was persisting since 2006, a surface-intensified and large-scale warming sharply emerged in 2016 following an ocean circulation shift that enhanced the northeastward penetration of warm and saline waters from the western subtropics. Driving mechanisms and ramification for deep ocean heat uptake will be discussed.

How to cite: Desbruyères, D., Chafik, L., and Maze, G.: Shifting ocean circulation warms the Subpolar North Atlantic since 2016, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-940, https://doi.org/10.5194/egusphere-egu21-940, 2021.

13:40–13:42
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EGU21-1982
Helene Asbjørnsen, Helen Johnson, and Marius Årthun

The inflow across the Iceland-Scotland Ridge determines the amount of heat supplied to the Nordic Seas from the subpolar North Atlantic (SPNA). Variability in inflow properties and volume transport at the ridge influence marine ecosystems and sea ice extent further north. The predictability of such downstream impacts depends on how variability at the ridge relate to large-scale ocean circulation changes in the North Atlantic. Here, we identify the upstream pathways of the Nordic Seas inflow, and assess the mechanisms responsible for interannual inflow variability. Using an eddy-resolving ocean model hindcast and a Lagrangian analysis tool, numerical particles are released at the ridge during 1986-2015 and tracked backward in time. Overall, 64% of the mean inflow volume transport has a subtropical origin and 26% has a subpolar or Arctic origin. The local instantaneous response to the NAO is important for the overall transport of both subtropical and Arctic-origin waters at the ridge. In the years before reaching the ridge, the subtropical particles are influenced by atmospheric circulation anomalies in the gyre boundary region and over the SPNA, forcing shifts in the North Atlantic Current (NAC) and the subpolar front. An equatorward shifted NAC and westward shifted subpolar front correspond to a warmer, more saline inflow. Wind stress curl anomalies over the SPNA also affect the amount of Arctic-origin water re-routed from the Labrador Current toward the Nordic Seas. A high transport of Arctic-origin water is associated with a colder, fresher inflow across the Iceland-Scotland Ridge. The results thus demonstrate the importance of gyre dynamics and wind forcing in affecting the Nordic Seas inflow properties and volume transport.

How to cite: Asbjørnsen, H., Johnson, H., and Årthun, M.: Linking variable Nordic Seas inflow to upstream circulation anomalies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1982, https://doi.org/10.5194/egusphere-egu21-1982, 2021.

13:42–13:44
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EGU21-8089
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ECS
Sam Jones, Stuart Cunningham, Neil Fraser, and Mark Inall

Circulation at the boundary of the subpolar North Atlantic influences both the horizontal (gyre) and vertical (overturning) components of the flow structure. While boundary current transport projects directly onto subpolar gyre strength, recent modelling studies have highlighted that buoyancy fluxes between the basin interior and the boundary, followed by rapid buoyancy export by boundary currents, are crucial steps in projecting air-sea interaction onto the strength of the Atlantic Meridional Overturning Circulation (AMOC). This work seeks observational insights into these key boundary processes.
To achieve this, we have constructed a robust boundary climatology from quality controlled CTD and Argo hydrography since the turn of the millennium. Following the 1000 m isobath north of 47 °N and aggregating data into 100 km bins, we build a picture of the typical large-scale temperature and salinity structure for each month.
This product will allow us to identify where and when important interior-boundary buoyancy fluxes take place over a seasonal cycle. A first step is to evaluate geostrophic flow into the boundary, and hence describe the vertical structure of advective buoyancy exchange. By appealing to satellite altimetry and Argo trajectories, we can also estimate turbulent eddy fluxes both at the surface and 1000 m depth. Models indicate these parameters are key in dictating the pathways for the AMOC lower limb, and we will place our observational findings in the context of these results.
Boundary current strength is another key parameter dictating the export of dense water from the subpolar gyre. We will appeal to satellite altimetry to build corresponding climatologies for barotropic boundary flow. Furthermore, along-slope density gradients give rise to a baroclinic boundary current forcing term, which we aim to investigate here. Water density generally increases as we follow the gyre counter-clockwise, with the notable exception of the West Greenland Current section, and our product allows us to partition the spatially-varying contribution of temperature and salinity towards these density gradients. For example, we can evaluate the impact of cooling along the eastern boundary, or surface freshening around southern Greenland, on the dynamics of boundary flow. Ultimately, we would like to understand the evolution of the dynamical balance experienced by a hypothetical fluid parcel traversing the entire subpolar gyre.

How to cite: Jones, S., Cunningham, S., Fraser, N., and Inall, M.: A climatology of the North Atlantic subpolar gyre boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8089, https://doi.org/10.5194/egusphere-egu21-8089, 2021.

13:44–13:46
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EGU21-15972
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ECS
D. Gwyn Evans, N. Penny Holliday, and Marilena Oltmanns

The OSNAP (Overturning in the Subpolar North Atlantic Program) array at ~60°N has provided new and unprecedented insight into the strength and variability of the meridional overturning circulation in the subpolar North Atlantic. OSNAP has identified the region of the subpolar North Atlantic east of Greenland as a key region for the water mass transformation and densification that sets the strength and variability of the overturning circulation. Here, we will investigate the drivers of this water mass transformation and their roles in driving the overturning circulation at OSNAP. Using a water mass analysis on both model-based and observational-based datasets, we isolate diathermal (across surfaces of constant temperature) and diahaline (across surfaces of constant salinity) transformations due to air-sea buoyancy fluxes, and mixing. We show that the time-mean overturning strength is set by both the air-sea buoyancy fluxes and the strength of subsurface mixing. This balance is apparent on a seasonal timescale, where we resolve large seasonal fluctuations in the both the air-sea buoyancy fluxes and mixing. The residual of this seasonal cycle then corresponds to the mean overturning strength. On interannual timescales, mixing becomes the dominant driver of variability in the overturning circulation. To determine the location of these water mass transformations and the dynamical processes responsible for the mixing-driven variability, our water mass analysis is projected onto geographical coordinates.

How to cite: Evans, D. G., Holliday, N. P., and Oltmanns, M.: Drivers of the water mass transformations that set the overturning circulation in the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15972, https://doi.org/10.5194/egusphere-egu21-15972, 2021.

13:46–13:48
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EGU21-847
Ilaria Stendardo, Bruno Buongiorno Nardelli, and Sara Durante

In the subpolar North Atlantic Ocean, Subpolar Mode Waters (SPMWs) are formed during late winter convection following the cyclonic circulation of the subpolar gyre. SPMWs participate in the upper flow of the Atlantic overturning circulation (AMOC) and provide much of the water that is eventually transformed into several components of the North Atlantic deep water (NADW), the cold, deep part of the AMOC. In a warming climate, an increase in upper ocean stratification is expected to lead to a reduced ventilation and a loss of oxygen. Thus, understanding how mode waters are affected by ventilation changes will help us to better understand the variability in the AMOC. In particular, we would like to address how the volume occupied by SPMWs has varied over the last decades due to ventilation changes, and what are the aspects driving the subpolar mode water formation, their interannual variations as well as the impact of the variability in the mixing and subduction and vertical dynamics on ocean deoxygenation. For this purpose, we use two observation-based 3D products from Copernicus Marine Service (CMEMS), the ARMOR3D and the OMEGA3D datasets. The first consists of 3D temperature and salinity fields, from the surface to 1500 m depth, available weekly over a regular grid at 1/4° horizontal resolution from 1993 to present. The second consists of observation-based quasi-geostrophic vertical and horizontal ocean currents with the same temporal and spatial resolution as ARMOR3D.

How to cite: Stendardo, I., Buongiorno Nardelli, B., and Durante, S.: Long-term ventilation changes of Subpolar Mode Waters in the North Atlantic Ocean and its impact on the oxygen distribution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-847, https://doi.org/10.5194/egusphere-egu21-847, 2021.

13:48–13:50
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EGU21-1594
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ECS
Diana Iakovleva and Igor Bashmachnikov

The Irminger Sea is one of the key region in the North Atlantic where deep winter convection develops.

We use ARMOR-3D dataset (0.25×0.25˚, from 1993) for calculating of heat (and freshwater) content, oceanic heat fluxes in the upper 500-m layer and the vertical heat exchange with the lower layers through the 500 m level. The air-sea heat exchange was derived from the OAFlux dataset (1×1˚), the radiation balance was obtained from the ERA-Interim reanalysis (0.25×0.25˚).

Computation of the heat balance was done over a closed region covering the central and western Irminger Sea (58-62˚ N and 36-44˚ W). The computations were repeated for several similar rectangular areas to analyze sensitivity of the analysis to the choice of boundaries of the region. However, the results of the analysis were largely independent from these variations.

The upper ocean heat advection in the study region was 37 TW (integrated along all boundaries of the region, sign «+» means that flux is directed to the study region), and was the dominant term in the annual mean heat balance. The annual mean latent heat flux (-21 TW) and sensible heat flux (-5 TW) were directed from the ocean. The annual mean radiation balance was 8 TW (to the ocean), while vertical heat exchange with the lower layers was low (-0.1 TW). On average, heat balance of upper 500-m layer was positive, and not all heat fluxes might be considered. For example, the contribution of horizontal mesoscale eddy exchange could be important. However, the amplitude of the interannual variability of the heat balance of about 15 TW was close to that of the heat content (about 20 TW), while the correlation between the parameters was significant and high (0.79) (after removal of the quadratic trend 0.80). This suggests that the main heat fluxes, which affect the interannual variability of the heat content in the upper 500-m layer were taken into account.

Interannual variability of maximum convection depth in the central Irminger Sea was found to significantly correlate with the upper ocean heat content mean over September-November (-0.73); both parameters showed a similar long-term tendency. The correlation of the convection depth with the freshwater content (September-November) was significantly less and positive (0.49). The latter is counterintuitive, as we expect a decrease of the convective depth with an increase of the upper ocean freshwater contents. It can be assumed this correlation was induced by a high negative correlation between the upper ocean heat and freshwater contents in the region (-0.64). The analysis, thus, suggests that the long-term variability of deep convection in the Irminger Sea was shaped by variability of the main heat fluxes, entering the region.

How to cite: Iakovleva, D. and Bashmachnikov, I.: The heat balance shapes deep convection in the Irminger Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1594, https://doi.org/10.5194/egusphere-egu21-1594, 2021.

13:50–13:52
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EGU21-2017
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ECS
|
Kevin Niklas Wiegand, Dagmar Kieke, and Paul G. Myers

In this study we analyze the exchange processes between the West Greenland shelf and the Labrador Sea. This region is affected by warm and saline waters originating from the subtropical North Atlantic, as well as cold and fresh waters from the Arctic and the Greenland Ice Sheet. Heat and freshwater both impact the local formation of Labrador Sea Water (LSW) that itself is a major contributor to the Atlantic Meridional Overturning Circulation.

We use the ARMOR3D large-scale hydrographic data set from the Copernicus Marine Environmental Monitoring Service (CMEMS) and validate it with ship-based measurements in the period between 1993 to 2018. By extracting cross-shelf sections from ARMOR3D for various locations around Greenland, we reconstruct time series of local water masses like the Irminger Water (IW) for the past three decades. Previous studies from the West Greenland shelf have shown that IW properties are locally anti-correlated to changes in LSW. We analyze the interannual and decadal variability of these IW time series and compare them towards hydrographic changes observed in the interior Labrador Sea.

Since ARMOR3D allows us to investigate interannual and decadal changes along cross-shelf sections, the goal of this study is to unravel the complex connection between changes in the shelf regions around Greenland and the interior Labrador Sea, especially the local water mass production.

How to cite: Wiegand, K. N., Kieke, D., and Myers, P. G.: Using reconstructed Irminger Water changes within the past three decades to connect the West Greenland shelf to the production of Labrador Sea Water, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2017, https://doi.org/10.5194/egusphere-egu21-2017, 2021.

13:52–13:54
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EGU21-2395
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ECS
Sotiria Georgiou, Stefanie L. Ypma, Nils Brüggemann, Juan-Manuel Sayol, Carine G. van der Boog, Paul Spence, Julie D. Pietrzak, and Caroline A. Katsman

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53oN in the boundary current and traced backwards in time in a high‐resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53oN originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary‐interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin, and part follows a longer route towards Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary‐interior exchanges are important for the pathways and the properties of water masses arriving at 53oN. It reveals a complex three‐dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53oN and thus for our understanding of the AMOC.

How to cite: Georgiou, S., Ypma, S. L., Brüggemann, N., Sayol, J.-M., van der Boog, C. G., Spence, P., Pietrzak, J. D., and Katsman, C. A.: Direct and indirect pathways of convected water masses and their impacts on the overturning dynamics of the Labrador Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2395, https://doi.org/10.5194/egusphere-egu21-2395, 2021.

13:54–13:56
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EGU21-3070
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ECS
Eva Prieto, Damien Desbruyères, and Virginie Thierry

Temperature and salinity seasonal to interannual variability of Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) is investigated by combining two in-situ datasets in the Irminger Sea for the period 1997-2020: 12-yr of repeated hydrography (1997-2018) provided by the FOUREX, OVIDE and RREX sections and 4-yr of data (2016-2020) from 8 Deep Argo floats deployed in the region between 2016 and 2018.

In order to enable a consistent analysis of ocean temperature and salinity variability from unevenly distributed vertical profiles (both in space and time), it is necessary to estimate the appropriate regional climatology to be removed from every observation. Two independent procedures are followed to compute anomalies and quantify uncertainties related to the choice of climatology: First, the global 1°-resolution World Ocean Atlas 2018 (2005-2017 averages) climatology is retrieved from every observed profile (Deep Argo, hydrography). Second, the well-known and sampled OVIDE transect (2002-2018 average) is used to build a reference section of geographical anomalies that are subsequently propagated along potential vorticity contours in the Irminger Sea. Neutral density surfaces 28.02 kgm-3 and 28.12 kgm-3 are then chosen from mean OVIDE 2002-2018 gridded fields as representative of ISOW and DSOW levels, respectively. Significant decadal trends in water mass properties are revealed by repeated hydrography, whereas some striking boundary-interior spatial patterns are captured by Deep Argo floats. Property changes of ISOW and DSOW are discussed in terms of changes of source waters in the Nordic Seas, entrainment of Atlantic waters into the overflow waters and cascading events from the Greenland slope.

 

How to cite: Prieto, E., Desbruyères, D., and Thierry, V.: Interpreting the observed variability of deep waters in the Irminger Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3070, https://doi.org/10.5194/egusphere-egu21-3070, 2021.

13:56–13:58
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EGU21-3500
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ECS
Tillys Petit, M. Susan Lozier, Simon A. Josey, and Stuart A. Cunningham

Convection in the North Atlantic Ocean is a key component of the global overturning circulation (MOC) as it produces dense water at high latitudes. Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of the North Atlantic deep waters. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the MOC lower limb. Air-sea fluxes and the surface density field are both key determinants of the buoyancy-driven transformation. To better understand the connection between atmospheric forcing and the Atlantic overturning circulation, we analyze the contributions of the air-sea fluxes and of the density structure to the transformation of surface water over the eastern subpolar gyre. More precisely, we consider the densification of subpolar mode water (SPMW) in the Iceland Basin that ‘pre-conditions’ the dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that variability in transformation is only weakly sensitive to changes in the heat and freshwater fluxes. Instead, changes in SPMW transformation are largely driven by the variance in the surface density structure, as expressed by the outcropping area for those isopycnals that define SPMW.This large influence of the surface density on the SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–15 over the Iceland Basin.  

How to cite: Petit, T., Lozier, M. S., Josey, S. A., and Cunningham, S. A.: Role of the density structure and air-sea fluxes on subpolar transformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3500, https://doi.org/10.5194/egusphere-egu21-3500, 2021.

13:58–14:00
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EGU21-4769
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ECS
Nora Fried and M. Femke de Jong

As an extension of the North Atlantic Current, the Irminger Current is an important component of the overturning in the subpolar North Atlantic. It contains warm, saline Subpolar Mode Water and cold, dense North East Atlantic Deep Water flowing northward along the western flank of the Reykjanes Ridge. As part of OSNAP (Overturning in the Subpolar North Atlantic Project) the Irminger Current has been monitored since 2014 with a mooring array consisting of five moorings, all equipped with current meters, ADCPs and CTDs.

Preliminary results from the recent 6-year mooring time series until summer 2020 give new insights into the interannual transport variability of the Irminger Current. The mean volume transport is 11.3 ± 8.8 Sv with a clear maximum of the yearly mean transport in 2019 (15.7 Sv). The Irminger Current experienced a decrease in salt transport by 50% from 2016 – 2018 compared to 2014 – 2016. This signal originates from a freshwater anomaly in the eastern subpolar North Atlantic.

For an investigation of the longer-term variability we used monthly mean reanalysis data (CMEMS) from 1993 - summer 2019 and the analysis and forecast up to summer 2020 along the Irminger Current mooring array across the Irminger Sea. The reanalysis data compares well with the mooring results both in mean transport and structural representation of the Irminger Current. Volume transport in the eastern Irminger Sea and sea surface height gradient are significantly correlated by r = 0.82 on interannual time scales. The 28-year time series shows a significant negative trend in volume transport over the eastern Irminger Sea, concomitant with a significant negative trend in the sea surface height and density gradient. Hydrographic changes over the top of the Mid Atlantic Ridge are dominating the trend in density gradient as changes in the central Irminger Sea are smaller and mostly density compensating.

How to cite: Fried, N. and de Jong, M. F.: 28-year volume transport decrease in the Irminger Sea: Results from mooring and reanalysis data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4769, https://doi.org/10.5194/egusphere-egu21-4769, 2021.

14:00–14:02
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EGU21-6112
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ECS
Jannes Koelling, Dariia Atamanchuk, Johannes Karstensen, and Douglas W.R. Wallace

Most of the life-sustaining oxygen found in the global deep ocean is supplied in one of only a handful of key regions around the globe, such as the Labrador Sea in the subpolar North Atlantic. Here, oxygen is supplied directly to the deep ocean during the formation of Labrador Sea Water (LSW), when convective mixing continuously brings low-oxygen deep water towards the surface and into contact with the atmosphere. The continuous exchange between the surface and deep ocean during convection can bring newly oxygenated waters as deep as 2000m. Although the associated oxygen uptake has been observed and quantified, and the resulting oxygen-rich water mass in the deep ocean is readily detected throughout the Atlantic Ocean, relatively little is known about the exact mechanisms and timing of its export out of the basin.

In this talk, we will present a novel dataset of oxygen sensors deployed within the boundary current at the exit of the Labrador Sea to investigate oxygen variability in the deep ocean. This is the first time that a continuous time series of oxygen has been collected in the boundary current of the Labrador Sea, with a total of 10 sensors deployed on 4 moorings from 2016 to 2020. The sensors at 600m depth show a sudden change in oxygen, temperature, and salinity in the spring, which we discuss in relation to deep convection in the interior. We also use data from Argo floats to analyse export pathways from the convection region to the location of the moorings. Our results give new insights into how the oxygen taken up in the central Labrador Sea subsequently spreads into the global deep ocean, and lay the basis for future work on quantifying variability of oxygen transport at the exit of the Labrador Sea.

How to cite: Koelling, J., Atamanchuk, D., Karstensen, J., and Wallace, D. W. R.: Export of newly oxygenated Labrador Sea Water at 53N, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6112, https://doi.org/10.5194/egusphere-egu21-6112, 2021.

14:02–14:04
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EGU21-7999
Heather Furey, Amy Bower, Bill Johns, Andree Ramsey, and Adam Houk

Iceland Scotland Overflow Water (ISOW), a component of the deep limb of the Atlantic Meridional Overturning Circulation (AMOC), is the equilibrated product of dense overflow into the eastern North Atlantic basin.  Modeling results and recent observations have suggested that a significant westward transport of ISOW (~1x106 m3s-1) may occur through the Bight Fracture Zone (BFZ) near 57°N, the first major channel through the Reykjanes Ridge where ISOW can cross into the Irminger Sea.  The remaining denser (and deeper) ISOW has been shown to leave the Iceland Basin westward via the Charlie-Gibbs Fracture Zone near 53°N, or southward into the West European Basin. Until now, there have been no measured time series in the BFZ to validate model results. Single moorings placed in the north and south channels of the BFZ from summer 2015 to summer 2017 were used to estimate a mean combined transport across the fracture zone of 0.8 ± 0.4 x106 m3s-1 westward, with each channel contributing about half of the mean transport. Variability between the two channels on shorter (month-long) times scales can be extreme: in March of 2016, for example, north channel transport was ~0.4 x106 m3s-1 eastward, while south channel transport was ~0.8 x106 m3s-1 westward.  For this 2-year period, transport is stronger in the summer (0.9-1.2 x106 m3s-1) than in winter (0.5-0.7 x106 m3s-1), where large fluctuations including complete reversals suggest transport variability may be affected by winter storms.  This mooring record also shows a fresh anomaly in ISOW beginning in early 2017, which has been shown by others to originate from the surface waters near the Grand Banks region of the western north Atlantic.  Transport variability in this two-year record is examined in the context of the transport variability of the OSNAP mooring arrays on the east and west flanks of the Reykjanes Ridge just north of BFZ during the same time period.  An observationally-based understanding of how the Iceland and Irminger basins communicate with each other via the deep limb of the AMOC through the BFZ will provide fundamental insight into the pathways and processes that define the subpolar AMOC system.

How to cite: Furey, H., Bower, A., Johns, B., Ramsey, A., and Houk, A.: Iceland-Scotland Overflow Water Transport Variability through the Deep Bight Fracture Zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7999, https://doi.org/10.5194/egusphere-egu21-7999, 2021.

14:04–14:06
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EGU21-8250
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ECS
Ivane Salaün, Virginie Thierry, and Herlé Mercier

Located south of Iceland, the Reykjanes Ridge is a major topographic structure of the North Atlantic Ocean that strongly influences the spatial distribution and circulation of the North Atlantic Subpolar Gyre water masses. Around the ridge, 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 the along Reykjanes Ridge flow variability and the inter-basin connection through the ridge and connections with the interior of each basin, volume and water mass transports over the Reykjanes Ridge during summer 2015, 2016 and 2017 are analyzed. Data used are velocity and hydrographic measurements carried out along and perpendicular to the crest of the Reykjanes Ridge during the RREX (Reykjanes Ridge Experiment Project) cruises in June–July 2015 and June–July 2017 and BOCATS cruise in July 2016. The new circulation scheme in the area described in 2015  by Petit et al. (J. Geophys. Res., 2018) with flows connecting the ERRC and IC branches at specific locations set by the bathymetry of the ridge is again observed  in 2016 and  2017, with variations concerning the connections with the interiors of the basins. The data set reveals remarkable changes in the hydrological properties and transports of the ERRC, IC and cross ridge flows. The westward transport across the ridge, which represents the subpolar gyre intensity, was estimated at -19.6±3.4 Sv in 2015 and -35.2±3 Sv in 2017. A freshening and a decline in density mainly affecting the Subpolar Mode Water was observed in 2017. It was associated with a lower mode water  transport partly compensated by a higher transport of intermediate and Arctic waters. We further document each water mass contribution to the westward flow of the gyre and the structure of the ERRC and IC.

How to cite: Salaün, I., Thierry, V., and Mercier, H.: The circulation near the Reykjanes Ridge in summers 2015, 2016 and 2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8250, https://doi.org/10.5194/egusphere-egu21-8250, 2021.

14:06–14:08
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EGU21-2562
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ECS
Elena V. Novoselova, Tatyana V. Belonenko, and Aleksandr M. Fedorov

The Lofoten Basin in the Norwegian Sea is a real reservoir of the Atlantic Waters. The shape of the Basin in the form of a bowl and a great depth with its monotonous increase to the centre results in the Atlantic Water gradually deepen and fill the Basin. The deepening of the Atlantic Waters in the Lofoten Basin determines not only the structure of its waters but also the features of the ocean-atmosphere interaction. Flowing through the transit regions, the Atlantic Waters lose heat to the atmosphere, mix with the surrounding water masses and undergo a transformation, which causes the formation of deep ocean waters. At the same time, the heat input with the Atlantic waters significantly exceeds its loss to the atmosphere in the Lofoten Basin.

We study isopycnal advection and diapycnal mixing in the Lofoten Basin. We use the GLORYS12V1 oceanic reanalysis data and analyze four isosteric δ-surfaces. We also calculate the depth of their location. We establish that δ-surfaces have the slope eastward with maximal deepening where the quasi-permanent Lofoten Vortex is located. We analyze the temperature distribution on the isosteric δ-surfaces as well as the interannual and seasonal variability of their location depth.

The maximal depth on the isosteric surfaces is observed in 2010, which is known as the year of the largest mixed layer depths in the Lofoten Basin according to the ARGO buoys. We demonstrate the same correspondence to in 2000, 2010, 2013.

The maximal depth on the isosteric surfaces is observed is reached in summer. The maximal areas with the greatest depths also are observed in summer in contrast to a minimum in winter. This means certain inertia of changes in the thermohaline characteristics of Atlantic Waters as well as a shift of 1-2 seasons of the influence of deep convection on isosteric surfaces.

It is shown that isopycnal advection in the Lofoten Basin makes a significant contribution to its importance as the main thermal reservoir of the Nordic Seas.

How to cite: Novoselova, E. V., Belonenko, T. V., and Fedorov, A. M.: Analysis of the isopycnal advection in the Lofoten basin (the Norwegian sea), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2562, https://doi.org/10.5194/egusphere-egu21-2562, 2021.

14:08–14:10
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EGU21-10729
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ECS
Siren Rühs, Eric Oliver, Arne Biastoch, Claus W. Böning, Michael Dowd, Klaus Getzlaff, and Paul G. Myers

Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing evidence that deep convection also occurs in the eastern SPG south of Cape Farewell and in the Irminger Sea. In particular, observations indicate gyre-scale intensified convection in 2015-2018. Here we assess this recent event in the context of the temporal evolution of the spatial deep convection pattern in the SPG since the mid-twentieth century, using realistic eddy-rich ocean model simulations. These reveal large interannual variability, including several periods with intensified deep convection in the eastern SPG. Notably, this happened in 2015-2018, but to a lesser degree in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation. Our analyses further suggest that deep convection in 2015-2018 occurred with an unprecedented high (low) relative contribution of the eastern (western) SPG to the total deep convection volume. This is partly linked to a considerable smaller north-westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of strong near-surface freshening in the Labrador Sea associated with Greenland melting.

How to cite: Rühs, S., Oliver, E., Biastoch, A., Böning, C. W., Dowd, M., Getzlaff, K., and Myers, P. G.: Changing spatial patterns of deep convection in the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10729, https://doi.org/10.5194/egusphere-egu21-10729, 2021.

14:10–15:00
Break
Chairperson: Bablu Sinha
15:30–15:32
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EGU21-7817
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ECS
Emma Worthington, Ben Moat, David Smeed, Jennifer Mecking, Robert Marsh, and Gerard McCarthy

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID array with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic re-search suggests that the AMOC may have been declining for decades or even centuries before this, however direct observations are sparse prior to 2004, giving only ‘snapshots’ of the overturning circulation. Previous studies have used linear models based on upper layer temperature anomalies to extend AMOC estimates back in time, however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher fidelity empirical model of AMOC variability based on RAPID data, and associated physically with changes in thickness of the persistent upper, intermediate and deep water masses at 26°N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC toapproximately annual shows multi-annual variability in agreement with RAPID observations, and that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline asindicated by other proxies and high resolution climate models. Our results reinforce that adequately capturing changes to thedeep circulation is key to detecting any anthropogenic climate change-related AMOC decline

How to cite: Worthington, E., Moat, B., Smeed, D., Mecking, J., Marsh, R., and McCarthy, G.: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7817, https://doi.org/10.5194/egusphere-egu21-7817, 2021.

15:32–15:34
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EGU21-7789
Claus W. Böning, Arne Biastoch, Klaus Getzlaff, Patrick Wagner, Siren Rühs, Franziska U. Schwarzkopf, and Markus Scheinert

A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20°) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4°-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by “sinking” (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.

How to cite: Böning, C. W., Biastoch, A., Getzlaff, K., Wagner, P., Rühs, S., Schwarzkopf, F. U., and Scheinert, M.: Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7789, https://doi.org/10.5194/egusphere-egu21-7789, 2021.

15:34–15:36
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EGU21-5324
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ECS
Levke Caesar and Gerard McCarthy

While there is increasing paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened over the last one to two hundred years (Caesar et al., 2018; Thornalley et al., 2018), this is not confirmed by climate model simulations. Instead, the new simulations from the 6th Coupled Model Intercomparison Project (CMIP6) show a slight strengthening of the multimodel mean AMOC from 1850 until about 1985 (Menary et al., 2020), attributed to anthropogenic aerosol forcing. Arguing for a recent weakening of the AMOC, some studies attribute the emergence of the North Atlantic warming hole as a sign of the reduced meridional heat transport associated with a weaker AMOC (e.g. Caesar et al., 2018), yet this cold anomaly has also been interpreted as being aerosol-forced (Booth et al., 2012) and therefore not necessarily a sign of a weakening AMOC but rather a possible driver of a strengthening of the AMOC.

Looking beyond temperature, a fresh anomaly has recently emerged in the subpolar North Atlantic (Holliday et al., 2020). While a strengthening AMOC has been linked with an increase in salinity in the subpolar gyre region (Menary et al., 2013), an AMOC weakening would, due to the salt-advection feedback, likely lead to a reduction in salinity in the North Atlantic region. To shed some light on the question of whether the cold anomaly is internally (AMOC) or externally (aerosol-forced) driven we consider the co-variability of salinity and temperature in the North Atlantic in respect of changes in surface fluxes or alternate drivers.

 

References

Booth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N., 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393): 228–232.

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. and Saba, V., 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700): 191-196.

Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., Hátún, H., Johns, W., Josey, S.A., Larsen, K.M.H., Mulet, S., Oltmanns, M., Reverdin, G., Rossby, T., Thierry, V., Valdimarsson, H. and Yashayaev, I., 2020. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1): 585.

Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., Müller, W.A., Matei, D. and Lee, S.-K., 2013. Mechanisms of aerosol-forced AMOC variability in a state of the art climate model. Journal of Geophysical Research: Oceans, 118(4): 2087-2096.

Menary, M.B., Robson, J., Allan, R.P., Booth, B.B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L. and Zhang, R., 2020. Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophysical Research Letters, 47(14): e2020GL088166.

Thornalley, D.J.R., Oppo, D.W., Ortega, P., Robson, J.I., Brierley, C.M., Davis, R., Hall, I.R., Moffa-Sanchez, P., Rose, N.L., Spooner, P.T., Yashayaev, I. and Keigwin, L.D., 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature, 556(7700): 227-230.

How to cite: Caesar, L. and McCarthy, G.: Co-variability of salinity and temperature changes in the North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5324, https://doi.org/10.5194/egusphere-egu21-5324, 2021.

15:36–15:38
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EGU21-9989
Leon Hermanson, Doug Smith, Nick Dunstone, and Rosie Eade

The Atlantic Meridional Overturning Circulation (AMOC) at 26N has been measured since 2004 by the RAPID-MOCHA array. On a multi-year timescale it shows a decline with signs of a recovery since around 2012. This variability is likely to be part of longer decadal variability. We examine here the decadal variability of the AMOC and its drivers in a coupled model run nudged to observations from 1960-2017. Temperature and winds are nudged throughout the atmosphere and potential temperature and salinity are nudged in the ocean, but the ocean velocities are allowed to vary freely. We nudge an ensemble of 10 ocean analyses into the ocean model to get an ensemble of responses, the mean of which reproduces the observed AMOC. We use these ocean-atmosphere re-analyses to study the drivers of the AMOC. The North Atlantic Oscillation (NAO) is well known to have an impact on the AMOC and is an important driver here. We find that the tropical Pacific also has a strong impact on the subtropical AMOC on multi-annual to decadal timescales. Together these two factors can explain more than half of all variability of the AMOC at 26N through wind forcing associated with Rossby waves and western boundary waves. This Pacific impact, not reported on before, is from windstress curl anomalies close to the East Coast of the southern US due to changes in the Pacific storm track and the Walker Circulation. As both the NAO and tropical Pacific variability is associated with solar and volcanic forcing, it is possible that solar and volcanic forcing are important for multi-annual to multi-decadal AMOC variability. We use observations of the NAO and tropical Pacific to reconstruct the AMOC from 1870 to present day and predict a continued recovery in the future.

How to cite: Hermanson, L., Smith, D., Dunstone, N., and Eade, R.: A New Pacific Influence on the Atlantic Meridional Overturning Circulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9989, https://doi.org/10.5194/egusphere-egu21-9989, 2021.

15:38–15:40
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EGU21-2855
Renske Gelderloos, Thomas W. N. Haine, and Mattia Almansi

Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from the OSNAP array and other mooring records shows that the circulation in this region displays substantial subinertial variability, typically with periods of several days. For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.

How to cite: Gelderloos, R., Haine, T. W. N., and Almansi, M.: Coastal Trapped Waves along the Southeast Greenland Coast in a realistic numerical simulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2855, https://doi.org/10.5194/egusphere-egu21-2855, 2021.

15:40–15:42
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EGU21-15886
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ECS
Alejandra Sanchez-Franks, Eleanor Frajka-Williams, Ben Moat, and David Smeed

The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude.

High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26°N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83% of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26°N lays down the starting point for monitoring large-scale circulation at all latitudes.

How to cite: Sanchez-Franks, A., Frajka-Williams, E., Moat, B., and Smeed, D.: A dynamically based method for estimating the Atlantic overturning circulation at 26°N from satellite altimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15886, https://doi.org/10.5194/egusphere-egu21-15886, 2021.

15:42–15:44
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EGU21-3122
Ben Moat, Eleanor Frajka-Williams, Joanne Williams, and Chris Meinen

Pressure Inverted Echo Sounders, sited on the seabed, indirectly measure the density of the water above them by combining pressure and travel time of an echo-sound pulse to the surface. Where the approximate structure of the water column is locally known, they can be used to select between a number of typical TS profiles (a gravest empirical mode or GEM field), providing temperature and salinity. But how accurate is this profile, and can such an instrument replace the expensive tall moorings currently used to monitor the MOC? We evaluate PIES deployments at 26N on the western boundary of the Atlantic between 2006 and 2018. We find that high-frequency (around weekly) variations in temperature are well captured by this technique, and the geostrophic part of the AMOC could be estimated in this way. However the GEM databases don't account for all low frequency variations in temperature and salinity profiles. At 26N we see for example, the results from PIES with cold bias above the thermocline and with a compensatory warm bias below it, and these biases lasting months or years. The profiles are also inaccurate at the surface, although seasonally-varying GEM fields may be helpful here. However the technique shows promise, and if it is developed further incorporating additional data sources such ARGO or as sea-surface temperature it may be possible to use it for long term monitoring of the Atlantic at 26N.

How to cite: Moat, B., Frajka-Williams, E., Williams, J., and Meinen, C.: Can PIES (Pressure Inverted Echo Sounders) replace tall moorings to monitor the AMOC?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3122, https://doi.org/10.5194/egusphere-egu21-3122, 2021.

15:44–15:46
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EGU21-11132
Eleanor Frajka-Williams, William E. Johns, Harry L. Bryden, David A. Smeed, Aurelie Duchez, and Lisa Holton

The Antilles Current is a narrow, northward flowing boundary current in the western Atlantic just east of the Bahamas. Its role in the larger scale circulation has been debated: alternately thought to be part of the western boundary closure of the gyre circulation or the northward flowing limb of the meridional overturning circulation (MOC). From 19 years of moored current meter observations (1987--1991, 2004--2018), we define the strength of the Antilles Current by the net transport between the Bahamas and 76.5°W (spanning about 45 km zonally) and in the thermocline (0–1000 m). We find a mean northward transport of 3.5 Sv, substantial interannual variability, and no discernable trend since 1987. The interannual variability of the AC transport is independent of the variability of the Florida Current (the Gulf Stream through the Florida Straits). Instead, the Antilles Current contributes to the interannual variability of the MOC at 26°N, while the trend in the strength of the gyre circulation (defined as the transbasin thermocline transport minus the AC) is responsible for the trend in the MOC. In particular, the 2009/10 slowdown of the MOC resulted from a weaker northward AC transport, rather than an intensified gyre transport. Using the recent 14 years of in situ transport records, we compare the interannual variability of the gyre circulation to that of wind stress curl forcing via a Sverdrup transport calculation, identifying a potential role for wind stress curl (WSC) forcing at 26°N with a ~2 year lag until 2016.  From 2016, the predicted gyre circulation using WSC diverges from the measured gyre strength.

How to cite: Frajka-Williams, E., Johns, W. E., Bryden, H. L., Smeed, D. A., Duchez, A., and Holton, L.: The Antilles Current and wind-driven gyre circulation at 26oN, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11132, https://doi.org/10.5194/egusphere-egu21-11132, 2021.

15:46–15:48
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EGU21-2824
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ECS
Isabela Le Bras, Fiamma Straneo, Morven Muilwijk, Lars Henrik Smedsrud, Feili Li, Susan Lozier, and Penny Holliday

Fresh Arctic waters flowing into the Atlantic are thought to have two primary fates. They may be mixed into the deep ocean as part of the overturning circulation, or flow alongside regions of deep water formation without impacting overturning. Climate models suggest that as increasing amounts of fresh water enter the Atlantic, the overturning circulation will be disrupted, yet we lack an understanding of how much fresh water is mixed into the overturning circulation's deep limb in the present day. To constrain these fresh water pathways, we build steady-state volume, salt, and heat budgets east of Greenland that are initialized with observations and closed using inverse methods. Fresh water sources are split into oceanic Polar Waters from the Arctic and surface fresh water fluxes, which include net precipitation, runoff, and ice melt, to examine how they imprint the circulation differently. We find that 65 mSv of the total 110 mSv of surface fresh water fluxes that enter our domain participate in the overturning circulation, as do 0.6 Sv of the total 1.2 Sv of Polar Waters that flow through Fram Strait. Based on these results, we hypothesize that the overturning circulation is more sensitive to future changes in Arctic fresh water outflow and precipitation, while Greenland runoff and iceberg melt are more likely to stay along the coast of Greenland.

How to cite: Le Bras, I., Straneo, F., Muilwijk, M., Smedsrud, L. H., Li, F., Lozier, S., and Holliday, P.: How much Arctic fresh water participates in the subpolar overturning circulation?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2824, https://doi.org/10.5194/egusphere-egu21-2824, 2021.

15:48–15:50
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EGU21-4927
Dagmar Kieke, Oliver Huhn, Christian Mertens, Monika Rhein, Reiner Steinfeldt, and Kevin N. Wiegand

Melting of the Greenland Ice Sheet is one of the major causes that adds to the ice sheet mass loss and subsequently to the global sea level rise. The accelerated melting observed in recent decades is mainly caused by surface melting due to atmospheric warming and submarine melting caused by the increased inflow of warm Atlantic Water into the glacier-inhabited fjords of Greenland. This water reaches the front of marine terminating glaciers or the base of floating ice tongues inducing submarine melting. However, knowledge about submarine melt rates is limited and often inferred from indirect or remote sensing methods. Open questions exist regarding the processes that control the interaction of the oceans with marine terminating glaciers and the subsequent pathway of glacially modified water. The increasing release of this meltwater into the ocean is expected to have an impact on the deep water formation in the North Atlantic causing it to decrease. Since the deep water formation and spreading contribute to the deep limb of the Atlantic Meridional Overturning Circulation, identifying, tracking, and quantifying the oceanic submarine meltwater content and its variability is of high interest. The noble gases helium and neon provide a useful tool to identify and to quantify the fraction of glacially modified water in the oceanic water column. In this study we evaluate hydrographic, velocity and noble gas measurements from a number of cruises conducted across the boundary current system around Greenland between 2015 and 2019. With focus on the East and West Greenland Current systems, we aim at obtaining a large-scale view on the submarine meltwater distribution around Greenland and discuss the different regional regimes in two Greenlandic fjord systems and the boundary current around Greenland.

How to cite: Kieke, D., Huhn, O., Mertens, C., Rhein, M., Steinfeldt, R., and Wiegand, K. N.: Inference of Submarine Meltwater in the Boundary Current System around Greenland in 2015-2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4927, https://doi.org/10.5194/egusphere-egu21-4927, 2021.

15:50–15:52
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EGU21-8225
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ECS
Sophie Stolzenberger, Roelof Rietbroek, Claudia Wekerle, Bernd Uebbing, and Jürgen Kusche

The impact of Greenland freshwater on oceanic variables in the North Atlantic has been controversially discussed in the past. Within the framework of the German research project GROCE (Greenland Ice Sheet Ocean Interaction), we present a comprehensive study using ocean modelling results including and excluding the Greenland freshwater flux. The aim of this study is whether signatures of Greenland ice sheet melting found in ocean model simulations are visible in the observations. Therefore, we estimate changes in temperature, salinity, steric heights and sea level anomalies since the 1990s. The observational database includes altimetric and gravimetric satellite data as well as Argo floats. We will discuss similarities/differences between model simulations and observations for smaller regions around Greenland in the North Atlantic. As these experiments are available for two different horizontal resolutions, we will furthermore be able to assess the effects of an increased model resolution.

How to cite: Stolzenberger, S., Rietbroek, R., Wekerle, C., Uebbing, B., and Kusche, J.: Greenland melting signatures in model simulations and oceanic observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8225, https://doi.org/10.5194/egusphere-egu21-8225, 2021.

15:52–15:54
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EGU21-16262
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ECS
Svetlana Radionovskaya, Luke Skinner, and Mervyn Greaves

MIS 4, a key paleoclimatic interval for the last glacial inception, is characterized by a rapid CO2 drop of approx. ~28ppm and a large drop in temperature (as seen in Antarctic ice cores). SSTs in the North Atlantic are thought to be coupled to AMOC strength, whereby various proxies suggest a weaker and shoaled AMOC during the transition from MIS5a to MIS4. Furthermore, several millennial events also occurred during MIS 4, including Heinrich Stadial 6 and DO events 16-19. MIS 4 is thus an ideal interval to study and eventually to disentangle, glacial-interglacial and millennial variability.

Here, we present high resolution planktonic and benthic foraminifera geochemical data from several marine sediment cores from the Iberian Margin (including stable isotope and trace element data). The Iberian Margin is a prime location to study millennial-scale climate variability as isotope records of planktonic and benthic foraminifera simultaneously recorded rapid climate change expressed in Greenland and Antarctic ice cores, respectively, during the last glacial period. However, our results so far, suggest that surface ocean response at this site does not track Greenland temperature, as would be expected for this region of the North Atlantic. Perhaps the most striking, our planktic Mg/Ca record shows a late onset of rapid MIS 4 cooling at the start of Heinrich 6, and no clear millennial variability signal. This is in agreement with SST reconstructed using alkenones (Pailler and Bard, 2002) and planktonic foraminifera faunal assemblages (Salgueiro et al., 2010) from nearby core sites. Local d18O seawater reconstructions imply major hydrological changes in the region, which is supported by the “dry event” seen in speleothems from North Eastern Iberia (Perez-Mehias et al., 2019) and Italy (Columbu et al., 2020), just before Heinrich 6. We propose that the observed changes may reflect changes in regional ocean and atmospheric circulation patterns such as the interaction of the strength and position of the Azores Current, Iberian Poleward Current and the Subtropical Gyre, which in turn could depend on the larger scale AMOC and wind driven surface ocean changes due to glacial-interglacial and millennial variability. Further links to moisture transport, ice sheet growth and carbon cycle are yet to be investigated.

 

References

Columbu, A., Chiarini, V., Spötl, C., Benazzi, S., Hellstrom, J., Cheng, H. and De Waele, J., 2020. Speleothem record attests to stable environmental conditions during Neanderthal–modern human turnover in southern Italy. Nature Ecology & Evolution, 4(9), pp.1188-1195.

Pailler, D. and Bard, E., 2002. High frequency palaeoceanographic changes during the past 140 000 yr recorded by the organic matter in sediments of the Iberian Margin. Palaeogeography, Palaeoclimatology, Palaeoecology, 181(4), pp.431-452.

Pérez-Mejías, C., Moreno, A., Sancho, C., Martín-García, R., Spötl, C., Cacho, I., Cheng, H. and Edwards, R., 2019. Orbital-to-millennial scale climate variability during Marine Isotope Stages 5 to 3 in northeast Iberia. Quaternary Science Reviews, 224, p.105946.

Salgueiro, E., Voelker, A., de Abreu, L., Abrantes, F., Meggers, H. and Wefer, G., 2010. Temperature and productivity changes off the western Iberian margin during the last 150 ky. Quaternary Science Reviews, 29(5-6), pp.680-695.

How to cite: Radionovskaya, S., Skinner, L., and Greaves, M.: Decoupling of surface ocean hydrology and Greenland ice core records in the eastern North Atlantic during the last glacial inception , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16262, https://doi.org/10.5194/egusphere-egu21-16262, 2021.

15:54–15:56
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EGU21-10059
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ECS
Sijia Zou, Amy Bower, Heather Furey, Robert Pickart, Loïc Houpert, and N. Penny Holliday

Abundant cyclonic eddies are observed to travel along the Deep Western Boundary Current around Greenland by Lagrangian floats, hydrographic stations and moorings. Most of the cyclones have intensified rotations below the surface (700-1000 dbar), with maximum azimuthal velocities of ~30 cm/s at radii of ~10 km. The swift rotation and small radius lead to a relatively large Rossby number (~0.4), suggesting important contributions from the ageostrophic terms. The subsurface rotational core is also characterized with a local (both vertically and horizontally) potential vorticity (PV) maximum, which is associated with the pinching of isopycnals towards the mid-depths (i.e. high stratification). The PV structure suggests the origin of the cyclone as the Denmark Strait Overflow Cyclone. The latter is known to be formed by vortex stretching southwest of the Denmark Strait, where outflow waters with high PV from the sill descends the continental slope into the low PV Irminger Sea. Finally, we show that these cyclones can influence the boundary currents around Greenland by introducing property anomalies that originate from the Denmark Strait.

How to cite: Zou, S., Bower, A., Furey, H., Pickart, R., Houpert, L., and Holliday, N. P.: Observed Denmark Strait Overflow Cyclones around Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10059, https://doi.org/10.5194/egusphere-egu21-10059, 2021.

15:56–15:58
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EGU21-12690
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ECS
Mattia Almansi, Thomas Haine, Renske Gelderloos, and Robert Pickart

Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.

How to cite: Almansi, M., Haine, T., Gelderloos, R., and Pickart, R.: Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12690, https://doi.org/10.5194/egusphere-egu21-12690, 2021.

15:58–16:00
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EGU21-13295
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ECS
Astrid Pacini, Robert S. Pickart, Isabela A. Le Bras, Fiammetta Straneo, N. Penny Holliday, and Michael A. Spall

The Labrador Sea is an important site for deep convection, and the boundary current surrounding the Sea impacts the strength of this convection and the subsequent restratification. As part of the Overturning of the Subpolar North Atlantic Program, ten moorings have been maintained on the West Greenland shelf and slope that provide hourly, high-resolution renderings of the boundary current. These data reveal the presence and propagation of abundant mid-depth intensified cyclonic eddies, which have not previously been documented in the West Greenland boundary current system. This study quantifies these features and their structure and demonstrates that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. Using the mooring data, the statistics of these features are presented, a composite eddy is constructed, and the velocity and transport structure are described. A synoptic survey of the region captured two of these features, and provides further insight into their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea, and their presence, propagation, and transport must be accounted for in order to assess their contribution to the heat and freshwater budgets of the Labrador Sea interior.

How to cite: Pacini, A., Pickart, R. S., Le Bras, I. A., Straneo, F., Holliday, N. P., and Spall, M. A.: Cyclonic eddies in the West Greenland boundary current system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13295, https://doi.org/10.5194/egusphere-egu21-13295, 2021.

16:00–16:02
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EGU21-13159
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ECS
Ilona Goszczko, Eleanor Frajka-Williams, Louis Clement, and N. Penny Holliday

The Labrador Sea’s surface circulation remains important for the large-scale thermohaline circulation due to its fast response to atmospheric forcing and strong links to the North Atlantic and the Arctic Ocean’s counterparts. Its role in redistribution of heat and momentum, as well as for the biochemical exchange with the atmosphere is crucial in several time and space scales. The region is characterised by advection of freshwater originating from the combined melt of the Arctic Ocean’s sea-ice and Greenland’s glaciers around and towards the interior of the Labrador Sea. The fate of surface freshwater is an important factor that modifies ocean stratification, deep water convection and thus, ocean climate. Despite the major role of surface freshwater in the Labrador Sea, the dominant mechanism responsible for its offshore transport remains debatable, whether it is due to wind-induced Ekman transport, particularly strong in winter, or to eddy advection.

To explore this disagreement, we use surface drifters deployed in three seasons: 50 in December 2019, 50 in March 2020 and 50 in August 2020 in the shelf/slope location off Cape Desolation and near Qaqortoq, a town in the south-west Greenland. The drifters are equipped with temperature sensors and underwater drogues allowing them to follow the cyclonic surface currents: first, the along-shelf, coastal current and along-slope, boundary current west of Greenland; then, if they are able to detach from the shelf edge, the interior circulation of the central Labrador Sea that directs them south-westward from the Davis Strait; eventually, joining the coastal and along-slope boundary currents east of Labrador before circulating into the Labrador Sea’s central basins or eventually leaving the study area.

To investigate the dominant force responsible for the surface transport we use a wind product (ERA5) in a combination with daily SST (OSTIA). Detachment from boundary current is defined as crossing of the 2500 m isobath. The number of crossings varies depending on the season and weather conditions, e.g. an abrupt change in wind direction. This, in turn, may create upwelling of deep-water masses near the shelf-break. However, trajectories of drifters superimposed on SST maps indicate that besides Ekman transport, eddies carry shelf-originating water offshore as well. Auxiliary data from below (Argo floats and other CTD profiles collected near the drifters) allow to distinguish how deep both processes can leave their signature or whether they can drive a return flow.

If any substantial changes in the North Atlantic wind field occur in the future, the fate of the surface water transport in the Labrador Sea will also change, both in respect to its volume and direction. This could potentially affect the balance between Ekman transport and eddies revealed by our analysis of surface drifters data.

How to cite: Goszczko, I., Frajka-Williams, E., Clement, L., and Holliday, N. P.: Wind driven Ekman transport vs eddies – ultimate fight or peaceful cooperation in the Labrador Sea?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13159, https://doi.org/10.5194/egusphere-egu21-13159, 2021.

16:02–16:04
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EGU21-485
Mathilde Jutras, Carolina Dufour, Alfonso Mucci, Frédéric Cyr, and Denis Gilbert

Oxygen concentrations in the deep waters of the Lower St. Lawrence Estuary, in eastern Canada, have decreased by 50% over the past century, reaching hypoxic levels. To study the causes of this deoxygenation, we applied a mixing model (an extended multi-parameter analysis - eOMP) to data collected in the St. Lawrence Estuary since the 1970s and from the late 1990s to 2018. This method accounts for diapycnal mixing and can distinguish between the physical and biogeochemical causes of deoxygenation. The eOMP reveals that, in recent years, most of the deoxygenation of deep waters of the St. Lawrence Estuary is due to a change in the circulation pattern in the western North Atlantic. Since 2008, the Slope Sea and the deep waters of the St. Lawrence Estuary are fed by an increasing amount of oxygen-poor North Atlantic Central Waters (NACW), transported by the Gulf Stream, at the expense of oxygen-rich Labrador Current Waters (LCW). The oxygenation level of the St. Lawrence Estuary therefore reflects what is happening in the western North Atlantic. In contrast, the eOMP shows that, from the 1970s to the late 1990s, biogeochemical changes such as local eutrophication and variations in oxygen consumption rates in the North Atlantic dominated the deoxygenation. 

Further analyses suggest that the variability in the LCW:NACW ratio in the Slope Waters is mainly controlled by the Scotian Shelf-break Current, an extension of the Labrador Current, and not by the position or strength of the Gulf Stream, as often suggested. When the Labrador Current is strong, little of the southward flowing Labrador Current waters follow the coast all the way to the Scotian Shelf, and most of these waters are deviated east towards the North Atlantic. The opposite is true when the Labrador Current is weak. We will present some analysis of LCW trajectories in different conditions and discuss their potential drivers, based on a high resolution model. Overall, our results highlight the primary role of the Labrador Current in determining (i) the oxygen concentration and other water properties on the western North Atlantic continental shelf and slope, and (ii) the advection of fresh Labrador Current Water into the subpolar North Atlantic, with possible implications on the thermohaline and gyre circulation.

How to cite: Jutras, M., Dufour, C., Mucci, A., Cyr, F., and Gilbert, D.: Variability of the circulation in the western North Atlantic and its impact on deep-water oxygen concentrations in the St. Lawrence Estuary on the western continental shelf: evidence from observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-485, https://doi.org/10.5194/egusphere-egu21-485, 2021.

16:04–16:06
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EGU21-10113
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ECS
David Bonan, Andrew Thompson, Emily Newsom, Shantong Sun, and Maria Rugenstein

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic climate change remains poorly understood in part, due to the computational expenses associated with running fully-coupled climate models to equilibrium. Here, we use a collection of millennial-length simulations from multiple state-of-the-art climate models to examine the transient and equilibrium responses of the AMOC to an abrupt quadrupling of atmospheric carbon-dioxide. All climate models exhibit a weakening of the AMOC on centennial timescales, but they disagree on the recovery of the AMOC over next millennia, despite the same greenhouse-gas forcing. In some models, the AMOC recovers after approximately 200 years, while in others the AMOC does not fully recover even after approximately 1000 years. To explain the behavior of the AMOC we relate the overturning circulation in the North Atlantic to the meridional density difference between the basin interior and the region of deep-water formation. This scaling both reproduces the initial decline and gradual recovery of the AMOC, and explains the inter-model spread of the AMOC responses. The initial shoaling and weakening occurs on centennial timescales and is attributed to the warming of the northern convection region. We argue that the AMOC weakens on a timescale linked to a combination of its initial depth and the global surface heat flux sensitivity. The recovery of the AMOC results from a pile-up of salinity in the Atlantic basin, when the AMOC is weakened, that propagates northward and reinvigorates convection. A weaker AMOC recovery is associated with a smaller salinity anomaly. We further show through surface water mass transformation that Southern Ocean processes may impact the salinity anomaly in the Atlantic basin. These results highlight the importance of considering the evolution of the AMOC and ocean heat transport beyond the 21st century as short-term changes are not indicative of long-term changes.

How to cite: Bonan, D., Thompson, A., Newsom, E., Sun, S., and Rugenstein, M.: Transient and equilibrium responses of the Atlantic meridional overturning circulation to warming in coupled climate models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10113, https://doi.org/10.5194/egusphere-egu21-10113, 2021.

16:06–17:00