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 Eastern North Atlantic, including the Northwest European shelf, experienced unprecedented surface temperature anomalies in June 2023 (anomalies up to 5 °C locally, north of Ireland). We show the shelf average underwent its longest recorded category II marine heatwave (16 days). With state-of-the-art observation and modelling capabilities, we show the marine heatwave developed quickly due to strong atmospheric forcing (high level of sunshine, weak winds, tropical air) and weak wave activity under anticyclonic weather regimes. Once formed, this shallow marine heatwave fed back on the weather: over the sea it reduced cloud cover and over land it contributed to breaking June mean temperature records and to enhanced convective rainfall through stronger, warmer and moister sea breezes. This marine heatwave was intensified by the last 20-year warming trend in sea surface temperatures. Such sea surface temperatures are projected to become commonplace by the middle of the century under a high greenhouse gas emission scenario.
How to cite: Berthou, S. and the Met Office, Plymouth Marine Laboratory, National Oceanography Centre, Scottish Association for Marine Science, Marine Institute, Marine Scotland: Exceptional atmospheric conditions in June 2023 generated a northwest European marine heatwave which contributed to breaking land temperature records, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17741, https://doi.org/10.5194/egusphere-egu25-17741, 2025.
During 2023, record high, widespread sea surface temperature (SST) anomalies of up to 1.5 °C developed in the North Atlantic, with a core located in the eastern subtropics. In a warming climate, extreme temperature events like this are increasing in frequency and intensity, having profound implications for marine life. The 2023 SST anomalies may have been driven by a combination of elements, such as weak winds, increased heat fluxes into the ocean, and basin-scale changes in circulation and heat transport. Yet, the relative importance of these factors has not been investigated. We use observational datasets, both in-situ and remotely sensed, as well as atmospheric reanalysis to provide long-term context and examine how different potential drivers contributed to the recent SST anomalies. Preliminary results show that throughout spring 2023, the eastern subtropical North Atlantic experienced anomalously high heat fluxes from the atmosphere to the ocean. Interestingly, large SST anomalies appeared almost instantly in regions of weaker wind speeds across the subtropics. We further explore connections between a general increase in upper ocean heat content, potential oceanic preconditioning over the prior two decades, and the North Atlantic Oscillation, contributing to favorable forcing conditions. We highlight the relative roles played by regional forcing and large-scale variability in the study region. Understanding the mutual importance of these roles is necessary when studying temperature extremes in the North Atlantic, especially as these events become more common and intense.
How to cite: Ryan, S. and Pihlaja, H.: Unraveling the 2023 record high temperatures in the eastern subtropical North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13209, https://doi.org/10.5194/egusphere-egu25-13209, 2025.
The Labrador Sea open-ocean deep convection was often thought to play an important role in AMOC changes. For example, in some previous water hosing experiments, the prescribed external freshwater flux is released broadly and covers the entire subpolar North Atlantic, which causes the weakening and shutdown of the Labrador Sea open-ocean deep convection and associated subsurface warming. The simulated subsurface warming over the subpolar North Atlantic due to the shutdown of the Labrador Sea open-ocean deep convection further drives the AMOC weakening. However, the importance of the Labrador Sea open-ocean deep convection in the AMOC has been challenged by theoretical, modeling, and observational analyses. In this study, an ensemble of water hosing experiments is conducted to examine mechanisms of AMOC weakening and its subsequent impact on the Labrador Sea open-ocean deep convection. The results show that the subpolar AMOC decline in response to the external freshwater flux released over the southern Nordic Sea is dominated by that across the eastern subpolar North Atlantic, and the largest subpolar AMOC decline is at the relatively dense level. The AMOC decline is associated with subsurface cooling in the subpolar North Atlantic and the decline in the deep ocean west–east density contrast across the subpolar basin. Contrary to previous studies showing that the AMOC decline is caused by subsurface warming through the shutdown of the Labrador Sea open-ocean deep convection, our results reveal a novel response, i.e., a strengthening of the Labrador Sea open-ocean deep convection, which is not a cause of the AMOC decline. We illustrate the key mechanisms causing the strengthening in the Labrador Sea open-ocean deep convection and the relationship with the AMOC weakening. This convection strengthening is mainly due to the relatively stronger freshening in the deep Labrador Sea associated with the freshening/weakening of the Iceland-Scotland Overflow, and thus reduced vertical stratification in the central Labrador Sea.
Wei, X., & Zhang, R. (2024). Weakening of the AMOC and strengthening of Labrador Sea deep convection in response to external freshwater forcing. Nature Communications, 15(1), 10357.
How to cite: Wei, X. and Zhang, R.: Weakening of the AMOC and Strengthening of Labrador Sea Deep Convection in Response to External Freshwater Forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7365, https://doi.org/10.5194/egusphere-egu25-7365, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is a critical component of the Earth’s climate system, playing a key role in global heat and carbon transport and influencing both regional and global climate patterns. While the AMOC’s importance for climate regulation is well-recognized, its future evolution under anthropogenic forcing remains highly uncertain due to incomplete understanding of how background mean states and internal variability influence its response. This study investigates the AMOC’s response to abrupt 4xCO₂ forcing compared to a control climate, isolating the roles of background mean state and internal variability. Using CMIP6 models, we classify models based on subsurface (2000 m depth) temperature and salinity characteristics. Models with warmer subsurface conditions exhibit a stronger initial response to abrupt CO₂ forcing within the first decade. Over the subsequent century (years 101–150), models with a warm-salty background mean state show a 50% AMOC decline, while those with a warm-fresh state experience a dramatic 90% decline. This highlights the influence of the background salinity mean state and how its influence shapes Arctic sea ice loss and deep convection in the North Atlantic, with most models indicating a northward shift toward the Barents Sea and Arctic regions—except in warm-fresh states. Single-model large ensembles further underscore the role of internal variability and model resolution in these dynamics. However, observational uncertainties in present-day subsurface salinity and temperature fields present challenges in precisely constraining the current mean state, complicating efforts to determine which classifications best align with reality. Our findings emphasize the influence of background mean states on AMOC’s response to extreme forcing, offering insights to refine projections of climate change using coupled models and inform future climate mitigation strategies.
How to cite: Ferster, B., Msadek, R., and Terray, L.: The role of model background mean state and internal variability in modulating the AMOC response to abrupt CO2 forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10529, https://doi.org/10.5194/egusphere-egu25-10529, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component of our climate system, influencing water mass formation and transformation. It is driven by buoyancy fluctuations and mixing within the water column. The AMOC is often studied using climate models by calculating strength indexes based on constant depth intervals (z-AMOC). However, at high latitudes, where deep water forms in the Atlantic, isopycnals are much steeper than in subtropical regions. This means that the z-AMOC framework may not fully capture the processes involved in interior ocean ventilation due to its failure to consider density gradients. To address the potential biases of the z-AMOC approach, we calculate the AMOC using density surfaces (ρ-AMOC). We compare the z-AMOC and ρ-AMOC frameworks under three scenarios: Pre-Industrial (PI), historical, and quadrupled PI CO2 concentrations (4xCO2). The PI and historical simulations serve as a testbed for evaluating the frameworks, while the 4xCO2 scenario is crucial for assessing climate sensitivity and natural variability in response to extreme CO2 levels. We also analyze water mass transformations driven by surface-induced and interior-mixing processes.
Our findings reveal that both the location and strength of AMOC maxima are significantly influenced by the choice of framework. Under constant depth coordinates, the AMOC reaches a maximum transport of 21 Sv at approximately 35oN, while it achieves around 25 Sv at 55oN when calculated from density surfaces for both PI and historical climates. In the 4xCO2 scenario, both frameworks show an abrupt weakening of the AMOC, linked to sea-ice melting and reduced deep convection, followed by a gradual recovery to maximum values of 10-15 Sv due to increased evaporation and salt export to the North Atlantic. Furthermore, we find that the z-AMOC maxima time series correlates more closely with those at 26oN (r ~ 0.7) than with ρ-AMOC maxima (r ~-0.3). This discrepancy arises from the flatter isopycnals in the z framework, even in the subpolar North Atlantic where isopycnals are actually steeper. Based on these results, we argue that the density framework better represents the physics of AMOC by directly incorporating water mass transformations and their density structure.
We indicate that including the density framework in climate model output configurations enhances our understanding of uncertainties regarding future climate change impacts. The AMOC is a critical climate tipping point, and there is currently no consensus on its future behavior. Calculating ρ-AMOC also becomes especially relevant when considering the 4xCO2 scenario as the AMOC shutdown and recovery in both frameworks driven by different processes indicates that the z-AMOC depicts the right patterns based on incorrect underlying mechanisms. This inconsistency introduces additional uncertainties to conclusions draw in studies addressing future AMOC strength and variability derived from the z-AMOC framework. Finally, we suggest that analysis across timescales and under different conditions must be performed with density surface outputs as much as possible, to enable a more comprehensive evaluation of these two frameworks and their applications.
How to cite: Oliveira Matos, F. D. A., Sidorenko, D., Shi, X., Ackermann, L., Streffing, J., Pereira, J., Stepanek, C., Lohmann, G., and Gierz, P.: Diagnosing the Atlantic Meridional Overturning Circulation under density surfaces is critical in the context of abrupt climate change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1563, https://doi.org/10.5194/egusphere-egu25-1563, 2025.
Despite numerous model-based analyses indicating a significant decline in the Atlantic Meridional Overturning Circulation (AMOC) in recent decades, robust, long-term evidence from multi-latitudinal in-situ observations remains limited. This study analyzes observational data from four mooring arrays, positioned along the western boundary of the North Atlantic (from 42.5°N to 16.5°N), to produce time series of the deep western boundary contribution to AMOC below and relative to 1000 m. Comparisons of such a transport time series at 26.5°N at the Rapid-MOCHA array confirms the viability of using the deep western boundary contribution transport to represent long-term trends and interannual variability of the AMOC. Overall, we detect linear trends of the deep overturning transports at all four latitudes, corresponding to a meridionally widespread decline of the AMOC over the past 20 years.
How to cite: Xing, Q., Elipot, S., Johns, W., Smeed, D., Moat, B., Lankhorst, M., and Loder, J.: Significant and Widespread Decline of the Observed Atlantic Meridional Overturning Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7046, https://doi.org/10.5194/egusphere-egu25-7046, 2025.
The RAPID-MOCHA array monitors the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N by combining contributions from wind-driven Ekman transport, the Florida Current, and the mid-ocean geostrophic flow derived from tall moorings. While the Florida Current transport has been remarkably steady over the past decades and the Ekman transport has been strengthening since 2004, the AMOC has been weakening since 2004 when the RAPID-MOCHA observations started. This study investigates the role of buoyancy anomalies along the deep western boundary (DWB) on the observed AMOC decline. The DWB presents density features typical of both upper and lower North Atlantic Deep Water (uNADW and lNADW, respectively), which are water masses formed in the subpolar North Atlantic and tend to flow southward along the North Atlantic western boundary until reaching 26.5°N. The uNADW can be divided into upper Labrador Sea Water (uLSW) and classical LSW (cLSW). Since 2004, the DWB has been getting lighter, largely due to warming, but with varying effects of salinity on the uNADW and lNADW layers. To isolate the influence of these water mass changes on the AMOC, we recalculated the AMOC by substituting the western boundary density profiles in a given layer (e.g., uNADW, lNADW, uLSW, cLSW) with monthly climatological values and compared the resulting estimates to those derived from the full RAPID-MOCHA data set. Between 2004 and 2023, the observed AMOC weakened at a rate of −0.8±0.7 Sv/decade, with DWB density anomalies accounting for 77% of this trend (−0.6±0.1 Sv/decade). Further breakdown reveals that the lNADW contributes 47% (−0.39±0.07 Sv/decade) and the uNADW contributes 33% (−0.27±0.07 Sv/decade) to the overall AMOC decline. When the uNADW is subdivided, the cLSW influences the AMOC weakening by −0.38±0.09 Sv/decade, similar to the influence of the lNADW, while uLSW acts to strengthen the AMOC by 0.11±0.03 Sv/decade. Experiments isolating temperature and salinity anomalies indicate that temperature anomalies drive approximately two-thirds of the DWB-induced AMOC weakening, with salinity playing a secondary but important role in the lNADW. These findings suggest that southward advection of buoyancy anomalies formed in the Labrador and Nordic Seas account for about 75% of the AMOC weakening observed at 26.5°N between 2004 and 2023, particularly highlighting the influence of cLSW and lNADW water mass changes. The trend of the residual signal (joint influence of Florida Current, upper ocean, eastern boundary and Ekman transports) is not statistically significant, whereas the DWB and individual water mass influence trends are significant at the 99% confidence level.
How to cite: Pita, I., Elipot, S., Johns, W., Smeed, D., and Moat, B.: Changing properties of NADW cause AMOC weakening at 26.5°N, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19557, https://doi.org/10.5194/egusphere-egu25-19557, 2025.
Direct measurements of the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport (MHT) are necessary to better understand the impact of anthropogenic greenhouse gas emissions on the global climate system. The RAPID-MOCHA-WBTS array at 26°N is the only trans-Atlantic observing system to provide 20 years of continuous measurements of the AMOC and MHT. While the design of the array has continuously evolved as our understanding of the AMOC has advanced, and as new technologies have become available, a goal now is to design a lower-cost and more sustainable observing system to continue AMOC estimations at high accuracy. Using the RAPID array data and ocean reanalyses, we evaluate the sensitivity of the AMOC to the choice of data included in its estimation. We find that the variability of the volume transport in the upper 3000-m of the water column exceeds what can be captured by synoptic hydrographic data or ocean reanalysis. However, the deep interior moorings along the eastern boundary and Mid-Atlantic ridge can be replaced by hydrographic data from repeat trans-Atlantic sections to reliably estimate the AMOC. A high-resolution ocean model is used to quantify the long-term uncertainty of using hydrographic data at the RAPID array on the AMOC estimation. It shows that the uncertainty is small as compared to the RAPID AMOC accuracy and that using hydrographic data does not change the significance of the observed AMOC trend.
How to cite: Petit, T., Smeed, D., Kajtar, J. B., Sinha, B., Blaker, A., Rayner, D., Elipot, S., Johns, W., Volkov, D. L., Smith, R. H., Higgs, N., and Moat, B.: Sensitivity on the AMOC estimate to the choice of data used at the RAPID 26N array, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20507, https://doi.org/10.5194/egusphere-egu25-20507, 2025.
Using 102 repeat hydrographic sections and a machine-learning approach, we define the water masses within Florida Straits and their variability since 2001. We find significant changes in the temperature and salinity properties of the Straits´ constituent water masses, which further drives significant variability in the transport of heat and freshwater into the North Atlantic across 26N. We further combine transport in the Florida Straits from hydrography with the transport across the RAPID array ordered in salinity space (a new framework for transport calculations). This approach highlights the relative contributions of the subtropical and subpolar/polar systems to the heat and freshwater divergence and water mass transformation north of 26N.
How to cite: McDonagh, E., Baringer, M., Frajka-Williams, E., Jebri, F., Volkov, D., Smith, R., Smeed, D., and Moat, B.: Water mass changes in Florida Straits since 2001, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9752, https://doi.org/10.5194/egusphere-egu25-9752, 2025.
The Gulf Stream serves both as the return flow for the North Atlantic’s wind-driven subtropical gyre and carries the warm limb of the Atlantic Meridional Overturning Circulation poleward. Here we consider the causes and consequences of low frequency variability in the western North Atlantic detected using a stream coordinates approach to identify the meandering Gulf Stream front which separates cold fresh waters and the shallow thermocline in the Slope Sea from the warm salty waters and the deep thermocline in the Sargasso Sea. This analysis capitalizes on observations collected from the CMV Oleander – a ship of opportunity that regularly crosses the Gulf Stream along a transect between New Jersey and Bermuda – with expendable bathythermographs (XBTs) deployed since the late 1970s and velocity profiles collected since the early 1990s using a hull-mounted acoustic Doppler current profiler (ADCP). Additional information from Argo floats, shipboard hydrographic casts and satellite altimetry provides spatial and temporal context for the Oleander measurements. Observations show that the Slope Sea area is shrinking as the Gulf Stream axis (identified by the location of the velocity maximum at 55 m depth along the Oleander Line) shifts northward and that the ambient Slope Sea waters are experiencing surface-intensified warming that reaches to about 750 m depth. This upper-ocean warming may reflect increased mixing of waters carried into the Slope Sea by warm core rings shed from the Gulf Stream and is consistent with a reported regime shift in 2000 when the average number of warm core rings shed annually from the Gulf Stream nearly doubled. The shrinking area inferred from the Oleander Line velocity measurements is consistent with maps of the kinetic energy based on altimetry, which suggest that this region of northward shifted Gulf Stream stretches from Cape Hatteras (~75°W) to about 69°W. In the Sargasso Sea, warming is concentrated within the “Eighteen Degree Water” (the subtropical North Atlantic mode water, STMW) which has warmed to 19°C. This warming of STWM is accompanied by a decrease in thickness of the STMW layer and deepening of the top of this layer. Global mean sea level is reportedly increasing by about 2.9 mm/yr, however, the increase in sea surface height (SSH) in the North Atlantic is not uniform in the region spanning the Gulf Stream between 68° and 58° W. In the Slope Sea, SSH is increasing by 1.6 mm/year while in the Sargasso Sea SSH is increasing by 4.0 mm/yr. The CMV Oleander observations, together with occasional velocity transects to 1000-m depth from an ADCP mounted on the Explorer of the Seas cruise ship, provide valuable benchmarks for comparison to ocean reanalysis products and state estimates. CMV Oleander continues to make regular measurements with an expanded sensor suite that includes meteorological and biological sampling.
How to cite: Andres, M., Harris, W., Perez, E., and Rossby, T.: Capitalizing on Observations from the CMV Oleander and Explorer of the Seas to Examine Causes and Consequences of Long-Term Variability in the Northwestern North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4875, https://doi.org/10.5194/egusphere-egu25-4875, 2025.
The meridional overturning circulation in the North Atlantic supplies oxygen to a large part of the world ocean's interior via the formation of mode waters and North Atlantic Deep. Whether human activities have altered this ventilation system remains uncertain. To assess the temporal changes of ocean ventilation in the North Atlantic, we calculated the "age" of seawaters, that is the duration since its last contact with the ocean surface, from both observed and climate models simulated chlorofluorocarbon-12 and sulfur hexafluoride concentrations. Results suggest enhanced ventilation in the intermediate waters and slowed-down ventilation in the deep waters over the past three decades. We propose such ventilation change is a climate change signal because (i) observed ventilation evolution pattern, although likely influenced by the major driver of natural variability in the region, the North Atlantic Oscillation, consistently emerges in historical simulations across different Earth System models, each representing different states of natural climate variability, (ii) the pattern intensifies with ongoing climate change in model projections under a high-emission scenario, indicating it is an anthropogenically forced signal, and (iii) observed and simulated ventilation changes in the North Atlantic seem to be part of a broader global trend, with enhanced upper-ocean ventilation, and slowed deep-ocean ventilation also in other ocean basins.
How to cite: Guo, H., Koeve, W., Kriest, I., Frenger, I., Tanhua, T., Brandt, P., He, Y., Xue, T., and Oschlies, A.: Changes of ventilation in the North Atlantic over the past three decades - a climate change signal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13814, https://doi.org/10.5194/egusphere-egu25-13814, 2025.
We have developed a freshwater flux (FWF) time series aimed at providing a benchmark data set for testing the sensitivity of ocean and coupled GCMs to realistic, plausible future FWF forcing alongside a 70 year reconstruction of past fluxes. Here we build on previous work that reconstructed the FWF from Arctic glaciers and the Greenland Ice Sheet (GrIS) from reanalysis (Bamber et al., 2018). First, we use ERA5 reanalyses, a regional climate model and satellite observations to reconstruct the FWF for all Arctic land ice from 1950-2021, partitioned into solid and liquid phases around the coastline of glaciated sectors of the Arctic. We then project the FWF forward until 2300 using estimates of GrIS melt derived from a structured expert judgement assessment for two temperature scenarios that approximate business as usual and a Paris Agreement limit to warming (Bamber et al., 2022). Fluxes from glaciers and ice caps are derived from projections for equivalent temperature scenarios. We develop projections for both the median and 95th percentile melt estimates to provide FWF forcings that encompass the plausible future range from Arctic land ice. We assumed a linear increase in mass loss from 2021 such that the integral up to 2100 matches the projection for the GrIS by that date. The geographic distribution of melt anomalies are scaled according to the present-day spatial “fingerprint” of mass loss. For the high end case (business as usual, 95th percentile) this equates to a FWF anomaly from the GrIS of about 0.16 Sv by mid century and 0.3 Sv by 2100, representing an unlikely but plausible FWF entering, primarily, the sub-polar North Atlantic.
We use the historical time series and ensemble of projections to examine their influence on the hydrography of the North Atlantic in a suite of sensitivity studies using the moderate resolution coupled model HadCM3, tuned to present-day transport at 34 degs south. We present preliminary findings of these forcing experiments compared to a control run with a climatological mean FWF. Even for the most extreme FWF scenario (~ 0.3 Sv) we do not see an AMOC collapse but a monotonic decline that is approximately a linear response to forcing. Interestingly, we do observe a modest cooling compared to the control, of about 0.3 degs for the historical period (1950-2021) in the sub-polar North Atlantic, which appears to be driven by recent ice sheet melt.
Bamber, J. L., M. Oppenheimer, R. E. Kopp, W. P. Aspinall, and R. M. Cooke (2022), Ice Sheet and Climate Processes Driving the Uncertainty in Projections of Future Sea Level Rise: Findings From a Structured Expert Judgement Approach, Earth's Future, 10(10), e2022EF002772, doi:https://doi.org/10.1029/2022EF002772.
Bamber, J. L., A. J. Tedstone, M. D. King, I. M. Howat, E. M. Enderlin, M. R. van den Broeke, and B. Noel (2018), Land Ice Freshwater Budget of the Arctic and North Atlantic Oceans: 1. Data, Methods, and Results, Journal of Geophysical Research: Oceans, 123(3), 1827-1837, doi:10.1002/2017jc013605.
How to cite: Bamber, J., Zhang, Z., and Lunt, D.: The past and projected freshwater flux from Arctic land ice and its impact on ocean circulation and climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10196, https://doi.org/10.5194/egusphere-egu25-10196, 2025.
The potential collapse of North Atlantic subpolar gyre (SPG) deep convection under global warming has emerged as an increasingly important research topic and a significant source of public concern within the context of climate risk. While both conceptual and coupled climate models have indicated the possibility of abrupt changes in SPG circulation, a comprehensive understanding of the mechanisms behind convection shutdown remains incomplete, despite existing dynamical interpretations. Pre-industrial control simulations from coupled climate models, designed to simulate a stable pre-industrial climate state over time periods of the order of 10^3 years, have been shown to provide meaningful insights about the behavior of SPG in absence of anthropogenic global warming. In this study, we investigate the potential collapse of SPG deep convection in the pre-industrial control simulation of the Community Earth System Model 2 (CESM2) developed by the National Center for Atmospheric Research (NCAR). By analyzing the time series of mixed layer depth, we identify 15 events of winter SPG shallow convection. The temporal evolution of the SPG states leading to convection shutdown exhibits common features across different events. Notably, a positive sea ice cover anomaly east of Greenland emerges four years before the event, coupled with a negative anomaly west of Greenland and strong negative phase of the North Atlantic Oscillation the year of the event, with an abrupt sea surface cooling in the Labrador sea. Defining a causal chain, as presented in this work, could be valuable for spoiling the major feedback mechanisms involved in the process as well as for detecting dynamical early warning signals, with a possible improvement in the predictability of such convection collapse events. This working hypothesis will be tested in other models for cross-validation and compared with similar events in forced simulations to explore parallels between the autonomous (pre-industrial) and non-autonomous case.
How to cite: Buccellato, M., Bellucci, A., Ruggieri, P., Corti, S., and Zappa, G.: Abrupt shifts in Subpolar Gyre deep convection under stable climate conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3155, https://doi.org/10.5194/egusphere-egu25-3155, 2025.
Understanding human-induced transformation in the multi-decadal variability of ocean circulation is important for predicting and interpreting current system change. This study illustrates the role the Subpolar Gyre (SPG) plays in modulating the variability of the Atlantic Meridional Overturning Circulation (AMOC), emphasising the interplay between surface and deep circulation and their sensitivity to atmospheric fluxes. Despite its significance, the dynamics of the SPG and its interaction with the AMOC during the historical period remain poorly understood due to limited observational data and palaeo reconstructions.
We investigate the mechanisms behind the historical variations in overturning using the CESM2 100-member ensemble (CESM2-LE). We focus on the density overturning strength at 55°N, which captures the combined effects of both overturning and gyre circulation. Employing change point analysis and composite analyses to various hydrographic and circulation metrics, we describe the mechanics of abrupt shifts in subpolar overturning.
Our analysis complements earlier studies on the CMIP6 historical AMOC strengthening and provides additional details on the associated internal variability based on the single-model ensemble used. We describe the precursing hydrography and atmospheric forcing to abrupt changes in subpolar overturning and the propagation of density anomalies associated with circulation shifts. Furthermore, our results show how variability before 1985 differs from the simulated variability during the period of AMOC decline driven by greenhouse gas forcing in recent decades. These results improve our understanding of the climate system's interactions and its sensitivity to external forcing over time.
How to cite: Nagler, I., Asbjørnsen, H., Ninnemann, U., and Born, A.: North Atlantic Circulation Shifts under Historical Anthropogenic Forcing in CESM2-LE , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15579, https://doi.org/10.5194/egusphere-egu25-15579, 2025.
The subpolar North Atlantic (SPNA) exhibits rapid cooling events on the decadal timescale in some climate model projections, but the processes driving these events, particularly their link to projected AMOC decline, are not fully understood. This study examines changes in decadal variability in the SPNA associated with AMOC weakening using 2200 years of output from a freshwater hosing experiment performed with the Community Earth System Model (CESM) under pre-industrial radiative forcing (Van Westen et al. 2024). We analyze North Atlantic sea-surface temperature (SST) variability during a long period of gradual AMOC weakening preceding its full collapse. Results show that, during a 500-year period before the AMOC collapse when the AMOC has weakened by about 15%, decadal SST variability in the Norwegian Sea increases by an order of magnitude. Evidence is shown that the enhanced variability is linked to a strengthened convection–salinity feedback driven by gradual changes in the ocean mean state associated with AMOC weakening and increased freshwater forcing. These findings align with a recent study showing similar mechanisms contribute to increased internal variability of SPNA SST under global warming (Gu et al. 2024), although the specific location of the enhanced variability differs. Our results suggest that relatively minor changes in long-term AMOC strength can be associated with major changes in SPNA variability and hence add to our understanding of AMOC–SPNA interactions, with implications for future climate impacts and potential early warning signals of AMOC collapse.
How to cite: Patrizio, C., Dijkstra, H., von der Heydt, A., and Bastiaansen, R.: Enhanced decadal variability in Norwegian sea with AMOC weakening in CESM, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2281, https://doi.org/10.5194/egusphere-egu25-2281, 2025.
Internal variations of climate can significantly influence global warming trends, especially at the continental scale, and could contribute to the recent abnormal observed warming over Europe. Model-based studies highlight that centennial variability of the North Atlantic can strongly affect this sector. However, a lack of high-resolution paleoclimate data does not allow a proper evaluation of the real existence of such a variability mode nor its amplitude. Here, we compile a series of annual proxy-based reconstructions over Europe from diverse sources to demonstrate and confirm the presence of such multi-centennial climate variability mode and quantify its amplitude. We show that this mode is closely tied to the internal variability of the Atlantic overturning circulation (AMOC) both in proxy-based reconstructions and climate models. When combined with instrumental observations, we show that the phase of this mode is crucial to be known. Indeed, results indicate that an internally-generated strengthening of the AMOC can explain a large part of the warming in the early 20th century and the relative cooling in the second half of this last century. A change in phase of this mode since the early 2000s is able to explain the observed amplified warming over Europe, which is projected to persist until the 2050s. According to an observational-constraint approach, this mode of variability could amplify the forced projected warming in Northern Europe by more than 58% in the next three decades. These results underscore the importance of considering internal climate variability when assessing regional warming trends, in order to develop consistent adaptation strategies.
How to cite: Al-Yaari, A., Swingedouw, D., Braconnot, P., Boyall, L., Lincoln, P., Marti, O., Caley, T., Extier, T., and Martin-Puertas, C.: Multi-Centennial internal variability in the North Atlantic will lead to additional warming over Europe in the next decades, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1954, https://doi.org/10.5194/egusphere-egu25-1954, 2025.
We investigate the low-frequency variability of the Atlantic Meridional Overturning Circulation (AMOC) using the CESM coupled model with an eddy-resolving resolution of 0.1°. In the piControl simulation, two dominant modes of AMOC variability are identified: a basin-wide mode spanning the entire Atlantic, with a periodicity of ~45 years; the other is a subpolar-gyre mode confined to the region between 20°N and 60°N, exhibiting a periodicity of ~40 years. The scenario simulation reveals that significant changes occur in AMOC variability under global warming, with an overall shift of AMOC variability towards higher frequencies and a reduction in its low-frequency amplitude. We will describe the impact from the relative contribution by the basin- and subpolar-gyre modes on the future AMOC response and discuss possible underlying processes involving density anomalies and changes in the large-scale atmospheric forcing.
How to cite: Jiang, W. and Li, F.: AMOC multi-decadal variability under global warming in high-resolution model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10740, https://doi.org/10.5194/egusphere-egu25-10740, 2025.
In our previous study, we found that the multicentennial variability (MCV) of the Atlantic Meridional Overturning Circulation (AMOC) in a CESM1 pre-industrial control simulation is primarily influenced by processes in the North Atlantic, especially the anomalous advection of mean salinity and the mean advection of salinity anomaly. In this study, we extend this finding to the IPSL-CM6A-LR and EC-Earth3-LR model simulations, revealing that the role of North Atlantic-originated processes in AMOC MCV is consistent across these models. Specifically, the anomalous advection of mean salinity and the mean advection of salinity anomaly remain key drivers of AMOC MCV in both models. Previous research has focused on the mean advection of salinity anomalies driven by Arctic sea-ice in the IPSL-CM6A-LR and EC-Earth3-LR models, leading to the partial conclusion that the Arctic Ocean dominates AMOC MCV. However, processes originating from the Arctic Ocean also include changes in the surface freshwater flux in the subpolar deep-water formation region. As Arctic sea ice melts, it reduces the amount of sea ice available to melt in the subpolar region. These two Arctic Ocean processes—one weakening and the other enhancing the AMOC anomaly—largely balance each other out, so the net effect of the Arctic Ocean on AMOC MCV is nearly neutral. Therefore, we conclude that the primary driver of AMOC MCV is processes originating from the North Atlantic, not the Arctic Ocean.
How to cite: Yang, K. and Yang, H.: Dominance of North Atlantic Ocean processes on the AMOC multicentennial variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2069, https://doi.org/10.5194/egusphere-egu25-2069, 2025.
The Holocene climate has experienced multicentennial variability (MCV), which is suggested to be significantly influenced by the Atlantic Meridional Overturning Circulation (AMOC)’s MCV, as evidenced in proxy records. However, the AMOC MCV’s origin, mechanism, and climatic impact particularly on the ocean, remain not well studied. Utilizing the ocean-only MIT general circulation model (MITgcm), we conducted 40 experiments with varying setups of small-amplitude stochastic surface freshwater forcing, each exceeding 5000 years in duration. The simulated AMOCs generally exhibit MCV, primarily driven by salinity variability in the upper-ocean of the North Atlantic. This salinity variability is dominated by meridional salinity advection between the subtropical and subpolar North Atlantic, rather than processes related to the Arctic Ocean or South Atlantic. Consequently, this MITgcm study identifies a North Atlantic Ocean-originated AMOC MCV, largely attributed to meridional salinity advection.
In terms of climatic impact, the modeled AMOC MCV induces MCV in the magnitude of Antarctic Bottom Water formation and the northward Atlantic meridional oceanic heat transport. This suggests that in the context of global warming and potential change in AMOC variability, low-frequency oceanic climate variability may shift in magnitude, period, or both. Such changes could potentially influence the centennial-scale anthropogenic climate change, which can overlap with a certain phase of multicentennial natural variability.
How to cite: Yan, C. and Yang, H.: MITgcm study of AMOC multicentennial variability’s origin, mechanism, and oceanic climate impact, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2401, https://doi.org/10.5194/egusphere-egu25-2401, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in regulating the global climate system by regulating meridional heat transport and facilitating deep-sea carbon uptake. Theoretical and modeling studies indicate that the AMOC may weaken—or even collapse—under anthropogenic warming, yet robust observational records remain limited. Continuous measurements at ~26.5° N (RAPID array) have been available only since 2004, restricting our knowledge of long-term AMOC variability and the associated regional and global feedback mechanisms. Here, we integrate outputs from the isotope-enabled Community Earth System Model Last Millennium Ensemble (iCESM-LME) to identify and isolate a primary mode of AMOC variability that strongly aligns with the RAPID array observations. To investigate potential teleconnections between the AMOC and globally distributed isotopic (δ18O) proxy records, we employ a suite of Proxy System Models to simulate four primary pseudoproxy archives—corals, speleothems, wood cellulose, and ice cores—forced by iCESM output. These pseudoproxies mimic the spatial distribution, seasonality, and time span of our compiled δ18O proxy database. Through a series of pseudoproxy experiments, we illustrate that the 'RAPID' mode manifests across several leading modes of variability in all four types of pseudoproxies. This coupling is further validated using the actual δ18O proxy records. Building on these findings, we apply a nested-PCA approach to extract the first globally distributed AMOC ‘RAPID’ mode signal from real isotopic proxy records spanning the past four centuries. The reconstructed signal reveals a gradual weakening of North Atlantic meridional heat transport strength beginning in the early 20th century—slightly lagging the onset of regional warming observed in temperature reconstructions. Notably, our results also share key temporal features (r = 0.6+) with previous AMOC reconstructions derived from individual proxies and ‘fingerprints’. Overall, this study introduces a novel method of reconstructing historical AMOC variability from globally distributed isotopic proxies by focusing on a single sub-component of the circulation that is directly linked to observational data.
How to cite: Chen, S., Osman, M., and Muschitiello, F.: 20th-Century Weakening of North Atlantic Meridional Heat Transport: Evidence from Global δ18O Proxies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11102, https://doi.org/10.5194/egusphere-egu25-11102, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth's climate. However, there are few long series of observations of the AMOC and the study of the mechanisms driving its variability depends mainly on numerical simulations. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyze the seasonal to decadal variability of the subpolar AMOC across the Greenland–Portugal OVIDE line since 1993. We decompose the MOC strength variability into a velocity-driven component due to circulation changes and a volume-driven component due to changes in the depth of the overturning maximum isopycnal. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs (linked to the seasonal cycle of density in the East Greenland Current) and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs. This compensation means that the decadal variability of the AMOC is weaker and therefore more difficult to detect than the decadal variability of its velocity-driven and volume-driven components, which is highlighted by the formalism that we propose.
How to cite: Mercier, H., Desbruyères, D., Lherminier, P., Velo, A., Carracedo, L., Fontela, M., and Pérez, F.: Variability of the eastern subpolar North Atlantic meridional overturning circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6208, https://doi.org/10.5194/egusphere-egu25-6208, 2025.
The North Atlantic Current (NAC) is a major source of heat towards the subpolar gyre and northern seas. However, its variability and drivers are not well understood. Here, we evaluated 8 years of continuous daily measurements as part of the international programme Overturning in the Subpolar North Atlantic Program (OSNAP) to investigate the NAC in the Iceland Basin. We found that the NAC volume and freshwater anomaly transport and heat content were highly variable, with significant variability at time scales of 16-120 days to annual. The shorter time scales were associated with mesoscale features abundant in the region. Composites analysis revealed that strong NAC periods were associated with a westward migration of the eastern boundary of the subpolar North Atlantic (SPNA) gyre and less eddy kinetic energy in the Iceland Basin, which was consistent with the presence of frontal-like structures instead of eddy-like structures. Stronger zonal wind stress triggers a fast response that piles water up between the SPNA and subtropical gyres which increases the sea surface height gradient and drives the acceleration of the NAC. The strengthening of the NAC increases the heat and salt transport northward. During our study period, both heat and salt increased across the moorings. These observations are important for understanding the heat and freshwater variability in the SPNA, which ultimately impact the Atlantic Meridional Overturning Circulation.
How to cite: Dotto, T., Holliday, N. P., Fraser, N., Moat, B., Firing, Y., Burmeister, K., Rayner, D., Cunningham, S., Worthington, E., and Johns, W. E.: Dynamics and Temporal Variability of the North Atlantic Current in the Iceland Basin, North Atlantic (2014 to 2022), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5722, https://doi.org/10.5194/egusphere-egu25-5722, 2025.
The Overturning in the Subpolar North Atlantic Program (OSNAP) provides key insights into the dynamics of the North Atlantic by delivering observational estimates of the Meridional Overturning Circulation (MOC) and Oceanic Heat Transport (OHT) along a section extending from southern Greenland to Scotland (OSNAP East). Despite its valuable contributions, OSNAP observations are subject to limitations, particularly in spatial resolution, potential measurement and post-processing errors, and a still relatively short but growing observational record of just over six years. This study compares OSNAP-derived transport estimates with those from state-of-the-art ocean reanalyses (ORAs), identifying key discrepancies and investigating their underlying causes.
While mean transport values for 2015–2020 agree well across datasets, notable differences emerge in spatially and temporally resolved analyses. Discrepancies in temporal variabilities are found to be linked to the limited spatial coverage of observations. When mimicking OSNAP’s observational coverage in ORAs by prescribing the mean annual temperature cycle in unobserved regions, the agreement in both mean values and variability of OHT improves, highlighting the role of observational sampling limitations in these differences.
This intercomparison highlights the mutual benefits of integrating observational and modeling approaches. It not only validates ORA performance but also exposes potential limitations in observational products. Ultimately, this work emphasizes the importance of synergistic efforts to capture the variability and dynamics of the North Atlantic, advancing our understanding of its role in climate regulation and improving predictions on regional to global scales.
How to cite: Winterer, I., Winkelbauer, S., Mayer, M., and Haimberger, L.: A Comparison of OSNAP Observations and Ocean Reanalyses in the Subpolar North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8925, https://doi.org/10.5194/egusphere-egu25-8925, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is important for the climate in Western Europe and models predict a decrease in the overturning circulation due to climate change. It is the fundamental mechanism for the transports of heat, freshwater, and dissolved gases. In the region east of the Grand Banks of Newfoundland and Flemish Cap, the Deep Western Boundary Current (DWBC) is part of the lower limb of the AMOC and brings cold and fresh water to the south, which is balanced by the North Atlantic Current (NAC), which brings warm and salty water to the north. In a long-term study we investigated the transport variability along a transect at 47°N that includes DWBC and NAC and lies in the transition zone between the subpolar and subtropical gyres, where decadal changes in the AMOC are transferred southward. The interactions between the DWBC and the NAC and its influence to the AMOC variability are subject to current research. We use 6 years of moored current meter observations between 2014 and 2020 within the DWBC as well as shipboard hydrographic and current meter measurements from 15 cruises between 2003 and 2020. The shipboard and mooring data were used to calculate volume transports. The shipboard data show that the DWBC consists of two cores, one in close proximity to the continental slope with maximum velocities at mid-depth and a bottom-intensified core further offshore. The correlation of both the hydrographic properties and absolute current measurements with sea surface height was used to reconstruct time series of geostrophic transport variability from satellite altimetry alone. The sea surface height and moored current meter volume transport time series are compared to estimate the reliability of the sea surface height time series. For the offshore core a higher correlation between the time series is found than for the slope core. To estimate the relation between the DWBC cores and the NAC, the correlation between their volume transport time series is examined. The two DWBC cores are not correlated, while a comparison with a NAC time series shows that the offshore core is significantly correlated with the NAC. A combination of these two DWBC time series was constructed to cover the entire DWBC. The entire DWBC is significantly anti-correlated with the NAC, which leads to larger volume transport of the NAC when the transport of the DWBC is smaller and vice versa. Overall, the sea surface height time series show no long-term trend in DWBC volume transport. When comparing the reconstructed monthly mean DWBC transports with a time series of AMOC variability at 47°N, a significant anti-correlation is found. This indicates that AMOC variability could be characterized to a large extent by the variability of the DWBC-NAC system.
How to cite: Aschenbeck, L. J., Mertens, C., Bracamontes-Ramírez, J., Steinfeld, R., and Wett, S.: Long-term variability of NAC and DWBC volume transports and their relation to the AMOC at 47°N, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13010, https://doi.org/10.5194/egusphere-egu25-13010, 2025.
The subpolar North Atlantic is a known hotspot for anthropogenic carbon and oxygen uptake, however the detailed pathways for dissolved gasses into the deep North Atlantic remain poorly understood. While it is clear that high-oxygen waters formed by deep convection in basin interiors are exported southward by boundary currents, it has also been hypothesized that significant water mass transformation and associated gas exchange occurs near or within boundary currents themselves. Here we present novel year-round oxygen observations mounted on the Overturning in the Subpolar North Atlantic Program (OSNAP) moorings within the Irminger Sea’s western boundary current. These observations are the first to show direct boundary current ventilation to over 1000m depths following down-front wind events, revealing a previously underappreciated pathway for oxygen into the deep North Atlantic.
How to cite: Le Bras, I., Miller, U., Palter, J., Straneo, F., Atamanchuk, D., Fogaren, K., Nagao, H., Nicholson, D. (., Park, E., Palevsky, H., and Yoder, M.: Observations of a direct boundary current ventilation pathway, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13025, https://doi.org/10.5194/egusphere-egu25-13025, 2025.
The Atlantic Meridional Overturning Circulation (AMOC), carrying warm, salty water to high latitudes, is a key component of the global ocean circulation with profound impacts on climate. To sustain the AMOC, dense-water formation at high northern latitudes, such as in the Nordic Seas and Arctic Ocean, is a requirement. Here, we use the high-resolution (1/12°) ocean reanalysis GLORYS12, corroborated by observations and other reanalyses, to show that a poleward expansion of warm Atlantic waters and corresponding sea-ice loss has caused a poleward shift of the dense water source regions in recent decades (1993-2020). This is manifested in enhanced surface water mass transformation in the Arctic Ocean, compensating for a reduction in the Nordic Seas. The associated strengthening of the Arctic Ocean overturning circulation has ensured that the transport of dense overflow waters across the Greenland-Scotland Ridge to the AMOC’s lower limb has remained stable. Our results thus provide evidence for a resilient northern overturning circulation in a warming climate.
How to cite: Årthun, M., Brakstad, A., Dörr, J., Johnson, H. L., Mans, C., Semper, S., and Våge, K.: Poleward shift of AMOC source regions maintains stable supply of dense overflow waters to the North Atlantic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1458, https://doi.org/10.5194/egusphere-egu25-1458, 2025.
Formed by deep convection in the Nordic Seas, cold, oxygen-rich Denmark Strait Overflow Water (DSOW) is the densest water mass in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). As DSOW spreads southward from the Denmark Strait into the Irminger Sea, the overflow water entrains ambient water masses. This entrainment is critical for the transfer of climate signals and biogeochemical tracers from the surface to the deep ocean. Although the impact of entrainment on DSOW properties in the immediate vicinity of the Denmark Strait (< 100 km downstream of the sill) has been studied extensively, the entrainment dynamics contributing to DSOW transformation further downstream in the Irminger Sea and its influence on variability in DSOW properties remain unclear. Here, we use observations from BGC-Argo floats and moorings along with an idealized numerical model of entrainment mixing to investigate the dynamics contributing to the observed DSOW transformation in the western Irminger Sea. We find that DSOW experiences continual warming and deoxygenation as it spreads along the Deep Western Boundary Current in the Irminger Sea. Furthermore, novel dissolved oxygen measurements from moored instruments offshore of Cape Farewell, Greenland reveal seasonality in the oxygen content of DSOW for the first time. Numerical model simulations of supercritical entrainment dynamics predict DSOW properties offshore Cape Farewell that are cooler and more oxygenated than in the observations. This disagreement suggests that sub-critical entrainment influences DSOW transformation in the western Irminger Sea, which we test by accounting for this process in the model. Model sensitivity experiments further suggest that the observed seasonal variation in the oxygen content of DSOW likely originates from the source overflow water upstream of the Denmark Strait. Overall, this work is the first to explore the entrainment dynamics of DSOW in the western Irminger Sea and its influence on the oxygen content of the overflow water, with implications for the response of the AMOC structure and dissolved gas uptake to changes in the climate system.
How to cite: Nagao, H., Le Bras, I., Miller, U., Palter, J., Bower, A., Furey, H., Koman, G., Atamanchuk, D., Fogaren, K., Nicholson, D., Park, E., Palevsky, H., and Yoder, M.: Influence of entrainment on the mean and seasonal variability in the Denmark Strait Overflow Water (DSOW) properties in the West Irminger Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13071, https://doi.org/10.5194/egusphere-egu25-13071, 2025.
Transient tracer observations from GLODAPv2 and more recent data are used to
compute transit time distributions (TTDs) for Labrador Sea Water. These TTDs are then integrated
basin wide over the subpolar, subtropical and tropical Atlantic. This allows to infer ventilation,
export and formation rates of LSW. We further devide the LSW density range into an upper (ULSW) and a
deeper part (DLSW). The results reflect the known variability of LSW formation, with high formation rates
of DLSW in the 1990s and after 2015, and periods with increased ULSW formation in between.
Astonishingly, the DLSW formation rate is always significantly larger than zero,
even in years without direct DLSW ventilation.
We also compare the formation rates derived from the TTDs with those calculated
from CFC/SF6 inventories. This shows, that the formation rates inferred from tracer inventories
depend more strongly on the integration regions (subpolar North Atlantic only
or including subtropics (and tropics)) than the TTD derived formation rates.
How to cite: Steinfeldt, R., Rhein, M., and Kieke, D.: Decadal variability of Labrador Sea Water formation , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16176, https://doi.org/10.5194/egusphere-egu25-16176, 2025.
An important component of the deep limb of the Atlantic Meridional Overturning Circulation – the Deep Western Boundary Current (DWBC) – forms in the North Atlantic subpolar gyre from the combination of cold, dense water from the Nordic Seas and the ambient water within the gyre. Much of this water merges south of the Denmark Strait along the eastern flank of Greenland and flows southwestward toward the tip of Cape Farewell where the Overturning in the Subpolar North Atlantic Program (OSNAP) has continually monitored the DWBC since 2014 using moorings consisting of current meters, acoustic doppler current profilers, and temperature-salinity recorders. Previous estimates of the DWBC (σθ > 27.8 kg m-3) at this location found 9-13 Sv of transport, including 10.8 Sv from the first two years of OSNAP data. This presentation extends the OSNAP estimates of the DWBC through 2022 and finds a 22% decrease in transport over the eight-year record from a thinning of the traditionally defined DWBC layer (σθ > 27.8 kg m-3) and weakening velocities. This also results in a 27% decrease in transport of the densest water mass of the DWBC, Denmark Strait Overflow Water (σθ > 27.88 kg m-3). Overall, the eight-year transport mean of the DWBC is 8.4 Sv. This presentation will also consider alternative methods for evaluating the DWBC that find a transport reduction of only 12-17% over the eight-year observation period.
How to cite: Koman, G., Bower, A., Furey, H., and Holliday, P.: Eight years of continuous observations of the Deep Western Boundary Current from Cape Farewell, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14226, https://doi.org/10.5194/egusphere-egu25-14226, 2025.
The Mediterranean Outflow Water (MOW) is a warm and saline water mass originating in the Gulf of Cadiz from the mixing of the Mediterranean Water flowing out of the Gibraltar Strait and the overlying North Atlantic Central Water. As it flows downstream from Gibraltar, this water mass gradually loses part of its high salinity content and becomes neutrally buoyant at a depth of approximately 1200 meters near Cape St. Vincent. From this point, two distinct cores of MOW have been identified: one spreads westward into the open North Atlantic Ocean, while the other flows northward along the Iberian continental slope as an eastern boundary current, with its signal detected as far north as 50°N (around Porcupine Bank).
This water mass is considered an important source of heat and salt for the North Atlantic basin and its northward branch in particular has drawn much interest due to the hypothesis that MOW could play an active role in the deep convective processes that occur in the Subpolar North Atlantic, supplying salt to high-latitude waters.
Recent studies identified the Irminger Sea and the Iceland Basin as the key regions for the formation of dense waters in the Subpolar North Atlantic. However, the extent to which MOW influences the dynamics of these regions remains largely unexplored and the fate of this water mass beyond the region of Porcupine Bank is still characterized by high uncertainty.
In this contribution, a comprehensive dataset of 22 years of Argo float profiles, acquired from 2001 to 2022 all over the North Atlantic basin, is utilized to bring new insights into the northward spread of MOW towards the Subpolar regions of the North Atlantic.
The Argo floats were chosen due to their extensive spatial and temporal coverage of the North Atlantic basin. Moreover, by sampling the depth range influenced by the presence of MOW (600-1300 m), these devices provide valuable in-situ data to identify and track the Mediterranean Outflow Water along its northward path.
The main expected outcomes of this work include the identification of the thermohaline properties of MOW, their evolution over time and space and the subsequent tracking of its northward flow. Specifically, Argo float data are employed to derive the θ-S relationship that defines MOW, enabling a precise identification of water masses along its path. This approach facilitates the analysis of the mixing processes affecting MOW, thereby allowing insight into the spatial and temporal evolution of its thermohaline properties.
How to cite: Calvo, E., Malanotte-Stone, P., Menna, M., Martellucci, R., and Zambianchi, E.: Tracking the Mediterranean Outflow Water: insight from 20 years of Argo data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11854, https://doi.org/10.5194/egusphere-egu25-11854, 2025.
The Faroe Bank Channel (FBC) transports dense, cold overflow water that contributes to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). By comparing the Copernicus GLORYS12 reanalysis data (1/12°) with 27 years of CTD and ADCP data in the FBC (Larsen et al., 2024), we find that GLORYS12 well-represents an observed warming and a lagged salinification trend since the mid-1990s. To investigate the sources and pathways of warming dense bottom waters upstream of the FBC, we re-release Lagrangian particles every 5 days, randomly distributed between 650 and 1100 m, and backtrack them in time for 27 years using the ocean reanalysis data. Our analysis reveals a basin-wide warming trend of approximately 0.1°C per decade in the Greenland and Iceland Seas. However, the primary flow pathways of FBC overflows appear to originate from the Arctic via an extension of the East Greenland Current and from a previously ignored source in the Lofoten Basin, with strong mixing and recirculation between these deep current pathways. Fewer particles traced back to the Greenland Sea gyre deep water. We explore the upstream effects and locations of the warming sources and their impact on variability in the overflows within the FBC, which may improve our understanding of AMOC dynamics.
How to cite: Schiller-Weiss, I., Hátún, H., Olsen, S. M., Larsen, K. M., and Schulz, H.: Multiple pathways of deep waters in the Nordic Seas elucidate the warming and salinification of eastern overflow waters, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13896, https://doi.org/10.5194/egusphere-egu25-13896, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is vitally important for regulating global climate through the redistribution of heat, salt, carbon and other tracers across latitudes, yet the precise role of its governing physical processes in the subpolar North Atlantic (SPNA) remains poorly understood. This knowledge gap is significant to address, given the AMOC’s sensitivity to anthropogenic climate change and its potential for dramatic weakening or collapse, with profound global implications. Here, we adopt a three-dimensional dynamical perspective, focusing on lateral exchange between the boundary current (BC) and basin interiors, ridge exchange over sills bordering the north of the SPNA, and shallow convection within the BC itself. We do so by analysing water mass transformation in density space, using segmented volume transport budgets in the eddy-resolving global ocean reanalysis GLORYS12 (1/12°). Our findings reveal that: (1) during the BC’s circumvention around the subpolar gyre alongstream intensification and densification takes place, and overflow waters from the Iceland–Scotland Ridge and Denmark Strait are added to the system; (2) vertical recirculation cells due to lateral exchange are not immediately evident; (3) residual water mass transformation can be partly explained by shallow convection driven by surface buoyancy fluxes; and (4) substantial spatial variability in local overturning contributions exists. These insights highlight the importance of further quantification of the relative contribution of each governing process to water mass transformation and subsequent overturning in the SPNA.
How to cite: Vermeulen, D. H. A., Gelderloos, R., and Katsman, C. A.: Process-based decomposition of North Atlantic local water mass transformation using volume transport budgets , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3724, https://doi.org/10.5194/egusphere-egu25-3724, 2025.
The Irminger Sea is one of the few places in the North Atlantic where dense water masses are formed through deep convection. Next to atmospheric forcing, wintertime convection in the Irminger Sea interior can be impacted by the extent of restratification in the preceding year(s). In the Irminger Sea, the central basin is cold contrasted to the Irminger Current (IC), its cyclonic boundary current that carries warm and saline waters of subtropical origin. In this study, we investigated the potential impact of the IC on restratification of the Irminger Sea’s convection area, using a high-resolution regional model combined with Lagrangian particle tracking. We released particles over the upper 1500 meters of the IC in the eastern Irminger Sea and tracked them forward in time.
Of those the majority stayed within the Irminger Sea: 38% followed the boundary current circulation and 61% entered the interior Irminger Sea. Only one percent of the particles left the Irminger Sea through Denmark Strait and to the Iceland Basin. Of those entering the interior, about one half reaches the deep convection area (DCA). The seeded particles reach the DCA from the eastern side, seemingly steered by mesoscale variability. On their way to the DCA, the IC waters loose part of their buoyancy but on average remain lighter than waters in the DCA. This westward spread of warm and saline IC waters likely limits the region of deep convection to the western Irminger Sea by adding to the stratification in the eastern part of the basin. Understanding the processes driving the lateral buoyancy fluxes to the basin’s interior is important to understand variability in deep convection in the Irminger Sea.
Considering the Irminger Sea’s importance in overturning future changes in water mass properties could influence the variability in convection and with impact subpolar overturning.
How to cite: Fried, N., Gelderloos, R., Tooth, O. J., Katsman, C. A., and de Jong, M. F.: Can the Irminger Current impact restratification in the Irminger Sea?A Lagrangian model study on the fate of the Irminger Current water, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1716, https://doi.org/10.5194/egusphere-egu25-1716, 2025.
In the subpolar North Atlantic, the strength of the Meridional Overturning Circulation is linked to rates of North Atlantic Deep Water formation, a water mass partially composed of Nordic Seas Overflow Waters. While Denmark Strait Overflow Water takes a relatively direct route out of the Irminger basin via the cyclonic boundary current, exit pathways of Iceland-Scotland Overflow Water (ISOW) from the Iceland Basin are less understood and more complex. Here, ISOW pathways and their interannual variability are explored in a Lagrangian framework using particles seeded within the 45-year 1/12° eddy-resolving North Atlantic HYCOM simulation. Our analysis reveals significant depth-dependent variability in ISOW pathways. Upper layers preferentially cross into the Irminger Basin through gaps in the Reykjanes Ridge while deeper layers take the more traditional route to the Charlie-Gibbs Fracture Zone (CGFZ). At the CGFZ, we observe a strong anticorrelation in the percentage of particles that end up in the western vs. eastern basin which varies on a timescale of ~2.5 years and is likely associated with the position of the North Atlantic Current (NAC). This anticorrelation however is much stronger in the upper layers as the influence of the NAC appears to decrease with depth. Of the approximately 55% of particles that translate through the CGFZ, those in the upper layers are more likely to follow the cyclonic boundary current while lower layer particles diffuse northwestward towards the Labrador Sea. These depth-dependent patterns, identified from simulated particle trajectories, are corroborated by observations from RAFOS floats deployed during the OSNAP campaign. These findings illustrate the importance of depth-dependent dynamics and interannual variability of the NAC in shaping ISOW pathways, with implications for deep circulation patterns in the subpolar North Atlantic and the rate of large-scale overturning.
How to cite: Johnson Exley, A., Bower, A., Xu, X., Zou, S., Pinckney, A., and Furey, H.: Interannual variability and depth-dependence in pathways of Iceland-Scotland Overflow Waters exiting the Iceland Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3860, https://doi.org/10.5194/egusphere-egu25-3860, 2025.
Away from continental boundaries, the variability of the global ocean is often dominated by eddies. Despite this interior turbulence, ocean boundary pressures on opposing sides of a basin can vary coherently over interannual to decadal timescales, while exhibiting large-scale (∼10⁴ km) spatial structure. As part of the OceanBound project, we investigate how boundary pressure differences reflect meridional transport anomalies in the Atlantic and use an adjoint model to track potential sources of variability.
In the ECCO state estimate, we find that boundary pressure differences across the Atlantic account for 60–90% of the meridional transport variability, both on interannual and subannual timescales. This result is consistent at most latitudes, excluding the equatorial region.
Adjoint model simulations allow us to quantify the linear sensitivity of across-basin pressure differences to surface forcing. We focus on two latitude ranges where boundary pressure estimates of meridional transport variability are particularly robust. The first is centred at 26.5°N, aligning with the RAPID array, and the second at 26.5°S, overlapping with the SAMBA array. In both cases, we identify surface winds above the continental shelf as the dominant driver of boundary pressure variability, whereas surface buoyancy forcing plays a negligible role.
Sensitivity fields derived from the adjoint model are used to reconstruct the Atlantic boundary pressure differences and, consequently, the significant geostrophic component of meridional transport variability. Forward perturbation experiments further reveal potential mechanisms underlying these sensitivities, as well as any non-linear behaviours.
How to cite: Styles, A., Boland, E., Hughes, C., Gururaj, S., and Jones, D.: Boundary Pressure: A Unique Window into Atlantic Transport Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3701, https://doi.org/10.5194/egusphere-egu25-3701, 2025.
The consequences of grid resolution refinement of both ocean and atmosphere components in and over the North Atlantic Ocean are examined in this study. The Flexible Ocean and Climate Infrastructure (FOCI) model is used to run three simulations carried out under constant 1950 forcing. Ocean and sea ice models run on the ORCA05.L46 grid; in two simulations, regional AGRIF grid refinement from 1/2˚ to 1/10˚ (VIKING10) is applied to the North Atlantic between 30˚ and ~80˚N. We also use two different atmospheric resolutions, Tco95.L91 (100km) and Tco319L137 (31 km) applied globally. The three combinations studied are: (1) coarse ocean and atmosphere, (2) refined ocean but coarse atmosphere, and (3) both components in the high-resolution configuration. Ocean grid refinement in regions of complex dynamics, such as the Gulf Stream and North Atlantic Current, is essential to capture mesoscale variabilities. Air-sea interaction and in particular surface heat fluxes only benefit from the explicit representation of mesoscale eddies, when also using the high-resolution atmosphere capturing a wider range of, for instance, the temperature distributions.
We examine the ocean and atmospheric mean state changes over the North Atlantic and find significant changes: The Atlantic Meridional Overturning Circulation strengthens as we move to higher ocean and atmospheric resolution. Most parts of the North Atlantic Ocean surface become warmer and saltier in the high-resolution ocean configuration, but with the finer atmospheric grid, this warming reduces and transforms into a colder and fresher mean state. The surface cooling with increasing atmospheric resolution is a result of a reduced TOA imbalance. The mid-depth (500-1200m) ocean experiences strong cooling of more than 2˚C especially in the subtropics with the grid refinement in the ocean, a difference that is considerably weaker when also refining the atmosphere. The cooling in the subtropics has two reasons, a stronger gyre-gyre interaction mixing more subpolar water into the subtropical gyre and intensified deep mixing in the Labrador Sea (associated with a stronger overturning) in the high-resolution ocean compared to the coarser one. Meridional volume and heat transports in the subtropical North Atlantic exhibit significant differences of up to 2-2.5 Sv with the change in resolution, which also suggests an influence by model resolution in the ocean and the air-sea interaction processes. With a higher ocean resolution, the oceanic heat transport into the Arctic increases by 0.15 PW at 62°N, causing a reduction of Arctic sea ice. Increasing the atmospheric resolution causes an expansion of Arctic sea ice, consistent with the overall surface cooling, but also changes the distribution of sea-ice thickness with thicker ice north of Greenland and thinner ice toward Russia, consistent with observations. Overall, the changes in the mean state of the ocean and atmosphere, as well as the feedback processes involved, will be discussed in this presentation, which we hope also benefits other modelling groups.
How to cite: Ojha, S., Kjellsson, J., and Martin, T.: Meridional heat transport in the North Atlantic region: Effects of ocean and atmosphere grid resolutions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8668, https://doi.org/10.5194/egusphere-egu25-8668, 2025.
Deep cyclones (DCs) were observed in the western North Atlantic under meander troughs of the Gulf Stream (GS) west of the New England seamounts during the Synoptic Ocean Prediction (SYNOP) field campaign. Although subsequent dedicated observations have been sparse, DCs appear common underneath major surface-intensified western boundary current extensions. Recent model studies with idealised domains suggest that DCs are important sources of eddy kinetic energy in the deep ocean, key sites of energy dissipation, and potential contributors to episodes of strong near-bottom currents and sediment resuspension known as “benthic storms”. In the western North Atlantic, DCs that form within GS meander troughs could play a role in the Atlantic meridional overturning circulation by providing a path for recirculation of water between the Deep Western Boundary Current (DWBC) and the adjacent oceanic basins. However, most numerical ocean models lack the vertical resolution that is needed to simulate in detail both the vertical structure of DCs and the near-bottom flows.
In this study, we configure the MIT general circulation model to produce eddy-rich simulations of western North Atlantic circulation at a horizontal resolution of 1/20o and with high vertical resolution (550 levels with uniform Dz = 10 m). Emphasis is placed on the role of DCs in the time-mean abyssal circulation and on their contribution to Lagrangian transport, particularly to the exchange of water between the DWBC and the adjacent basins and between the bottom mixed layer and the stratified interior. In the simulations, deep cyclones are found to form west of the New England seamounts, consistent with field observations from SYNOP. They also form in the Sohm abyssal plain – east of the seamounts – although observations are lacking to confirm or refute this result. In our simulations, the DCs typically persist for 30-90 days and move eastward at a speed of ~1.5 cm s-1 in tandem with the GS meander troughs near the surface. The time-averaged horizontal velocity field in the Sohm abyssal plain depicts a large-scale cyclonic circulation cell that is reminiscent of the Northern Recirculation Gyre proposed from sparse observations of deep current meters. Preliminary results of Lagrangian transport suggest that water parcels in the DWBC may enter the interior through entrainment of DCs near 68ºW (west of the New England seamounts). Further Lagrangian analyses of the simulated deep circulation are ongoing to elucidate the temporal and spatial scales of particle transport associated with the DCs and benthic storms.
How to cite: Chen, S.-Y. (., Marchal, O., Andres, M., and Gardner, W.: Deep Cyclones in the western North Atlantic: Insight from a Regional Numerical Model with High Vertical Resolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5256, https://doi.org/10.5194/egusphere-egu25-5256, 2025.
The subpolar gyre (SPG) is one of the regions in the North Atlantic ocean where convection takes place. It has been indicated as one of the earth system’s tipping elements that is closest to crossing its threshold due to global warming, which also could impact the Atlantic Meridional Overturning Circulation (AMOC). Therefore, understanding the mechanisms of its variability is of key importance to learn about possible changes under global warming. We use causal inference to study the mechanisms of convection in the SPG region in CMIP6 models. Causal inference goes beyond correlation, by taking into account common drivers and other possible confounding factors, to establish causal links between variables. We find that the interaction between convection and density is well represented in most models, whereas a link of both to the circulation strength of the gyre is captured by fewer models. These results provide valuable information on the capability of CMIP6 in representing SPG variability, and form a starting point for investigating possible links with the AMOC.
How to cite: Falkena, S. K. J. and von der Heydt, A. S.: Mechanisms of subpolar gyre variability in CMIP6, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8813, https://doi.org/10.5194/egusphere-egu25-8813, 2025.
The biological carbon pump includes all the biological processes involving the production of organic carbon in the euphotic layer and its export towards the deep ocean, where it will be stored and isolated from the atmosphere for centuries. The main export pathway is via the sinking of particles by gravity, known as the gravitational pump. Another important one is the migrant pump, sustained by a specific behaviour called zooplankton Diel Vertical Migration (DVM). Migrant organisms feed in the euphotic layer at night and hide from predators at depth during daytime, thus actively transferring carbon from the surface to the migration depth. The export flux of carbon is attenuated along its way to the dark ocean, mainly by heterotrophic processes linked to prokaryotes and zooplankton. The strongest decline occurs in what is called the mesopelagic zone (approximately 100-1000 m). The study of Kwon et al. (2009) demonstrated the crucial control of this attenuation on atmospheric CO2 concentrations and thus on Earth’s climate. Yet, the processes at work in the mesopelagic realm remain poorly understood, as it was underlined by Wilson et al. (2022).
In the present study, we address this issue through a modelling approach. We perform realistic 3D coupled physical-biogeochemical simulations at an unusually high horizontal resolution (2 km). We use CROCO (Coastal and Regional Ocean Community, Mason et al., 2010) and PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies, Aumont et al., 2015) models, respectively for the physical and biogeochemical compartments. The simulations take place in the Northeast Atlantic Ocean during the years 2020-2024. This work is part of the APERO ANR (Assessing marine biogenic matter Production, Export and Remineralization: from the surface to the dark Ocean), which is based on a cruise that took place during the summer of 2023 near the PAP station (Porcupine Abyssal Plain, located southwest of Ireland).
We aim to shed light on the main mechanisms driving carbon flux attenuation in the mesopelagic realm, and to understand the role of small scales on carbon export and storage in the deep ocean. More specifically, using results from the APERO cruise, we will focus on how to improve the parametrisation of zooplankton DVM and particle sinking velocities in the PISCES model.
How to cite: Barge, F. and Mémery, L.: High-resolution physical-biogeochemical modelling in the Northeast Atlantic Ocean: mechanisms driving carbon flux attenuation in the mesopelagic realm, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1054, https://doi.org/10.5194/egusphere-egu25-1054, 2025.
The Eastern Tropical North Atlantic Oxygen Minimum Zone (ETNA OMZ) is located in a region of northward Sverdrup transport, opposite to that of the subtropical gyre. This means that its dynamics are unlikely to be associated with the shadow zone associated with the Ventilated Thermocline Theory, as has traditionally been assumed. Rather, we argue an important role for the latitudinally alternating zonal jets associated with the mesoscale eddy field. To illustrate this, we use an advection-diffusion model coupled to a simple dynamical ocean model. The advection-diffusion model carries a passive tracer with a source at the western boundary and a Newtonian damping term to mimic oxygen consumption. The dynamical model is a non-linear 1 1/2 layer reduced-gravity model. The latter is forced by an annually oscillating mass flux confined to the near-equatorial band that, in turn, leads to the generation of mesoscale eddies and latitudinally alternating zonal jets at higher latitudes. The model uses North Atlantic geometry and develops a tracer minimum zone remarkably similar in location to the ETNA OMZ. Although the model is forced only at the annual period, the model nevertheless exhibits decadal and multidecadal variability in its spun-up state. The associated trends are comparable to observed trends in oxygen within the ETNA oxygen minimum zone. Notable exceptions are the multi-decadal decrease in oxygen in the lower oxygen minimum zone, and the sharp decrease in oxygen in the upper oxygen minimum zone between 2006 and 2013.
How to cite: Greatbatch, R., Köhn, E., Brandt, P., and Claus, M.: A simple model for the Eastern Tropical North Atlantic Oxygen Minimum Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3340, https://doi.org/10.5194/egusphere-egu25-3340, 2025.
The ocean fronts, such as the Gulf Stream North Wall (GSNW) and the Shelf-Slope Front (SSF), along the eastern coast of North American continent in the Northwest Atlantic, at both surface and subsurface levels, significantly influence regional climate, oceanic energy and mass exchanges, marine ecosystems and fisheries. Many previous studies have examined the variability in the positions of the GSNW or SSF separately, usually based on oceanographic variables at a single depth. In this study, we investigate the leading mode (Northwest Atlantic Temperature Gradient Mode, NATG) of the sea temperature gradient at multiple depths from observations and models, which reflects the combined long-term variability of the SSF and GSNW. The NATG index shows a significant spectral peak at ~100 years in the power spectrum and underwent a significant multi-decadal trend turning around early 1960s. We find that the velocity difference between the Gulf Stream and Labrador Current has a strong positive correlation with the NATG. Additionally, the NATG may be associated with an anomalous surface atmospheric cyclone-anticyclone configuration along the east coast of the North American continent.
How to cite: Xiong, H. and Li, J.: Long-term variability of ocean fronts in the Northwest Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4240, https://doi.org/10.5194/egusphere-egu25-4240, 2025.
The ocean, as the largest carbon reservoir, plays a crucial role in regulating the global climate by absorbing atmospheric carbon dioxide and heat generated by carbon emissions. It achieves this by transferring water masses from the surface to the ocean interior. Among the various features influencing ocean circulation, subtropical mode water (STMW) in the North Atlantic has received significant attention due to its formation through direct ocean-atmosphere interactions and its substantial impact on the ventilation of the upper ocean. In this study, we explore the mechanisms underlying the seasonal cycle of STMW from a dynamical perspective.
We use an ensemble of 48 partially coupled North Atlantic Ocean simulations, with mesoscale-permitting resolution, and apply an ensemble-averaging approach to separate the mean flow (residual-mean) and eddy fluctuations, both of which are temporally and spatially dependent. We quantify STMW by analyzing the evolution of a pool of low Ertel potential vorticity (PV). Our results indicate that the annual cycle of STMW can be explained by the interactions of three primary transports: (1) the ensemble-mean flux, representing the large-scale Eulerian-mean flow shared by all ensemble members, (2) bolus eddy transport driven by strong baroclinic instabilities within the PV pool, and (3) residual eddy transport due to the non-linear fluctuations among the ensemble members. During the winter when STMW begins to form, the ensemble-mean flow plays a dominant role, deflating the PV pool by transporting low-PV water from the north into the pool. Meanwhile, bolus transport actively mixes the PVs within the pool along isopycnal surfaces, leading to a PV homogenization. As the season progresses, residual eddy transport begins to counteract the ensemble-mean flow, creating a balance that results in the inflation of the PV pool and, consequently, the erosion of STMW.
How to cite: Poje, A., Uchida, T., Penduff, T., Dewar, W., Deremble, B., Wienders, N., and Sun, L.: On the Dynamics of the Subtropical Mode Water from an Ensemble Simulation Viewpoint, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14381, https://doi.org/10.5194/egusphere-egu25-14381, 2025.
The North Atlantic Ocean has a crucial role in the regional and global climate, with significant socio-economic impacts related to droughts, hurricanes and changes in the weather pattern. The Atlantic Meridional Overturning Circulation (AMOC) is the main component of ocean heat transport to the North Atlantic as part of the Earth’s major circulation system. The AMOC is considered a tipping element of the planet’s climate, and abrupt and strong changes in this global-scale circulation could lead to shifts in European and North American climates and warming in the Southern Ocean and Antarctica. Hence, it is critical to better comprehend the drivers influencing the AMOC state and its variability.
The direct connection between the AMOC and the deep-water formation is being challenged by recent studies, suggesting this relationship cannot explain alone the broad spectrum of AMOC variations. Moreover, the Gulf Stream is responsible for a substantial part of meridional heat transport and is considered part of the AMOC system in the North Atlantic. Recent studies show the latitudinal displacement of the current could affect the AMOC variability. Furthermore, the advection at intermediate level (800-1200m) of the Mediterranean outflow is suggested to be a possible key contributor to the strength and variability of the AMOC, although large uncertainty remains concerning the pathway of these waters into the North Atlantic and its contribution in the precondition phase of the deep-water formation in the Labrador and Greenland Island Norwegian sea. Understanding the influence of each of these drivers is vital.
The scarcity of direct observational records in space and time poses an important challenge to disentangle the AMOC drivers and their variability in the North Atlantic, especially since the North Atlantic Ocean presents unusual variability at decadal and multi-decadal scales. Previous studies have used either short observation records, low-resolution numerical models or indirect (proxy) observations to investigate the AMOC. Long-term and three-dimensional datasets are needed to understand the ocean’s changes at large space and time scales. Therefore, the aim of our work is to investigate the multi-drivers of the AMOC variability using a large ensemble (32 members) of global historical ocean reanalyses covering the period from 1961 to 2022. We focus on the deep-water formation, Gulf Stream shift and Mediterranean Outflow Water (MOW) as multi-drivers of the AMOC in the North Atlantic. It is the first time that a large ensemble of 60-year observation-based ocean dataset is used to investigate multiple drivers responsible for changes in the AMOC and the North Atlantic’s decadal climate. We calculated indexes for each driver and used multivariate regression methods (e.g. LASSO) to explore the role of multi-drivers. Preliminary results show that at the interannual time scale, the variability of the Gulf Stream is well correlated with the AMOC variability and at a longer time scale, the deep-water formation and Mediterranean outflow have a close correlation with the AMOC transport. Further investigation is ongoing to quantify the relative contribution from these drivers to the AMOC variability at different time scales.
How to cite: Araujo, J., Yang, C., Artale, V., De Toma, V., Simoncelli, S., Finlayson, M., and Storto, A.: Multi-drivers for Atlantic Meridional Overturning Circulation variability in an ensemble of historical ocean reanalyses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16680, https://doi.org/10.5194/egusphere-egu25-16680, 2025.
By acting as a global heat buffer and water supply, the ocean plays a critical role in influencing climate variability. For instance, the Atlantic Meridional Overturning Circulation (AMOC) has often been connected to climate variability on timescales ranging from decades to millennia. However, AMOC variability on centennial timescales has often been overlooked due to the limited availability of long climate model simulations as well as the scarcity of suitable paleoclimate proxies. Models that do simulate centennial variability all show a salinity anomaly being transported to the deepwater formation regions in the North Atlantic Ocean; the underlying mechanism differs between models however, further complicating the understanding of centennial variability. These mechanisms can be broadly categorized into two groups: either the salinity anomaly originates in the subtropics following changes in local precipitation, or it derives from the Arctic Ocean as a result of anomalies in sea ice concentration. In the case of CESM1, which is one of the few CMIP5 models that show clear centennial variability, regression analysis suggests that the former process dominates. To provide more detail on the origin of centennial AMOC variability in CESM1, we will present its complex spatial-temporal patterns using Multi-channel Singular Spectrum Analysis (MSSA), which is a technique enabling the identification of propagating patterns of variability in time series of spatial fields. MSSA will be applied to multi-millennial simulations and focus of the analysis will be on the identification of the propagation mechanisms, e.g. associated phase differences between tracer fields, responsible for centennial variability.
How to cite: Schuring, I. H. M., Bense, T., Bakker, P., and Dijkstra, H. A.: Mechanisms of Centennial AMOC Variability in CESM1, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6030, https://doi.org/10.5194/egusphere-egu25-6030, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is an important set of large-scale ocean currents that impacts regional climates on a wide variety of timescales due to its transport of heat to high northern latitudes. Understanding internal AMOC variability on (multi)centennial timescales is important for accurate attribution of recent changes in AMOC strength to internal variability or to anthropogenic forcing, and to improve future climate projections. However, much remains unknown about the spatiotemporal structure and underlying physical mechanisms of (multi)centennial AMOC variability (CAV). Recent research with various Earth System Models (ESMs) and Earth System Models of intermediate complexity (EMICs) have shown several different potential mechanisms of CAV across models, with the dominant mechanisms taking place in different regions, such as the Arctic, subtropical North Atlantic, or Southern Ocean. Here, we take a different approach and analyse how mechanisms of CAV vary within a perturbed parameter ensemble of the EMIC iLOVECLIM. Specifically, we investigate whether parameter choice alters spatio-temporal aspects of a model specific mechanism of CAV, or whether parameter choice alters the dominance of different mechanisms of CAV within the model. The ensemble is analysed using the spatiotemporal pattern recognition technique Multi-channel Singular Spectrum Analysis (MSSA).
How to cite: Bense, T., Schuring, I., Bakker, P., and Dijkstra, H.: Centennial AMOC variability in a perturbed parameter ensemble of the EMIC iLOVECLIM, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6398, https://doi.org/10.5194/egusphere-egu25-6398, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component in the Earth's climate system. Despite extensive research on the AMOC response to climate forcings, substantial discrepancies persist across models. These discrepancies may partly stem from differences in the representation of North Atlantic deep convection, particularly in the location of primary convection regions. This study investigates how difference in North Atlantic deep mixing patterns influence the AMOC response to CO2 forcing, using pre-industrial control and abrupt-4×CO2 experiments from 14 CMIP6 models. Based on winter mixed-layer depth (MLD) climatologies, we identified three main regions of North Atlantic deep mixing: the Labrador Sea, Irminger & Iceland Basins, and the Greenland-Iceland-Norwegian (GIN) Seas. Utilizing principal component analysis and k-means clustering, we identify two groups of models: (1) the LII cluster, with stronger mixing in the Labrador Sea and Irminger & Iceland Basins and (2) the GIN cluster, exhibiting stronger mixing concentrated in the GIN Seas. We find that the two clusters have similar mean-state AMOC in the pre-industrial scenario despite significant differences in regions of deep mixing. However, their projected responses to abrupt forcing diverge significantly. The LII cluster exhibits much stronger weakening and shoaling of the AMOC compared to the GIN cluster. Preliminary analyses of sea ice fraction indicate notable differences in the Labrador Sea. In the LII cluster, large parts of the Labrador Sea are ice-free, typical of models that are relatively warm and salty in the North Atlantic, whereas the GIN cluster demonstrates relatively high sea ice concentration, with its southern edge extending further. Our results suggest a possible link between deep convection representations and AMOC responses to greenhouse gas emissions, offering a potential reference for assessing model accuracy in projecting AMOC changes based on their climatological representation of deep mixing.
How to cite: Hu, M., Müller, J. M., and Jnglin Wills, R.: North Atlantic Deep Mixing Patterns Affect AMOC Responses to Abrupt-4xCO2 forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17351, https://doi.org/10.5194/egusphere-egu25-17351, 2025.
The northwards transport of heat through the North Atlantic Ocean is a crucial part of the present climate system. Ocean heat loss at high latitudes warms the atmosphere and cryosphere, influencing weather and melting ice, while simultaneously contributing to deep water formation integral to the Atlantic Meridional Overturning Circulation. Farther north, rising temperatures influenced by this oceanic heat transport has also driven Arctic atlantification. Thus accurate knowledge of ocean heat transport pathways will enable us to identify the hotspots of air-sea heat loss, and investigate the drivers of variability in heat transported along these pathways. When defining the heat transport pathways through the North Atlantic, many studies primarily take into account surface level variables, such as sea surface temperature, surface currents, or sea surface height. We investigate the utility of ocean heat content integrated from level of deep convection, 1000m depth, in identifying the heat transport pathways within the subpolar North Atlantic. This study uses data from the GLORYS12V1 Global Reanalysis dataset, spanning a 28 year period, to demonstrate that a dimensionless product of heat content and current speed provides a heat transport proxy that is more effective in determining the key pathways than using either heat content or speed alone. As expected, this method reveals that the poleward oceanic heat transport primarily follows topography northwards, but also indicates the presence of returning flows and recirculations that comprise the larger heat transport pathway system. Using data from the ERA5 Climate Reanalysis dataset over the same time period, we also show that the regions of the Arctic that exhibit the greatest rate of near-surface atmospheric warming do not perfectly correspond with the pathways themselves, but can be seen to correlate with the trends in heat content in the water column. This implies that atmospheric models that rely only on SST without taking water column stratification and heat content into account may be underestimating ocean heat fluxes.
How to cite: Stewart, K. and Lenn, Y.: Tracing the heat signature of Atlantic water through the GIN seas and its impact on Arctic ice and climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2743, https://doi.org/10.5194/egusphere-egu25-2743, 2025.
The North Atlantic is an important region when considering the global ocean’s absorption of anthropogenic carbon emissions. The North Atlantic carbon sink has variability which is linked to the Atlantic Meridional Overturning Circulation (AMOC). Currently, continuous observational data of the AMOC is limited to the RAPID and OSNAP mooring arrays at specific latitudes which have been in situ since 2004 and 2014, respectively. Ocean models can be used to produce data with greater spatial and temporal coverage, but these are often not constrained by observations. This leads to a great uncertainty in how AMOC variability in a changing climate may impact carbon uptake in the North Atlantic. The EXPLANATIONS project aims to tackle this problem by increasing our understanding of the North Atlantic carbon sink and the ocean interior carbon transports. We utilize an inverse water mass model, the Optimal Transformation Method (OTM) to estimate the transports and mixing of tracers between and within ocean basins. OTM simultaneously solves budgets of heat, freshwater, and carbon in a manner consistent with ocean reanalysis and carbon product data. We compare the observed data at RAPID and OSNAP with the OTM solution by defining a North Atlantic domain between the two mooring arrays. We use the data collected from these arrays to further constrain the inverse model, thus giving a better representation of the AMOC and its impact on carbon. Output from OTM informs upon the transports and mixing of volume, heat, freshwater, and carbon between and within the ocean basins. It allows for the construction of carbon budgets within the North Atlantic, improving our understanding of how these may change with variability in the AMOC.
How to cite: Hutton, T. and Mackay, N.: Modelling the impact of AMOC variability on carbon uptake and transport in the North Atlantic Ocean using an inverse water mass model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3725, https://doi.org/10.5194/egusphere-egu25-3725, 2025.
The northward transport of warm, salty waters within the extended Gulf Stream system along the borders of the subtropical and subpolar gyres is climatically important. However, the role of variable gyre circulation in modulating the northward ‘throughput’ of water between subtropical and subpolar latitudes remains unknown. Here, we use the Lagrangian analysis tool TRACMASS with the 1/12° ocean reanalysis GLORYS12 to quantify variability in the northward throughput between the gyres. Lagrangian particles are seeded in the Gulf Stream at 30°N between 1993 and 2017, and tracked forward in time to quantify the volume of water recirculating within the subtropical gyre versus the throughput to the subpolar gyre. On average, 64% of the Gulf Stream water recirculates within the subtropical gyre while 36% is transported north of 45°N into the subpolar gyre within the four years of tracking. The subtropical recirculation is strongly correlated to the net volume transport at the Gulf Stream seeding section on interannual time scales, indicative of an overall stronger/weaker subtropical gyre with a strong/weak western boundary current. The subtropical-subpolar throughput is not significantly correlated to the net volume transport at the seeding section. This indicates that regional wind patterns and/or stratification exist that favours enhanced subtropical-subpolar throughput. Variable subtropical-subpolar throughput is potentially a mechanism contributing to reducing meridional coherence in the AMOC strength between subtropical and subpolar latitudes.
How to cite: Asbjørnsen, H.: Variable northward throughput between the North Atlantic gyres and implications for overturning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5602, https://doi.org/10.5194/egusphere-egu25-5602, 2025.
The North Atlantic Subpolar region is a critical region for global climate dynamics, serving as a gateway between the subtropical and Arctic zones. This region is characterized by intense atmosphere-ocean interactions, where water masses meet and undergo transformation, influencing ocean circulation. Recent studies have highlighted a phenomenon of freshening in the Subpolar North Atlantic, with potential implications for regional climate and ocean circulation. Given the complex interactions between the ocean, atmosphere, and cryosphere, uncertainties remain regarding the underlying causes and future climate impacts. In this study, we utilize new historical simulations from a state-of-the-art global climate model, to investigate how variations in salinity are linked to changes in large-scale atmospheric circulation. Our composite analysis of extreme years reveals that freshwater anomalies in the subpolar gyre are closely associated with cooler sea surface temperatures and atmospheric pressure anomalies over Western Europe. These findings suggest that freshwater fluxes could have significant, far-reaching effects on both regional climate and the broader North Atlantic climate system.
How to cite: Ayres, H. and Oltmanns, M.: The Role of Freshwater Variability in North Atlantic Subpolar Climate Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9235, https://doi.org/10.5194/egusphere-egu25-9235, 2025.
We perform modelling experiments designed to decompose the historical buoyancy and momentum flux contributions to observed AMOC variability. Using NEMO, we have forced the model with interannually varying momentum fluxes, while keeping the buoyancy fluxes fixed with a repeat year forcing (momentum experiment), and then with interannual buoyancy forcing and repeat year momentum forcing (buoyancy experiment). These are compared to a full interannually forced control.
Our analysis focuses on comparisons with the RAPID and OSNAP arrays to assess how well each of the momentum and buoyancy experiments matches AMOC variability at RAPID/OSNAP across different timescales. One key result is that decadal variability of AMOC is most dependent on buoyancy forcing, and that both momentum and buoyancy forcing are important for reproducing interannual variability.
How to cite: Walsh, A., Mecking, J., Hirschi, J., and Blaker, A.: Assessing AMOC sensitivity to buoyancy and momentum forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10417, https://doi.org/10.5194/egusphere-egu25-10417, 2025.
In the sub-polar North Atlantic, perturbations to climatological mean freshwater fluxes can impact the strength of the climatically important Atlantic Meridional Overturning Circulation, providing an impetus for us to try and understand the pathways of freshwater in the region. In recent years, our understanding of the climatological mean horizontal pathways of freshwater through Greenland’s boundary current systems and out of the sub-polar North Atlantic have increased; however, we lack a thorough understanding of where freshwaters are destroyed by processes such as diahaline mixing.
In this study, I describe and use the freshwater transformation framework (based on the water mass transformation framework pioneered by Walin (1982)) to quantify how rates of diahaline mixing vary around Greenland with both season and region. I demonstrate the framework using an eddy resolving coupled configuration of the ICON earth system model (5 km ocean, 10 km atmosphere). Two patterns emerge:
- the destruction of fresh waters by mixing is stronger during wintertime than summertime;
- the destruction of fresh waters by mixing is stronger off the coast of southern Greenland than Northern Greenland.
Using the freshwater transformation framework, I also explore the interplay of diahaline mixing with the salinification of boundary currents, surface sources of freshwater, and the storage of low salinity water in different regions. I find that different processes dominate the climatological mean freshwater balance depending upon the season and region under consideration.
How to cite: Goldsworth, F.: Exploring the mixing of freshwater around Greenland in a high-resolution climate model using the freshwater transformation framework, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13158, https://doi.org/10.5194/egusphere-egu25-13158, 2025.
The Atlantic Meridional Overturning Circulation (AMOC) is a vital mechanism of heat transport in the climate system, but it has been suggested that its strength will change in the coming decades. This strength depends in part on water mass transformations in the North Atlantic, and understanding the factors that contribute to this variability is crucial to predicting the future behaviour of the AMOC. This research aims to deepen our understanding of one such factor: the role that cyclonic storms play in priming and initiating deep water formation in the Labrador Sea and Nordic Seas. We study cyclone statistics for both regions, but primarily study the ocean’s response in the Labrador Sea. To do this, we used pressure and wind fields from two atmospheric datasets (ERA5 and the Canadian Meteorological Centre’s Global Deterministic Prediction System Reforecasts, or CGRF) to identify and track cyclones. We then look at output from a very-high resolution NEMO (Nucleus for European Modelling of the Ocean) model configuration, run over 2002-2019 and forced with the above atmospheric datasets to evaluate what effects passing cyclones exert on upper ocean properties.
Preliminary results indicate that the passage of individual cyclones noticeably cools and deepens the mixed layer, likely via an associated increase in surface heat loss. The other key points we still aim to investigate are the linkage between deep convection and cyclones, and how the presence of this cyclone forcing affects the properties of the resulting deep water masses. We also aim to quantify the contribution of cyclones to deep convection over the study period relative to the amount contributed by the background environmental conditions. We explore whether the cyclones have a positive contribution to deep water formation, particularly after multiple cyclones transit in relatively short succession.
How to cite: Whincup, R., Pennelly, C., and Myers, P.: Investigating the role of extratropical cyclones in North Atlantic deep water formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13754, https://doi.org/10.5194/egusphere-egu25-13754, 2025.
Representing mesoscale eddies and understanding their impact on large-scale ocean circulation are critical challenges in climate research. High-resolution models are necessary to resolve eddies explicitly, while parameterisations, most commonly those of Gent and McWilliams (1990) and Redi (1982), are used in lower-resolution models to simulate their effects. Mesoscale eddies can affect the Atlantic Meridional Overturning Circulation (AMOC) by flattening isopycnals. Additionally, mesoscale eddies can also affect AMOC by modifying turbulent diapycnal mixing, which is parameterised by e.g. the mixing scheme of Gaspar et al. (1990). The modification of turbulent mixing can occur when horizontal density gradients and with them, vertical velocity shear and the turbulent shear production are reduced as a result of flattened isopycnals.
In this study, we analyse the sensitivity of AMOC to parameterised eddy diffusivity using a coupled model. To isolate the role of eddies in diapycnal mixing, we separate the buoyancy tendency forcing produced by the eddy parameterisations (GM-Redi) from that produced by diapycnal mixing. Previous studies with uncoupled models have shown how eddy-induced isopycnal flattening affects the upwelling of North Atlantic Deep Water (NADW) in the Southern Ocean (Marshall et al., 2017). Our results from a coupled model show that the impact on diapycnal mixing is most pronounced in the downwelling regions in the Subpolar North Atlantic, with increasing eddy diffusivity causing a shift in the dominant location of deep water formation from the Labrador Sea northeastward to the Irminger and Iceland Basins.
By investigating the interplay between eddy-induced mixing and the AMOC, this work provides new insights into how spatially variable mixing processes shape large-scale ocean circulation patterns.
How to cite: Mosso, A., Goldsworth, F., and von Storch, J.-S.: Impact of Eddy-Induced Mixing on AMOC: A Model Study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18209, https://doi.org/10.5194/egusphere-egu25-18209, 2025.
In the coming decades, climate change is expected to lead to increasing freshwater input to the subpolar North Atlantic from Greenland and the Arctic. This could affect the ocean circulation in the region, and potentially the Atlantic Meridional Overturning Circulation (AMOC).
Greenland and Arctic origin waters initially enter the coastal and shelf regions of Greenland and Canada, where they are transported by narrow boundary currents. Observational studies and regional simulations have shown that exchanges between the shelf region and the open ocean are restricted to a few areas. This constrains where, how much, and at which timescales, additional freshwater may affect the interior subpolar North Atlantic.
The potential effects of Greenland melt on the AMOC is often tested in climate models via hosing experiments, in which large volumes of freshwater are released across broad areas of the North Atlantic. While most hosing experiments release freshwater uniformly in that region to understand the mechanisms and impacts of an AMOC slowdown, a few targeted freshwater release experiments have also been carried out around Greenland to more realistically assess how Greenland melt might affect the ocean circulation.
It is however unclear whether the resolution of these models allows to adequately represent the pathways of the added freshwater. Moreover, the traditional understanding of a direct link between convection in the Subpolar Gyre and the AMOC has been challenged, highlighting the need to also assess the effect of the added freshwater on the circulation and hydrography of the subpolar North Atlantic.
Here we use results from hosing experiments carried out in the MPI-ESM coupled climate model as part of the NAHosMIP project (Jackson et al 2023) to examine how the added freshwater circulates in the subpolar North Atlantic and affects the region's circulation and hydrography. We compare the Greenland focused and uniform hosing experiments to investigate how the hosing location affects these results. We then evaluate these findings against observations and results from high-resolution regional simulations to determine whether targeted hosing experiments can indeed help understanding the impact of future increases in freshwater input to the subpolar North Atlantic, identify potential limitations, and how they could be addressed.
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Jackson, L. C., Alastrué de Asenjo, E., Bellomo, K., Danabasoglu, G., Haak, H., Hu, A., Jungclaus, J., Lee, W., Meccia, V. L., Saenko, O., Shao, A., and Swingedouw, D.: Understanding AMOC stability: the North Atlantic Hosing Model Intercomparison Project, Geoscientific Model Development, 16, 1975–1995, https://doi.org/10.5194/gmd-16-1975-2023, 2023.
How to cite: Duyck, E. and Goldsworth, F.: Pathways and impacts of increased Greenland and Arctic freshwater fluxes to the Subpolar North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19159, https://doi.org/10.5194/egusphere-egu25-19159, 2025.
As climate change impacts ocean properties, such as temperatures, salinity and stratification, the Atlantic Meridional Overturning Circulation (AMOC) may be at risk of collapse. A weakened AMOC can be connected to shifts in global weather patterns, such as more heatwaves in Europe. The outflow water from the Mediterranean Sea has properties that influence the North Atlantic Ocean hydrodynamics and the AMOC. The Mediterranean Outflow Water (MOW) properties vary due to climate change, affecting the Atlantic Ocean thermohaline characteristics.
Using a multi-data approach, this study compares four reanalyses of different horizontal grid resolutions in a seventy years time span (RR 1955-2015 DOI: 10.25423/MEDSEA_REANALYSIS_PHY_006_009, MEDREA16 1987-2018 DOI:10.25423/medsea_reanalysis_phys_006_004, MEDREA24 1987-2024 DOI: 10.25423/CMCC/MEDSEA_MULTIYEAR_PHY_006_004_E3R1, CIGAR-CS 1961-2022 http://cigar.ismar.cnr.it/) to the World Ocean Atlas 2023 climatology (DOI: 10.25921/va26-hv25) and mooring observations at the Espartel Sill. The study evaluates volume, heat and salt transports, temperature and salinity time series at Gibraltar and analyzes the MOW decadal variability in the North Atlantic from 1955 to 2024.
The goal of this research is to assess the consistency and accuracy of multiple reanalysis products in simulating the Mediterranean Sea and Atlantic Ocean interaction and to establish how the model resolution and geometry affect the MOW characteristics. Ultimately, this assists in understanding how MOW variability may impact the AMOC.
How to cite: Finlayson, M., Simoncelli, S., Yang, C., Araujo, J., and Sammartino, S.: Mediterranean Outflow Water analysis through multiple reanalysis and observational data products, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19637, https://doi.org/10.5194/egusphere-egu25-19637, 2025.
