We welcome contributions from observers and modellers on the following topics:
-- climate relevant processes in the North Atlantic region in the atmosphere, ocean, and cryosphere
-- variability in the ocean and the atmosphere in the North Atlantic sector on a broad range of time scales
-- interpretation of observed variability in the atmosphere and the ocean in the North Atlantic sector
-- response of the atmosphere to changes in the North Atlantic
-- dynamics of the Atlantic meridional overturning circulation
-- role of water mass transformation and circulation changes on anthropogenic carbon and other parameters
-- changes in adjacent seas related 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)
-- comparison of observed and simulated climate variability in the North Atlantic sector and Europe
-- linkage between the observational records and proxies from the recent past
The Atlantic Meridional Overturning Circulation (AMOC) plays a major role in shaping the Northern Hemisphere climate, and assessing the risk of a future slowdown has become a key challenge in ocean research. Over the past two decades, advances in observations and modeling have substantially refined our understanding of where and how deep waters are formed.
In this Award Lecture of the OS Division, I will review these developments to examine the drivers of North Atlantic dynamics and their representation in coupled climate models. First, I will focus on observation-based estimates of water mass transformation in the subpolar gyre, highlighting the dominant role of local buoyancy forcing in the Irminger and Iceland basins. Second, I will examine how deep water formation is simulated in coupled climate models, identifying key biases that lead to excessive formation in the Labrador Sea and assessing their implications for the AMOC at subpolar latitudes. Finally, I will discuss the southward propagation of deep waters and the coherence of AMOC variability across the North Atlantic, placing these results in the broader context of AMOC change at different timescale.
How to cite: Petit, T.: On the Role of Atmospheric Forcing on the North Atlantic Dynamic – Insights from Observations and Climate Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10992, https://doi.org/10.5194/egusphere-egu26-10992, 2026.
The subpolar North Atlantic exhibits pronounced variability on decadal and longer time scales, which has implications for decadal predictability of climate and marine biogeochemical fields. Yet, its driving mechanisms are still under debate. It has been shown from both observations and modeling that this variability is associated with anomalous deep water formation in the Labrador Sea generated by the surface heat fluxes associated with the North Atlantic Oscillation (NAO) and the consequent adjustment of thermohaline circulation, most evident during the 1990s when the NAO was persistently positive. However, the direct observations of overturning circulation along the sections east and west of Greenland (Overturning in the Subpolar North Atlantic Program; OSNAP) do not support this view, as observed overturning in the Labrador Sea is very weak under positive NAO conditions during the observed period from 2014 onward. In this study, we use high-resolution (0.1°) forced ocean–sea-ice simulations, which reasonably reproduce the mean overturning in density coordinates observed at the OSNAP line, to elucidate this contrasting overturning between the two periods under similar positive NAO conditions. Simulated deep water formation in the Labrador Sea is indeed weak during the 2010s, while it is very active during the 1990s. These signals are meridionally coherent, suggesting coherent changes in Atlantic meridional overturning circulation (AMOC). We also find that this anomalous overturning in the Labrador Sea takes place over densities far heavier than the density where maximum overturning occurs, thus the maximum overturning time series cannot accurately capture these signals. We have conducted sensitivity experiments to identify whether the weak overturning during the 2010s is due to oceanic or atmospheric conditions. These experiments reveal that the weaker overturning is largely generated by weak surface heat release due to a warmer air temperature over the Labrador Sea. We have further performed composite analyses using the CESM2 pre-industrial and transient (historical plus SSP370) simulations to investigate how such warm air conditions come about over the Labrador Sea. The composite analyses suggest that the warm air temperature is likely due to a warm SST condition in the Labrador Sea, internally generated, rather than externally forced. Conversely, the strong overturning during the 1990s was likely because of cooler conditions in the Labrador Sea.
How to cite: Kim, W. M., Yeager, S., Rosbson, J., and Walsh, A.: Subpolar North Atlantic overturning: the 1990s versus the 2010s, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17732, https://doi.org/10.5194/egusphere-egu26-17732, 2026.
A simplistic view of the Atlantic Meridional Overturning Circulation (AMOC) is that it is composed of a northward flow of warm, salty water that sinks at high latitudes as a result of wintertime surface buoyancy losses, subsequently returning southward as the cold, fresh Deep Western Boundary Current. Its strength is often expressed as the maximum of the overturning streamfunction in a depth and latitude range, the latter normally centred at around 26°N. In reality, however, the AMOC is partly “carried” by the largely wind-forced and near-barotropic horizontal gyre circulation, and the production of upper NADW has also been shown to depend on a chain of surface densification around the subpolar gyre, in addition to the deep convection localised in the Labrador and Irminger Seas. The overturning streamfunction in depth coordinates is therefore far from being a complete description of the AMOC.
We present a range of AMOC metrics using a set of centennial simulations of the HadGEM3-GC5 coupled model with a ¼° NEMO ocean component. These include a gyre index based on the barotropic streamfunction, surface-forced indices, regional mixed-layer volumes, and transport indices evaluated against a range of vertical axes. We compare these indices with the traditional overturning metric at the RAPID section at 26°N, and discuss the causal links between them. This work is carried out under the EU HORIZON25 project Explaining and Predicting the Ocean Conveyor (EPOC).
How to cite: Megann, A., Blaker, A., Hirschi, J., and Aksenov, Y.: Disentangling the AMOC: influences of the gyre circulation, surface density transformations and overturning circulation on AMOC variability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6942, https://doi.org/10.5194/egusphere-egu26-6942, 2026.
Water mass transformation in the Subpolar North Atlantic strongly influences the strength of the Atlantic Meridional Overturning Circulation (AMOC), oceanic heat and carbon uptake, and regional climate variability. Despite its importance, the variability of the Subpolar Gyre (SPG), its potential regime transitions, and its coupling to the AMOC remain poorly constrained, particularly regarding the role of mesoscale eddies. While advective-convective feedbacks have been proposed to lead to bistability in the SPG, it is unclear whether such behavior persists in strongly eddying ocean models.
Here, we examine SPG variability and deep water formation for distinct AMOC regimes in the stand-alone global ocean and high-resolution (0.1°) version of the Parallel Ocean Program. The POP was integrated for 600 years under a slowly increasing freshwater flux forcing over the North Atlantic, featuring a strong (20 Sv) AMOC state and, after the AMOC collapse, a weak (5 Sv) AMOC state. Monthly-averaged model output is used to construct composites that contrast SPG circulation and convective activity with particular emphasis on the role of resolved mesoscale variability.
There are pronounced contrasts in regional convection and SPG behavior between the strong and weak AMOC states. In the strong overturning regime, deep convection across the Labrador Sea and Irminger Basin exhibits relatively low variability, while mixed layer depth variability is more pronounced in the Nordic Seas. In the weak overturning regime, deep convection in the Labrador Sea and Nordic Seas is strongly reduced to shallow mixed layer depths (< 150 m). In contrast, the Irminger Basin exhibits enhanced decadal variability and increased mixed layer depths. Notably, an accompanying low-resolution (1°) simulation does not reproduce this feature and lacks a sustained weak AMOC state after its collapse, highlighting the potential importance of eddy processes that are parameterized in coarse-resolution models.
These results underscore the sensitivity of SPG dynamics and AMOC stability to model resolution and motivate further investigation into the representation of mesoscale processes in climate models and their role in shaping North Atlantic variability across distinct AMOC states.
How to cite: Ypma, S. L., van Westen, R. M., von der Heydt, A. S., and Dijkstra, H. A.: Deep convection variability across strong and weak AMOC states in an eddy-resolving ocean simulation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9655, https://doi.org/10.5194/egusphere-egu26-9655, 2026.
The Atlantic Multidecadal Variability (AMV) is a prominent mode of low-frequency climate variability, characterized by basin-scale sea surface temperature (SST) variations in the North Atlantic and strong global impacts. AMV can be forced externally by surface radiative fluxes or internally generated. The latter generation mechanism is commonly attributed to variations in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that a robust AMV can arise and be sustained by large-scale atmosphere–ocean interactions, even in the absence of a dominant role for AMOC variations, in a fully coupled model—the Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). The simulated AMV is driven primarily by surface shortwave and turbulent heat fluxes across the North Atlantic. Essentially, an initial warming over the Gulf Stream region strengthens and spreads by reducing low cloud cover and enhancing surface shortwave radiation. This mechanism is enabled by the relatively narrow width of the North Atlantic, compared to the North Pacific. Our results broaden the conceptual understanding of AMV physics and underscore the importance of atmosphere–ocean interactions in sustaining it.
How to cite: Hu, S., Li, X., Fedorov, A., and Van Roekel, L.: Atlantic Multidecadal Variability driven by the western current warming–eastern low cloud reduction mechanism, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6053, https://doi.org/10.5194/egusphere-egu26-6053, 2026.
There is substantial model uncertainty in how the AMOC evolves in CMIP6 historical simulations. A significant part of this uncertainty has been related to the substantial uncertainty in the strength of the historical Anthropogenic Aerosol forcing. However, there is also significant uncertainty in how models simulate the AMOC response to historical greenhouse gas forcing, which is not well understood. Therefore, this raises the question, how sensitive is the real-world AMOC to historical greenhouse gas forcing? Here, we use simulations from the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) to isolate the historically forced signal, and to understand the spread in AMOC response between models. By 2014, AMOC declined significantly in hist-ghg simulations as expected, but there is a very large spread in AMOC decline that is approximately equivalent to the uncertainty due to aerosol forcing. We find that the decline appears to be strongly related to changes in turbulent heat loss in the subpolar North Atlantic, which is itself related to the change in air-sea temperature and humidity contrasts (i.e., it is thermodynamically driven). The spread in hist-ghg simulations is also consistent with the spread in abrupt 4xCO2 simulations, and further analysis of those simulations supports a causal relationship between the spread in the initial forced changes in the air-sea temperature contrasts and the resultant AMOC decline.
How to cite: Madan, G., Robson, J., and Sutton, R.: Understanding the uncertainty in simulated AMOC changes to historical greenhouse gas emissions in CMIP6, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22473, https://doi.org/10.5194/egusphere-egu26-22473, 2026.
How to cite: Hermanson, L., Smith, D., and Seabrook, M.: Using climate forcings runs to attribute the decadal variability and predictability of the AMOC, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18744, https://doi.org/10.5194/egusphere-egu26-18744, 2026.
To understand the AMOC response to historical greenhouse gas (GHG) forcing including the role of resolution, new historical simulations with GHG forcing fixed at 1950s level (fixedGHG) are performed in EPOC project. We compare results from fixedGHG run with HighResMIP control run and historical run, to isolate the impact of GHG on historical changes in the AMOC. We assess AMOC variability and its associated dynamical sea level (SSH) fingerprint in two configurations of in MPI-ESM-1.2-HR (0.4º ocean) and MPI-ESM-1.2-ER (0.1º eddy-resolving). In MPI-ESM-1.2-ER, the historical run exhibits a significant negative trend of the AMOC, while fixedGHG run exhibits a significant positive trend of the AMOC. The results support the ideas that GHG forcing leads to slowdown of the AMOC and aerosol forcing lead to spin-up of the AMOC. The change of the AMOC is coherent across latitudes, with larger amplitudes of trend in the subpolar North Atlantic in MPI-ESM-1.2-ER. In MPI-ESM-1.2-HR, neither experiment shows a significant long-term trend, although a slight AMOC decline emerges after the mid-1990s in the historical run. We further evaluate the AMOC–SSH relationship at 26°N using AVISO altimetry and RAPID observations. Both observations and the ER historical run display a canonical Gulf Stream–related dipole: positive SSH anomalies south and negative anomalies north of the Gulf Stream SSH ridge, along with negative anomalies along the Labrador Current—consistent with a strengthened Gulf Stream and Deep Western Boundary Current during strong AMOC states. The HR configuration fails to reproduce this fingerprint, underscoring the importance of eddy-resolving simulations for capturing AMOC–SSH covariability. We are further analyzing SSH patterns in fixed-GHG simulations to isolate the effects of GHG forcing and to elucidate the underlying mechanisms.
How to cite: fan, H. and Barcelona Martín, J.: The AMOC Response to GHG Forcing and Its Fingerprint on Dynamical Sea Level, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17696, https://doi.org/10.5194/egusphere-egu26-17696, 2026.
Based on satellite altimetry, GRACE space gravimetry and ARGO-based steric data down to 2000m, recent studies have shown that the North Atlantic sea level budget of the past two decades is not closed, with strong regional residuals in the mid-latitudes. It was proposed that this results from salinity errors reported since 2015/2016 in some Argo float measurements. In this study, we revisit the North Atlantic sea level budget using altimetry, GRACE, different Argo products and ocean reanalyses. The reanalyses are used to estimate the manometric contribution for further comparisons with GRACE data, as well as for estimating the deep ocean contribution to the sea level budget, not yet sampled by Argo. We first find that using the CIGAR ocean reanalysis-based manometric component instead of GRACE reduces the residuals of the sea level budget in the North Atlantic (ie, altimetry-based sea level minus sum of components). We also find that accounting for the deep ocean (below 2000m) thermal expansion (from the CIGAR reanalysis) allows for the quasi closure of the North Atlantic sea level budget. The North Atlantic halosteric component in the upper 2000 m displays a small decrease since the early 2010s, significantly larger after 2016. The 2010–2016 halosteric decrease may reflect a real salinity increase in the region, but salinity measurement errors may have impacted the halosteric component after that date. The main result of this study is that deep ocean warming plays a non-negligible role in the North Atlantic and has to be accounted for in the sea level budget assessment.
How to cite: Song, Z., Cazenave, A., Llovel, W., Storto, A., and Bouih, M.: North Atlantic sea level budget revisited, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3385, https://doi.org/10.5194/egusphere-egu26-3385, 2026.
Ocean tides play an important role in shaping circulation, stratification, and mixing in the North Atlantic subpolar region, yet their impacts at fine spatial scales remain insufficiently quantified. In this study, we investigate the effects of ocean tides in the North Atlantic subpolar area using a km-scale high-resolution ocean model. Two numerical experiments are conducted: a control simulation including full tidal forcing and a sensitivity experiment in which tidal forcing is suppressed. By comparing these experiments, we isolate the tidal contributions to sea surface elevation, currents, and vertical mixing.
The results show that tides substantially enhance barotropic and baroclinic variability, particularly over complex topography such as continental slopes and ridges. Tidal currents intensify near-bottom shear and promote vertical mixing, leading to modifications in stratification and water mass properties. In addition, tide–topography interactions generate internal tides that propagate into the interior basin, influencing submesoscale circulation and energy redistribution. These tidal effects further modulate the mean flow and variability in the subpolar gyre, with implications for regional heat and salt transport. Meanwhile, natural variability also plays a role when distinguishing between tidal effects and internally generated variability.
Our findings highlight the importance of explicitly resolving tidal processes in high-resolution ocean models for accurately representing circulation and mixing in the North Atlantic subpolar region. This study emphasizes that tides are a key component of subpolar ocean dynamics and should be considered in studies of climate-relevant processes in this region.
How to cite: Lin, L.: The effects of ocean tides in North Atlantic subpolar area studied by high-resolution ocean model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4458, https://doi.org/10.5194/egusphere-egu26-4458, 2026.
The North Atlantic is a key region of the climate system, where ocean circulation redistributes heat across latitudes and drives pronounced variability on interannual to multidecadal time scales. Observational programs have provided valuable insights into Atlantic circulation and variability, but their spatial and temporal coverage remains limited. Global ocean reanalyses offer a complementary, spatially complete view of the ocean and therefore provide potentially valuable tools to investigate heat transports and their variability in the North Atlantic.
This contribution aims to provide a clear and quantitative assessment of the usefulness and reliability of ocean reanalyses for diagnosing ocean heat transport and its variability in the North Atlantic. It is carried out within the framework of the Marine Environment Reanalyses Evaluation Project (MER-EP), which aims to systematically evaluate and intercompare global marine reanalyses. By systematically comparing multiple reanalyses with observational and budget-based constraints, we aim to identify where and under which conditions reanalysis-derived transport estimates are robust and where important limitations remain. This assessment is essential for the appropriate use of ocean reanalyses in studies of North Atlantic variability and its role in climate change.
We evaluate ocean heat transport and related diagnostics in the North Atlantic using a large ensemble of global ocean reanalyses from different modelling centers. Transport calculations are performed using the newly developed StraitFlux (Winkelbauer et al. 2024) diagnostic framework, which enables consistent transport estimates across different model grids and vertical coordinate systems.
Our analysis focuses on the North Atlantic sector and its variability, with particular attention to major observing systems such as RAPID, SAMBA, OSNAP (Winkelbauer et al., preprint) and transports across the Greenland-Scotland Ridge, Fram Strait and the Barents Sea Opening. In addition to transports obtained from ocean reanalyses and insitu observations, we estimate meridional ocean heat transport indirectly from the ocean heat budget. These inferred transports are obtained by combining surface heat fluxes inferred from the atmospheric energy budget, ocean heat content tendencies and contributions from sea ice melt, and by imposing appropriate boundary conditions at basin chokepoints. This approach provides complementary ocean heat transport estimates that are largely independent of both the reanalysis circulation fields and the insitu observations. It allows to assess ocean reanalysis performance consistently across the entire North Atlantic, including regions and latitude bands where no insitu transport measurements are available.
Winkelbauer, S., Mayer, M., and Haimberger, L.: StraitFlux – precise computations of water strait fluxes on various modeling grids, Geosci. Model Dev., 17, 4603–4620, https://doi.org/10.5194/gmd-17-4603-2024, 2024.
Winkelbauer, S., Winterer, I., Mayer, M., Fu, Y., and Haimberger, L.: Subpolar Atlantic meridional heat transports from OSNAP and ocean reanalyses – a comparison, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-4093, 2025.
How to cite: Winkelbauer, S., Mayer, M., Forget, G., Song, Y., and Haimberger, L.: On the reliability of reanalysis-derived heat transport in the North Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3145, https://doi.org/10.5194/egusphere-egu26-3145, 2026.
The subpolar North Atlantic Ocean and adjacent Arctic Seas experience strong winter heat loss and surface water densification which play a key role in the large-scale ocean circulation. The balance of processes driving this heat loss is expected to change as the climate system heats up and ice cover declines with impacts on both the ocean and the atmosphere. Here, we use a 40-member ensemble of runs with the HadGEM3-GC3.1 model (termed the CANARI Large Ensemble) to investigate these changes. The runs span 1950-2014 and 2015-2100 using CMIP6 historical and SSP3-7.0 forcings, and model resolution is ¼ degree ocean – N216 atmosphere. In the sub-polar Atlantic, the winter heat loss initially strengthens through to the mid-1980s before weakening by of order 50% by 2100 due primarily to variations in the sea-air temperature gradient. In the Arctic, the winter heat loss is initially dominated by ice decline before becoming dominated by atmospheric conditions from mid-century onwards. Regional variations in the impacts of these changes on both the ocean and the atmosphere will also be explored.
How to cite: Josey, S., Blaker, A., Grist, J., Mecking, J., and Sinha, B.: Future Evolution of Subpolar Atlantic and Arctic Ocean-Atmosphere Interaction in the CANARI Large Ensemble , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5497, https://doi.org/10.5194/egusphere-egu26-5497, 2026.
Recent Arctic warming and melting sea ice are iconic features of global warming. Yet, it is unlikely that anthropogenic forcing is solely responsible for these changes. The Early-20th-Century Arctic Warming (ETCAW), comparable to the recent one, provides a benchmark for natural climate variability but remains poorly understood. Sparse sea-ice observations is a major issue—limiting also past modelling studies. Here, we use a new physically based sea-ice reconstruction and atmospheric model experiments to replicate, for the first time, the rapid ETCAW. We find that two-thirds of the strong winter warming is driven by increased ocean heat release, amplified further by the lapse-rate feedback. This response is linked to extensive sea-ice loss present in the reconstruction and to strengthened poleward Atlantic heat transport. These results clarify the role of sea-ice loss in the ETCAW and provide new insight into natural variability’s influence on future Arctic climate change.
How to cite: Li, F., Semenov, V., Keenlyside, N., Aldonina, T., Ha, K.-J., Chung, E.-S., and Yang, X.-Q.: Unveiling the Role of Sea-Ice Loss in Early-20th-Century Arctic Warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11351, https://doi.org/10.5194/egusphere-egu26-11351, 2026.
Since the post-industrial era, sea surface temperature (SST) has shown a consistent warming trend at a global scale, while chlorophyll-α (Chl-α) concentrations have generally exhibited declining trends in the open ocean. Some recent studies suggest that intensified coastal upwelling, driven by increased alongshore winds, may locally counteract these negative trends by enhancing biological productivity. This study assesses the evolution of long-term trends in SST, Chl-α, meridional wind stress and Saharan dust over the Northeast Atlantic, focusing on both open-ocean oligotrophic regions and coastal upwelling systems. We applied the methodology developed by Siemer et al. (2021), using the same satellite and in situ datasets, updated to include the most recent years, and defining additional subregions to better resolve smaller open-ocean areas of interest. Our results reveal a significant acceleration of SST warming across the entire study area during the last six years. In open-ocean regions, this acceleration is accompanied by a strengthening of negative Chl-α trends, indicating a continued decline in phytoplankton biomass. In contrast, coastal upwelling regions, particularly the Northwest African upwelling system, exhibit a slowdown in the decline of Chl-α and productive area. However, trends in upwelling-favourable wind stress over the African coast are predominantly negative, suggesting a weakening of the atmospheric forcing traditionally associated with enhanced coastal productivity. The inclusion of Saharan dust variability allows us to assess the combined role of atmospheric forcing and aerosol deposition in modulating recent biophysical trends in the region.
How to cite: Pereira, C., Fraile-Nuez, E., González-Vega, A., Machín, F., Puig Montellà, E., Martín- Díaz, J. P., and Ayuso- Candal, S.: Multi-decadal trends in SST, chlorophyll- a, NPP and atmospheric forcing across oligotrophic and upwelling regions of the Northeast Atlantic , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20957, https://doi.org/10.5194/egusphere-egu26-20957, 2026.
The Northwest Atlantic Ocean is a climatically sensitive region influenced by two major boundary currents—the warm Gulf Stream and the cold Labrador Current—which transport water masses of contrasting temperature and salinity, and it also hosts a crucial component of the Atlantic Meridional Overturning Circulation (AMOC). Recent studies indicate substantial changes in both currents, with potential implications for regional ocean dynamics. In this study, we investigate the evolution of stratification in the Northwest Atlantic over the period 1993–2023 using an eddy-permitting reanalysis dataset. Stratification is quantified through the Brunt–Väisälä frequency, and long-term trends are assessed. To diagnose the drivers of the observed stratification changes, we further examine the variability in current pathways using Lagrangian parcel tracking. Additionally, Optimum Multiparameter (OMP) analysis reveals that changes in circulation are redistributing water masses across the study domain, which likely contributes to the modulation of water-column stratification. Stratification in this region is a key regulator of ocean primary production, oxygen ventilation, vertical mixing, and deep convection, thereby influencing both ecosystem dynamics and large-scale ocean circulation.
How to cite: John, A. B., Pant, V., and Lahiri, S. P.: Do Changes in the Western Boundary Circulation Cause Stratification Changes in the Northwest Atlantic Ocean?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1151, https://doi.org/10.5194/egusphere-egu26-1151, 2026.
The Atlantic Ocean exhibits a persistent northward heat transport at all latitudes, providing a key source of heat to the relatively northerly located European continent. Consequently, variability in the North Atlantic circulation plays a central role in modulating regional climate patterns in Europe. However, the widely reported lack of meridional coherence in the Atlantic basin on interannual to decadal timescales impedes the detection of large-scale circulation changes and their separation from internal climate variability. The Subpolar Gyre (SPG) is particularly important because variability in its circulation strength and hydrographic properties impacts both local dense water formation and heat transport towards the Arctic. In this study, we examine the structure and variability of the SPG circulation and quantify the recirculation within the gyre versus the throughput towards the Nordic Seas across the Greenland–Scotland Ridge. We employ the Lagrangian trajectory tool TRACMASS to identify the dominant pathways of recirculation and throughput, and quantify the associated volume and heat transports. We utilise a 1/12° ocean hindcast as Eulerian input fields for the period 1979–2021, and seed Lagrangian particles in the North Atlantic Current at 53°N. The Lagrangian trajectories allow us to quantify the spatio-temporal variability of the circulation, and to localise the depth- and density-dependent connectivity between the SPG and the Nordic Seas. The results lay the groundwork for a better understanding of the SPG as a potential modulator of heat transport towards the Arctic.
How to cite: Unegg, J., Asbjørnsen, H., and Svendsen, L.: Variability in the Subpolar Gyre circulation and throughput towards the Nordic Seas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7013, https://doi.org/10.5194/egusphere-egu26-7013, 2026.
The Rockall Trough (RT), located in the northeastern Atlantic Ocean, is a dynamically complex region characterized by a strong tidal forcing, flow–bathymetry interactions, intense mesoscale activity and deep winter mixing. These processes promote enhanced vertical and lateral mixing, making the RT a key region for the transformation of intermediate and deep water masses of southern origin that subsequently feed high-latitude convective sites. As such, modifications occurring in the RT may have important implications for deep convection and large-scale circulation in the subpolar North Atlantic (SPNA).
The Mediterranean Outflow Water (MOW), originating in the Gulf of Cádiz and spreading northward along the European continental margin, is a major contributor to the heat and salt budgets of the North Atlantic. Although previous studies have identified the presence of the MOW within the RT, its pathways, transformation processes and interaction with surrounding water masses in and beyond this region remain poorly understood. In particular, the extent to which the MOW properties are modified before entering the SPNA is still uncertain.
In this study, we combine an extensive dataset of more than 20 years of Argo float observations with a set of simulated Lagrangian trajectories to investigate the pathways of the MOW in the RT, the evolution of its properties, and the interactions of the MOW with the resident water masses. Argo data are used to identify the MOW signal at intermediate depths and to quantify changes in temperature and salinity along its pathways, while Lagrangian simulations provide insight into the paths, residence times, and connectivity within and beyond the RT.
In addition, Copernicus reanalysis data are employed to characterize the persistent features of the intermediate circulation in the RT, allowing us to assess how these structures influence the transport, spreading, and mixing of the MOW in this key transition region.
The long-term Argo record further allows us to examine the interannual variability of the MOW pathways and properties, providing new insights on the processes that regulate its spreading further north, into the SPNA.
How to cite: Calvo, E., Malanotte, P., Menna, M., Martellucci, R., and Zambianchi, E.: Pathways and transformation of the Mediterranean Outflow Water in the Rockall Trough, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9291, https://doi.org/10.5194/egusphere-egu26-9291, 2026.
The Southwest Iberian margin is a critical oceanographic region, forming the boundary of an eastern boundary upwelling system and serving as the primary pathway for Mediterranean Outflow Water (MOW) into the Atlantic Ocean. An analysis of year-long (November 2023–December 2024) observations from moored instruments at two sites on the continental slope was conducted: a mid-slope location (PA-II; 1488 m depth) and a lower-slope site (PA-I; 2606 m depth), the latter located on the so-called Shackleton site. We investigated the natural variability of temperature, salinity, turbidity, and current velocity and direction at both subsurface and deep levels at each mooring.
Analysis of Temperature-Salinity (TS) diagrams revealed three distinct water masses. At 2606 m depth in PA-I, the North East Atlantic Deep Water (NEADW) was present, while more diluted NEADW and MOW were occasionally identified at ~ 1488 m depth in PA-II. Subsurface waters (~353 m depth in PA-I and ~418 m depth in PA-II) were characterized by the presence of the Eastern North Atlantic Central Water (ENACW) of subtropical and subpolar origins, respectively. The TS time series reveals that short-term fluctuations were more prominent than clear seasonal signals.
Current speeds were higher in subsurface waters (≥0.3 ms⁻¹) than in deep waters (0-0.3 ms⁻¹). After tidal removal, the dominant directions in PA-I were eastward in the subsurface level and north/northwestward at 2606 m depth. At PA-II, subsurface currents flow north/northeast, while deep waters move north/-northwest. Notably, currents at ~ 1488 m depth in PA-II were highly influenced by tidal components as indicated by a directional change from northeast/ southwest to north/northwest and maximum speed reduction from 0.4 ms⁻¹ to 0.3 ms⁻¹, a pattern not observed at other depths, after removing the tidal influence.
Persistent values ranging from 0.1–0.5 FTU (Formazin Turbidity Units) over extended periods were interpreted as long-lived increases in turbidity associated with upwelling, background sedimentation, and resuspension cycles. Short-lived turbidity peaks (≥0.5 FTU), lasting hours to days, are also recorded. Turbidity amplitudes were generally lower in deep waters compared to subsurface waters.
Based on surface winds, surface temperature, chlorophyll concentration, and the upwelling index, we interpret the subsurface, low-moderate turbidity signals at PA-I as offshore transport of particles along isopycnals during the peak upwelling phase (July- September). During this period, ENACW was upwelled, consistent with the subsurface current flow directions at both sites. The low-to-moderate deep-water turbidity variations, indicative of near-bottom resuspension events, coincided with the timing of local bottom trawling activities. A prominent short-lived event recorded in subsurface waters at PA-II is linked to a regional earthquake in August 2024 (~ 57 km to epicentre), while other short-lived events coincided with increased riverine sediment discharge driven by rainstorms in the west part of the peninsula.
Overall, these integrated hydrographic, currents, and turbidity observations underscore the strong coupling between water-mass structure, upwelling dynamics, and lateral transport pathways. They emphasize how both physical oceanographic processes and episodic natural and human-induced forcing are pivotal in shaping subsurface and deep-water environments in this dynamic boundary region.
How to cite: Hewage, P. L., Arjona-Camas, M., Sanchez-Vidal, A., J. Sierro, F., and Ausín, B.: Subsurface and deep-water mass characteristics and variability in the Southwest Iberian margin from year-long observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20033, https://doi.org/10.5194/egusphere-egu26-20033, 2026.
The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the climate system, transporting heat, freshwater, and momentum across the Atlantic basin and playing a critical role in regulating regional climate and global heat uptake. It is commonly portrayed as a conveyor belt, with warm saline waters travelling northward, losing heat to the atmosphere and freshening due to precipitation and ice melt. Sinking occurs at high latitudes once the water is sufficiently dense, and the newly formed dense waters travel southward. However, this picture is overly simplistic. The AMOC is not spatially uniform: transport anomalies at one latitude do not always map directly to anomalies elsewhere, because of internal recirculations, gyre-scale compensations, mixing, and local forcing.
Within the EU-funded EPOC (Explaining and Predicting the Ocean Conveyor) project we have examined transport variability and coherence on seasonal and longer timescales in observations and a range of numerical models. Starting from the Arctic gateways and progressing southward, transports and variability of volume, heat and freshwater are compared at key observational sections are compared. Meridional coherence of the AMOC is examined using latitude-correlation and EOF decomposition methods, and comparisons against recent Bayesian modelling heat and observation-based freshwater transports are made. In this poster we summarise the key analysis and work performed under WP1.
This work is funded by the UKRI (grant number 10038003) as part of the EPOC project (Explaining and Predicting the Ocean Conveyor; grant number: 101059547).
How to cite: Blaker, A., de Steur, L., Megann, A., Vallivattathillam, P., Aksenov, Y., Fredriksen, H.-B., and Hirschi, J. and the Contributors to EPOC WP1: AMOC transport variability and coherence from observations and models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19285, https://doi.org/10.5194/egusphere-egu26-19285, 2026.
The circulation of the North Atlantic Ocean plays a vital role in the Earth's climate system. Numerous studies, mainly through computer simulations, have examined the stability of the Atlantic Meridional Overturning Circulation (AMOC) in the context of a warming climate. Some of these studies predict a potential collapse of the AMOC in the foreseeable future, which would require a significant influx of freshwater into the subpolar North Atlantic (NA) and/or Nordic Seas. Paleoreconstructions of the NA circulation indicates a major shift in the position of the subpolar cold front either precedes or coincides with substantial changes in AMOC dynamics. These changes imply a significant alteration in circulation patterns, beginning with noticeable restructuring of the subtropical and subpolar gyres. This would lead to modifications in the Gulf Stream system and the North Atlantic Current (NAC), affecting the thermohaline fields as well as the position and strength of these two current systems. Although some models predict a significant slowdown or even collapse of the AMOC, recent observational studies offer a more cautious perspective. For instance, the Gulf Stream system exhibits high resilience to perturbations from ongoing sea-surface warming. In this study, we analyze the decadal variability of temperature and salinity from in situ observations, along with upper-ocean currents in the subpolar NA (SPNA). We find that the thermohaline pattern of the upper ocean layers in the SPNA and Nordic Seas has remained resilient for over 70 years. The deceleration of the AMOC is evident but relatively modest, with average velocities in the upper layers decreasing by less than 10-15% over 30 years. This deceleration is also not consistent throughout the NAC region. Furthermore, the subpolar front migration over 70 years is a maximum of 3° of latitude, with the spatial variability of the yearly 10°C isotherms substantially less than that. Overall, the conclusion about the resilience of the NAC aligns well with that of the Gulf Stream, with no substantial changes in the position or intensity of the subpolar gyre. We conclude that while the AMOC is susceptible to some deceleration due to ongoing surface warming and/or freshening at high latitudes, it may also be sufficiently resilient to withstand these changes. Although it cannot be entirely ruled out that the AMOC may reach its tipping point within this century, an analysis of data on decadal variability in the upper arm of the AMOC suggests that such a collapse is unlikely.
How to cite: Mishonov, A., Seidov, D., and Reagan, J.: Resilience of the North Atlantic Circulation on Decadal Timescales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1758, https://doi.org/10.5194/egusphere-egu26-1758, 2026.
The North Atlantic subpolar gyre is a highly dynamic region where ocean–atmosphere interactions are shaped by variations in freshwater export from the Arctic, import of subtropical waters via the Gulf Stream, mixing, in addition to large-scale atmospheric circulation patterns such as the NAO and the strength and position of the North Atlantic jet. Building on our earlier analysis using the CANARI Large Ensemble (HadGEM3-GC3.1), we previously showed that winters with anomalously fresh surface waters systematically exhibit shallower mixed layers, cooler SSTs, and weaker surface heat loss. These conditions imply enhanced freshwater-driven stratification, reduced deep convection, and a tendency for heat to be trapped below the surface, features consistent with the structure and persistence of the North Atlantic Warming Hole (NAWH).
In this study, we extend that framework to examine how this wintertime surface cooling and associated changes in the surface heat fluxes interact with the overlying atmosphere across a range of background circulation states. Using ensemble member–specific sea level pressure anomaly patterns and a regime-classification approach, we identify multiple atmospheric response modes that differ in the strength and latitude of the North Atlantic pressure gradient. These regimes reveal that the atmospheric response to subpolar cooling is not uniform. The background fields play a decisive role in determining how the surface cooling interacts with the large-scale atmospheric circulation.
Together, our results highlight a dynamically consistent pathway linking freshwater import, ocean stratification changes, regional winter SST cooling, heat flux responses, and large-scale atmospheric circulation shifts. This work provides new insight into the range of possible atmosphere–ocean climate feedbacks associated with ongoing and future freshening of the subpolar North Atlantic.
How to cite: Ayres, H. and Oltmanns, M.: Freshwater-driven subpolar gyre cooling and atmospheric regime responses using a large ensemble climate model., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5455, https://doi.org/10.5194/egusphere-egu26-5455, 2026.
The North Atlantic re-emergence phenomenon is an intermittent event in which winter sea surface temperature (SST) anomalies subduct under the seasonal thermocline and re-emerge the following winter when the mixed layer deepens. This means that the ocean acts as a `memory' for North Atlantic winter climate on interannual scales. Previous studies of the North Atlantic re-emergence phenomenon are limited by short observational records, forced-ocean models, or poor resolution. The CANARI Large Ensemble (65 years x 40 members of the UK Met Office Climate Model (HadGEM3) at N216 atmosphere and 1/4 degree ocean resolution) provides an opportunity to robustly analyse these events and their mechanisms. We have tested existing mechanistic theories, and are answering other pertinent questions, such as; does stratospheric preconditioning occur and can we harness predictability from it? Are there multi-year impacts and cascading effects? Can we quantify the relationship between Atlantic meridional overturning circulation (AMOC) and re-emergence?
How to cite: Collingwood, E., Sinha, B., Marsh, R., Blaker, A., Marshall, G., Scaife, A., and King, J.: The North Atlantic Re-emergence Phenomenon in a Coupled Large-ensemble Climate Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6755, https://doi.org/10.5194/egusphere-egu26-6755, 2026.
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of Earth’s climate system, regulating large-scale ocean heat and nutrient transport. Paleo reconstructions indicate that the AMOC varied substantial during late Quaternary climate transitions. Sedimentary 231Pa/230Th has been widely used as a tracer for reconstructing past AMOC strength. However, recent studies have questioned its applicability, as 231Pa/230Th is also sensitive to particle fluxes. In particular, variations in export productivity and metalliferous particles emitted from hydrothermal vents may overprint the circulation signal.
Here, we investigate the sensitivity of sedimentary 231Pa/230Th to past AMOC variability by compiling new and revised down-core 231Pa/230Th records spanning the last 30,000 years from a geographically confined sector of the mid–North Atlantic, covering water depths from 2,102 to 4,110 m. This justifies the assumption of very similar particles fluxes for all core locations in this pelagic environment. However, despite their close spatial proximity, the down-core 231Pa/230Th records exhibit two clearly distinguishable trends, with increasing 231Pa/230Th at shallower sites and decreasing trends at deeper sites. These trends are unlikely to be the result of changes in particle scavenging alone: biogenic opal concentrations reveal similar down-core trends throughout all sites, while the absolute concentrations remain consistently below 10 wt% and bulk sediment Fe/Ti and Cu/Ti ratios at most sites provide no evidence for significant local inputs of metalliferous particles associated with enhanced hydrothermal activity despite the region's proximity to the mid-ocean ridge. The only exception is one core closest to multiple active hydrothermal vents showing intermittent intervals of elevated Fe/Ti and Cu/Ti ratios, which are associated with elevated 231Pa/230Th ratios.
By incorporating the 231Pa/230Th records from this geographically confined study area into a basin-wide North Atlantic compilation, we show that the inverted 231Pa/230Th trends observed over the last 30,000 years are coherent North Atlantic-wide features. To investigate the underlying mechanisms, we conducted a set of conceptual Holocene and LGM AMOC simulations using the 231Pa/230Th-enabled Bern3D model. The simulations show that during the LGM a weaker AMOC, relative to the Holocene, can reproduce the observed depth-depend 231Pa/230Th response. This pattern is most likely caused by the spatiotemporally variable balance between particle-mediated scavenging and lateral advection of 231Pa. Importantly, changes in both processes are governed on basin scale by the AMOC. These findings indicate that shallow to intermediate-depth sediment cores capture signals of past circulation strength, even when their 231Pa/230Th response is inverse to the conventional deep-ocean interpretation of higher 231Pa/230Th reflecting a weaker AMOC and vice versa.
How to cite: Gerber, L., Lippold, J., Repschläger, J., Friedrich, O., Testorf, P., Ehnis, M., Blaser, P., Pöppelmeier, F., and Jaccard, S. L.: Processes driving 231Pa/230Th in the mid-North Atlantic Basin over the last 30,000 years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10057, https://doi.org/10.5194/egusphere-egu26-10057, 2026.
