The strength of the ocean carbon sink is maintained by the physical, biological, and chemical processes. Any change in these processes may alter the carbon-climate feedbacks and affect the rate of global change. Our predictive understanding of the susceptibility of these feedbacks to global change is heterogeneous: some responses are well quantified, whilst for some, even the direction of change is unclear. This makes representing ocean biogeochemical cycles as an interactive component of Earth system models (ESMs) a key scientific challenge. This challenge unfolds in resolving the critical marine biological and physical processes, as well as their feedbacks in high spatial resolutions on climate-relevant time scales. Thereby, advancements in ocean biogeochemical ESM components need to embrace emerging observational and laboratory evidence, together with novel computational technologies. This lecture will discuss the current progress, challenges and opportunities in addressing knowledge gaps in our predictive understanding of the changing ocean carbon sink, its variability and feedbacks in the Earth system.

How to cite: Ilyina, T.: Predictability and feedbacks of the changing ocean carbon sink – insights from Earth system models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6396, https://doi.org/10.5194/egusphere-egu25-6396, 2025.

The Atlantic Meridional Overturning Circulation (AMOC) is crucial for global ocean carbon and heat uptake, and controls the climate around the North Atlantic. Despite its importance, quantifying the AMOC’s past changes and assessing its vulnerability to climate change remains highly uncertain. Understanding past AMOC changes has relied on proxies, most notably sea surface temperature anomalies over the subpolar North Atlantic. Here, we use 24 Earth System Models from Coupled Model Intercomparison Project Phase 6 (CMIP6) to demonstrate that these sea surface temperature anomalies cannot robustly reconstruct the AMOC on annual, decadal or centennial timescales. Instead, we find that the net air-sea heat flux anomaly between the Arctic and any given latitude between 26.5°N and 50°N in the North Atlantic is tightly linked to the AMOC anomaly at that latitude on decadal and centennial timescales. On these timescales, the air-sea heat flux proxy works through the conservation of energy, in which energy transferred laterally into the region is typically released to the atmosphere through surface heat fluxes. On annual timescales, however, air-sea heat flux anomalies are modulated more so by atmospheric variability and less by AMOC anomalies. Based on the here identified relationship and observation-based estimates of the past air-sea heat flux in the North Atlantic from reanalysis products, we show the decadal averaged AMOC at 26.5°N has not weakened from 1963 to 2017 although substantial variability exists. Furthermore, we find no decline in the AMOC at any other latitude, though the decadal variability appears distinct between subtropical and subpolar latitudes. This result aligns with previous work that has shown the lack of meridional coherence in the AMOC, and the presence of distinct overturning cells.

How to cite: Terhaar, J., Vogt, L., and Foukal, N. P.: Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8323, https://doi.org/10.5194/egusphere-egu25-8323, 2025.

Climate Change is expected to increase the intensity and frequency of extreme rainfall events in the coastal areas of Patagonia (Southwest Atlantic Ocean, SWAO). These events carry heavy loads of terrestrial materials and nutrients, and minor components such as kaolin and ash, into coastal areas through riverine inputs. The Chubut River estuary was used a reference coastal ecosystem in the SWAO. In its lower course, the river is diverted into irrigation channels that supply water for agricultural activities. These channels are open from spring to early autumn, increasing the runoff of terrestrial material, and are closed during the rest of the year. Furthermore, kaolin mines are located in the upper course of the river and ash deposition coming from volcanos have been registered. A monitoring of terrestrial material of the Chubut River estuary was conducted and the attenuation coefficients of the different components were evaluated, including terrigenous material, kaolin, and ash. The findings show that the terrestrial material, estimated as dissolved organic carbon (DOC), doubles during rainfall conditions and when irrigation channels are open. During extreme rainfall events, DOC concentrations increased by up to fivefold compared to normal conditions, being the main attenuator in the river. This resulted in a PAR attenuation coefficient variable between 1.3 m^-1 under baseline conditions (closed channels, no rainfall) to over 8 m^-1 following extreme rainfall events in the outer regime (seawater side) of the estuary. Further monitoring of the different under-studied estuarine components in the SWAO and their effects on the attenuation coefficient is crucial for primary productivity studies.

How to cite: Vizzo, J. I., Helbling, E. W., and Villafañe, V. E.: Inputs of Terrestrial Material, Kaolin and Ash into Coastal Patagonian Waters and their Effects on the Attenuation Coefficient of the Chubut River Estuary (Argentina), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-434, https://doi.org/10.5194/egusphere-egu25-434, 2025.

A wide range of problems in oceanic mass and energy transport involve learning submesoscale surface flow fields from diurnal geostationary satellite observations. Yet, traditional methods, such as the Maximum Cross-Correlation (MCC) algorithm, suffer from limited spatiotemporal resolution and extensive post-processing. Here, we present the RAFT-Ocean architecture, a deep neural network-based approach for learning submesoscale flow fields in pixel-to-pixel manner, to retrieve submesoscale surface flow fields from geostationary satellite data. Compared to the MCC algorithm, the RAFT-Ocean architecture significantly improves these methods, reducing the end-point error (EPE) uncertainty by more than 65% and the absolute angular error (AAE) by more than 55%. The RAFT-Ocean architecture, when transferred to the geostationary ocean color satellite (GOCI/CMOS and GOCI-II/GK2B) sea surface chlorophyll-a products for diurnal hourly flow field retrieval, produced more realistic, continuous, and refined sea surface flow field data compared to geostrophic flow data from altimeter data. The refined diurnal hourly flow field matched well with the filamentous structure of surface phytoplankton, demonstrating an advantage in spatiotemporal resolution for kinetic energy transfer across scales. This approach enhances flow field retrieval quality and opens new avenues for real-time marine environment monitoring and modeling.

How to cite: Ding, X., Zhao, M., and Li, H.: Deep learning for submesoscale surface flow retrieval from geostationary satellite observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2306, https://doi.org/10.5194/egusphere-egu25-2306, 2025.

This study is part of a doctoral research project aimed at characterizing coastal sediments in relation to the presence of microplastics and marine litter. Within this framework, the present research seeks to establish an up-to-date knowledge base regarding the geochemical characterization of sediments across different seasons along the Ferrara coastal area, specifically at Lido degli Estensi (Ferrara, Italy). The objective is to identify potential vulnerabilities and/or critical aspects related to environmental pollution that require further investigation. Building upon the methodology of Aquilano et al. (2023) and adapting it to the experimental requirements of the current study, a research area was selected at Lido degli Estensi, outside zones allocated for tourism-related public concessions. This site is located on the southern side of the Porto Garibaldi navigation channel (Comacchio municipality, Ferrara), in a coastal section experiencing accretion due to the construction of artificial jetties at the port-channel entrance. These jetties trap sediment transported from the south as a result of longshore drift. Given the beach's width (approximately 150 m), a cross-shore profile was divided into five zones based on specific geomorphological characteristics: swash zone, lower backshore, upper backshore, dune scarp, and dune. Along this beach profile, variations in carbonate content, major oxide composition, and heavy metal concentrations were investigated across different seasons using eight sampling points per season. To evaluate sediment quality in terms of heavy metal contamination, the following indices were employed: Enrichment Factor (EF; Reinmann and De Caritat, 2005), Geoaccumulation Index (Igeo; Buccolieri et al., 2006), Contamination Factor (CF; Loska et al., 2004), and Pollution Load Index (PLI; Ferreira et al., 2022). Furthermore, heavy metal concentrations detected in the samples were compared with the limits established by current Italian legislation (Legislative Decree 152/06). This study was conducted as part of the ECS_00000033_ECOSISTER project, funded under the National Recovery and Resilience Plan (NRRP), Mission 04 Component 2 Investment 1.5 – NextGenerationEU (Call for Tender No. 3277, dated 30/12/2021).

How to cite: Buoninsegni, J., Marrocchino, E., Tassinari, R., Tessari, U., and Vaccaro, C.: Geochemical characterization of coastal sediments: a preliminary study of seasonal variations at Lido degli Estensi (Ferrara, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3582, https://doi.org/10.5194/egusphere-egu25-3582, 2025.

Climate change significantly impacts the hydrology and water resources of any region especially high mountain areas including cryosphere that consist of glaciers. Numerous studies report that glaciers are retreating and losing volume with time causing serious concerns over freshwater availability in the basins they feed water to. Assessment of these changes and their relationship with various climatic aspects are crucial to understand and tackle such challenges. Long-term trends in temperature and precipitation and their spatio-temporal distribution, for the mountainous state of Uttarakhand in India were assessed, utilizing the India Meteorological Department’s gridded precipitation and temperature datasets for the period 1951-2023. Mann-Kendall trend test was performed at 90% significance level, for each grid, to check monthly trends, which gave critical insights upon shifts in seasonal meteorology. Results reveal notable changes in the monthly distribution of precipitation with many grids reporting a decreasing winter precipitation (Oct-Jan) and many showing an increasing precipitation for May and August. Global warming impact is much visible through changes in minimum temperatures for almost all the grids, reporting a strong positive trend for February, March, August, September and November. Importantly, these changes are more prominent for the high-altitude areas, which highlights elevation dependent climate change pattern. Evidently, the precipitation is shifting from winters to summers and the minimum temperatures are increasing towards the end of ablation season (Aug-Sep), decreasing the chances of receiving solid precipitation or snowfall. Consequently, a decrease in snow cover is expected in the future, which from a hydrological perspective, would lead to a reduction in snowmelt discharge and its contribution to streamflow of the Himalayan perennial rivers. Moreover, the increasing temperature and precipitation during summers can generate huge discharges from glacierized catchments due to increased simultaneous contribution of glacier-melt and rainfall, causing destructive flash floods and debris flow events, as being witnessed in the recent past. Combination of decreased precipitation in winter months and increased temperatures overall, can prove detrimental to glaciers’ health as they will melt more, whereas their replenishment will be lesser, leading to negative mass balances. Climate change is certainly having an adverse effect on the mountain hydrology, especially that of the Himalayan cryosphere. The glaciated catchments are expected to have more glacier-melt and rainfall-runoff contribution and less snow-melt contribution in the near-future. The glaciers, present in the region, are expected to retreat and lose mass more rapidly, considering the meteorological changes in the high elevation areas. Small glaciers might deplete faster, which would lead to problems of freshwater availability in the nearer downstream areas dependent on the melt-runoff water. While there seems no immediate solution to the prevailing scenario of climate change, community-based measures can be adopted to tackle problems of water availability. Water conservation and springshed management in the mountainous regions are some focus areas to work upon, in order to ensure water security under the changing climate.

How to cite: Thapliyal, M., Singh, S., and Patel, L.: Climate Change and its Impact on the Hydrology of a Glaciated Mountainous Region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4832, https://doi.org/10.5194/egusphere-egu25-4832, 2025.

The coastal ecosystems, including estuaries and mangroves, are highly vulnerable to anthropogenic intervention, particularly due to activities such as urbanization, wastewater discharge, and industrial development, which can alter their ecosystem services and affect habitat quality. In order to evaluate the impact of these interventions through the carbon and nitrogen isotopic composition of two macroalgae Boodleopsis verticillata and Bostrychia spp in four coastal ecosystems of the Colombian Pacific (Valencia - VAL, San Pedro - SPE, Chucheros – CHU with low intervention, and Piangüita - PIA with high intervention) were used to understand the sources of these elements. δ¹⁵N values is a commonly used to providing information about nitrogen sources in primary producers. δ¹³C values is used to investigate carbon sources i.e. terrestrial or marine. Samples were collected during 2014, 2015, and 2016, and analyzed by isotope ratio mass spectrometer. The results show that the δ¹³C values ranged from -33.97 to -31.93 ‰ in VAL, -33.78 to -30.09 ‰ in SPE, -31.12 to -28.45 ‰ in CHU, and -33.32 to -21.71 ‰ in PIA. δ¹⁵N values ranged from 0.32 to 3.18 ‰ in VAL, 0.57 to 5.47 ‰ in SPE, 1.82 to 3.39 ‰ in CHU, and 2.32 to 10.16 ‰ in PIA. Significant differences were found among the four areas with mean δ¹³C values by locality (VAL -30.21 ‰, SPE -31.71 ‰, CHU -30.09 ‰, and PIA -30.52 ‰) and δ¹⁵N values (VAL 1.74 ‰, SPE 2.30 ‰, CHU 2.40 ‰, and PIA 4.47 ‰) reflecting the impacts of human activities on the coastal ecosystems. This work contributes to understanding the effects of anthropogenic intervention on pollution and wastewater discharge in coastal ecosystems, providing key tools for the development of environmental management policies that support conservation in the Colombian Pacific.

How to cite: Arce-Sánchez, R. S., Medina-Contreras, D., and Sánchez-Gonzalez, A.: Primary producers as indicators of anthropogenic intervention in the Colombian Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7292, https://doi.org/10.5194/egusphere-egu25-7292, 2025.

The Mediterranean Sea, with its unique characteristics as a semi-enclosed and highly stratified basin, serves as a natural laboratory for studying oceanic processes of global relevance. Vertical mixing is a fundamental process regulating the transfer of mass, heat, and nutrients between water column layers, influencing dynamical and biogeochemical processes, and controlling the exchange with the overlying atmosphere. Due to its turbulent nature acting on small spatial and temporal scales, vertical mixing remains challenging to simulate in modern ocean circulation models. Moreover, the interaction between vertical mixing and horizontal diffusion/advection is essential in shaping the transport and distribution of heat, nutrients, and pollutants in marine environments. Finding the optimal vertical mixing parameterizations alongside horizontal advection and diffusion schemes in an ocean circulation model, able to simulate the available observations, presents significant challenges due to the need for consistent scaling, numerical stability, and accurate representation of multi-scale processes.

Here, we use the same system setup as the Mediterranean Forecasting System of the Copernicus Marine Service, that is NEMO (v4.2) general circulation model, including tides, coupled with the WaveWatch III wave model. The model features a horizontal resolution of 1/24° (approximately 4 km) and 141 unevenly spaced vertical levels. We investigate the performance of different numerical vertical closure schemes – a Richardson-number-dependent, a one-equation and a two-equation models – as well as the effect of different lateral advection and diffusion schemes. The role played by the enhanced vertical diffusion due to Camarinal Sill at the Strait of Gibraltar in controlling the exchange of water masses between the Atlantic Ocean and the Mediterranean Sea is also investigated. We validate our model by assessing our ability to reproduce physical processes and by comparing it with in-situ data throughout the Mediterranean basin, across varying seasons and years.

How to cite: Gualtieri, L., Borile, F., Burchard, H., Oddo, P., Miraglio, P., Clementi, E., Goglio, A. C., and Pinardi, N.: Investigating vertical mixing and lateral diffusion parameterizations in the Mediterranean Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8649, https://doi.org/10.5194/egusphere-egu25-8649, 2025.

Breaking internal gravity waves cause small-scale turbulent mixing, which changes water mass properties, affects biogeochemical cycles, and contributes to driving the large-scale overturning circulation. Ocean general circulation models do not resolve this process and thus rely on a parameterization. The state-of-the-art IDEMIX (Internal wave Dissipation, Energy and MIXing) model predicts the propagation and dissipation of internal wave energy based on external forcing functions that represent the main generation mechanisms, notably the internal tide generation at the sea floor and the near-inertial wave generation at the sea surface. By linking small-scale mixing to internal wave energetics, IDEMIX allows the consistent parameterization of wave-induced mixing in ocean models. Its basic incarnation treats all internal waves as part of a horizontally homogeneous continuum and was shown to successfully reproduce observed turbulent kinetic energy dissipation rates and internal wave energy levels. In a newer configuration (IDEMIX2), the internal wave field is compartmentalized, distinguishing between a high-mode continuum on the one hand and low-mode near-inertial wave and internal tide compartments, whose horizontal propagation is explicitly resolved in wavenumber angle space, on the other hand. We present the evaluation of the IDEMIX2 model with a particular focus on the impact of applying an anisotropic internal tide forcing. So far, parameterizations of internal tide-driven mixing have not taken the strong anisotropy of the internal tide generation process into account. We demonstrate the need for doing so, showing a notable impact on the modeled internal wave energetics and predicted mixing when changing from the previous isotropic to the new anisotropic tidal forcing in IDEMIX2. 

How to cite: Pollmann, F., Eden, C., Olbers, D., Nycander, J., and Zhao, Z.: Anisotropic internal tide forcing in the consistent internal wave mixing scheme IDEMIX, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8764, https://doi.org/10.5194/egusphere-egu25-8764, 2025.

This study investigates the spatio-temporal variability and long-term warming trends in sea surface temperature (SST) across the Arabian Sea from 2000 to 2019, using daily AVHRR satellite observations with a 1°x1° spatial resolution. Seasonal and interannual SST dynamics reveal patterns shaped by monsoonal processes and global climate phenomena, such as El Niño and La Niña. Wavelet spectrum analysis highlights periodic fluctuations and dominant frequencies associated with interannual climate variability, further emphasizing the influence of seasonal processes. Spring (MAM) exhibits the most pronounced interannual warming, particularly in the central and northern regions, while autumn (SON) demonstrates significant warming trends, especially in the southern basin. Monsoonal processes influence seasonal variability, with winter (DJF) cooling in the northern Arabian Sea and summer (JJA) upwelling along Oman and Somalia, resulting in localized cooling amidst broader warming trends in central and southern regions. Wavelet power spectra from critical regions, including the Gulf of Oman, Balochistan Coast, and Mumbai, indicate dominant periodicities of interannual warming, with variations corresponding to regional oceanographic processes. For instance, the Balochistan Coast displays the highest warming rate (0.0519°C/year), underscored by strong wavelet power at periodicities tied to El Niño–Southern Oscillation (ENSO) cycles. Similarly, the Gulf of Oman and Mumbai exhibit distinct spectral peaks, reflecting localized climate dynamics and variability. Regionally, the warming trend varies significantly. The Gulf of Aden (0.0181°C/year), Gulf of Oman (0.0164°C/year), and Gulf of Kutch (0.0269°C/year) exhibit moderate warming rates, while areas like the Balochistan Coast and South of Salalah (0.023°C/year) highlight significant localized warming. Southwestern Arabian Sea regions west of Kochi (0.0209°C/year) and Mangalore (0.0323°C/year) also demonstrate notable trends. In contrast, regions like Minicoy (0.0162°C/year) and the Male-Maldives area (0.0073°C/year) show relatively weaker warming. These findings underscore the critical role of spatial and seasonal variability in shaping SST changes and their implications for regional climate patterns, monsoonal behavior, marine ecosystems, and fisheries. The pronounced warming in key regions, coupled with insights from wavelet spectrum analysis, highlights the influence of localized oceanographic processes, such as upwelling, heat transport, and climate-induced variability. These results necessitate further study to assess future impacts and develop mitigation strategies for sensitive marine biodiversity and economic resources in the Arabian Sea . 

How to cite: Saha, S. and Mukherjee, A.: Unraveling the Arabian Sea’s Thermal Pulse: Seasonal and Interannual SST Variability Amidst Climate Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12429, https://doi.org/10.5194/egusphere-egu25-12429, 2025.

We have previously developed a 12-sample environmental DNA (eDNA) sampler designed for use in the marine surface. The sampler can collect and store eDNA samples on filter cartridges according to scheduled sequences. Communicating via mobile phone networks also makes it possible to collect samples on demand. For the underwater eDNA sample-return missions, we have designed and developed a compact eDNA sampler with an oil-filled (pressure-balanced) configuration, enabling its deployment at various depths. Field trials for the underwater eDNA sampler were performed using underwater platforms such as deep-sea landers. Here, we introduce the newly developed compact eDNA sampler and discuss its potential applications in mid- to deep-ocean layers, focusing on eDNA sample-return missions targeting jellyfish and other marine species.

How to cite: Fukuba, T. and Lindsay, D.: Development of an underwater eDNA sampler and its potential application in jellyfish eDNA detection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14703, https://doi.org/10.5194/egusphere-egu25-14703, 2025.

Between Sept. 4, 2023, and Sept. 12, 2023, a cyclogenesis develops close to the Greek coast in the Ionian Sea. The evolution of this cyclone is divided into two phases: a strongly baroclinic one with intense orographic precipitation in Greece, and a final barotropic phase with the formation of an intense tropical-like cyclone (TLC) impacting Libya. In this work, we investigate this TLC (named “Daniel”) initially using the standalone WRF model with different sea surface temperature sources,  untile the use of the coupled atmosphere-ocean models. Preliminary results show that SST plays a crucial role in the intensification and tropicalization of the cyclone, with a strong impact not only along the cyclone track but especially in the neighboring areas, where high values of heat transport a precipitable water are found. We also observe how the use of a coupled model as a digital twin, shows strengths in the quality of the simulation and the physics of the process, but highlights some critical issues in the configuration of the marine model, which at small technical variations produces intense changes in the structure of the ocean and atmosphere.

How to cite: Ricchi, A., Ferretti, R., Pantillon, F., Dafis, S., Menna, M., Martellucci, R., Serafini, P., and Carrió, D. S. C.: On the role of air-sea-wave interaction in developing destructive Tropical-Like Cyclones DANIEL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17499, https://doi.org/10.5194/egusphere-egu25-17499, 2025.

Ice crevasses are pervasive features across the Arctic and Antarctic ice sheets. These deep, open fractures in the ice surface serve as critical conduits for transporting surface meltwater into the englacial system, significantly impacting ice sheet hydrology and stability. Accurate mapping of the spatial and temporal distribution of ice crevasses is vital for advancing our understanding of ice sheet dynamics and their evolution. Remote sensing technology provides a robust platform to achieve this purpose, while the rapid advancement of machine learning algorithms offers substantial benefits for automated crevasse detection, facilitating efficient and large-scale mapping. This study conducts a comprehensive comparison of the performance of various machine learning models, including CNN, U-Net, ResNet, and DeepLab, for ice crevasse extraction. Through quantitative evaluation metrics and visual inspection, the optimal machine learning model was selected to map ice crevasses on Antarctic ice shelves using multi-source remote sensing data, such as SAR and optical satellite imagery. Furthermore, this work explores the strengths and limitations of various machine learning in detecting ice crevasse and proposes potential solutions for further refinement. This study aims to contributes to enhancing ice crevasse detection and offering robust ice crevasse datasets, which is crucial for reliable analyzing the dynamic of the Antarctic ice sheet in the future.

How to cite: Liang, S. and Xiao, X.: Antarctic ice shelf crevasse detection using multi-source remote sensing data and machine learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19031, https://doi.org/10.5194/egusphere-egu25-19031, 2025.

SnowMapPy is a Python-based package developed to streamline the collection, preparation, and analysis of MODIS NDSI data, specifically from the Terra and Aqua satellite products. By automating essential steps (data clipping, reprojection, filtering, and time series generation), SnowMapPy improves the efficiency and precision of snow hydrology research. The protocol allows users to work with both local and Google Earth Engine cloud-based datasets, enabling flexible data acquisition and processing tailored to the needs of snow hydrology, water resource management, and climate change studies. Designed for accessibility and flexibility, SnowMapPy supports large-scale, high-resolution snow cover analysis with minimal configuration. The package facilitates customized workflows through its modular structure, making it a valuable tool for researchers aiming to understand snow dynamics and their impact on seasonal water resources. 

How to cite: Elyoussfi, H., Boudhar, A., Belaqziz, S., Bousbaa, M., Bechri, H., Sproles, E. A., and Benzhair, F.: SnowMapPy v1.0: A Python Package for Automated Snow Cover Mapping and Monitoring in Mountain Regions , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20632, https://doi.org/10.5194/egusphere-egu25-20632, 2025.

This study explores the seasonal and lagged correlations between Chlorophyll-a (Chl-a) concentrations and vertical velocity (wT) to elucidate upwelling's role in driving phytoplankton productivity. In Oman (Region III), an immediate response to upwelling was observed, with the strongest correlation (r = 0.7) at lag 0 during peak upwelling months (June–July). In contrast, Iranian regions (I & II) exhibited delayed responses, with maximum correlations (r = 0.7) at lag 1 (occurring about a month later). This delay may result from processes like nutrient mixing and remineralization. Seasonal trends revealed sustained Chl-a concentrations in Oman, peaking at 2.39 mg m^-3 in September, while Iran showed a steady decline after a July peak of 1.37 mg m^-3. Stratification and horizontal currents modulated Chl-a distributions, with weaker stratification in Oman enabling efficient nutrient delivery. These findings reveal the intricate dynamics of upwelling-driven productivity across both semi -enclosed and open marine ecosystems. By examining regional variations in the context of broader oceanographic processes, this study offers valuable insights for the sustainable management of upwelling systems and for anticipating their responses to climate change.

How to cite: A. Ismail, K. and Salim, M.: Unveiling the Impact of Upwelling on Phytoplankton Productivity in the Arabian/Persian Gulf and Sea of Oman, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20989, https://doi.org/10.5194/egusphere-egu25-20989, 2025.

The warm and saline Atlantic Water in the Nordic Seas serves as a conduit for poleward oceanic heat transport and plays  a crucial role in regulating the Northern Hemisphere climate. However, the impact of mesoscale eddies on this heat transport remains unclear, owing to a lack of in situ observations and numerous ocean modeling challenges. Our study aims to improve  the model representation of eddies and investigate their role in  oceanic heat transport in the Nordic Seas. Using a novel configuration of the MITgcm ocean-ice model,  with a resolution ranging from 1 to 4 km, we analyze 21 years of simulation. We show that oceanic heat transport anomalies are predominantly driven by velocity variations along Norwegian Atlantic Current, while lateral eddies play a significant role in leaking heat westward along a few key pathways, most notably near the Lofoten Escarpment. Further investigation on the linkage between ocean's temporal variability with the atmosphere is underway. Our study emphasizes the significant role of eddies in modulating poleward heat transport toward the Arctic by diverting heat laterally. 

How to cite: Jian, D., Zhai, X., Renfrew, I., and Stevens, D.: Oceanic heat transport along the Norwegian Atlantic Current and the role of eddies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-487, https://doi.org/10.5194/egusphere-egu25-487, 2025.

How to cite: Oglethorpe, K., Lanham, J., Reiss, R., Boland, E., and Mashayek, A.: A Dataset of Arctic Ocean Water Masses from 40 Years of Hydrographic Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1303, https://doi.org/10.5194/egusphere-egu25-1303, 2025.

State estimates like the Arctic Subpolar gyre sTate Estimate (ASTE, Nguyen et al., 2021) are powerful tools that combine observational data and numerical models to reconstruct the ice and ocean’s physical state over time. Unlike sequential data-assimilated reanalysis products, state estimates minimize misfit to a large set of various observations by adjusting model input and parameters rather than altering the model’s physical state, thereby consistently obeying physical laws and ensuring all source and sink terms can be identified. 

In this talk, I will explain the methodology behind a state estimate and present the first release of ASTE, which provides complete estimates of the Arctic sea ice and ocean states spanning 2002-2017 at a spatial resolution of about 15 km. I will highlight how ASTE has informed studies ranging from the analysis of Atlantic Water properties in the Arctic to the characterization of beneficial environmental conditions for high-latitude benthic habitats, and how ASTE’s adjoint model, i.e., the capability of running the model backwards in time to track which processes have influenced a chosen variable, provides a powerful method to unambiguously identify causal connections in the coupled Arctic system.

Towards the next release of ASTE, I will present a study of the impact of tides on Arctic sea ice, based on a higher, 3.5 km resolution version of ASTE that has been run for one full seasonal cycle, in a configuration including and excluding tides. While the study shows an overall decrease in sea ice volume in the presence of tides associated with increased vertical mixing and the upward flux of heat from deeper layers of the Arctic Ocean in line with previous findings, it also reveals an unexpected result, pointing to a new mechanism resulting in delayed sea ice melt in summer.

How to cite: Schulz, K., Nguyen, A., Pillar, H., and Heimbach, P.: The Arctic Subpolar gyre sTate Estimate (ASTE): A Gateway to Understanding Ice-Ocean Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1425, https://doi.org/10.5194/egusphere-egu25-1425, 2025.

The Arctic is a complex system in which ocean, sea ice, land, and atmosphere all interact. Poleward energy transport is crucial for climate variability in the Arctic and is controlled by atmospheric transport at the middle-high latitudes. The ocean has been rising non-uniformly under global warming. The future state of the ocean on a regional scale is uncertain. Coupled Model Intercomparison Project Phase 6 (CMIP6) provides different scenarios (SSPs 1.26, 2.45, 5.85) that give insights into this uncertainty across the chosen regions under different global warming thresholds. Here, we show that with every degree change in the global warming threshold, there is a corresponding change in the ocean dynamic sea level (DSL). The Arctic, Irish and Norwegian coasts respond at different scales under the global warming thresholds. This study provides insight into the Irish-Nordic-Arctic Sea's future state, which is necessary for policy formulation and planning.

How to cite: Eresanya, E., McCarthy, G. D., Chinta, V., and Nnamchi, H. C.:  Future projection of the Ocean Dynamic Sea Level over the Irish-Nordic-Arctic Seas under different global warming thresholds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2008, https://doi.org/10.5194/egusphere-egu25-2008, 2025.

The Arctic Ocean has increasingly drawn widespread attention in global climate change system. However, due to the high-latitude air-sea characteristics and the seasonal distribution of sea ice, the on-site marine environment surveys are more challenging than other oceans.

To understand the ice‒sea thermal dynamic processes, we built the in-situ observation dataset based on a series of international in-situ observation plans carried out in the Arctic Ocean and Chinese Arctic Research Expedition. With the support of polar icebreakers Xuelong and Xuelong-2, China has carried out a series of scientific investigations in Arctic Ocean for special phenomena, and accumulated many first-hand in-situ observations.

We used quality control and data processing methods to analyze and re-arrange the data mentioned above and obtained nearly a million thermohaline profiles from1983 to 2023. Meanwhile, a monthly climatology dataset is established with a horizontal resolution of 0.25×0.25° and 57 vertical layers. The datasets can serve as a standard reference for future observation data quality control, and can also be used to correct the thermohaline results of existing ice-ocean coupled models.

In order to evaluate the quality of the in-situ observations dataset, we selected typical water exchange areas for water mass analysis and partial thermohaline profile analysis，the result shows a significant seasonal variation and has a high quality and effectively reflects the overall hydrological characteristics of the Arctic Ocean. Meanwhile we compared the climatology datasets with WOA18, and find out there is clearly positive feedback by using Chinese Arctic Research Expedition data in the climatology datasets we built. And the thermohaline has stronger continuity and more stable structure. In the key of Chinese Arctic Research Expedition area, the analysis can reflect the high temperature Pacific water flowing into the Arctic Ocean, with a clear meridional temperature stratification, and temperature gradually decreasing from south to north.

Evaluating Ocean Heat Content (OHC) with in-situ observations climatology datasets show that the climatology dataset reflects the accurate state of the OHC, and can be used to verify and evaluate the OHC calculated from different model.

Next step, for studying the thermohaline structure of the Arctic ocean, we will use AI models for training with reanalysis data to get the prediction field by using the observation datasets we built.

How to cite: Wu, X. and Li, J.:  Construction and Evaluation of In-situ Observation Dataset and Its Climatology in Arctic Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2085, https://doi.org/10.5194/egusphere-egu25-2085, 2025.

Integrated retrieval using the optimal estimation (OE) method iteratively finds a set of geographical parameters that best match the observations. However, this method becomes more challenging over the ice surface due to the highly sensitive parameters such as sea ice concentration (SIC) and multiyear ice concentration (MYIC). In this study, a new time constraint that captures the distinct temporal characteristics of SIC and MYIC is incorporated into the OE method. The integrated retrievals, using both the original and time-constraint OE method (referred to as OE and OE-Z, respectively), were conducted based on FengYun-3D (FY-3D) microwave radiation imager (MWRI) data. Compared to other radiometer-based SIC and MYIC products, OE-Z outperforms OE, with the correlations increasing from 0.91 to 0.96 for SIC and from 0.41 to 0.49 for MYIC. The time constraint in OE-Z effectively mitigates the anomalous retrievals in SIC and MYIC, resulting in smoother and more reasonable time series than OE. Improvements in SIC and MYIC lead to enhanced simulation of surface microwave emission, thus improving the retrieval of atmospheric parameters. In comparison with the MOSAiC total water vapor (TWV) measurements, the RMSE in OE-Z reduces from 1.72 to 1.66 kg/m², and the correlation increases from 0.46 to 0.50. The simulated brightness temperature (TB) biases in OE-Z reduce from 0.71 to 0.31 K at 36 GHz and from −8.95 to −7.72 K at 89 GHz. This emphasizes the importance of imposing suitable constraints on highly sensitive parameters in integrated retrieval.

How to cite: Yan, Z., Ye, Y., Heygster, G., Zhang, X., Chen, Z., and Xiao, C.: Integrated Retrieval of Surface and Atmospheric Variables in the Arctic From FY-3D MWRI With a Time-Constraint Optimal Estimation Method, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2262, https://doi.org/10.5194/egusphere-egu25-2262, 2025.

Sea ice drift has significant impacts on climate change and navigation safety. Currently, various approaches have been employed to address quantization error and achieve subpixel precision in sea ice drift extraction using maximum cross-correlation (MCC). However, limited research has been conducted to compare these approaches. This study compares the performance of three approaches: image oversampling, subpixel similarity estimation, and the combination of both, for MCC-based Arctic sea ice drift extraction with subpixel precision at different time intervals. The research findings indicate that the combined approach of image oversampling and subpixel similarity estimation outperforms any single approach in terms of the accuracy of extracted sea ice drift. Additionally, this study provides recommended combinations of spatial resolutions (achieved through image oversampling) and subpixel similarity estimation methods for retrieving sea ice drift based on Fengyun-3D (FY-3D) Microwave Radiation Imager (MWRI) data at different time intervals.

How to cite: Wang, X. and Xu, Z.: Sub-Pixel Precision Image Matching for Sea Ice Drift Retrieval Using Maximum Cross-Correlation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3482, https://doi.org/10.5194/egusphere-egu25-3482, 2025.

The stratification of the Arctic Ocean plays a central role in regulating the impact of climate change on the Arctic. Though the stratification in the eastern Eurasian Basin halocline is known to have weakened since the 2000s, the variability over the full AW depth range in the whole Eurasian Basin has been little explored.

Our analysis aims to combine available in-situ observations to characterize the regional changes in stratification in the Eurasian Arctic Ocean over the past four decades. We find that, in both the Nansen and Amundsen basins, the variability of the temperature and salinity is most pronounced in the thermocline that separates the Atlantic Water (AW) core from the stratified halocline. This variability is affected by both warm and salty pulses entering through the Fram Strait, and by long-term trends. Positive temperature and salinity anomalies in the thermocline are associated with a destratification of the thermocline down to the AW core. In these layers, the stratification is estimated to have decreased by up to 50% across the Eurasian Arctic over the past 40 years, implying the possibility of enhanced vertical salt and heat fluxes up to the base of the halocline. In contrast, the stratification of the halocline has remained approximately constant or increased. Using a conceptual advective-diffusive model which takes into account the impact of stratification changes on vertical diffusion, we further show that the observed structure of changes is well reproduced by vertical diffusion of anomalies travelling from the Fram Strait around the Eurasian Basin. Our approach, using clustering techniques to divide the Eurasian Basin into several regions with coherent temperature, salinity and stratification profiles, provides new insights on the regional evolution of the Eurasian Arctic stratification, in particular in regions where few long-term studies are available like the Amundsen Basin.

How to cite: Challet, F., Herbaut, C., Houssais, M.-N., and Meneghello, G.: Large-scale destratification in the Eurasian Basin thermocline driving Atlantic Water shoaling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3536, https://doi.org/10.5194/egusphere-egu25-3536, 2025.

Fjord ice, that includes both sea and glacier ice, is an important part of the fjord microclimate that impacts e.g. water-atmosphere energy transfer, habitat conditions, ocean wave transformation and coastal processes. It also plays a role in ship and snowmobile operations. Understanding the trends in fjord ice extent, duration and timing aids understanding the impact of changing climate on the magnitude of natural hazards (such as coastal flooding and erosion) and improving future predictions.

Satellite images provide high-frequency large-area information on the state of the fjord ice, with Synthetic Aperture Radar (SAR) imagery being unaffected by polar night and weather conditions. Few studies have attempted automating fjord ice detection from satellite imagery, likely due to problems related to the topography influence on the sea state, mixed land/water pixels, presence of rocks and islands and wave breaking in the nearshore.

This study builds on the recent progress of Johansson et al. (2020) who adapted the semi-automated binary ice/open water classification method of Cristea et al. (2020) to Svalbard fjord environment, and Swirad et al. (2024a) who created a near-daily dataset of binary ice/open water maps at 50 m resolution for Hornsund fjord from the entire Sentinel-1 A/B dataset spanning Oct 2014 – Jun 2023. Swirad et al. (2024a) did not find direct relationships between fjord-scale ice coverage and air and water temperatures. Nonetheless, temporal peaks in ice coverage existed in March for the main basin, April for the inner bays and locally in October. The authors associated these with the arrival of pack ice from the Greenland Sea, formation of in situ fast ice and intensification of tidewater glacier calving, respectively.

Speculating that stronger relationships can be found between climate and ice coverage if fjord ice is unpacked into ‘drift ice’, ‘fast ice’ and ‘glacier ice’ we developed an algorithm that splits the ‘ice’ from the binary classification into the three classes using pixel and polygon properties such as continuity in time, location, size, shape and timing. We then explored relationships between ice, meteorological and hydrographic conditions. The dataset was also extended back to Jan 2012 using RADARSAT-2 imagery (Swirad et al., 2024b).

References:

Cristea, A., van Houtte, J., and Doulgeris, A. P.: Integrating Incidence Angle Dependencies Into the Clustering-Based Segmentation of SAR Images, IEEE J. Sel. Top. Appl., 13, 2925–2939, https://doi.org/10.1109/JSTARS.2020.2993067, 2020.

Johansson, A. M., Malnes, E., Gerland, S., Cristea, A., Doulgeris, A. P., Divine, D. V., Pavlova, O., and Lauknes, T. R.: Consistent ice and open water classification combining historical synthetic aperture radar satellite images from ERS-1/2, Envisat ASAR, RADARSAT-2 and Sentinel-1A/B, Ann. Glaciol., 61, 40–50, https://doi.org/10.1017/aog.2019.52, 2020.

Swirad, Z. M., Johansson, A. M., and Malnes, E.: Extent, duration and timing of the sea ice cover in Hornsund, Svalbard, from 2014–2023, The Cryosphere, 18, 895–910, https://doi.org/10.5194/tc-18-895-2024, 2024a.

Swirad, Z. M., Johansson, A. M., and Malnes, E.: Ice distribution in Hornsund fjord, Svalbard from RADARSAT-2 (2012-2016) [dataset], PANGAEA, https://doi.org/10.1594/PANGAEA.969031, 2024b.

How to cite: Swirad, Z., Johansson, M., and Malnes, E.: Unpacking fjord ice in Hornsund, Svalbard, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4070, https://doi.org/10.5194/egusphere-egu25-4070, 2025.

The Arctic Ocean plays an important role in the global climate system as it acts, for example, as major reservoir of anthropogenic carbon. Despite its global significance, data on physical parameters and tracers in the Arctic Ocean are still sparse and thus carbon inventory estimates only weakly constrained, for which insights into Arctic air-sea-ice gas exchange and ventilation need to be enhanced. Noble gases, with their biological and chemical inertness and constant atmospheric abundance history, fill this gap, as their concentrations in water are set by the conditions of last atmospheric contact. In light of this, water samples taken during the Synoptic Arctic Survey (SAS) expedition to the Central Arctic Ocean with the Swedish icebreaker Oden in summer 2021 (SAS-Oden 2021) at six stations from the surface to the seafloor were analyzed for their noble gas content. This first application of the full set of the five stable noble gases (helium, neon, argon, krypton and xenon) to the Arctic Ocean marks a new step towards a comprehensive understanding of Arctic Ocean dynamics.

The measured profiles show a strong influence of rapid cooling, excess air injection and brine rejection from sea ice formation, which affect the light and heavy noble gases differently, depending on their size, solubility and diffusivity. Building upon work from groundwater hydrology and extensions to cave calcites, as well as previous ocean applications of noble gases, the concepts of recharge temperatures, excess air terms and ice fractions or freezing rates are transferred to the Arctic Ocean, enabling the development of new parameterizations of the air-sea-ice exchange processes. We present two “static” model approaches, differing in the sea ice parameterization, and a “dynamic” mixed reactor-type model for two limits (steady state and quasi-steady state), resulting in different parameterizations of rapid cooling. The fit results from a least-squares regression for all four models are able to reproduce the measured concentrations both accurately and precisely and thus allow for predictions for other gases. In our study, these are the anthropogenic transient tracers sulfur hexafluoride (SF₆) and dichlorodifluoromethane (CFC-12), which were also measured during the SAS-Oden 2021 expedition and are used to determine water ages, a task for which the intitial surface saturations need to be known. We suggest a relative oversaturation of around 6% of SF₆ to CFC-12 due to the deviating impact of excess air, compatible with previous estimates from noble gas measurements.

How to cite: Hubner, C.-M., Scott, S., Arck, Y., and Aeschbach, W.: Using Noble Gases to Constrain Parameterizations of Arctic Air-Sea-Ice Gas Exchange Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4579, https://doi.org/10.5194/egusphere-egu25-4579, 2025.

Starting from May 2023, a global anomaly event led to the highest recorded sea surface temperature (SST) in history, underscoring the urgency of understanding how warming oceans impact polar and subpolar regions. Against this backdrop, our study focuses on the Sea of Okhotsk, where data from the U.S. National Snow and Ice Data Center and sea surface height measurements revealed an unprecedented, ice-free zone—measuring 50 to 80 kilometers in radius—near the Gulf of Patience (たらいかわん), east of Sakhalin Island, during Feb. – Mar. 2023. This phenomenon stands in stark contrast to observations in previous years and appears closely linked to sea surface height anomalies (SSHA). The role of such localized oceanic features, including eddies, in shaping late-spring sea ice melting patterns is of interest.

During the SOYA cruise in February 2023, National Central University (Taiwan) and Hokkaido University deployed eleven Taiwan-made drifting wave buoys. These buoys captured high-resolution data on waves, ocean currents, and sea temperatures, revealing robust mesoscale ocean eddy activity within the region. This study integrates buoy-based observations, satellite remote sensing, and numerical model outputs to explore the dynamic relationship between mesoscale eddies and the rapid formation of the ice-free zone. It is the aim to investigate how eddies influence springtime sea ice melting and distribution in the Okhotsk Sea. The preliminary findings may have implications for climate modeling, marine ecosystems, and regional socioeconomic activities, and will be shown in detail in the poster.

How to cite: Chien, H., Wang, A.-S., and Lin, L.-C.: Observations of an Emerging Ice-Free Zone in the Sea of Okhotsk during the Spring Sea-Ice Melting Period amid the 2023 Global SST Warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4756, https://doi.org/10.5194/egusphere-egu25-4756, 2025.

The near-surface wind speed in the Arctic plays an increasingly critical role in shaping local air-sea interactions and ensuring the safety of trans-Arctic shipping. However, its potential changes under a warming climate and the underlying mechanisms driving these changes remain unclear. By analyzing reanalysis data and model simulations, we demonstrate that Arctic surface wind speed has significantly increased since the 1960s, with the most pronounced acceleration occurring over the Arctic Ocean basins adjacent to the North Atlantic and the North Pacific. Historical simulations from CMIP6 models indicate that this acceleration is primarily driven by greenhouse gas induced warming, which is particularly prominent during the cold seasons. On one hand, the rapid surface warming in the Arctic disrupts the temperature inversion over sea ice, reducing atmospheric stability in the lower troposphere and enhancing thermal turbulence in the Arctic boundary layer. On the other hand, Arctic warming raises the height of the boundary layer, allowing stronger turbulence to mix high-altitude wind speed down to the surface, thereby intensifying near-surface wind speeds. Furthermore, CMIP6 models project a robust increase in Arctic NWS under various warming scenarios throughout the 21st century. This increase is especially prominent near the Kara Sea and the Beaufort Sea, with stronger wind speeds projected under higher SSP scenarios.

How to cite: Deng, K., Liu, W., Yang, S., and Chen, D.: Anthropogenic amplification of the Arctic near-surface wind speed, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4891, https://doi.org/10.5194/egusphere-egu25-4891, 2025.

Rapid changes in the polar marine environment, driven by climate change, are altering the variability of nutrient and light distribution, with significant impacts on primary producer growth. However, access to polar regions is limited, and satellite data from high-latitude areas are typically available only during the summer, complicating the acquisition of continuous in-situ data. To address this, we collected year-round chlorophyll-a (Chl-a) concentration data in polar regions using a mooring system and compared the results with reanalysis data. Unlike previous satellite-based studies that rely on surface measurements, we applied the annual vertical distribution of Chl-a to the Vertically Generalized Production Model (VGPM) to estimate annual primary production more accurately. Our findings reveal that phytoplankton exhibited a subsurface chlorophyll maximum (SCM) as sea ice retreated, with the SCM layer persisting for approximately four months—contrary to the gradually deepening SCM distribution predicted by model-based reanalysis data. This suggests that light and nutrient conditions within the SCM remained stable, supporting continuous phytoplankton growth. The estimated annual primary production, based on this vertical distribution of Chl-a, was 6.85 gC m⁻² yr⁻¹, which is more than ten times higher than estimates based on satellite data alone, highlighting significant underestimation by satellite-based approaches. Furthermore, this value was comparable to the average satellite-derived primary production of surrounding coastal and shelf areas (15.80 ± 10.65 6.85 gC m⁻² yr⁻¹). These results emphasize the importance of incorporating vertical distribution of phytoplankton and light in polar marine ecological models to enhance our understanding of carbon cycling and food web dynamics in these regions.

How to cite: Park, J., Ko, E., Lee, Y., and Yang, E. J.: Reassessing primary production in polar ocean: A novel approach using mooring systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5304, https://doi.org/10.5194/egusphere-egu25-5304, 2025.

The presence or absence of sea ice introduces a substantial perturbation to surface‒atmosphere energy exchanges. Comprehending the effect of varying sea ice cover on surface‒atmosphere interactions is an important consideration for understanding the Arctic climate system. The recurring North Water Polynya (NOW) serves as a natural laboratory for isolating cloud responses to a rapid, near-step perturbation in sea ice. In this study, we employed high-resolution Arctic System Reanalysis version 2 (ASRv2) data to estimate turbulent heat fluxes over the NOW and nearby sea ice (NSI) area between 2005/2006 and 2015/2016. The results indicate that the average turbulent heat fluxes in the polynya are about 87% and 86% higher than in the NSI area over the 10 years during the entire duration of the polynya and during polar night, respectively. Enhanced turbulent heat fluxes from the polynya tend to produce more low-level clouds. The relationship between the polynya and low cloud in winter was examined based on Cloud‒Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The low-cloud fraction (0–2 km) was about 7–34% larger over the polynya than the NSI area, and the ice water content below 200 m was about 250%–413% higher over the former than the latter. The correlation between cloud fraction and turbulent heat fluxes in the polynya peaks around the altitude of 200–300 m. These results suggest that the NOW affects the Arctic boundary layer cloudiness and structure in wintertime. Furthermore, higher horizontal resolution reanalysis data can advance our understanding of the cloud-polynya response.

How to cite: Hui, F., Ren, H., Shokr, M., Zhang, T., Zhang, Z., and Cheng, X.: Turbulent heat fluxes in the North Water Polynya and ice estimated based on ASRv2 data and their impact on cloud, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5349, https://doi.org/10.5194/egusphere-egu25-5349, 2025.

The Beaufort Gyre is an important feature of the Arctic Ocean. By accumulating or releasing freshwater, it influences ocean properties both within the Arctic and as far as the North Atlantic. Yet, its future remains uncertain: the gyre could strengthen as sea ice declines and allows increased wind stress on the ocean, or weaken along with the Beaufort High pressure system. Here, we provide a first evaluation of the Beaufort Gyre in historical and climate-change simulations from 27 available global climate models. We find that the vast majority of models overestimate the gyre area, strength, and northward extent. After discarding the models with too inaccurate a gyre and its drivers – namely, the sea ice cover and Beaufort High – we quantify changes in the Beaufort Gyre under two emission scenarios: the intermediate SSP2–4.5 and the high-warming SSP5–8.5. By the end of the 21st century, most models simulate a significant decline or even disappearance of the Beaufort Gyre, especially under SSP5–8.5. We show that this decline is mainly driven by a simulated future weakening of the Beaufort High, whose influence on the Beaufort Gyre variations is enhanced by the transition to a thin-ice Arctic. The simulated gyre decline is associated with an expected decrease in freshwater storage, with reduced salinity contrasts between the gyre and both Arctic subsurface waters and freshwater outflow regions. While model biases and unresolved processes remain, such possible stratification changes could shift the Atlantic-Arctic Meridional Overturning Circulation northward.

How to cite: Athanase, M., Köhler, R., Heuzé, C., Lévine, X., and Williams, R.: The Arctic Beaufort Gyre in CMIP6 Models: Present and Future, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5642, https://doi.org/10.5194/egusphere-egu25-5642, 2025.

Numerical models serve as an essential tool to investigate the causes and effects of Arctic sea ice changes. Evaluating the simulation capabilities of the most recent CMIP6 models in sea ice volume flux provides references for model applications and improvements. Meanwhile, reliable long-term simulation results of the ice volume flux contribute to a deeper understanding of the sea ice response to global climate change.

In this study, the sea ice volume flux through six Arctic gateways over the past four decades (1979–2014) were estimated in combination of satellite observations of sea ice concentration (SIC) and sea ice motion (SIM) as well as the Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS) reanalysis sea ice thickness (SIT) data. The simulation capability of 17 CMIP6 historical models for the volume flux through Fram Strait were quantitatively assessed. Sea ice volume flux simulated from the ensemble mean of 17 CMIP6 models demonstrates better performance than that from the individual model, yet IPSL-CM6A-LR and EC-Earth3-Veg-LR outperform the ensemble mean in the annual volume flux, with Taylor scores of 0.86 and 0.50, respectively. CMIP6 models display relatively robust capability in simulating the seasonal variations of volume flux. Among them, CESM2-WACCM performs the best, with a correlation coefficient of 0.96 and a Taylor score of 0.88. Conversely, NESM3 demonstrates the largest deviation from the observation/reanalysis data, with the lowest Taylor score of 0.16. The variability of sea ice volume flux is primarily influenced by SIM and SIT, followed by SIC. The extreme large sea ice export through Fram Strait is linked to the occurrence of anomalously low air temperatures, which in turn promote increased SIC and SIT in the corresponding region. Moreover, the intensified activity of Arctic cyclones and Arctic dipole anomaly could boost the southward sea ice velocity through Fram Strait, which further enhance the sea ice outflow.

How to cite: Ye, Y., Kuang, H., Sun, S., Wang, S., Bi, H., Chen, Z., and Cheng, X.: An assessment of the CMIP6 performance in simulating Arctic sea ice volume flux via Fram Strait, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6241, https://doi.org/10.5194/egusphere-egu25-6241, 2025.

The Arctic Ocean is undergoing rapid transformations due to the loss of sea ice, shifts in its heat budget and physical structure, and the “greening” of the polar surface ocean. These changes have profound implications for ocean biogeochemistry, the carbon cycle, and ocean acidification (OA). As part of the U.S. Synoptic Arctic Survey (SAS), we conducted a transect from the Chukchi Sea shelf to the North Pole during late summer 2022, enabling comprehensive sampling of the ocean carbon cycle in the seldom-sampled high Arctic. Discrete samples of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) were collected from CTD-hydrocasts spanning surface to deep waters, complemented by higher-frequency underway measurements of DIC, TA, and pH. These observations establish a critical baseline for tracking future changes in Arctic carbon dynamics, biogeochemistry, and acidification. Additionally, the 2022 US SAS dataset allows for comparison with earlier observations, including the 1994 Arctic Ocean Section (AOS), the 2005 Beringia expedition, and the 2015 GEOTRACES Arctic cruise. Our synthesis reveals significant and ongoing changes in the Arctic Ocean carbon cycle, including: (1) substantial uptake of anthropogenic CO₂; (2) alterations in the driving force for air-sea CO₂ exchange; (3) a decreasing capacity of the Arctic Ocean to absorb atmospheric CO₂; and (4) intensified impacts on surface pH and ocean acidification. These findings underscore the accelerating pace of carbon cycle changes in the high Arctic and highlight the importance of sustained monitoring.

How to cite: Garley, R. and Bates, N.: Arctic Ocean inorganic carbon and acidification changes from 1994 to 2022 across the Chukchi Sea to the North Pole: A US contribution to the International Synoptic Arctic Survey Program, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6782, https://doi.org/10.5194/egusphere-egu25-6782, 2025.

 The Arctic is a hot spot of climate change. Sea ice and snow, in particular, act as an insulator that prevent heat exchange between the ocean and the atmosphere and have been an important factor in mitigating temperature increases in the Arctic. However, the reduction of sea ice over the past 40 years has led to an increase in ocean-atmosphere heat exchange, contributing to Arctic Amplification. Despite its importance, obtaining observational data beneath sea ice in the Arctic during winter has been challenging due to the unique conditions of ice coverage, especially in winter. Several studies have been able to make use of recent advances in autonomous instrumentation to calculate wintertime ocean to ice heat flux (OHF). However, there remain considerable discrepancies in OHF estimates, even when examining the same time periods and research areas, primarily due to variations in calculation methods.

 In this study, we used observational data from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) to calculate OHF from October 2019 to May 2020. The observations were made by Woods Hole Oceanographic Institution Ice-tethered Profilers and Microstructure profilers drifting with sea ice along the Transpolar Drift. Here, we assess the applicability of an OHF parameterization from observational data, relying on the temperature difference between the mixed layer and the freezing temperature.

 The results in winter predominantly show negative (downward) OHF. We consider those results physically implausible, and they seem to be related to the ubiquitous presence of supercooled water in the mixed layer. When applying near surface temperature rather than freezing temperature to assess the heat content in the boundary layer, the wintertime OHF values are closed to zero until mid-March 2020. This result is in line with direct (dissipation based) measurements of OHF from the stratified ocean into the mixed layer during the same period. This study, therefore, suggests limitations in the applicability of the OHF parameterization in supercooled conditions. By opting for ocean surface temperature observations from the Arctic winter of 2019-2020, which were consistently lower than the freezing temperature, we anticipate that these refined calculation methods will yield more accurate results for assessing heat flux in future Arctic winters.

 From mid-March to early May, the OHF increased significantly, and so did the upward heat flux into the mixed layer. Our results suggest this shift occurred once the sea ice had drifted southward across the Gakkel Ridge toward Fram Strait. Analyzing the hydrographic properties of the upper ocean, we conclude that not only seasonal but also regional changes contributed to this shift.  

How to cite: Choi, Y., Kanzow, T., Rabe, B., and Reifenberg, S.: Ocean-to-Ice Heat Flux in the Central Arctic: Results from the MOSAiC Expedition (2019-2020), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6867, https://doi.org/10.5194/egusphere-egu25-6867, 2025.

Arctic eddies are important for mixing and heat exchange between sea ice and ocean. The effect carries over to the ecosystem to cause spatial patterns of primary production up to fish distribution. Strong stratification makes the Rossby radius of eddies on Arctic shelve very small resulting in spatial gradients in eddy sizes over the Arctic. Limited resolution of models in the past has been preventing correct representation. We present eddy statistics in a kilometric Arctic Ocean NEMO-SI3 model, using NEMO version 5.0 with the RK3 advection scheme and the TKE mixing scheme. The sea ice rheology is aEVP. We aim to validate the number of eddies as well as eddy sizes with available data from satellite and moorings. This simulation was done as part of the CANARI project, which includes examination of future sea ice loss impact on mixing and the possibility of accelerated sea ice decline. This work was funded by the Natural Environment Research Council (NERC) project CANARI NE/W004984/1. This work used the ARCHER2 UK National Supercomputing Service (https://www.archer2.ac.uk).

How to cite: Rynders, S., Aksenov, Y., Coward, A., and Harle, J.: First look at Arctic eddies in a kilometric NEMO5 simulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7121, https://doi.org/10.5194/egusphere-egu25-7121, 2025.

Understanding the distribution and variations of sea and glacier ice coverage is critically important for assessing both the impacts and drivers of climate change, particularly in the Arctic. Sea ice responds dynamically to both ocean and atmospheric motion, implying variability on very short timescales which is challenging to monitor. In this study we assess the ability of PlanetScope satellite imagery, offering both high spatial and temporal resolutions, to analyse temporal variability. The study area is Isbjørnhamna-Hansbukta area in north-western Hornsund, Svalbard -  a fjord characterised by both in situ formed sea ice (fast ice and drift ice broken from the fast ice e.g. by waves), pack ice drifting into the fjord from south-west with Sørkapp Current that brings cold water masses from Barents Sea, and glacier ice from calving Hansbeen. 

We selected cloud-free images over a 4.5 ✕ 4.9 km AOI, large enough to depict the spread of sea ice to ensure accuracy in the analysis. From ten images captured in 2023, we collected sample reflectance data for three categories: thin ice, thick ice, and water. Thin ice in the AOI is typically grey and grey-white sea ice (10-30 cm) as well as brash ice and growlers, while thick ice is often snow-covered young and first-year sea ice (>30cm) as well as bergy bits and icebergs. Using these data, we calculated normalised difference spectral indices for both 8-band and 4-band imagery. Coastal Blue-Green 1 and Blue-Red indices were determined to be the most effective for discriminating between the different categories, and optimum thresholds were identified. Applying these indices and thresholds in QGIS, we generated 233 maps covering the months of March to August for the years 2018 to 2023.

From  the initial visual interpretation, the results showed credible classification of the images and revealed continuous seasonal patterns for all years of the study, with minimal ice coverage observed in March, May, and July through August, a peak in sea ice coverage in April, and a resurgence of thin ice in June. However, no observable multi-year trends could be identified from a preliminary analysis of the maps, other than a sharp decline in ice coverage in 2023. Quantitative analysis of the maps allows estimates of the sea and glacier ice extent within the AOI to be made. 

This research enhances our understanding of seasonal and interannual sea and glacier ice distribution in the nearshore and coastal zone of Svalbard. These findings have the potential to inform future studies about sea ice distribution, with the PlanetScope Imagery maps to be made publicly available through the Svalbard Integrated Arctic Earth Observing System data portal at the end of the study. Future research will compare the relative advantages of PlanetScope and SAR imagery.

How to cite: Makhotina, E., Rees, G., Swirad, Z., and Tutubalina, O.: Mapping of sea and glacier ice distribution in 2018-2023 in the Hornsund fjord, Svalbard with PlanetScope imagery, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7127, https://doi.org/10.5194/egusphere-egu25-7127, 2025.

Fengyun satellites have now developed the capability to retrieve multiple polar sea ice parameters based on active and passive microwave payloads. This includes the operational production and release of four types of polar sea ice products, including the FY-3 MWRI radiometer sea ice concentration, the FY-3E WindRAD scatterometer sea ice edge and type, and the FY-3 GNOS-R sea ice thickness. The monitoring capabilities of Fengyun satellites in the polar regions are continuously improving. This study will systematically introduce the inversion and validation of polar sea ice parameters mentioned above, focusing on the research of sea ice edge and type inversion from the WindRAD scatterometer, which is the world's first dual-frequency, dual-polarization, fan-beam rotating scanning system. The release and application of operational sea ice parameter products from Fengyun satellites can further enhance the polar sea ice monitoring capabilities and provide a scientific and reliable new data source for research related to polar and global climate change, such as climate numerical models and the monitoring of extreme climate events.

How to cite: Zhai, X., Tian, S., Yin, C., Huang, K., Cao, G., Zheng, Z., Shang, J., Wu, S., Chen, L., and Hu, X.: Research Progress and Applications of Polar Sea Ice Products Based on Multi-Source Remote Sensing Payloads of Fengyun Satellites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7556, https://doi.org/10.5194/egusphere-egu25-7556, 2025.

The rise in air temperature due to global warming has significantly reduced the extent and thickness of sea ice, a phenomenon with profound implications. Sea ice loss results from complex factors, including changes in heat and momentum fluxes and internal feedbacks within the Arctic air-ocean system. This loss influences atmospheric circulation and mid-latitude weather patterns. Declining sea ice volume increases seasonal variability in ocean-atmosphere heat exchange, emphasizing the need to accurately estimate ocean heat flux at the sea ice base. Ocean heat flux, crucial for sea ice formation and melting, is challenging to measure directly. This study addresses this by using observational data to estimate ocean heat flux through the interplay of conduction (analyzed using Fourier series to reduce noise) and latent heat. The resulting Arctic buoy data map enhances predictions of sea ice dynamics.

How to cite: Hwang, I. and Moon, W.: Ocean heat flux and a buoy data map with noise eliminated, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7857, https://doi.org/10.5194/egusphere-egu25-7857, 2025.

Significant uncertainties in projections of various ocean and sea ice variables stem from a variety of sources, including different modeling approaches, imperfect representations of physical processes, and natural variability. Multi-model ensembles like CMIP6 are essential for assessing the range of uncertainty, however they rely on "model democracy," which assumes all models are equally plausible and independent of one-another.

Various constraining and weighting approaches are in use to minimize model uncertainties. Most of these approaches focus on state quantities, often relying solely on historical simulations of the target variable itself as the primary diagnostic. Here, we want to use more process-based diagnostics to incorporate physical mechanisms and interactions that govern the system dynamics. Previous assessments of the historical Arctic's energy budget in CMIP6 have shown tight connections between oceanic heat transports and key Arctic state quantities like sea ice and the ocean's warming rate, with substantial biases prevailing from the ocean to the Arctic surface. Using our new StraitFlux tools, which enable fast and precise calculations of oceanic transports for diverse climate models, we can quite efficiently incorporate oceanic transports into existing model weighting algorithms. By evaluating model performance against observational data and assessing their independence of one-another, we aim to identify and mitigate biases in Arctic projections. We use this approach to weight and constrain key Arctic variables, such as sea ice, for a large ensemble of CMIP6 models. For example, weighting the Arctic September sea ice extent ensemble reduces the spread in the first year of an ice-free Arctic and indicates a general tendency to an earlier ice-free Arctic than when using model democracy. Those results agree very well with past studies using different weighting diagnostics, demonstrating the robustness of the weighting approach. 

How to cite: Winkelbauer, S., Mayer, M., and Haimberger, L.: Constraining Arctic Climate Projections: A Process-Based Approach to Model Weighting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9085, https://doi.org/10.5194/egusphere-egu25-9085, 2025.

New hydroacoustic measurements combined with old data reveal the widespread occurrence of contourite drift deposits - indicative of persistent strong bottom currents -  at rather great water depths in the northern Baltic Sea and  Eastern Canadian coastal waters (Foxe Basin, Hudson Bay, Gulf of St. Lawrence). In addition, lag deposits suggest that strong bottom currents temporary eroded sediments most likely during the cold Little Ice Age. For example, the Little Ice Age lag deposits are found to a water depth of  ~ 300 m in Foxe Basin and to ~ 150 m in the Baltic Sea. In all ecosystems the depositional environment changed drastically with the onset of climate warming after the Little Ice Age: calm conditions prevailed leading to the accumulation of fine-grained sediments. A possible mechanism to explain the strong bottom currents during the Little Ice Age is an enhanced deep-water formation caused by accelerated convection and/or brine formation (Eastern Canadian waters) during colder winter conditions. Attempts to model the enhanced winter-time deep-water formation / convection remain inconclusive and do not match the hydroacoustic and sedimentological evidence. However, solving this issue is critical as it could allow to, e.g., reconstruct past winter temperatures based on sedimentological grain-size studies. Yet, most proxies used in paleo-oceanographic temperature reconstructions only relate to spring and summer (growing season) conditions. Our results indicate that winter temperature changes (strength and length of sea-ice season) are of critical importance for the depositional environment and bottom water ventilation in the Eastern Canadian and Baltic Sea ecosystems.

How to cite: Moros, M., Kotilainen, A., Neumann, T., Kolling, H., Papenmeier, S., Brembach, K., Lenz, K.-F., De Vernal, A., Lajeunesse, P., St-Onge, G., Kienast, S. S., Damste, J. S., Meier, H. E. M., and Schneider, R.: Strong winter-time deep-water formation during the Little Ice Age in subarctic semi-enclosed formerly glaciated marginal seas (Baltic Sea and Eastern Canadian coastal waters) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9800, https://doi.org/10.5194/egusphere-egu25-9800, 2025.

The Arctic Ocean is a hot spot of climate change, with enhanced warming and freshening of near-surface waters and a rapid decline of sea ice in recent decades. Climate model projections suggest that the Arctic Ocean may be ice-free in summer as early as 2030-2050, accompanied by an intensified seasonal cycle of sea ice characterized by earlier melting and later growth seasons. This transition will enhance interactions between the ocean, atmosphere and sea ice, likely altering the stratification of the Arctic Ocean during summer. The projected retreat of summer sea ice in the coming decades raises the question of how the seasonal cycle of the ocean may change, which is critical in regulating chemical, biological and physical processes in the region. Given the non-uniformity of sea ice loss across the Arctic, pan-Arctic averages fail to capture the spatial variability of these changes. In this study, we analyze 36 climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the SSP5-8.5 scenario to characterize regional changes in the Arctic Ocean seasonal cycle in the near future. Our results reveal an intensified seasonal cycle of sea surface temperature and a weakened seasonal cycle of sea surface salinity with significant regional variability and model dependence. Changes at depth are primarily confined to the mixed layer. By analyzing the mixed layer temperature and salinity budget for each region, we identify the key processes driving these changes. These insights enhance our understanding of the evolving seasonal dynamics of the Arctic Ocean and their broader implications in a rapidly changing climate.

How to cite: Le Gloannec, C., Msadek, R., and Lique, C.: The seasonal cycle of the Arctic Ocean in a summer ice-free climate : changes, driving processes and consequences., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10639, https://doi.org/10.5194/egusphere-egu25-10639, 2025.

Seasonal and interannual variations in Arctic Ocean stratification significantly influence the vertical exchange of heat, salt, nutrient fluxes and the surface ice cover. On the seasonal scale, Arctic stratification is mainly influenced by ice melting/freezing processes. We used a one-dimensional (1D) coupled sea ice-ocean model to understand the effects of ice melting/freezing processes on stratification and their feedback on the ice itself. This 1D model can accurately simulate observed seasonal changes in the vertical structure of the upper Arctic Ocean. Then, we prevent the model from releasing meltwater into the ocean or maintaining a constant ice cover during the melting season, in a series of decoupling experiments, which reveal the following points: In summer, meltwater has negative feedback on ice melting by insulating a portion of the solar radiation into the Near Surface Temperature Maximum (NSTM); sea ice changes primarily manifest as the well-known albedo feedback. In winter, meltwater has minimal impact in strongly stratified regions, however, in weakly stratified regions, meltwater promotes freezing by hindering the heat upward mixing from Atlantic warm water (AWW); In regions with less ice cover, if there is no meltwater to counteract the stronger mixing due to the winter atmosphere-ocean energy exchange, the AWW can mix dramatically upwards, and even melt the ice in winter. In contrast, if there is enough ice cover to insulate the atmosphere from the ocean, strong mixing will not occur, even without meltwater. The 1D-model study demonstrates that, as Arctic sea ice diminishes and Atlantification intensifies in the future, the impact of meltwater on the ice-ocean system will become increasingly significant. For multiyear scales, we utilized CIGAR historical ocean reanalysis (1961-2022) data and extensive in situ observations from the Arctic Ocean to investigate the long-term variations in Arctic Ocean stratification. The results show a strong correlation between stratification strength and freshwater content in the Arctic Ocean. However, over the past decade, while the freshwater content in the Beaufort Sea has remained regionally stable, stratification strength has shown a decline. This suggests that, with the retreat of sea ice, atmospheric energy input is becoming increasingly significant in influencing stratification.

How to cite: Zhang, H., Storto, A., Bai, X., and Yang, C.: Impacts of Seasonal and Interannual Sea Ice Changes on Arctic Ocean Stratification, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10970, https://doi.org/10.5194/egusphere-egu25-10970, 2025.

How Arctic waters end up in the North Atlantic? We have examined ocean connectivity for neutral density surfaces by developing Montgomery Potential for the NEMO ocean model. The method is coded in Python, enabling calculating geostrophic flow on pseudo-neutral density surfaces. We have analysed global NEMOv4.2 at 1/12 degree runs for the 2008-2021 period for ocean connectivity from the Arctic to the North Atlantic. We have also mapped water-mass pathways by releasing on neutral density surfaces 25.8-28.2 in the Laptev Sea, in the Denmark and Davis Straits, near the Flemish Cap and on the West European Shelf, then by tracking particles forward and backward. The transient times from the Laptev Sea to the Great Banks are of about 6 years; across the Atlantic – another 6 yrs; and the Laptev Sea to the West European Shelf is of about 16 years in total. The model transient times were compared to those from the observed Technetium spread to the Western Barents Sea and to the regions around Greenland. This presented work has been funded from the European Union's project EPOC, EU grant 101059547 and UKRI grant 10038003, EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes”, grant 101081193 and UKRI grant 10039429, and from the UK NERC projects LTS-M BIOPOLE (NE/W004933/1), CANARI (NE/W004984/1) and UK LTS-S Atlantic Climate & Environment Strategic Science –ATLANTIS. For the EU projects the work reflects only the authors' view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. We acknowledge the use of ARCHER UK National Supercomputing and JASMIN.

How to cite: Aksenov, Y., Rynders, S., Nurser, G. A. J., Megan, A., Kelly, S., and Coward, A.: Arctic to the North Atlantic connectivity using Montgomery Potential on neutral density surfaces, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11030, https://doi.org/10.5194/egusphere-egu25-11030, 2025.

How to cite: Wei, X., Wilson, C., and Bacon, S.: Arctic spin-up under melting sea ice , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12218, https://doi.org/10.5194/egusphere-egu25-12218, 2025.

Freshwater plays an important role in the Arctic Ocean, where stratification and circulation are dominated by salinity. River runoff is an important piece of the Arctic freshwater budget, and it is changing rapidly with climate change. River runoff into the Arctic Ocean has been increasing in both amount and temperature, a trend which is expected to continue into the future. We look at forcing a state of the art ocean model with future runoff projections for the Arctic Ocean, to understand how this increase in runoff temperature and flow impacts the changing Arctic. Runoff projections are produced using the A-HYPE hydrological model, over the Arctic drainage basin, giving both runoff and river temperature data. These are used to force a regional configuration of the Nucleus for European Modelling of the Ocean (NEMO) framework 4.2, with a nested 1/12 degree Arctic Ocean. As opposed to traditional methods of linearly scaling runoff for future projections, combining hydrological model output with ocean models gives a more complete spatially and temporally varying picture of runoff. Changes in river runoff has implications for sea ice futures, circulation patterns, freshwater storage and release of freshwater to lower latitudes.

How to cite: Weiss-Gibbons, T., Pennelly, C., Stadnyk, T., and Myers, P.: Future Changes in Arctic River Runoff and its Impact on the Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12510, https://doi.org/10.5194/egusphere-egu25-12510, 2025.

The pathways and time scales of Atlantic Water (AW) transport to the Arctic Ocean (AO), and its subsequent return to the North Atlantic, are critical for understanding the ocean’s role in modulating heat, salinity, and the sequestration of anthropogenic trace gases.

To quantify the time scales of AW transport by advective and diffusive processes, we applied the Inverse-Gaussian Transit-Time Distribution (IG-TTD) method, utilizing a suite of radionuclide datasets. The IG-TTD parameters—mean transit time (Γ), representing advection, and width (Δ), characterizing diffusion—were derived from radionuclides such as Iodine-129 (I-129), Technetium-99 (Tc-99), and Uranium-236 (U-236). These radionuclides originate primarily from two European nuclear reprocessing facilities. To complement observational data, idealized tracers from an ocean general circulation model (OGCM) were incorporated, including Boundary Impulse Response (BIR) tracers and dilution tracers. BIR tracers constrained the mixing ratio (Δ/Γ) in the IG-TTD, while the dilution tracer refined source functions for improved accuracy.

Preliminary results indicate a transit time of approximately 25 years from the Iceland-Scotland Ridge to the central Arctic Ocean, with mixing ratios (Δ/Γ) ranging between 0.2 and 0.4—significantly lower than the typical value of ~1 observed for CFCs/SF6 tracers transitioning from surface ventilation to the ocean interior. A dilution factor on the order of 1000 was necessary to scale source functions and avoid unrealistically high mean ages. Transit times showed substantial variability within the same region, depending on radionuclide type and sampling period, highlighting the impact of strong synoptic variability in ocean currents on measurement uncertainties. Additionally, dual-tracer constraints on mixing ratios, comparisons of transit times derived from radionuclides versus ventilation tracers, and assessments against model-simulated BIR tracers are discussed.

How to cite: He, Y., Lin, M., and Jeansson, E.: Estimated Transport of Atlantic Water to the Arctic Ocean Using Observed and Simulated Radionuclides, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13327, https://doi.org/10.5194/egusphere-egu25-13327, 2025.

In the Arctic, ocean surface waves are becoming more energetic. This is due to the larger wind fetch caused by decreased sea ice cover in summer and delayed sea ice formation in fall. These changes, driven by global climate change and regional warming, are projected to be more extreme in the future. Surface gravity waves are a key factor in coastal erosion and flooding, which are already negatively affecting coastlines in the Arctic (Casas-Prat & Wang 2020). Understanding and quantifying surface waves evolution is therefore particularly important for the communities that live along the coasts of the Canadian Arctic Archipelago (CAA), yet it has not been investigated with modeling.

We used the spectral wave model Wavewatch III® (Tolman 1997, 1999a, 2009) to simulate gravity waves formation and propagation for the entire Arctic and the North Atlantic over 2002-2022, using output from a regional 1/4° NEMO simulation. Simulations reveal a positive wave height trend in Baffin Bay and locations near the sea ice margin in the Barents, Kara and East Greenland Seas. A positive trend is found in Baffin Bay from June to October (max 0.25 m/y), where peak wave heights of 4-6 m are also observed during fall, in the second decade of the run. This highlights the importance of combined delay in seasonal sea ice formation and storm activity in the CAA, with storms more likely to produce high waves conditions during fall.

Further ongoing work will: 1) analyze the impact of waves on coastal erosion; 2) project ocean and wave conditions under CMIP6 forcing: the numerical ocean model NEMO, at 1/4° resolution, and a nested grid over the CAA will allow WW3 wave simulations to be projected over 2100.

How to cite: Pochini, E. and Myers, P.: Simulated wave evolution and coastal erosion in the Arctic and the Canadian Arctic Archipelago (2002-2022), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13701, https://doi.org/10.5194/egusphere-egu25-13701, 2025.

Arctic sea ice has undergone massive changes in the latest decades. Not only has the ice extent seen a great reduction over the satellite era, sea ice thickness is also strongly altered as a result of changing climate conditions. In this study, we look at sea ice volume in the Arctic and show how its response to increasing temperatures has changed in recent years. Using a Bayesian statistical framework, we look at reanalysis data of sea ice volume and detect changepoints in trends. We have identified an abrupt change in Arctic sea ice volume relative to global mean temperature. Spatial analysis shows that this signal of abrupt change primarily stems from loss of sea ice thickness in the Canadian Basin and Beaufort Gyre region. We compare these findings with CMIP6 Earth system models and find similar behaviour in several models. Further, we have conducted experiments with the NorESM model to better describe the mechanisms of this abrupt change, and to see how the sea ice volume behaves if global warming is later reversed.

How to cite: Steene, R. J. and Rypdal, M.: Changing trends in Arctic sea ice volume, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16814, https://doi.org/10.5194/egusphere-egu25-16814, 2025.

The ongoing rapid decline in Arctic sea ice is considered as a tipping element of our climate system. It is exposing a warmer and more acidified ocean directly to the atmosphere, permitting greater light penetration and enhanced exchange of heat, momentum, and gases across the air-sea interface. Earth system models project that these thermal and biogeochemical changes will dramatically perturb Arctic Ocean carbonate chemistry. As one of the consequences, the projections indicate that the seasonal maximum in surface ocean pCO₂ generally shifts from winter to summer during this century. Yet, it is unknown whether such biogeochemical changes in the Arctic would be reversible, if we managed to reduce atmospheric carbon dioxide concentrations. Here we analyse the reversibility of Arctic biogeochemistry changes using idealised 1pctCO2-cdr simulations from six earth system models. These model experiments simulate a 140-year period of 1% annual atmospheric CO₂ increase (rampup to 4x preindustrial levels), followed by a 140-year period of 1% annual CO₂ decrease (rampdown). Our results indicate that the present day pCO₂ cycle is largely recovered when atmospheric CO₂ returns to preindustrial levels. However, most models exhibit substantial hysteresis, particularly during summer, where surface ocean pCO₂ remains more elevated during the rampdown phase relative to the rampup phase (difference in Arctic average up to 60 𝜇atm pCO₂ for the same atmospheric CO₂ levels). Despite model differences, their projections consistently show pronounced regional variability in the pCO₂ hysteresis, with high hysteresis occurring for example in the Nordic Seas and the Barents Sea. Our results indicate that the pCO₂ hysteresis is particularly sensitive to sea surface temperature and net primary productivity, both of which show regionally varying hysteresis as well. These findings underscore the complex impacts of Arctic sea ice loss on biogeochemical cycles, emphasising the importance of accounting for hysteresis in CO₂ overshoot scenarios and climate mitigation strategies.

How to cite: Köhn, E. E., Kwiatkowski, L., Orr, J. C., Gastineau, G., and Mignot, J.: How reversible are carbonate chemistry changes triggered by future Arctic sea ice loss?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17101, https://doi.org/10.5194/egusphere-egu25-17101, 2025.

In the Arctic, where the effects of the changing climate are occurring faster than anywhere else on Earth, warming, sea-ice decline and changes in ocean circulation have already resulted in an overall increase of the marine primary productivity. According to global climate projections, the increased productivity is expected to continue in this region due to greater open-water habitats and larger growing seasons. Significant shifts in phytoplankton composition and an increasingly unstable community structure are also expected through the 21st century in response to climate change. Nevertheless, high uncertainties still exist in future net primary productivity (NPP) and the overall response of phytoplankton to climate change in the Arctic and subarctic regions.

This study assesses the effect of changing physical characteristics in the Nordic and Barents Seas on nutrient distribution and phytoplankton dynamics over the 21st century using the high-resolution NORWegian ECOlogical Model system (NORWECOM.E2E). The results show two distinct pathways of the phytoplankton response, differentiating Arctic conditions (i.e., Barents Sea) and Atlantic conditions (i.e., Nordic Seas). The Barents Sea, a shallow and well-mixed basin with persistent nutrient supply from the deep ocean to the surface, experiences a gradual intensification of the phytoplankton blooms towards the end of the century. This response is consistent with increasing temperatures, sunlight availability due to reduced sea-ice extent and the intensification of the vertical mixing.

In contrast, the Nordic Seas experience an abrupt change in the phytoplankton dynamics, with a sudden shift in the phytoplankton communities from a diatom-dominated to a flagellate-dominated bloom, according to the simulations. The rapid change in phytoplankton bloom dynamics is caused by an interplay between a shallowing mixed layer depth and changing nutrient consumption patterns by phytoplankton. These changes are consistent across climate scenarios SSP2-4.5, SSP3-7.0 and SSP5-8.5. However, the timing and magnitude of the changes vary significantly, with SSP3-7.0 showing the most abrupt changes.

As Arctic conditions continue at an accelerated pace, major implications for local and regional ecosystems are expected. These impacts will, most probably, not be limited to the Arctic region given its crucial role in the Earth’s system. Changes in phytoplankton bloom dynamics have the potential to impact the global carbon cycle by altering primary productivity and carbon export into the deep ocean, ultimately affecting the global climate.

How to cite: Gutierrez-Loza, L. and Lauvset, S. K.: Changes in phytoplankton bloom dynamics in the future Arctic Ocean from a Regional Ecological Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17889, https://doi.org/10.5194/egusphere-egu25-17889, 2025.

The Arctic Ocean is projected to become ice-free by the middle of the century, accompanied by changes in freshwater input, stratification, and warming of the upper ocean. The marine ecosystem is predominantly influenced by the availability of light and nutrients for phytoplankton, which form the base of the food web. With the projected changes of the physical environment throughout the course of the century, CMIP6 models suggest a general increase in Arctic net primary production. It is anticipated that phytoplankton shift from a light-limited state to nutrient limitation across large areas of the Arctic Ocean, potentially leading to increased exudation of organic carbon into the water column.

We briefly discuss the mechanisms driving the dynamics of organic carbon in the upper Arctic Ocean before focusing on long-term trends in Arctic biogeochemistry projected until 2100. Using an ocean general circulation sea-ice biogeochemistry model based on the IPCC Shared Socio-economic Pathway high-emission scenario SSP3-7.0, we observe regionally varying increases in exuded organic carbon, alongside enhanced formation of particulate organic carbon in the upper water column. These particles can either be transferred from the ocean to the atmosphere, acting as precursors to primary marine organic aerosols, or sink in the water column, contributing to carbon export. Our findings align with other recent studies, showing a shift from light to nutrient limitation in phytoplankton growth, particularly in regions experiencing retreat of the marginal ice zone. Our simulation indicates that diatoms are the primary contributors to organic carbon exudation and subsequent particle aggregation. However, some regions do not exhibit an overall increase in particulate organic carbon due to elevated remineralization rates. Overall, our projection provides an assessment of the impact of changes in the physical environment on phytoplankton dynamics and, consequently, on organic carbon pools in the upper Arctic Ocean. This work is part of the DFG Transregional Collaborative Research Centre 172 on Arctic Amplification.

How to cite: Zeising, M., Oziel, L., and Bracher, A.: Projected Increase of Phytoplankton Carbon Exudation and Particle Formation in the Arctic Ocean until the End of the Century, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18826, https://doi.org/10.5194/egusphere-egu25-18826, 2025.

Atlantification—the northward inflow of anomalous waters and biota from the Atlantic into the polar basins—has wide-ranging climatological ramifications.  Sustained observations demonstrated that, contrary to the global climate model projections, atlantification has already advanced into the Amerasian Basin of the Arctic Ocean, having a significant impact on the physical and ecological components of the climate system. The primary example is the rapidly diminishing sea ice in the Siberian Arctic Ocean (SAO), which is caused by the weakened ocean stratification and amplified heat fluxes. These sea ice thickness anomalies caused by atlantification persist across the Arctic region and are prevalent along the entirety of the Transpolar Drift. Furthermore, we observe the transition of the central SAO to conditions resembling those in the eastern SAO 5-7 years ago and the emergence of a powerful ocean-heat/ice-albedo feedback, which accelerates sea-ice losses. The eastern SAO is still strongly stratified but collaborative international observations demonstrate that the atlantification-driven shoaling of warm, salty, and nutrient-rich intermediate waters already has important ecological consequences there. Disentangling the role of atlantification in multiple and complex high-latitude changes should be a priority in future modeling and observational efforts.

How to cite: Polyakov, I.:  Atlantification advances into the Amerasian Basin of the Arctic Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20584, https://doi.org/10.5194/egusphere-egu25-20584, 2025.

Recent studies of Arctic Amplification (AA) suggest that the Arctic has been warming between three to four times faster compared to the global average. The loss of sea ice in the Arctic has been one of the most evident manifestations of the warming climate over the past several decades. This decline has been most pronounced in the Barents/Kara seas during winter and in the western Arctic during summer

Changes in the Arctic sea ice cover can be both a cause and a consequence of anomalous atmospheric and oceanic warming. In the case of the winter trend, some earlier studies have suggested that factors other than atmospheric forcing, e.g., ocean heat transport and storage, are responsible for the observed sea ice retreat. Moreover, results from models participating in Phase 6 of the Coupled Model Intercomparison Project (CMIP6) suggest an emergent constraint linking oceanic heat convergence to declining sea ice cover in the Arctic Ocean. At the same time, significant biases in individual simulated sea ice states persist, resulting in the continued large CMIP model spread. The limited skill in the historical simulations of the Arctic climate system hinders the interpretation of their results and affects the reliability of their future projections.

In this presentation, we will address some of these limitations, focusing on the importance of oceanic heat transport from the Nordic Seas, its convergence and impact on the sea ice over the Barents Sea, and the remaining outflow into the central Arctic. Results from the Regional Arctic System Model (RASM), at varying spatial resolutions and forced with an atmospheric reanalysis or fully coupled, will be evaluated to demonstrate a relatively wide range of the simulated volume fluxes into the Barents Sea. Apparent coupled linkages between oceanic volume and heat fluxes, sea ice cover, and the oceanic heat convergence over the Barents Sea will be demonstrated. The importance of spatial resolution in representing some critical processes related to ocean mesoscale, sea ice characteristics, and air-sea coupling in the region will be discussed. Finally, the need for expanded long-term measurements to reduce uncertainties in the observational estimates of oceanic fluxes in and out of the Barents Sea will be rationalized.

How to cite: Maslowski, W., Lee, Y., Osinski, R., Clement Kinney, J., and Seefeldt, M.: On the importance of air-sea ice-ocean coupling in the Barents Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20633, https://doi.org/10.5194/egusphere-egu25-20633, 2025.

The annual sea ice minimum extent in the Arctic Ocean has decreased almost two-fold since the advent of satellite observations in the 1970s, leaving more open water before the fall freeze-up.  Here, we leverage a combined dataset from the 2022 NASA Salinity and Stratification at the Sea Ice Edge (SASSIE) field program to elucidate the central hypothesis that drove SASSIE: Does surface salinity stratification due to sea ice melt, precipitation, and riverine inputs lead to changes in the rates or extent of autumnal sea ice advance? The SASSIE study region in the Beaufort Sea is stratified both by melting sea ice in the summer and riverine discharge. We leverage measurements of oxygen isotopes as well as colored dissolved organic matter (CDOM) to trace the origins of fresher water at the surface.

In addition to an in-depth analysis of in situ data, we use the General Ocean Turbulence Model (GOTM) for individual profiles as well as the Regional Ocean Modeling System (ROMS) initialized and forced with observations from the SASSIE field campaign. These observations include temperature and salinity from the salinity snake instrument at 1-2cm depth, shipborne thermosalinograph (4m) and underway conductivity-temperature-depth (uCTD) measurements (5-100m), acoustic Doppler current profiler (ADCP) data, as well as meteorological and net heat flux observations. In realistically forced runs, we re-create the observations during the month-long cruise. We then modify the stratification to both increase and decrease salinity stratification to assess the importance of salinity stratification on the autumnal sea ice advance. We compare these model outputs to satellite-derived freeze-up data as well as in situ observations from autonomous platforms in the area. Preliminary results show a strong control of salinity on rapid sea ice advance, in which areas that are highly stratified freeze significantly faster than areas of deeper or weaker stratification.

Based on this hypothesis, we present a novel way of modelling the autumnal Arctic Sea Ice advance using a Convolutional Long-Short-Term Memory (LSTM) Neural Network model. In this machine learning approach, we demonstrate that the inclusion of the experimental merged salinity OISSS v3 dataset derived from the Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites significantly improves forecast accuracy of sea ice concentration in our study area, which encompasses the East Siberian, Chukchi, and Beaufort Seas. The model is based on 8 years of training data and tested using 3 years of evaluation data. Using this 60-day forecast, we show that the spatial forecasting pattern of sea ice concentration is significantly improved. This is further illustrated in an ablation study, in which we find sea surface salinity to be the 4^th most important predictive term after sea surface temperature, net heat flux, and sea ice concentration.

Through these studies, we show the connection between the terrestrial water cycle, oceanic freshwater fluxes, and sea ice formation in the Arctic, and present a novel technique of sea ice prediction that will become increasingly useful as the Arctic becomes more ice free.

How to cite: Schanze, J., Springer, S., Anderson, J., Town, M., Lim, E. Q., Treadwell, D., Zhou, Z., Zhou, S., and Melnichenko, O.: The Effects of Salinity and Stratification on Rapid Sea Ice Advance in the Arctic Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21413, https://doi.org/10.5194/egusphere-egu25-21413, 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 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.

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 35^oN, while it achieves around 25 Sv at 55^oN 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 26^oN (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.

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.

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 20^th 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.

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 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.

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 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 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.

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.

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 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 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.

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.

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 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.

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 1/20^o 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 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 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.

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 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 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.

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 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 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.

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 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 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.

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.

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 95^th 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, 95^th 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.

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.

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.

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.

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 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 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.

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.

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.

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 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.

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.

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.

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 (σ_θ > 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 (σ_θ > 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 (σ_θ > 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 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.

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.

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.

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.

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 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.

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.

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.

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.

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.

The eastern equatorial Atlantic hosts a productive marine ecosystem that depends on upward supply of nitrate, the primary limiting nutrient in this region. The annual productivity peak, indicated by elevated surface chlorophyll levels, occurs in the Northern Hemisphere summer, roughly coinciding with strengthened easterly winds. For enhanced productivity in the equatorial Atlantic, nitrate-rich water must rise into the turbulent layer above the Equatorial Undercurrent. Using data from two trans-Atlantic equatorial surveys, along with extended time series from equatorial moorings, we demonstrate how three independent wind-driven processes shape the seasonality of equatorial Atlantic productivity: (1) the nitracline shoals in response to intensifying easterly winds; (2) the depth of the Equatorial Undercurrent core, defined by maximum eastward velocity, is controlled by an annual oscillation of basin-scale standing equatorial waves and (3) mixing intensity in the shear zone above the Equatorial Undercurrent core is governed by local and instantaneous winds. The interplay of these three mechanisms shapes a unique seasonal cycle of nutrient supply and productivity in the equatorial Atlantic, with a productivity minimum in April due to a shallow Equatorial Undercurrent and a productivity maximum in July resulting from a shallow nitracline coupled with enhanced mixing.

How to cite: Brandt, P., Körner, M., Moum, J. N., Roch, M., Subramaniam, A., Czeschel, R., Krahmann, G., Dengler, M., and Kiko, R.: Seasonal productivity of the equatorialAtlantic shaped by distinct wind-drivenprocesses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-337, https://doi.org/10.5194/egusphere-egu25-337, 2025.

Variability of sea surface temperature (SST) in the north tropical Atlantic (NTA) exerts a substantial impact on Atlantic hurricane activity. Referred to as the NTA mode, its positive phase features warm SST anomalies, conducive to increased intensity and frequency of North Atlantic hurricanes. The period 2023-2024 saw two consecutive positive NTA events, featuring a broad warm anomaly pattern in 2024 following the 2023/24 strong El Niño, but a localized SST warm anomaly in the coastal region off northwest Africa in 2023 following a La Niña. Whether there exists inherent diversity in NTA dynamics and impact is unclear. Here we find that the NTA possesses two distinctive flavors: the basin-wide (BNTA) mode and coastal (CNTA) mode. Such diversity is underpinned by an asymmetric response of air-sea heat flux at the SST anomaly centers of the two NTA modes. The BNTA has an overall stronger impact on Atlantic hurricane activity due to its more westward and persistent warm anomaly pattern. Furthermore, since 1990s, the well-known impact from El Niño-Southern Oscillation on the north tropical Atlantic is felt through its influence on the BNTA mode. Our finding highlights the importance of distinguishing and understanding NTA flavors in assessing and predicting their climatic impacts.

How to cite: Liu, Y., McPhaden, M., Cai, W., Zhang, Y., Zhao, J., Nnamchi, H., Lin, X., Li, Z., and Yang, J.-C.: Two flavors of north tropical Atlantic climate variability with distinct  impact on Atlantic hurricanes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2963, https://doi.org/10.5194/egusphere-egu25-2963, 2025.

The Atlantic Niño is the primary interannual variability mode in the tropical Atlantic, with far-reaching impacts on global climate. A recent study identified two types of the Atlantic Niño, each with its maximum warming centered in the central and eastern equatorial Atlantic, respectively. Through analysis of observational data and numerical model experiments, we find that the two Atlantic Niño types have distinct climatic impacts on Europe. This is because the central Atlantic Niño is associated with a pronounced increase in precipitation in the western tropical Atlantic, while the positive precipitation anomalies during the eastern type are mainly located in the eastern basin with weaker amplitudes. Consequently, compared to the eastern Atlantic Niño, the extra-tropical atmospheric waves and the associated precipitation and temperature anomalies in Europe during the central type are stronger and shifted westward. Therefore, distinguishing between the two Atlantic Niño types may help improve seasonal climate predictions in Europe.

How to cite: Chen, B., Zhang, L., and Wang, C.: Distinct Impacts of the Central and Eastern Atlantic Niño on the European Climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3073, https://doi.org/10.5194/egusphere-egu25-3073, 2025.

The Atlantic Niño/Niña is a dominant climate variability, exerting substantial climate impacts. Besides interannual variability, the observed Atlantic Niño/Niña also demonstrates robust variations at decadal timescale (decadal ATL). The underlying mechanisms, however, remain unclear. Here, we conduct a 300yr picontrol experiment using CESM that reasonably captures mean climate of the Atlantic cold tongue and decadal ATL. A warming of the Atlantic cold tongue weakens St. Helena anticyclone via triggering atmospheric Rossby wave, which decreases the subtropical cell and suppresses the equatorial upwelling, amplifying the initial warming. Meanwhile, the weakened anticyclone enhances wind speed over the southwestern Atlantic and cools local SST. Such cooling propagates with mean current toward east, driving an eastward propagation of negative wind stress curl anomalies and thus a cooling along thermocline over 5S-12S, with a cross basin time of 6yr. This cooling is further advected with mean current at thermocline to reach the equator, after which it develops following the Bjerknes feedback and shifts the phase of decadal ATL.

How to cite: Yang, Y., Wu, L., Wang, H., Chen, Y., and Yang, C.: On the mechanisms of Atlantic Niño/Niña decadal variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4268, https://doi.org/10.5194/egusphere-egu25-4268, 2025.

The rainfall variabilities of the West African and South American summer monsoons, pivotal for local and global climate systems, are strongly influenced by tropical Atlantic sea surface temperature anomalies. This study investigates the impacts of two recently identified Atlantic Niño types, central and eastern Atlantic Niño (CAN and EAN), on these monsoon systems using observational data and numerical experiments. During boreal summer, EAN events exhibit increased rainfall over West Africa compared to CAN events, indicating a strengthened West African summer monsoon. Enhanced moisture flux convergence from eastern Atlantic warming drives these wetting conditions during EAN events. Conversely, CAN events have a more pronounced influence on South American monsoon rainfall during austral summer, causing a rainfall anomaly dipole between the Amazon and eastern Brazil, suggesting an eastward shift in the South American summer monsoon rainfall belt. These rainfall changes are linked to cyclonic circulation anomalies over the southwest Atlantic Ocean, attributed to central Atlantic warming during CAN events. Furthermore, a statistical model assesses hindcast skills of rainfall variability in the two summer monsoon regions, affirming the benefits of separating Atlantic Niño into CAN and EAN events for improved seasonal climate predictions.

How to cite: Xing, W., Wang, C., and Zhang, L.: Influences of Central and Eastern Atlantic Niño on the West African and South American Summer Monsoons, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4950, https://doi.org/10.5194/egusphere-egu25-4950, 2025.

Atlantic Niño, the dominant climate mode in the equatorial Atlantic, is known to remotely force a La Niña-like response in the Pacific, potentially affecting seasonal climate predictions. Here, we use both observations and large-ensemble simulations to explore the physical mechanisms linking the Atlantic to the Pacific. Results indicate that an eastward propagating atmospheric Kelvin wave from the Atlantic, through the Indian Ocean, to the Pacific is the primary pathway. Interaction of this Kelvin wave with the orography of the Maritime Continent induces orographic moisture convergence, contributing to the generation of a local Walker Cell over the Maritime Continent-Western Pacific area. Moreover, land friction over the Maritime Continent dissipates Kelvin wave energy, affecting the strength of the Bjerknes feedback and thus the development of the La Niña-like response. Therefore, improving the representation of land–atmosphere–ocean interactions over the Maritime Continent may be fundamental to realistically simulate Atlantic Niño's impact on El Niño-Southern Oscillation.

How to cite: Liu, S., Chang, P., Wan, X., Yeager, S. G., Richter, I., and Zhang, R.: Role of the Maritime Continent in the remote influence of Atlantic Niño on the Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5072, https://doi.org/10.5194/egusphere-egu25-5072, 2025.

The Atlantic Niño controls the boreal summer tropical Atlantic variability at interannual time scales, with pronounced climate impacts in adjacent and remote areas. Changes in the spatial configuration of the Atlantic Niño has been reported during the observational record, coinciding with a modification of the background state and associated teleconnections. The driving mechanisms of the Atlantic Niño have been also changed in recent decades.

The aim of the present study is to explore the role of the ocean background state in the Atlantic Niño diversity and associated air-sea mechanisms. For such purpose, we will use two twin 30-year high-resolution simulations performed in the H2020-EU NEXTGEMS project. Both simulations have the same horizontal resolution (10km) and only differ in the vertical stratification of the upper 20m: 2m layers for the THIN simulation and 10m layers for the THICK one.

To this aim, the Bjerknes feedback and ocean wave propagation are analyzed, and a complete heat budget analysis will be computed and compared in both simulations. Finally, the role of the background state in the modification of air-sea interactions and thus, in Atlantic Niño diversity will be also investigated.

How to cite: Martín-Rey, M., Rodríguez-Fonseca, B., Losada, T., Prigent, A., Polo, I., Abi, A., Mohino, E., Montoya-Carramolino, L., Calvo-Miguélez, E., Wu, J., and Putrasahan, D.: Driving mechanisms of Atlantic Niño under different vertical ocean resolutions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8915, https://doi.org/10.5194/egusphere-egu25-8915, 2025.

How to cite: Deppenmeier, A.-L., Bryan, F., Kessler, W., and Thompson, L.: How alike are diabatic processes in the tropical Atlantic to the Pacific? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9323, https://doi.org/10.5194/egusphere-egu25-9323, 2025.

The upwelling region off northwest Africa exhibits pronounced seasonal variability and high productivity, playing a critical role in supporting fisheries. The sea surface temperature (SST) difference between the coast and offshore areas serves as a key proxy for upwelling intensity. Using observational data, we found distinct regional dependencies in the response of SST differences to atmospheric forcing. In the permanent upwelling region (21°N-30°N), both upwelling-favorable winds and heat flux enhance the coastal-offshore SST difference, leading the variations by about 70-100 days. In contrast, in the seasonal upwelling region (12°N-19°N),  changes in SST differences precede wind variations by less than one month, particularly during the transition to the downwelling season. Heat flux in this region acts to dampen SST gradients, contrasting with its role in the permanent upwelling zone. Additionally, our results indicate that the response of the SST difference to atmospheric forcing is faster and stronger when the mixed layer is shallower. These results highlight the spatial variability and complexity of air-sea interactions in the northwest African upwelling system, with implications for understanding coastal upwelling dynamics and informing fisheries management.

How to cite: Chen, L. and Juricke, S.: Seasonal cycle of sea surface temperature and air-sea interactions in the Northwest African upwelling region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11504, https://doi.org/10.5194/egusphere-egu25-11504, 2025.

El Niño-Southern Oscillation (ENSO) is one of the most globally relevant modes of climate variability, playing a crucial role for tropical and extratropical seasonal predictions. During certain decades, specifically in the early and late 20th century, ENSO is coupled with the Atlantic Niño. Since the Atlantic Niño exhibits its maximum variability during the boreal summer (JJA), while ENSO peaks in winter (DJF), it has been shown that the Atlantic Niño can act as a predictor for ENSO in certain decades. This linkage operates on an interannual scale by alterations in the Atlantic Walker cell and, at decadal scales it has been related with changes in certain patterns, such as the Atlantic Multidecadal Variability (AMV), and an increase in pantropical oceanic variability. Nevertheless, further research on the mechanisms of this connection is needed.

This work analyzes this Atlantic-Pacific connection in SEAS5-20C, as well as the decadal and interannual mechanisms that underpin this connection. Furthermore, it discusses the influence of this connection on the decadal variability of ENSO and Atlantic Niño predictive skill. It is found that decadal changes in tropical basin interactions coincide with changes in the predictability of the tropical Atlantic and Pacific. These findings reveal how the connection between tropical basins is associated with improvements in ENSO and Atlantic Niño predictions.

How to cite: Robles Fernández, A. J., Rodriguez-Fonseca, B., Losada Doval, T., Weisheimer, A., and Alonso Balmaseda, M.: Impact of the equatorial Atlantic on ENSO prediction in SEAS5-20C re-forecast, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11559, https://doi.org/10.5194/egusphere-egu25-11559, 2025.

The purpose of this work is to reveal structural features of waters in the tropical Atlantic in the deep and intermediate layers. Based on the data set expanded in recent years, the content of deep and intermediate waters was calculated from conservative chemical variables.

The work includes data obtained from 1873 to 2023 (GLODAPv2.2022, eWOCE, WODB18 databases). Data from expeditions of the MSU Faculty of Geography from 2019 to 2023 were also used.

The following parameters were used to calculate the water mass content:

Results:

1) Broecker calculated the fraction of deep water in the Atlantic using the PO₄*. It was found that the best agreement with the content calculated by PO₄* was shown by the PO parameter with a deviation of 5-10%.

a)  b)   

Fig.1. North Atlantic Deep Water (NADW) distributions calculated by PO₄*(a), PO (b).

2) Deep water contents calculated using PO₄* on the sections were compared with water mass boundaries determined mainly using hydrophysical parameters.

NADW in the western tropical Atlantic is divided into three components: Upper NADW, Middle NADW and Lower NADW. It was found that in most of the analyzed sections, the lower boundaries of MNADW and LNADW practically coincide with the isolines of 85% and 60% of the NADW content.

In addition to large gradients of hydrophysical characteristics, the upper boundary of Antarctic Bottom Water (AABW) is determined by the Si/P=33 ratio (Arzhanova, Artamonova, 2014). In the western Atlantic it most often passes along the isoline of 25% AABW content, in the eastern Atlantic - along the isoline of 15% AABW content.

3) The distribution of AABW is of particular interest because it is transformed as it flows from the western basin to the eastern basin through the Mid-Atlantic Ridge faults. It was decided to refer to the transformed AABW as Northeast Atlantic Bottom Water (NEABW). It has been shown that NEABW is 50% composed of waters entering the eastern Atlantic through the Vema Fracture Zone, and 30% of these waters are “pure” AABW.

4) The PO parameter was used to determine the fraction of Antarctic Intermediate Water (AAIW) and Mediterranean Water (MW):

Fig. 2. Examples of obtained distributions of intermediate waters.

This work was supported by the Russian Science Foundation grant № 23-17-00032.

References:

1) Broecker et al. Radiocarbon decay and oxygen utilization in the Deep Atlantic Ocean // Global geochemical cycles. 1991. V 5, №1. Pp 87-117.

2) Broecker W. "NO" a conservative water-mass tracer // Earth and Planetary Science Letters. V 23. Pp 100-107.

3) Broecker et al. Sources and Flow Patterns of Deep-Ocean Waters as Deduced From Potential Temperature, Salinity, and Initial Phosphate Concentration // J. Geophys. : Oceans.1985. V 90, № C4. Pp 6925-6939.

4) V. Arzhanova, K. V. Artamonova. Hydrochemical structure of water masses in areas of the Antarctic Krill (Euphausia Superba Dana) fisheries // Proceedings of VNIRO. 2014. V 152. Pp. 118-132.

How to cite: Samborskaia, I. and Demidov, A.: Structure of intermediate and deep waters in the tropical Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13437, https://doi.org/10.5194/egusphere-egu25-13437, 2025.

Submesoscale coherent vortices (SCVs) have been frequently observed in the eastern tropical Atlantic (between 12°S and 12°N) based on moored and shipboard observations. They are located well below the mixed layer with no surface signature and, thus, undetectable by remote sensing making in-situ observations and modeling indispensable. The SCVs persist and are relatively long-lived and coherent, despite the increasing suppression of geostrophic balance and the rapid change in the Coriolis parameter (ß-effect) near the equator. These factors typically suggest predominant wave-like structures in this region. Additionally, the energetic zonal current system, which stretch and shear the vorticity fields, further complicate the formation of closed vortex structures. Ship-based oxygen measurements conducted in the area between 6°-12°N, 24°-18°W reveal that approximately two-third of these SCVs are associated with low oxygen cores with dissolved oxygen concentrations less than 60 µmol kg^-1 (minimum 40 µmol kg^-1). These values are significantly lower than the climatological averages for this depth range (> 80 µmol kg^-1). Both, observed water mass characteristics and the analysis of an eddy-resolving ocean-biogeochemistry model indicate that the majority of SCVs originate from the eastern boundary and may last for longer than half a year. While propagating westward into a higher potential vorticity environment, anticyclonic SCVs with a low PV core are more effectively isolated and feature longer life times than cyclonic SCVs with a high PV core. The vertical structure of the dominating anticyclonic SCVs is characterized by higher baroclinic modes 4-10, associated with a Rossby radius of 34 -13 km respectively, which is in agreement with the observed eddy radius and well below the 1^st baroclinic Rossby radius of deformation in the region (> 100 km). This study does not only increase our understanding of submesoscale dynamics in equatorial regions, but also how SCVs contribute to the formation of hypoxic zones in the open ocean due to their association with low-oxygen extremes. These hypoxic regimes have the potential to directly impact pelagic fish, biodiversity, and biogeochemical cycles.

How to cite: Schuette, F., Hahn, J., Frenger, I., Bendinger, A., Dilmahamod, F., Schulz, M., and Brandt, P.: Tropical low oxygen extreme events caused by persistent submesoscale coherent vortices, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13553, https://doi.org/10.5194/egusphere-egu25-13553, 2025.

Tropical Atlantic Variability (TAV) exerts a significant influence on the climate of different regions. Understanding these teleconnections and their impacts can improve predictability, particularly in the North Atlantic-European (NAE) region. The Atlantic Niño (ATLN) or Equatorial Mode is known for being the dominant pattern of TAV. This study aims at exploring the atmospheric response to winter ATLN, as it has been much less documented than the summer ATLN. Coupled simulations and atmosphere-only experiments with the CMIP6 version of the climate model EC-EARTH (T255L91) have been performed and analysed to revisit the ATLN-NAE teleconnection and further improve process understanding. The coupled simulation consists in a 250-year long integration, after spin-up, with fixed radiative forcing at present conditions; the atmospheric response is estimated by linear regression onto the winter ATLN index defined by Okumura&Xie. The atmosphere-only experiments comprise two 150-year long integrations keeping again the radiative forcing fixed, a control run with climatological SSTs and a sensitivity run prescribing the observed ATLN with climatology elsewhere; the atmospheric response is evaluated by comparing both experiments. Results show a local Gill-type structure, symmetrically straddling the equator, whose amplitude increases from November-December to January-February. In the extratropics, the upper-tropospheric circulation displays a dipolar structure with cyclonic anomalies at mid-latitudes and anticyclonic anomalies at subpolar latitudes, which is different from the North Atlantic Oscillation (NAO). The associated precipitation anomalies show a robust and approximately-linear signal on the European continent.

How to cite: Gil Reyes, L., García-Serrano, J., and Kucharski, F.: Teleconnection of the winter Atlantic Niño to the North Atlantic-European atmospheric circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16700, https://doi.org/10.5194/egusphere-egu25-16700, 2025.

Westerly wind anomalies over the equatorial Atlantic and Pacific play an important role in initiating the Atlantic Zonal Mode (AZM; also known as Atlantic Niño) and El Niño/Southern Oscillation (ENSO). Both the AZM and ENSO tend to develop in boreal spring and may interact by influencing the Walker circulation. For example, it has been suggested that a negative AZM event can contribute to the development of El Niño in the Pacific. Conversely, it has also been suggested that an early developing El Niño event can trigger a negative AZM event. Disentangling these causalities has proven difficult because both variability patterns tend to develop around the same time.

Here we use multi-centennial atmosphere-only simulations with prescribed climatological SSTs to evaluate the role of atmospheric internal variability as a precursor to both the AZM and ENSO. Despite the absence of SST anomalies, the simulations produce substantial wind variability over the equatorial basins, highlighting the role of internal atmospheric variability. Composite analysis indicates that westerly wind events over the equatorial Atlantic are accompanied by easterly anomalies over the maritime continent and western equatorial Pacific. These anomalies are preceded by a North Atlantic Oscillation (NAO)-like pattern during later winter. Our results suggest that atmospheric internal variability tends to produce opposite-signed wind anomalies over the western equatorial Pacific and Atlantic. This could mean that an external factor, such as the NAO, seeds the development of the AZM and ENSO. Their further evolution will be determined by interbasin interaction but also by the pre-conditioning of the two basins. To what extent such processes act in nature awaits further investigation.

How to cite: Richter, I.: Extratropical precursors to the Atlantic Zonal Mode and ENSO, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16986, https://doi.org/10.5194/egusphere-egu25-16986, 2025.

The Canary upwelling system, located along the Northwest African coast between approximately 10ºN and 35ºN, is among the most productive marine ecosystems globally. It supports rich marine biodiversity and sustains economically significant fisheries. Notably, the coastal regions off Mauritania and Senegal (9ºN–22ºN), comprising the southern part of this system, exhibit pronounced interannual variability in net primary production (NPP). This variability is influenced by extreme warm and cold events, known as Dakar Niños and Niñas, respectively. In this study, we analyze the physical mechanisms driving the interannual variability of NPP from 2003 to 2022, using a combination of satellite observations, reanalysis data, and ocean model outputs. Our results indicate that the interannual variability of NPP is closely linked to changes in sea surface temperature (SST), with the most pronounced effects occurring during February-March-April, i.e. the main upwelling season. A total of six previously undocumented episodes of strong anomalous coastal high and low NPP were identified, nearly all of which are associated with Dakar Niños and Niñas. Our findings suggest that these events are linked to both local and remote forcing mechanisms. The local forcing is associated with variations of the coastal alongshore winds. The remote forcing involves the propagations of coastal trapped waves, triggered by wind fluctuations in the Gulf of Guinea, or by wind-forced equatorial Kelvin waves originating in the western-to-central equatorial Atlantic. Additional remote influences may stem from large-scale climate modes, including the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Atlantic Meridional Mode (AMM).

How to cite: Imbol Koungue, R. A., Prigent, A., Lübbecke, J., and Brandt, P.:  Interannual variability of net primary productivity in the Northwest African coastal upwelling system and their relation to Dakar Niños and Niñas., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18521, https://doi.org/10.5194/egusphere-egu25-18521, 2025.

Previous studies have identified diverse spatial patterns of the Atlantic Niño (AN) linked to different teleconnections. The emergence of these structures coincides with different mean conditions and driving mechanisms. Here, we explore the role of the tropical Atlantic background state in changing the effectiveness of the dynamic mechanisms that generate the AN, and in shaping the distinct AN patterns.

To this aim, we use simulations from five models of the Extratropical-Tropical Interaction Model Intercomparison Project (ETIN-MIP), where changes in the background state are induced by perturbations in incoming solar radiation across three different latitudinal bands.

Our results reveal that modifying the ocean background state could induce the reported changes in the AN pattern through the alteration of ocean wave dynamics and air-sea coupling.

Mean thermocline slope and stratification in the equatorial Atlantic have a pronounced impact on the Bjerknes Feedback (BF), shaping the AN pattern. In particular, a less tilted equatorial mean thermocline (shallower in the west) in spring could strengthen wind-thermocline coupling under strong anomalous interannual westerlies. Additionally, a tilted mean thermocline, shallower in the east and less stratified in June-August, favors the thermocline-SST coupling. 

Consequently, the stronger BF produces an eastward AN, with SST anomalies confined to the east of the basin and the coast of Africa. Conversely, when the mean state is less favorable, a weaker BF, combined with less effective wave dynamics, results in a westward AN structure.

How to cite: Montoya-Carramolino, L., Losada, T., and Martín-Rey, M.: Influence of ocean background state in Atlantic Niño diversity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19250, https://doi.org/10.5194/egusphere-egu25-19250, 2025.

Canary Upwelling System (CUS) is, together with California, Humboldt, and Benguela, one of the four main Eastern Boundary Upwelling Systems (EBUS) across the globe. In particular, small pelagic fishes (hereinafter SPF) dominate the marine biomass in EBUS where they represent a vital intermediate connection between plankton and large predatory species. Regarding the CUS, SPF constitute in weight close to 70% of the total landings of northwest African countries, being the the Sardinella aurita (hereinafter sardinella) one of the dominant SPF species in terms of abundance. This species, for instance, represents the primary source of animal protein in Senegal. However, the absence of systematic observations of sardinella across northwest Africa largely constraint our understanding of how the environmental variability impacts the abundance and distribution of this species. In this work a novel end-to-end (here climate-to-fish) model-based strategy, including explicit representation of sardinella dynamics, has been designed. The results we are obtaining are enabling us to better understand interesting links (and related underlying processes) with well-known climate modes such as NAO and ENSO.

How to cite: López-Parages, J. and Sánchez-Garrido, J. C.: Analysing the climate influence on sardinella abundance in northwest Africa from a novel end-to-end model strategy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19465, https://doi.org/10.5194/egusphere-egu25-19465, 2025.

This study explores the oceanic connection between the equatorial dynamics and the coastal variability along the northern coast of the Gulf of Guinea on interannual timescales, based on experiments with a high-resolution tropical Atlantic Ocean model over 1958-2015. Equatorial Kelvin waves, forced by wind-stress anomalies in the west-central equatorial basin, significantly control the interannual fluctuations of the coastal sea-level and subsurface temperature near the thermocline (>70%), leaving only a marginal role for the local forcing contribution. The dynamical coastal response exhibits a clear propagative nature, with poleward propagations (0.75-1.2 m.s^-1) from Cameroon to Liberia. Because the northern coast of the Gulf of Guinea is close to the equatorial waveguide, the coastal variability is influenced by both equatorially-forced coastal trapped waves and reflected equatorial Rossby waves. Furthermore, remote equatorial forcing explains more of the surface temperature variance for the coastal systems associated with clear upwelling characteristics such as Côte d’Ivoire and Ghana, where subsurface/surface coupling is more efficient. The surface thermal amplitude and timing is shaped by the coastal stratification and circulation and exhibits a marked seasonal modulation, so that the timing of the SST anomalies relative to the dynamical signature lacks consistency, making SST a less reliable variable for tracking coastal propagations in the Gulf of Guinea. Our findings open the possibility of predicting interannual changes in coastal conditions off Côte d’Ivoire and Ghana a few months in advance, to anticipate impacts on fish habitats and resources, and to facilitate proactive measures for sustainable management and conservation efforts.

How to cite: Illig, S., Djakouré, S., and Mitchodigni, T.: Influence of the remote equatorial dynamics on the interannual variability along the northern coast of the Gulf of Guinea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19497, https://doi.org/10.5194/egusphere-egu25-19497, 2025.

The equatorial Atlantic plays a critical role in regional and global climates, yet the influence of Saharan dust in this region remains underexplored. While Saharan dust’s effects on sea surface temperature variability in the North Tropical Atlantic are well-documented, its impact near the equator, particularly during boreal winter, when dust transport reaches its southernmost extent, has received limited attention. Using observational and reanalysis data, we investigate the effects of Saharan dust on equatorial Atlantic variability. We observe a distinct and complex response contrary to the expected cooling from reduced solar radiation. Dust-induced warming in the lower troposphere drives significant sea surface temperature warming off northwestern Africa through changes in latent heat fluxes and Ekman convergence, leading to an off-equatorial warm front. This warm front generates cross-equatorial winds that shift the Atlantic rain belt northward, cool the equatorial region, and trigger wave activity, ultimately causing delayed warming. This study highlights the need to understand complex dust-climate interactions, identifying Saharan dust as a potential driver of equatorial Atlantic variability with broader climatic implications.

How to cite: Vallès Casanova, I., Adam, O., and Martín Rey, M.: Influence of winter Saharan dust on equatorial Atlantic variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20043, https://doi.org/10.5194/egusphere-egu25-20043, 2025.

Agulhas Current meanders, also known as Natal Pulses, are the dominant modes of variability within the Agulhas Current. These meanders significantly impact local hydrological dynamics and ecosystems. Previous observations and model outputs suggest that the influence of these meanders on Agulhas ring shedding is limited, while their effect on the Agulhas retroflection remains unclear. Models also imply that the Southern Ocean supergyre has more influence on Agulhas leakage variability than upstream drivers. Here, we developed an algorithm based on 28 years of daily-averaged Global Ocean Physics Reanalysis data with a spatial resolution of 0.083° × 0.083°. Using current velocities and sea surface height anomalies (SSHA), we tracked Agulhas Current meanders from the Agulhas Current Time-series Experiment (ACT) region to the Agulhas Return Current. Specifically, we examine the influence of the meanders on the westmost extent, i.e. minimum longitude, of the Agulhas retroflection on the basis that westwards excursions of the Agulhas current increase the probability of leakage over a longer path through this region. This is compared with the potential influence of the Subtropical Front (STF) which is the southmost front of the Southern Ocean supergyre. Our algorithm detects between 1-6 meanders per year in the ACT region, with an increasing trend over the 28-year period. These are more than reported by recent studies. More work is required to understand why we identify more meanders in the reanalysis than was found from in situ and satellite observations, however the increasing trend in the number of meanders is similar. The meanders correspond to negative SSHA propagating along the mean Agulhas Current path that is well defined down to Agulhas Bank at 22^O E. The maximum lagged correlation coefficient between the minimum longitude of the Agulhas retroflection and 22^O E SSHA is –0.23, which is three times higher than that between the minimum longitude of the retroflection and the subtropical front. This suggests that meanders exert a stronger influence on the zonal movement of the Agulhas retroflection compared to the STF. Additionally, the 10-day lagged correlation coefficient of SSHA between the Agulhas retroflection and the return current is 0.18, indicating that Agulhas meanders also have significant impact on the return current. These findings support the hypothesis that Agulhas meanders play an important role in shifting the position of the Agulhas retroflection. Finally, we quantify heat loss along the lagrangain trajectories in the Agulhas retroflection region to evaluate the likelihood of greater leakage for longer trajectories.

How to cite: Shen, S., Lenn, Y.-D., Donohue, K., and Beal, L.: Agulhas Meanders vs. Subtropical Front: Influence on Retroflection path over 28 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-358, https://doi.org/10.5194/egusphere-egu25-358, 2025.

Despite recognition of the South Atlantic Ocean as a significant CO₂ sink, the variability of carbon fluxes (FCO₂) at small temporal and spatial scales remains poorly understood. This gap is especially evident in transitional regions like the Cape Basin, where mesoscale oceanic features and localized atmospheric processes strongly influence ocean-atmosphere CO₂ exchange. Current observational and modeling approaches lack the resolution to capture these fine-scale fluctuations, potentially biasing global CO₂ flux estimates. To address this, our study examines short-term (1–10 days) and small-scale (0.1–10 km) drivers of CO₂ flux variability in the Cape Basin using 2-month high-resolution time-series data from a Wave Glider during late summer. By decomposing the carbon flux equation and applying Reynolds decomposition, we show that wind-driven gas transfer velocity (Kw) dominates the periods of enhanced FCO2, accounting for approximately 78% of flux variability on average in the Cape Basin. A secondary contribution of 11% comes from changes in the air-sea pCO₂ gradient (ΔpCO₂). . However, during localized periods - over the course of hours to days - mesoscale eddies and fronts enhance the ΔpCO₂ to the order of 60 µatm. During moderate winds (5 m s-1 < U10 < 15 m s-1), this sets ΔpCO2 as the dominant driver in FCO2 variability (>50%). However, when U10 < 15 m s-1, Kw dominates variability of FCO2 irrespective of ΔpCO₂. These findings underscore the importance of small-scale ocean processes in CO₂ exchange, their nuanced relationship with wind speed - dominated by large scale extratropical cyclones in the Cape Basin - and the need for high-resolution observations to improve global flux estimates.

How to cite: Ruiz Gomez, G., Swart, S., Du plessis, M., and Nicholson, S.-A.: Drivers of CO2 flux variability in the Cape Basin, South Africa., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-364, https://doi.org/10.5194/egusphere-egu25-364, 2025.

The Cape Basin is a highly dynamic region subjected to ocean ventilation and deepwater mass formation. Flowtopography interactions along the Agulhas Retroflection produce standing meanders with elevated Eddy Kinetic Energy and modified frontal structures. While numerical studies have shown that these features enhance deep water mass formation and tracer stirring, observational data on these processes have been limited due to their fine spatial and temporal scales. This study combines high-resolution Seaglider measurements of Apparent Oxygen Utilization (AOU) with backscatter data to map ventilation pathways. Results demonstrate the transport of low AOU values to depth via advection and stirring along isopycnals, as well as across-isopycnal transport near ocean fronts with strong buoyancy gradients and elevated diapycnal spiciness curvature. These findings provide critical observational evidence for the role of submesoscale processes in deep water mass formation and their broader implications for global climate dynamics and ocean circulation.

How to cite: Koets, R., Swart, S., du Plessis, M., and Donohue, K.: Drivers of ventilation in the CapeBasin using Apparent OxygenUtilization (AOU) as a tracer., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-429, https://doi.org/10.5194/egusphere-egu25-429, 2025.

Living benthic foraminifera were studied in the highly productive West African upwelling system off Walvis Bay, Namibia. The Benguela Current System, influenced by the prevailing southeastern trade winds, governs the region's oceanography and is significantly impacted by climate variability. This results in seasonal upwelling, leading to high primary productivion and the formation of a Diatomaceous Mud Belt (DMB).  Inside this DMB anoxic and/or hypoxic conditions prevail between 150 and 450 m depths, impacting benthic foraminiferal communities (Inthorn et al., 2006). Our aims were to establish the vertical distribution patterns of foraminiferal species in the sediment (microhabitats) at each sampling station and to examine the ecological responses of foraminifera to varying oxygen conditions.

Sediment samples from NIOZ cruises 64PE449 and 64PE450 obtained by multicorer were analysed for living foraminifera identified by Rose Bengal staining. Subsamples were collected at 0.5 cm intervals from the top 2 cm of each sediment core and at 1 cm intervals from 2 to 10 cm depth. Isotopic analysis (δ¹³C) was conducted using IR-MS on cleaned specimens (20–45 µg) after ultrasonic washing to remove clays, using NFHS1 and NBS19 standards for calibration.

At the two deeper stations, at 750 and 324 m depth, bottom water oxygen concentrations were 105 and 44 µMol/L, respectively. The benthic foraminiferal assemblages were diverse and were largely limited to the top 1.5 cm of the sediment. Interspecific differences in microhabitat were limited.

At the six stations positioned around the 100 m isobath, oxygen concentrations varied between 3 and 20 µMol/L. The faunal diversity was much lower, with only five species being recorded: Bulimina elongata, Bolivina pacifica, Fursenkoina complanata, Nonionella stella, and Virgulinella fragilis. The assemblages were always strongly dominated by one or two species. At these stations, faunal penetration into the sediment was much larger, at some stations until 10 cm, again with rather limited interspecific differences in microhabitat. Remarkably, at three stations one or more conspicuous density maxima were found at depth in the sediment. This suggests at present, repetitive deposition of cm thiick sediment deposits takes place, burying living foraminiferal assemblages, which remain preserved, and stained by Rose Bengal, for some time in the deeper sediment layers.

A key finding is the significantly lower δ¹³C values observed in Virgulinella fragilis compared to co-occurring species at similar depths, in accordance to Bernhard, 2003., attributed to bicarbonate release during sulphate reduction, indicating environments with high sulphate reduction rates. Further investigation will explore these metabolic and biomineralization variations and their relationship to foraminiferal assemblages, environmental parameters, and overall ecosystem functioning within the West African upwelling system. As δ¹³C is measurable in fossil tests, it gives hope in future to interpret outlying overly negative δ¹³C values as probable anoxic metabolic pathway in case of fossil foraminifera.

How to cite: Prasetyo, D. G., Báldi, K., Jorissen, F. J., Pacho, L., Jan De Nooijer, L., and Reichart, G.-J.: Living Foraminifera Assemblage of the West African Upwelling System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-933, https://doi.org/10.5194/egusphere-egu25-933, 2025.

The Cape Basin is a turbulent region off the west coast of South Africa where warm and salty Indian Ocean waters mix with cooler, fresher South Atlantic waters. This injection of heat and salt is known as Agulhas leakage and has been tied to the Atlantic Meridional Overturning Circulation’s (AMOC) strength, stability, and variability. Paleoclimate data and model hindcasts point to an increase in Agulhas leakage as the climate warms, but quantifying real-world leakage is very difficult owing to its turbulent nature. This is further complicated by the basin’s four dynamical regions: eddies, eddy-eddy interactions, topography, and filamentation which can influence and transform the water masses through the water column. Their properties and circulation are critical to understand because they can influence where and how this leakage is transported into the South Atlantic. Using data from the ARGO float array, we identified six water masses in the region using neutral density and calculated their heat and salt transports along the leakage corridor.  Tropical Surface Waters (TSW, <25.5), Subtropical Surface Waters (STSW, 25.5-26), South Atlantic Subtropical Mode Waters (SASTMW, 26-26.5), and the Upper North Atlantic Deep Waters (UNADW, 27.92-28.08) have a combined heat and salt transport of 6.85x10-11 PW and 11.8 kg/s respectively into the leakage corridor. South Indian Mode Waters (SICW, 26.5-27) and the Intermediate Waters (IW, 27-27.92) have a combined heat and salt transport of 3.36x10-11 PW and 47.6 kg/s respectively through the leakage corridor. As a result, approximately 30% of heat and 80% of salt are transported into the South Atlantic primarily from the SICW and the IW masses. Previous studies have primarily focused on the intermediate water masses as the contributor, yet our results show that Mode Waters can have a significant impact on Agulhas leakage’s transport and variability. To improve our understanding of Agulhas leakage and its impact on the AMOC, we must turn to improving our understanding on the basin’s seasonal variability and its potential mode water formation. 

How to cite: Sampson, R., Beal, L., and Novelli, G.: Water Mass Transformation in the Cape Cauldron, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1135, https://doi.org/10.5194/egusphere-egu25-1135, 2025.

The Eastern Boundary Upwelling system of the Southeastern Tropical Atlantic (SETA) is of great socioeconomic importance for local communities since it supports highly productive fisheries and diverse marine ecosystems. Comprehending the local processes that shape the physical characteristics of this system is thus crucial. The SETA is characterized by a strong meridional sea surface temperature (SST) gradient and is influenced by a large freshwater input from land mainly due to Congo River discharge. Here, we use high-resolution ocean model sensitivity experiments to show the impacts of the freshwater discharge from rivers on the mean state SST and the dynamics of this coastal region. By comparing experiments with and without river discharge we find that the freshwater presence increases the mean state coastal SST by up to 0.9ºC from 6ºS to 25ºS, while reducing the SST by more than 1ºC from 6ºS to 3ºS. These changes are associated with a halosteric effect of an elevated sea surface due to the lower sea surface salinity, leading to strong pressure gradients that drive upwelling and downwelling processes north and south of the Congo River mouth at 6ºS, respectively. Alongshore horizontal temperature advection also related to the sea surface height gradients plays likewise an important role in warming (cooling) the SST mean state south (north) of 6ºS. Ultimately, the change in coastal currents pushes the meridional temperature gradient further south. These results highlight the influence of freshwater input on SST and ocean surface dynamics, particularly relevant in the context of projected climate change, suggesting a future increase in Congo River discharge.

How to cite: Costa Aroucha, L., Lübbecke, J., Brandt, P., Schwarzkopf, F., and Biastoch, A.: Southeastern Tropical Atlantic Coastal Dynamics and Sea Surface Temperature affected by River Discharge: insights from modelling., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1289, https://doi.org/10.5194/egusphere-egu25-1289, 2025.

Agulhas Rings are anticyclonic, warm-core eddies that play a crucial role in the exchange of water masses between the Indian and Atlantic Oceans. Formed at the Agulhas Retroflection near the southern tip of Africa, these rings constitute an essential component of the global thermohaline circulation, transporting ocean properties such as heat, salt, and energy. Their movement and property transfer significantly influence regional climate systems and large-scale ocean dynamics. It is well established that Agulhas Rings differ significantly in their characteristics. However, certain types, such as subsurface-intensified Agulhas Rings, remain remarkably understudied and are poorly represented in most ocean models. Investigations of these features often require high-resolution observational and modelling approaches. In this study, we present high-resolution glider observations from March/April 2021 and 2022 of two types of Agulhas Rings (surface- and subsurface-intensified), highlighting their distinct characteristics. Both glider campaigns utilized a microstructure probe to enable detailed observations of energy dissipation and diapycnal mixing. Our analysis reveals that major differences between the two eddy types occur near the eddy centre, where the subsurface-intensified Agulhas Ring exhibits elevated energy dissipation (log₁₀(ε)= -7 W kg^-1) and diapycnal mixing (log₁₀ (κ_ρ)= -4 m²s ^-1) beneath the surface mixed layer. Further analysis shows that mixing length scales (up to 18 km), are also elevated within the sub-surface intensified eddy, suggesting enhanced vertical and lateral distribution of ocean properties. These findings indicate a faster decay of the sub-surface intensified eddy and thus suggest a more local impact compared with the potentially longer-lived surface intensified eddy. By highlighting the distinct oceanic and energetic characteristics of surface- and subsurface-intensified Agulhas Rings, this study contributes to a better understanding of their role in influencing thermohaline structure and redistributing energy. These findings provide valuable insights that can support the development of more accurate parameterizations in future ocean models.

How to cite: Oelerich, R., Walter, M., and Bachmayer, R.: Energy dissipation and mixing within surface- and sub-surface intensified Agulhas Rings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1410, https://doi.org/10.5194/egusphere-egu25-1410, 2025.

Small-scale variability is essential to understanding ocean circulation, air-sea interactions, and biogeochemical processes. Yet, current satellite-derived sea surface salinity (SSS) data can only resolve features larger than 40 km. This study aims to capture smaller scale variability (≤25 km) by reconstructing SSS data from satellite observations. The focus is on the Brazil-Malvinas Confluence (BMC), a biologically productive region characterized by intense mesoscale structures and associated with strong SSS gradients primarily due to the discharge of the La Plata River. A Lagrangian reconstruction method is employed to advect satellite SSS fields by altimetric geostrophic currents to capture smaller-scale details. These reconstructed fields are validated against in-situ salinity measurements from thermosalinographs. Results show that the reconstructed fields successfully capture smaller scale features observed in the region. This approach seeks to enhance the effective resolution of SSS data, overcoming the limitations of current satellite observations.

How to cite: Combes, V., Martí-Solana, C., and Barceló-Llull, B.: Detecting Salinity Fronts From Satellite Observations in the Brazil-Malvinas Confluence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1530, https://doi.org/10.5194/egusphere-egu25-1530, 2025.

The Argentine Basin hosts a unique oceanic feature: the Zapiola Anticyclonic Circulation (ZAC) located above a sedimentary deposit. Taking advantage of a high-resolution (1/12º) global ocean reanalysis (GLORYS12) we examine the ZAC over 27 years (1993-2019). The mean ZAC is bottom-intensified with bottom currents reaching 0.10 m s^-1. The ZAC volume transport ranges from -18.5 Sv to 268 Sv with a mean of 122 Sv. The strong negative peaks correspond to occasional ZAC collapses. During large transport events (>195.4 Sv) the ZAC shows a well defined  coherent gyre. Strong transport events are associated with high eddy kinetic energy (EKE) at the periphery of the ZAC (especially to the west and south). In contrast, during weak transport events (<49.8 Sv), EKE increases at the center of the ZAC and decreases at the ZAC periphery. 
A weak ZAC is more permeable to external mesoscale structures. Each weak event features a cyclonic eddy at the center of the ZAC carrying subantarctic cold and fresh waters. 
The ZAC exhibits a multi-year modulation, with periods of 4-5 years (1993-1997, 1998-2003 and 2004-2009) of low salinity corresponding to low transport, and high salinity to high transport.
Over the last 27 years, transport time series exhibit a significant negative trend of -15 Sv.decade^-1 associated with a  negative trend in EKE (-0.015 (m/s)².decade^-1) to the north west of the ZAC. Waters in the Zapiola region become warmer and saltier in the first 2000 m of the water column because of the southward migration of the subtropical front.  

How to cite: Poli, L., Artana, C., Provost, C., Sirven, J., and Le Blanc-Pressenda, R.: Collapses, maxima, multi-year modulation and trends of the Zapiola Anticyclonic Circulation: insights from Mercator Reanalysis., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2324, https://doi.org/10.5194/egusphere-egu25-2324, 2025.

The Cape Basin off the western coast of South Africa is characterized by rich mesoscale and submesoscale variability generated by the shedding of eddies and filaments from the Agulhas Retroflection. These features carry warm, salty water into the cooler, fresher South Atlantic Ocean. Thermohaline interleaving is common in this region due to strong lateral water-mass gradients and the presence of stirring processes. These intrusions are an important pathway to water-mass transformation because they result in an increased surface area over which diapycnal mixing can work to homogenize water property contrasts. We present the first observational study that can track interleaving features at high vertical and horizontal resolution over distances of O(100 km) in the Cape Basin by using diapycnal spiciness curvature to detect intrusions within data collected by the Wire Flyer towed profiling vehicle and EM-APEX profiling floats. These datasets show interleaving features present throughout thermocline waters (σ = 26.0 – 27.5), but their scales and slopes vary significantly. Wire Flyer sections highlight strong differences in interleaving characteristics and generation mechanisms over distances of 10s of km. Several of the transects exhibit signatures of internal waves, which appear to modulate the interleaving structure. EM-APEX floats provide an alternative, semi-Lagrangian sampling perspective and show the persistence of interleaving features over time scales of 1-15 days. We conclude that the thermohaline variability in this region is primarily driven by mesoscale stirring, although double diffusion may be acting to grow the features after their formation. This study showcases the variety of physical processes existing at different length and time scales that contribute to the formation and structure of interleaving in the Cape Basin.

How to cite: Grose, L., Donohue, K., and Roman, C.: Patterns of thermohaline interleaving in the Cape Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3338, https://doi.org/10.5194/egusphere-egu25-3338, 2025.

The upper cell of the Atlantic meridional overturning circulation (AMOC) comprises a deep lower limb exporting North Atlantic deepwater (NADW) southward, fed by a shallow upper limb carrying interocean waters northward from the southern South Atlantic and thereby enabling continued NADW production. On decadal and longer timescales, high-latitude density anomalies affecting the production of NADW are expected to generate coherent overturning transport changes across latitudes in the Atlantic basin. This process calls for compensating effects in upstream components of the northward AMOC upper limb, wherein upper-ocean meridional transports are adjusted according to flow continuity principles, tending to even out mass imbalances.

Besides the South Atlantic functioning as a mediator of interocean exchanges that are crucial for driving and maintaining the AMOC, the subtropical South Atlantic is the only region where the total northward component of the AMOC upper limb is subjected to ocean interior dynamics along its interhemispheric trajectory — before being settled over the Atlantic western boundary, a narrow crossroad along which it proceeds up to the subpolar North Atlantic by delineating the American coastline. This results from the AMOC upper limb being incorporated into the anticyclonic South Atlantic subtropical gyre (SASG) at its origins. By crossing the subtropical South Atlantic and meeting the South American coast, the AMOC upper limb is ultimately decoupled from the SASG and placed over the South Atlantic western boundary — as the westward flow along the SASG northern boundary bifurcates meridionally, redistributing waters between these two major large-scale circulation systems.

This work aims to demonstrate that the singular dynamic setting of the South Atlantic general circulation, compared to its North Atlantic counterpart, makes it more vulnerable to large AMOC changes. The findings from two recent studies based on transient deglacial simulation results will be discussed, which suggest that the upper-ocean flow redistributions taking place over the South Atlantic western boundary are highly responsive to AMOC changes under pre-industrial paleocean dynamics, and thus have the potential to provide future insights into the degree and timescales over which overturning in the North Atlantic impacts adjacent ocean basins and vice versa.

This model-based perspective elucidates the link between the South Atlantic western boundary current system and the AMOC through the establishment of meridional connectivity of AMOC variability. By definition, the steady-state AMOC system is meridionally coherent. Questions that naturally arise are: How is AMOC meridional coherence transiently modulated? And how will this modulation process evolve under modern climate change conditions? These questions are particularly relevant for upcoming research efforts dedicated to understanding how the South Atlantic circulation is to be shaped by climate change.

How to cite: Marcello, F., Wainer, I., M. de Mahiques, M., and C. Bicego, M.: South Atlantic upper-ocean flow redistributions and their connection to overturning in the North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3852, https://doi.org/10.5194/egusphere-egu25-3852, 2025.

Eastern Boundary Upwelling Systems, driven by wind-induced Ekman transport, bring cold, nutrient-rich deep waters to the surface, making them hotspots of biological activity with significant economic, ecological, and social value. Also contributing to the upwelling of deep waters, is the Ekman pumping, which is created by cyclonic surface wind stress curl (WSC). WSC further influences local circulation and the formation of upwelling hotspots, such as those found downwind of prominent capes. Accurately representing these processes is critical for predicting future changes in upwelling and their impacts on marine productivity, in particular for the Southern Benguela Upwelling (SBU), a region characterised by a complex coastal geometry and orography. Using a fine resolution (1 km), curvilinear grid regional numerical model of the SBU, based on CROCO, this work highlights the sensitivity of upwelling processes to the fine scale spatial wind variability, by using two different wind products which differ by their resolution: ERA5 (~30 km) and WASA3 (~3 km). The lack of coastal wind drop-off in the CROCO-ERA5 simulation results in more intense nearshore and less intense offshore upwelling than observed in the CROCO-WASA3 simulation. These differences in the spatial structure of the coastal upwelling impact the coastal circulation. The inshore branch of the equatorward flowing Benguela Jet is shifted offshore in the CROCO-WASA3 run, while a poleward coastal jet emerges with intermittency. A better understanding of these upwelling and current structures could provide new insights into the generation of harmful algal blooms in the SBU.

How to cite: Flügel, R., Fearon, G., Herbette, S., Treguier, A. M., and Veitch, J.: The Role of Fine-Scale Winds in Upwelling and Coastal Circulation in the Southern Benguela Upwelling System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10536, https://doi.org/10.5194/egusphere-egu25-10536, 2025.

We investigate mesoscale eddy effects on the ventilation timescales and pathways of South Atlantic Antarctic Intermediate Water (AAIW) using a high-resolution 1/10° eddy-resolving ocean model (Parallel Ocean Program) combined with a Lagrangian particle tracking algorithm (OceanParcels). AAIW sequesters a significant amount of anthropogenic carbon along its ventilation pathways through the eddy-rich Southern Ocean. The distribution of subduction zones along dynamic sections of the Antarctic Circumpolar Current, as well as the Malvinas Confluence Zone, indicates an influence of mesoscale eddies on ventilation, in addition to the zonally uniform Ekman transport. To identify eddy effects, we perform particle backtracking from the South Atlantic AAIW interior to the mixed layer, both with eddy-resolving model output velocity and its mean state, where mesoscale eddy velocities are absent. We characterise mean ages and ventilation pathways for South Atlantic AAIW originating from subduction zones located around the Drake Passage, the South Atlantic, and the South Indian Ocean. Eddy effects increase the contribution of Drake Passage waters to the South Atlantic AAIW and reduce mean age estimates, attributable to accelerated advection together with deeper and more southward subduction below the mixed layer, near the Malvinas Confluence Zone. We highlight the role of eddies in the ventilation of the South Atlantic pycnocline, which, if accurately represented, increase estimates of ventilation rates and strengthen cold water inflow, enhancing Southern Ocean carbon and heat uptake in ocean models.

How to cite: Schäfers, S., Griesel, A., and Chouksey, M.: Role of mesoscale eddies in ventilation pathways of South Atlantic AAIW using Lagrangian backtracking, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10739, https://doi.org/10.5194/egusphere-egu25-10739, 2025.

The ocean is a chaotic system where the presence of mesoscale eddies makes the understanding of basin or global scale flows difficult. However, the pressures on the continental shelves (boundary pressures) are only weakly influenced by eddies and, in most places, directly reflect the global scale processes. In this study, we use depth-integrated boundary pressure as a constraint to estimate the total upwelling in the Indian and Pacific oceans. The two main factors that determine the difference in the depth-integrated pressure between the east Pacific and the east Atlantic are the winds and the upwelling. Calculations from reduced-order theoretical models and diagnostics from 1/12th degree NEMO simulation show that, given the winds and boundary pressures, we can infer upwelling/downwelling in each ocean basin with errors of order 1 Sv. We apply this method using observations near eastern boundaries to put constraints on the total upwelling in the Pacific and Indian oceans.

How to cite: Gururaj, S., Hughes, C., and Bingham, R.: Constraining the overturning in the Pacific and Indian oceans by using boundary pressures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11518, https://doi.org/10.5194/egusphere-egu25-11518, 2025.

The Atlantic meridional overturning circulation (AMOC) is a key feature of the global ocean circulation and has a big impact on regional weather and global climate. To project future AMOC variability, it is vital to understand and properly assess the past variability. The AMOC transport and in particular its geostrophic component is measured at several latitudes by specific observing systems. However, the transport can be calculated using multiple combinations of instruments which complicates both the comparison of the observed AMOC at different latitudes and to model transports.

Here, we present results from a systematic comparison of methods to compute the upper branch of the AMOC at 11°S, utilizing data from the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) array. The TRACOS array comprises 10 years of observations, including data from Pressure Inverted Echo Sounders, tall moorings, and ship sections at the eastern and western boundaries as well as supplementary data from Argo floats and satellites. By subsampling the observational setup in two ocean models (INALT20 and VIKING20X, both based on NEMO), we quantify uncertainties in AMOC transport estimates on different time scales.

We find that bottom pressure measurements, despite being prone to sensor drifts, effectively capture the seasonal variability. The largest potential source of error lies in the choice of vertical structure between the measurement points. Longer-term variability assessments based on moored density measurements require particularly high vertical resolution in the upper 500m. For both methods, the error associated with replacing the eastern boundary data with a climatological seasonal cycle is small, indicating low uncertainty resulting from loss of instruments in that region. We also consider uncertainties in the Ekman transport, which has a magnitude of about one-third of the geostrophic transport at 11°S. Ekman transport estimates vary by a few Sverdrups depending on the choice of wind stress product or drag coefficient used. All in all, we find that the TRACOS array can assess AMOC signals but we also show potential improvements in the array design to reduce uncertainties regarding longer-term variability.

How to cite: Hans, A. C., Hummels, R., Brandt, P., Juricke, S., and Schwarzkopf, F.: Uncertainty evaluation of the AMOC transport calculation at 11°S, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13885, https://doi.org/10.5194/egusphere-egu25-13885, 2025.

The circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flows. The zonally integrated meridional flow is associated with the Atlantic Meridional Overturning Circulation (AMOC) — a major component of the global climate system. In the tropics, the northward, upper branch of the AMOC flow is superimposed by the shallower overturning associated with the wind-driven Subtropical cells (STC).

The western boundary observing system of the TRACOS (Tropical Atlantic Circulation & Overturning at 11°S) array consists of four tall moorings monitoring the strong western boundary current system – more specifically, the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) as part of the AMOC, the STC, and the wind-driven gyre system. More than 15 years of moored current meter observations, collected between 2000 and 2004 and continuously since 2013 within the NBUC and DWBC, are analyzed for their variability across different time scales. NBUC and DWBC transports are calculated from alongshore velocity data by regressing current meter observations onto variability patterns. These patterns are obtained from 15 ship sections, derived by combining underway vessel mounted ADCP measurements with on-station lowered ADCP measurements to provide full-depth alongshore velocity fields. The NBUC transport is dominated by a seasonal signal with only minor interannual variations and no obvious trend, while the DWBC is dominated by strong intra-seasonal variability induced by the passage of deep eddies. Despite these deep eddies being evident throughout the DWBC transport time series, longer term variability is also becoming evident as the time series is prolonging.

How to cite: Hummels, R., Brandt, P., Dengler, M., and Hans, A. C.: Variability of the Western Boundary Current System at 11°S, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13966, https://doi.org/10.5194/egusphere-egu25-13966, 2025.

Simple box models of the Atlantic Meridional Overturning Circulation (AMOC) often rely on ad-hoc scaling laws that link AMOC strength to the meridional density gradient. In contrast, Global Climate Models (GCMs) provide more comprehensive simulations but demand substantial computational resources. To evaluate the validity of these scaling laws, we develop a simplified AMOC model that represents the circulation as a geostrophically balanced flow confined to the western boundary of the Atlantic basin. Basin-wide pressure gradients are driven by mixing along continental boundaries and wind-driven upwelling in the Southern Ocean. We explore the limiting cases of quasi-adiabatic and diffusive overturning circulations, deriving corresponding scaling laws for AMOC strength. Given the critical influence of these scaling laws on AMOC stability, we examine how stability depends on the dominant driving mechanism—either diffusive mixing or adiabatic upwelling. This analysis aims to identify GCM biases that could significantly affect AMOC stability and must be addressed to accurately assess the risk of an AMOC collapse in the coming century.

How to cite: Vanderborght, E. and Dijkstra, H.: A Mechanical Model for the Inter-Hemispheric Overturning Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16235, https://doi.org/10.5194/egusphere-egu25-16235, 2025.

Cold-water corals and sponges form iconic and globally occurring benthic communities, provide important habitats for a diverse associated fauna and thrive in environmental conditions with often large temporal and spatial variations in near-bottom currents, food availability and other environmental parameters. We investigate the variability of near‐bottom currents and physical processes from simulations with a nested hydrodynamic modelling framework at two seamounts rich in benthic fauna along the Northeast Walvis Ridge, Valdivia Bank and Ewing Seamount. Our aim is to obtain new insights on physical drivers of observed occurrences and distribution of benthic suspension feeders (cnidarians and sponges) in this data‐poor area. We use dynamic downscaling of high-resolution implementations of the ROMS-AGRIF model in combination with high-resolution bathymetry and open boundary forcing from the basin-scale model INALT20 and the OSU inverse tidal model to explore the fine-scale physical processes and mechanisms that potentially drive a continuous or episodic food supply to the benthic communities.  Over a three-year period, we analysed how near-bottom currents vary in space and time and assess potential connections between the distribution of filter-feeding fauna and the surrounding physical marine environment. We identified a close link between flow dynamics, internal tide dynamics and faunal species distributions. We propose that physical processes such as kinetic energy dissipation and internal wave dynamics could serve as functional indicators of food supply and particle encounter rates in future species distribution and habitat suitability models for important deep-sea taxa, such as those that represent vulnerable marine ecosystems. Our results also show little impact of mesoscale eddies from the Agulhas Leakage as they propagate north-westward into the southeast Atlantic along a well-defined corridor, which only occasionally extends as far north as the Valdivia Bank and Ewing Seamount.

How to cite: Mohn, C., Schwarzkopf, F. U., Jiménez García, P., Orejas, C., Huvenne, V. A. I., Schumacher, M., Pérez-Rodríguez, I., Sarralde Vizuete, R., López-Abellán, L. J., Dale, A. C., Devey, C., Hansen, J. L. S., Møller, E. F., and Biastoch, A.: Dynamics of Near-Bottom Currents in Cold-Water Coral and Sponge Areas along the Walvis Ridge., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16401, https://doi.org/10.5194/egusphere-egu25-16401, 2025.

Antarctic Intermediate Water (AAIW) plays a crucial role in the global thermohaline circulation and is a vital component of the Atlantic Meridional Overturning Circulation (AMOC). It significantly contributes to the redistribution of heat, oxygen, and nutrients across the global ocean. Understanding the dynamics of intermediate water circulation over millennial timescales is essential for evaluating how changes in the AMOC affect ocean heat transport during abrupt climatic events. Despite its importance, the relationship between global intermediate water circulation and abrupt high-latitude climatic events such as the Younger Dryas (YD) and Heinrich Stadials (HS) remains partly understood, particularly in the Indian Ocean. To address this gap, we present a high-resolution ~29 kyr record of Neodymium isotopes (Ɛ_Nd) from the authigenic phases of a sediment core (SK-17) collected at 840 m depth in the eastern Arabian Sea, off Goa. Our Ɛ_Nd data shows significant temporal variations from -9.5 to -6.1 throughout the core. Climatic periods (such as YD and HS) with enhanced radiogenic Nd signatures indicate increased northward penetration of AAIW into the northern Indian Ocean during these intervals. These episodes correspond to colder periods in the Northern Hemisphere, suggesting a direct linkage between Northern Hemisphere climate dynamics and the formation of AAIW in the Southern Ocean. Specifically, the enhanced formation of AAIW during these times may have been driven by warming-induced deceleration of the AMOC, which likely triggered increased AAIW production in the Southern Ocean. This connection highlights the interplay between the North Atlantic Deep-Water formation and Southern Ocean climate processes governed by the "bipolar seesaw" mechanism. By linking AAIW variability in the Arabian Sea to global climatic events, our study underscores the importance of intermediate water masses in understanding the mechanisms driving past and potential future changes in the AMOC.

How to cite: Shukla, A., Mishra, T. K., Singh, S. K., and Singh, A. D.: Extensive Intrusion of Antarctic Intermediate Water into the Arabian Sea during Younger Dryas , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-270, https://doi.org/10.5194/egusphere-egu25-270, 2025.

The Arabian Sea (AS) hosts the world’s thickest and most intense oxygen minimum zone (OMZ), and previous studies have documented a dramatic decline of dissolved oxygen (DO) in the northeastern AS in recent decades. In this study, using the recently released data from Biogeochemical-Argo floats, we found a surprising trend of recovery in deoxygenation within the core region of the OMZ in the AS (ASOMZ) since 2013. The average DO concentration increased by approximately threefold, from ~0.63 μM in 2013 to ~1.68 μM in 2022, and the thickness of the ASOMZ decreased by 13%. We find that the weakening of Oman upwelling resulting from the weakening of the summer monsoon is the main driver of oxygenation in the ASOMZ. In addition, the reduction of primary production linked to warming-driven stratification reinforces deoxygenation recovery at depth.

How to cite: Liu, T., Qiu, Y., and Lin, X.: Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decade as observed by BGC-Argo floats, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2038, https://doi.org/10.5194/egusphere-egu25-2038, 2025.

Several paleoceanographic and climate modeling studies have shown both oceanic and atmospheric teleconnections between climate in the tropics and the high latitudes on timescales ranging from decadal to multi-centennial. The last glacial period is characterized by millennial-scale abrupt warmings (interstadials) followed by rather gradual coolings to colder (stadials) Dansgaard-Oeschger (DO) events. The more pronounced of these stadial phases coincide with occurrences of ice-rafted debris in sediments from the mid-latitude Atlantic Ocean, referred to as Heinrich events (HE). Climate oscillations associated with DOs and HEs are also recorded in tropical climate archives around the Indian Ocean and on the Asian continent. However, forcing and response mechanisms of the Indo-Asian monsoon system and ocean-atmosphere exchange processes in conjunction with these millennial-scale oscillations are still poorly understood. Here, we present high-resolution geochemical and micropaleontological data from a sediment core located at 571 m water depth offshore Pakistan, representing the past 80,000 years at millennial-scale resolution.

Alkenone unsaturation-derived sea surface temperature (SST) estimates show overall variations between 23 and 28°C. Millennial scale SST changes of 2°C are modulated by longer-term SST fluctuations. Interstadial intervals are characterized by higher organic carbon (TOC) concentrations, whereas sediments with low TOC contents mark stadials. Productivity-related and anoxia-indicating proxies show abrupt shifts with a 50-60 year duration at climate transitions, such as interstadial inceptions. Inorganic data consistently indicate that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River during interstadials. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. Stadials are characterized by an increased contribution of aeolian dust. HEs are especially dry, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.  

The stable oxygen isotope (δ¹⁸O) records of the surface-dwelling foraminifera G. ruber and of the thermocline-dwelling P. obliquiloculata both show a strong correspondence to Greenland ice core δ¹⁸O, whereas the δ¹⁸O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Distinct shifts in benthic δ¹⁸O during stadials indicate frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. The most pronounced oceanographic changes occur during the transition and the termination of HE 4, respectively. Mg/Ca ratios of G. affinis show a rapid increase (decrease) of bottom water temperatures during the onset (termination) of HE 4, which is in good agreement to modelling studies. The hydrogen isotopic composition of terrigenous plant waxes indicates that HE 4 is much drier than the surrounding DOs.

Overall, our results strengthen the notion that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, with significant impacts on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.

How to cite: Lückge, A., Hollstein, M., Kienast, M., Groeneveld, J., Schefuß, E., Mohtadi, M., and Steinke, S.: Southern Hemisphere water mass transport to the Arabian Sea linked to Greenland climate variability during Heinrich Event 4 , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3030, https://doi.org/10.5194/egusphere-egu25-3030, 2025.

The Equatorial Intermediate Current (EIC) impacts the distribution and the transport of biogeochemical tracers such as oxygen. The EIC in Indian Ocean, covering the range from 200 to 1000 m between 2°S and 2°N , has higher velocity and a lower-frequency variability in the central basin than in the east. The EIC variability is forced by the wind stress forming equatorial beams and is also strengthened by basin resonance. We use zonal current velocity timeseries of 2015-2023 obtained from different equatorial moorings and a continuous timeseries of 2000-2022 years derived from a global NEMO ocean model configuration at 0.25° horizontal resolution with 46 z-levels (ORCA025.L46) and apply the method of vertical mode decomposition aiming to characterize equatorial zonal velocity variability of the Indian Ocean by equatorial beams, baroclinic modes, and equatorial basin resonance.

From west to east, the Indian Ocean is divided by the topography into three subbasins. The west basin is from the western boundary to the Maldives Islands at 73°E; the central basin is from 73°E to the 90°E ridge; the east basin is from 90°E to the eastern boundary. The frequency – baroclinic mode decomposition of the velocity field shows that semiannual and annual signals are the most significant components. For semiannual signals, the second to fourth baroclinic modes contribute at the mooring locations at 80°E and 85°E, while the fifth to eighth modes dominate at 93°E, indicating the essential role of higher baroclinic modes in the eastern basin. For annual signals, lower baroclinic modes are more significant in the east than in the central subbasin. The model output agrees with the observed distribution of contributing baroclinic modes. Observations further reveal several strong EIC events occurring in 2015-2016 and 2020-2021. Atmospheric data showed corresponding strong anomalies in zonal wind stress and outgoing long-wave radiation. Sea surface temperature anomalies happened along with them. With the distribution of the contributing baroclinic mode, the equatorial beams could explain the strong current events at intermediate depths. The energy input from atmospheric forcing propagates along beams, which are predominantly formed by the second baroclinic mode in the central basin and by the superposition of several higher baroclinic modes in the eastern basin. Future research would focus on the role of equatorial beams in the deeper current variability with the knowledge of contributed baroclinic modes in the Indian Ocean.

How to cite: Zhong, Q., Brandt, P., Czeschel, R., and Schwarzkopf, F. U.: Intermediate circulation variability in the equatorial Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3617, https://doi.org/10.5194/egusphere-egu25-3617, 2025.

This study investigated the influence of interannual variations in tropical Indian Ocean tripole (IOT) on the surface air temperature (SAT) over the western Tibetan Plateau (TP) during boreal summer. During the positive phase of the IOT, two northward cross-equatorial airflows are induced over the tropical eastern and western Indian Ocean. These airflows reinforce the ascending motion over southern tropical Asia (80°–125°E, 15°–25°N), increasing local precipitation, as confirmed by observations and simulations by the Community Atmosphere Model. The upper-level Asian Continental Meridional Teleconnection (ACMT) pattern is excited by the latent heat released from precipitation and transmits signals from southern tropical Asia to the western TP, leading to the positive geopotential height anomalies and anomalous anticyclones over there. Upper-level circulation anomalies over the western TP enhance atmospheric thickness through adiabatic processes, consequently elevating local SAT. The ACMT associated with precipitation anomalies thus serves as an atmospheric bridge connecting the IOT and the SAT variations over the western TP.

How to cite: Zhang, Y., Zhu, M., and Li, J.: Physical connection between the tropical Indian Ocean tripole and western Tibetan Plateau surface air temperature during boreal summer , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6573, https://doi.org/10.5194/egusphere-egu25-6573, 2025.

Oxygen minimum zones (OMZ) contribute to 20 to 40 % of global fixed nitrogen loss despite occupying only about 1 % of the ocean. The Bay of Bengal (BoB) contains one of the most pronounced OMZ in intermediate waters worldwide, with oxygen levels near anoxic conditions. Understanding nitrogen cycling in OMZs is critical for comprehending and accurately modeling the global oceanic nitrogen cycle.

In this study, we examined nitrogen cycling in the East Equatorial Indian Ocean (EEIO) and the BoB using water column properties—including temperature, salinity, oxygen, nutrients, and dual stable isotopes of nitrate—collected during a cruise in April/May 2024. Potential temperature and salinity profiles revealed a clear separation between the BoB and the EEIO at 5°N, with distinct water mass distributions and limited mixing between the two regions.

Depth profiles of nitrate stable isotopes displayed notable variations. In waters below 300 m, isotopic signatures were influenced solely by water mass distribution. In contrast, isotope variations in the upper 200 m reflected active on-site fractionation. Surface waters (<100 m) exhibited significant nitrate isotope enrichment and a nitrate deficit, driven by phytoplankton uptake. Below this layer, nitrification was observed, primarily through regenerative production using previously assimilated biomass rather than newly fixed nitrogen from N₂ fixation. A regional decoupling of nitrate dual isotopes, with more enriched δ¹⁸O-NO₃^- in more northern samples of the central BoB, suggested increased nitrite reduction followed by re-oxidation without full assimilation into organic matter in the BoB.

Within the OMZ of the BoB, we identified a persistent nitrate deficit and slightly enriched nitrate isotopes, indicative of nitrogen loss. Given that oxygen concentrations remained slightly above the threshold for significant denitrification in most samples, anammox likely represents the dominant nitrogen loss pathway in the BoB's OMZ.

How to cite: Schulz, G., Dähnke, K., Sanders, T., Penopp, J., Bange, H. W., Czeschel, R., and Gaye, B.: Nitrogen Cycling in the East Equatorial Indian Ocean and Bay of Bengal: Insights from Nitrate Isotopes and Water Masses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8266, https://doi.org/10.5194/egusphere-egu25-8266, 2025.

The rapid warming of the oceans is increasingly recognized for its significant impacts on the climate system and is a central issue in the Climate Variability and Predictability (CLIVAR) research program. However, the effect of future ocean warming—particularly regional ocean warming—on the Hadley Circulation (HC) remains poorly understood. This study addresses the regional contributions of tropical ocean warming to future HC changes, focusing on the 1.5°C, 2°C, and 3°C warming scenarios outlined in the Paris Agreement. Through large ensemble numerical simulations, we demonstrate for the first time that the tropical Indian Ocean dominates future HC changes, while the tropical Pacific Ocean is the main source of uncertainty in HC projections. These results provide critical insights for improving Earth system models and enhancing the projection of tropical atmospheric circulation. Furthermore, they offer a scientific foundation for monitoring and forecasting climate risks associated with future HC shifts, supporting the development of key policy decisions.

Sun, Y., Ramstein, G., Fedorov, A.V., Ding, L., & Liu, B. (2025). Tropical Indian Ocean drives Hadley circulation change in a warming climate. National Science Review, 12(1), nwae375, https://doi.org/10.1093/nsr/nwae375

How to cite: Sun, Y., Ramstein, G., Fedorov, A., and Ding, L.: Tropical Indian Ocean Warming: A Key Driver of Future Hadley Circulation Changes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8405, https://doi.org/10.5194/egusphere-egu25-8405, 2025.

The Arabian Sea, a part of the north Indian Ocean shows a trend of increasing sea surface temperature (SST) over a decadal scale. This area is particularly important due to high phytoplankton growth which is mostly governed by atmospheric forcing and also a place for carbon burial. Hence it is imperative to understand the responses of phytoplankton to this warming trend. The summer monsoon (June-August) winds develop a low-level atmospheric jet (Findlater Jet) blowing across the central Arabian Sea. The positive wind stress curl in the north of this jet leads to open ocean upwelling with consequent nutrient enrichment and phytoplankton bloom. The negative curl in the south results in down-welling and deepening of the mixed layer depth. During the winter monsoon, the wind direction reverses and speed weakens, but in the northern part the cold convective mixing occurs due to the cooling and densification of surface waters and also fuels high phytoplankton growth. However, the south remains oligotrophic, low productive, and warmer compared to the north. We present here two data sets of phytoplankton taxonomy done by both marker pigment analyses by HPLC and light microscopy collected in two field campaigns during summer monsoons 2017 (August) and 2018 (August) along the central Arabian Sea (64°E, 11 -21° N in 1 ° interval). The northern part of the Findlater Jet was mostly occupied by the cooler waters and highest nutrient levels that promoted large diatom-dominated phytoplankton biomass. The southern part was oligotrophic with deep mixed layers, warm, and dominated by (~50%) picocynaobacteria and Prochlorococcus (containing zeaxanthin and DV-Chla) followed by smaller chain-forming diatoms, and heterotrophic dinoflagellates. We have observed that the upwelling strength was stronger in 2018 with cooler waters and higher nutrient levels compared to 2017. The occurrences of warmer waters in 2017 supported higher growth of picocynaobacteria. This is also consistent with other global analyses of long-term trends observed from the Indian Ocean. We have considered a box of 64-66°E and divided it into two sectors, south (18-21°N) and north (11-15°N). The satellite-derived SST data from 2000-2024 indicates a warming trend during summer monsoon both in the north and south. However, no such trend was noticed during winter. This observation suggests that warming during summer monsoon may directly influence the phytoplankton community and may affect carbon transfer and cycling in this dynamic basin.

How to cite: Biswas, H., Chowdhury, M., and Majumder, N.: Picocyanobacteria show warm water preference in the south-central Arabian Sea (North Indian Ocean) during the summer monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8568, https://doi.org/10.5194/egusphere-egu25-8568, 2025.

The Indian Ocean is affected by seasonal and interannual climate variability, with large consequences on the water cycle over adjacent landmasses. The northern Indian Ocean influences the African and Asian monsoon systems, and its seasonal and interannual variability interacts with El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, potentially triggering Indian Ocean Dipole (IOD) development. Future projections suggest that Indian Ocean modes of climate variability could be enhanced under warming scenarios, but a lack of past records precludes a deeper understanding of how the Indian Ocean modes of variability could change over evolving boundary conditions.

Due to low sediment accumulation rates and bioturbation in marine cores, bulk analyses cannot usually record past climate variability at seasonal or interannual timescales. In contrast, individual foraminifera analyses (IFA) can theoretically estimate the total variance in a population of foraminifera that experienced upper ocean variability at sub-centennial timescales. Geochemical records (δ¹⁸O, Mg/Ca) based on IFA have been used to reconstruct past climate variance including seasonality, ENSO in the Pacific Ocean, and IOD in the eastern Indian Ocean. However, using variance in a foraminifera population to pinpoint which mode of variability is the ultimate driver remains a challenge.

Here we model the range of variability of different key sites in the Indian Ocean to seasonal vs interannual climate variability using ORAS-5 re-analysis temperature and salinity data spanning the period from 1958 to 2018, and to assess the impact of changing the amplitude of these modes of variability on total δ¹⁸O variance of model foraminifera populations. In light of these results, we then analyzed IFA (δ¹⁸O and morphology) in three core-top samples, targeting two planktonic foraminifera species with different seasonalities and depth habitats: Ocean Drilling Program Site 722 in the Arabian Sea upwelling region, monsoon-influenced Site MD77-191 near the southern tip of India, and Site MD96-2060 core offshore Tanzania in the western Indian Ocean. Results will allow us to better evaluate how to interpret IFA in different oceanographic biomes of the Indian Ocean, ultimately leading to a better understanding of past climate extremes embedded in paleoclimate record.

How to cite: Lichterfeld, Y., Leduc, G., Thirumalai, K., Vidal, L., De Garidel-Thoron, T., Sonzogni, C., and Bolton, C.: Calibrating Individual Foraminifera Analysis for Climate reconstruction in the Western Indian Ocean: Assessing Seasonal and Interannual Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8783, https://doi.org/10.5194/egusphere-egu25-8783, 2025.

The Maldives archipelago has become one of the most famous touristic locations in the world and is under noticeable pressure from various human activities such as extensive fisheries and tourism.  Because of this, the Pristine Seas Project (National Geographic) included the area in its efforts to fill existing knowledge gaps such as its species diversity and distribution as part of a larger aim to establish science-based protected marine reserve zones. Foraminifera, unicellular marine marvels, are renowned for their use as an environmental bioindicator of broader conditions. The present study investigates the diversity and biogeographic patterns of recent shallow water foraminifera that inhabit three distinctive atolls in the Maldives archipelago. Sediment samples were collected from reef and lagoon environments of the 26 distinct localities across three southern Maldives atolls - Addu, Fuvahmulah, and Huvadhoo. The most abundant taxa are Amphistegina followed by Calcarina, Heterostegina and Sorites. A species richness and diversity varied among sampling sites, with higher richness observed in MV18 station (Addu Atoll). Cluster analysis revealed distinct foraminiferal assemblages associated with different reef zones and sediment types. Here we discuss environmental parameters such as depth, substrate characteristics, ocean current influence, foraminiferal distribution patterns within and between different atolls of Maldives and also in comparison to the greater Indian Ocean datasets. The empirical data generated in this study offers a better understanding of ecosystem biodiversity in this remote location which may act as a baseline for future experimental and ecological studies, assessing possible anthropogenic influences and provides valuable insights into the regional vulnerability to climate change. Present study highlights the importance of habitat, microhabitat conservation and contributes to our knowledge of Indian Ocean marine biodiversity and biogeography.

Keywords: Indian Ocean, assemblage, habitat, distribution, large benthic foraminifera

How to cite: Baroliya, H., Anagnostoudi, T., Oron, S., Sala, E., Freidlander, A., and Goodman- Tchernov, B.: A Baseline for Recognizing Change: Diversity and Biogeography of the Maldives Atolls' Shallow Water Foraminifera, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8934, https://doi.org/10.5194/egusphere-egu25-8934, 2025.

Mesoscale eddies have a significant impact on the biological productivity in the Bay of Bengal (Eastern Indian Ocean). However, we do not currently have a complete understanding of them. The Bay of Bengal is a region that is highly dependent on fishing for its population’s survival, with surrounding countries such as Bangladesh depending on fish from the bay for approximately 60% of their consumed animal protein, and hence understanding any changes in this biological productivity is important. In this work, using 25-42 years of satellite data and the py-eddy-tracker eddy detection algorithm (Mason et al., 2014), we observe a yearly recurring mesoscale eddy (called hereafter the Odisha Eddy) on the western coast of the Bay of Bengal, which was noted before but investigated here, for the first time, in terms of physics and biological response. We also examine the impact of a confluence zone that forms between the EICC as it reverses poleward and an equatorward flowing boundary during May to September on the Odisha Eddy variability, using a new algorithm that automatically identifies this confluence zone based on satellite altimetry currents data. We further investigated the impact that the EICC confluence zone has on the biological productivity of the Odisha Eddy. We found that the eddy has an increase in the concentration of chlorophyll-a by 42% as compared to the surrounding waters, and that when the eddy occurs alongside the confluence zone, the chlorophyll-a content within the Odisha Eddy is 36% higher than when the eddy occurs alone. The Odisha Eddy also presents a larger radius (31% increase), amplitude (47% increase) and faster rotational velocity (15% increase) when it occurs alongside the confluence zone. We conclude that the EICC confluence zone amplifies the positive effect that the Odisha Eddy has on the biological productivity of this region of the Bay of Bengal. We hypothesise that this is due to the confluence zone enhancing advection of nutrients within and around the eddy. More research is needed to fully examine this mechanism and other controls (e.g., impact of ocean planetary waves, wind field) influencing the Odisha Eddy productivity in the Bay.

How to cite: Luty, W., Jebri, F., Srokosz, M., Ross, A., Griffiths, S., and Jacobs, Z.: Impact of a newly observed recurring eddy on the western Bay of Bengal productivity , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9261, https://doi.org/10.5194/egusphere-egu25-9261, 2025.

The Indian Ocean Dipole (IOD) and Tripole (IOT) represent primary modes of interannual variability in the Indian Ocean, impacting both regional and global climate. Unlike the IOD, which is closely related to the El Niño-Southern Oscillation (ENSO), our findings unveil a substantial influence of the Australian Monsoon on the IOT. An anomalously strong Monsoon induces local sea surface temperature (SST) variations via the wind-evaporation-SST mechanism, triggering atmospheric circulation anomalies in the eastern Indian Ocean. These circulation changes lead to changes in oceanic heat transport, facilitating the formation of the IOT. Our analysis reveals a strengthening connection between the Australian Monsoon and the IOT in recent decades, with a projected further strengthening under global warming. This contrasts with the diminished coupling between ENSO and IOD in recent decades from observations and model projections, illustrating evolving Indian Ocean dynamics under the warming climate.

How to cite: Chen, M.: Emerging influence of the Australian Monsoon on Indian Ocean interannual variability in a warming climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10702, https://doi.org/10.5194/egusphere-egu25-10702, 2025.

Salinity plays a major role in the global hydrological cycle and climate by modulating upper ocean stratification and sea surface temperature. Past studies have revealed the salinity variation in different major ocean basins from interannual to decadal time scales. However, understanding of upper ocean salinity variation in the Indian Ocean is limited, especially on decadal time scale and thus its global impacts. Our study reveals the presence of a horseshoe pattern in the upper ocean salinity, which varies on a decadal time scale. This pattern is generated from air-sea interaction mechanisms and is linked with Ningaloo Niño. The sea surface temperature anomalies in the southeastern tropical Indian Ocean in relation to Ningaloo Niño triggers the circulation anomaly causing the variation in the precipitation pattern. As a result, the upper ocean freshens or gets more saline and forms as a horseshoe pattern in upper ocean salinity on a decadal time scale. This variation could be useful for better presentation of salinity variation in the Indian Ocean and its associated impacts.

How to cite: Sahu, L., Senapati, B., and Dash, M. K.: A horseshoe salinity pattern in the Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12271, https://doi.org/10.5194/egusphere-egu25-12271, 2025.

Sea Surface Temperature (SST) extreme, known as the Marine Heat Extremes (MHEs), has become more frequent and intense over the years in the Northern Indian Ocean (NIO), leading to increased uncertainty in monsoon and cyclones. In this study, we characterised the evolution of MHEs utilising the monthly Hadley Centre Sea Ice and SST (HadISST) for 1900–2020 over the NIO. For a comparative analysis of evolution of MHEs, the region was further divided into Eastern Equatorial IO (EEIO), Western Equatorial IO (WEIO), Arabian Sea (AS), and Bay of Bengal (BoB). A MHE event is defined when the SST crosses the monthly varying 98th percentile threshold corresponding to the fixed climatological baseline of 1901–1950. Two normalized indices, i.e., Normalized Extreme Frequency Index and Heat Index, have been utilized to understand the spatio-temporal characteristics of intensity and frequency, respectively. Both the indices show a non-linear exponential increment. Moreover, the area fraction experiencing MHEs was also found to increase swiftly, following a sigmoidal curve. Frequent mean regime-shifts in these quantities have been observed, increasing the unpredictability of the climate system. Moreover, statistical tests revealed that the MHE attributes are increasing because of the increasing mean SST rather than its variance.  A mixed layer heat budget analysis shows that the MHE attributes have been increasing more rapidly over the WEIO, followed by EEIO, AS, and lastly the BoB, majorly due to the net heat flux followed by the horizontal advection. These findings underscore the non-linear escalation of thermal stress on marine ecosystems and the broader climate, emphasizing the need to develop mitigation strategies.

How to cite: Gupta, H., Deogharia, R., and Sil, S.: Regime Shifts in Marine Heat Extremes in the Northern Indian Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15053, https://doi.org/10.5194/egusphere-egu25-15053, 2025.

With the developing observation system, our understanding of Indian Ocean circulation has advanced in recent years. We have noticed the rapid increase of the ocean content in the Indian Ocean, and its role in global climate changes. In the mean state, a relatively closed current loop is established by an eastward current along the equator and a westward current south of the equator, regarded as the Indian Ocean Tropical Gyre (IOTG). As an important component of Indian Ocean air-sea interaction, the essential impacts of IOTG have been discovered. Due to the monsoon, IOTG displays significant seasonal variations, characterized by the reversal of currents and associated heat-salt redistribution. Also, IOTG interacts with the climate modes. This paper summarizes the advances, including the multi-scale variations of IOTG, associated heat-salt transport, and its ecological impact.

How to cite: Du, Y., ZHao, Z., and Zhang, Y.: The Indian Ocean Tropical Gyre and associated heat-salt Transport and its ecological impact: A review, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15560, https://doi.org/10.5194/egusphere-egu25-15560, 2025.

Accurate representation and predictability of the Indian Ocean Dipole (IOD) in seasonal forecasts are crucial given its pronounced socioeconomic impacts on countries surrounding the Indian Ocean.  Using hindcasts from the coupled Met Office Global Seasonal Forecasting System (GloSea6), coupled mean state biases in the western and eastern equatorial Indian Ocean (WEIO and EEIO) and their impacts on IOD prediction are examined. 

Results show that GloSea6 exhibits a pronounced cold bias in the EEIO that rapidly develops after the onset of the monsoon in boreal summer (JJA, July-August) and persists through the autumn season (SON, September-November).  This cold bias, along with a dry bias, is linked to erroneous easterlies and a shallow thermocline, likely associated with the summer monsoon circulation.  The seasonal evolution and relative timing of the precipitation biases between the western and eastern IOD poles, such that the biases develop through JJA in the EEIO but follow in the WEIO in SON, suggests that the EEIO plays the leading role in the development of coupled feedbacks that result in an overall large dipole pattern of atmospheric and subsurface oceanic biases in SON.

Analysis of skill metrics for the IOD shows that GloSea6 achieves a high anomaly correlation coefficient at short lead times, though it tends to overestimate IOD ampltiude, indicating higher IOD variability compared to observations.  This overestimation is larger in the eastern IOD pole than in the western pole and is likely linked to the poor representation of the evolution of the SST anomalies in the EEIO during positive and negative IOD events in SON.  This aligns with the skill metrics of the individual poles, which show a lower anomaly correlation coefficient and higher prediction errors observed in the eastern pole compared to the west.

Results in this study highlight the crucial role of regional biases, particularly in the EEIO, in shaping IOD variability and suggest that addressing these regional biases in GloSea6 could improve IOD prediction skill, enhancing forecasts of climate impacts for countries surrounding the Indian Ocean.

How to cite: Turner, A., Gler, M., Hirons, L., Marzin, C., and Wainwright, C.: Systematic biases over the equatorial Indian Ocean and their influence on seasonal forecasts of the IOD, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19800, https://doi.org/10.5194/egusphere-egu25-19800, 2025.

The Great Whirl (GW), a prominent anticyclonic gyre in the northwest Indian Ocean, is crucial in regional circulation and energy dynamics during the summer monsoon. Using satellite observations and high-resolution ocean simulations, this study examines the mechanisms behind the growth and maintenance of Eddy Kinetic Energy (EKE) in the GW region. EKE peaks about 56 days after the summer monsoon’s peak, a delay caused by energy transfer processes. Southwest wind forcing during the monsoon initiates the EKE growth, with the barotropic energy conversions from mean flows eventually dominating the energy input. Enhanced stretching and shear effects of the Somali Currents (SC) intensify barotropic instabilities, maintaining EKE even as monsoon winds weaken. The baroclinic energy conversions act as a secondary energy input, exhibiting a positive eddy buoyancy work (potential energy to kinetic energy) at the upwelling wedge regions northwest of the GW. Our study highlights the importance of internal energy transfer processes in modulating ocean circulation and energy dynamics off the Somali Coast, emphasizing eddy-mean flow interactions and potential-to-kinetic energy transfer in the Somali upwelling system.

How to cite: Lin, J., Wang, M., Dai, L., Sasaki, H., and Du, Y.: Delayed Response of Eddy Kinetic Energy Build-up off Somali Coast During Summer Monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20979, https://doi.org/10.5194/egusphere-egu25-20979, 2025.

Climate variability in the Indian and Pacific Oceans exhibits strong coupling on interannual to decadal timescales. Since the early 2000s, however, synchronization of decadal climate modes between the two basins has decreased due to enhanced greenhouse gas forcing and anthropogenically driven warming of the Indian Ocean. Understanding mechanisms of decoupling is crucial for properly characterizing and predicting low-frequency (decadal-multidecadal) climate variations which have a large impact on regional water resources around the Indian Ocean rim and marine ecosystems.

Here we contextualize the recent inter-basin decoupling by reconstructing Indo-Pacific basin interactions over the past four centuries (1630-2000 CE) through leveraging a compilation of tropical paleoclimate archives and two reconstruction methods. Specifically, we employ a network of coral proxy records from the Indian Ocean, Maritime Continent, and Pacific Ocean, alongside select hydroclimatically-sensitive stalagmite and tree-ring records from the Indian Ocean rim to reconstruct the Indian and Pacific Walker circulations, as well as the Indian Ocean Basin Mode over the past four centuries.

Our results confirm that Indo-Pacific coupling was present throughout the preindustrial era, and was disrupted only by a series of strong tropical volcanic eruptions during the early 19th century. We find, based on last millennium climate model simulations and hemispheric temperature reconstructions, that the interhemispheric asymmetry of cooling in response to volcanic forcing as well as the Indian Ocean’s strong sensitivity to external forcings caused this anomalous decoupling of Indo-Pacific climate. Additionally, the mechanisms of past decoupling associated with volcanism provide insights into the source of inter-model spread on the magnitude of future Indo-Pacific trends.

How to cite: Wang, S., Ummenhofer, C., and Oppo, D.: Low-frequency coupling of the Indian and Pacific Walker circulation modulated by volcanic forcing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21348, https://doi.org/10.5194/egusphere-egu25-21348, 2025.

Chlorophyll concentrations in the ocean exhibit significant spatial and temporal variability, driven by a complex interplay of physical, chemical, and biological factors. Seasonal changes are primarily influenced by variations in light availability, temperature, and nutrient supply, while interannual fluctuations are often linked to large-scale climate phenomena such as the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). These variations have profound implications for oceanic primary productivity, marine ecosystems, and global biogeochemical cycles. The Arabian Sea, one of the most productive regions of the global ocean, experiences high biological productivity due to seasonal upwelling driven by the Southwest Monsoon and winter convective mixing. Upwelling brings nutrient-rich deep waters to the surface, stimulating phytoplankton growth and increasing chlorophyll concentrations, which in turn support a diverse marine ecosystem and economically significant fisheries. In our study, we analysed monthly and seasonal climatologies of chlorophyll(mg/m³) in the Arabian Sea, examining anomalies over a 24-year period (1998–2021). Our results revealed significant interannual variability, with notable peaks and dips in chlorophyll anomalies. For instance, anomalies exceeded a deviation from the climatological mean by 0.2 in 2005 but dropped as low as -0.2 in 2016. By analysing the standard deviation of log-transformed chlorophyll-a anomalies, we identified five regions exhibiting the highest variability. Further investigation into these regions revealed distinct patterns in upwelling dynamics across different years and seasons, emphasizing the diverse factors influencing upwelling processes in the Arabian Sea. Focusing on the western coast of India, we observed contrasting climatic behaviours between the northern and southern regions. In the northern part, wind anomalies did not directly correspond to chlorophyll anomalies, indicating a more complex interplay of factors. Conversely, in the southern region, a strong correlation between chlorophyll and wind anomalies suggests a dominant wind-driven upwelling mechanism. These findings enhance our understanding of the regional variability in upwelling processes and highlight the intricate interactions between oceanic and atmospheric drivers in this dynamic marine system. Our study provides valuable insights into the variability of chlorophyll concentrations in the Arabian Sea, offering a better understanding of its ecological and climatic significance. These findings contribute to improved modelling and prediction of primary productivity, which is crucial for both ecosystem management and climate studies.

How to cite: Joshi, S. and Prakash Dey, S.: Interannual Variability of Chlorophyll Concentrations in the Arabian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21671, https://doi.org/10.5194/egusphere-egu25-21671, 2025.

Pycnocline stratification is increasing across multiple ocean basins due to a warming surface ocean and changes in wind forcing. Pycnocline stratification plays a leading order role in tracer transport, shaping capacity for heat and carbon uptake, making it a key parameter of interest on timescales ranging from paleoclimate to plankton blooms. Part of the challenge in assessing the role of pycnocline stratification in global models is the two-way connection between physical processes at the mesoscale and submesoscale and stratification, with important implications for the resulting tracer transport. Using idealized runs of MITgcm, we find that the strength of pycnocline stratification influences the formation and evolution of submesoscale structure. When a constant isopycnal slope is initialized, tracers get efficiently transferred across the base of the mixed layer and get trapped in anticyclonic submesoscale vortices below the mixed layer. This leads to tracer concentrations below the mixed layer and fluxes through it to be stronger under decreased stratification conditions. In contrast, when the frontal lateral buoyancy gradient is held fixed while stratification changes, the vertical flux of tracers and the concentrations at depth stay constant across all examined stratification conditions. Understanding the relationship between pycnocline stratification and fine-scale physical motions is necessary to diagnose and predict trends in carbon uptake and storage, particularly in the Southern Ocean. 

How to cite: Dove, L., Freilich, M., Siegelman, L., Fox-Kemper, B., and Hall, P.: Frontal Subduction in an Increasingly Stratified Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-213, https://doi.org/10.5194/egusphere-egu25-213, 2025.

The Southern Ocean plays a major role in the global carbon cycle, with air-sea carbon dioxide (CO₂) exchanges influenced by phytoplankton phenology. However, during summer, global ocean biogeochemistry models struggle to capture the interplay between biological and physical processes and their combined effects on air-sea CO₂ fluxes. Improved constraints on phytoplankton seasonal dynamics and the mechanistic drivers of their spatial and temporal variability are crucial for refining these models. In this study, we used a 24-year (1998–2022) database of weekly vertical profiles of chlorophyll-a and particulate organic carbon concentrations. This observation-based 4D dataset was generated by matching up and merging satellite and hydrological data using a machine learning methodology trained on BioGeoChemical-Argo observations. A Functional Principal Component Analysis, coupled with a Gaussian Mixture Model, was applied to this dataset, enabling a revised regionalisation of the Southern Ocean based on the 3D seasonal variability of phytoplankton biomass and particulate organic carbon concentrations. Distinct latitudinal differences mostly align with physical oceanic fronts, while zonal differences emerge within the Antarctic Circumpolar Current region. These spatial patterns reflect regional disparities in key phytoplankton growth drivers, including light and nutrient availability, vertical mixing, and stratification. We further explore the interannual variability of this bioregionalisation, shedding light on the environmental drivers of phytoplankton seasonal dynamics and their potential biogeochemical impacts in the context of a changing climate.

How to cite: Mayot, N., Sauzède, R., Izard, L., Nerini, D., and Uitz, J.: Advancing understanding of Southern Ocean phytoplankton phenology with 4D data product, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-339, https://doi.org/10.5194/egusphere-egu25-339, 2025.

The Southern Ocean is a critical player in regulating Earth’s carbon cycle and climate, yet its role under future climate change remains uncertain. Studying the Southern Ocean under past climate states can help address this knowledge gap. For example, changes in the Southern Ocean, through modulating deep ocean carbon content, are widely thought to have played a driving role in the atmospheric carbon dioxide (CO₂) fluctuations of Earth’s past ice ages. Here we present three novel findings that advance our understanding of processes linking Southern Ocean changes to glacial CO₂ drawdown. First, a new proxy record reveals a tight coupling between atmospheric CO₂ levels and deep Southern Ocean carbon storage over the Last Glacial Cycle, providing clear evidence of the systematic transfer of carbon into the deep ocean during glaciation and its release during deglaciation. These results are used to quantify the deep Indo-Pacific’s remineralized carbon content at the Last Glacial Maximum and are found to explain a significant proportion of observed glacial CO₂ drawdown. Second, we demonstrate how improved ventilation of North Pacific mid-depths (evidenced by glacial proxy data) directly impacts Southern Ocean biogeochemistry by reducing the carbon and nutrient load of waters upwelling in the Southern Ocean. This process enhances biological pump efficiency, curtailing Southern Ocean CO₂ outgassing and highlighting a critical interhemispheric connection in glacial nutrient cycling. Finally, idealized numerical modelling experiments demonstrate cooling of the high northern latitudes associated with a large Northern Hemisphere ice sheet can, in isolation of any other forcing, remotely induce glacial-like changes in Southern Ocean sea ice, circulation, and biogeochemistry that work to enhance ocean carbon content. These changes include expanded southern sea ice and cooler, saltier, better-stratified bottom water, which together increase oceanic carbon storage via solubility- and disequilibrium-driven effects. Collectively, these findings underscore the Southern Ocean’s central role in mediating interhemispheric and glacial climate feedbacks, offering new insights into the processes that drove the Earth’s into low-CO₂ glacial periods.

How to cite: Shankle, M., MacGilchrist, G., Rae, J., and Burke, A.: Southern Ocean mechanisms of glacial CO2 drawdown and their links to global climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-427, https://doi.org/10.5194/egusphere-egu25-427, 2025.

Recent observations reveal that Antarctic Bottom Water is thinning, warming, and spreading northward more slowly into the abyssal ocean. The causes and consequences of these changes remain uncertain due to limited observations. Using historical data (1985-2024) and model projections (2041-2050), we assess abyssal ocean changes in the past, present, and future. Between 1985-2024, isopycnals below 3000~m descended at -149±5 m decade^-1, and are replaced by warmer water, causing warming of 0.03±0.02 °C decade^-1. Freshening of -0.004±0.003 g kg^-1 decade^-1 occurred due to meltwater-driven changes in continental shelf waters forming bottom water. Projections suggest bottom water thinning will continue, doubling the abyssal ocean’s contribution to Southern Ocean sea level rise by 2050. However, freshening has slowed or reversed, indicating a salinity turning point. This transition occurs as shelf waters become too fresh and light to reach the abyssal ocean, with important implications for overturning circulation, nutrient cycling, and regional sea level rise.

How to cite: Gunn, K., England, M., and Rintoul, S.: Persistent Abyssal Warming but Emerging Salinity Shifts in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1745, https://doi.org/10.5194/egusphere-egu25-1745, 2025.

The Amundsen Sea, located in West Antarctica, is undergoing rapid melting due to the intrusion of Circumpolar Deep Water. This intrusion results in ice sheet thinning and basal melting, which can have cascading effects on the biogeochemical cycle of dissolved organic matter (DOM) by introducing iron from sea ice and glaciers and by affecting ocean circulation. Therefore, it is crucial to understand the dynamics of the DOM in this region. Our study focused on assessing the optical properties of DOM in the oceanic areas adjacent to the West Getz Ice Shelf (WGIS) and the Dotson Ice Shelf (DIS). Notably, the WGIS regions exhibited relatively high dissolved organic carbon (DOC) and chromophoric DOM (CDOM) absorption coefficient at 350 nm (a₃₅₀). Molecular weight indices, including spectral slope coefficient (S_275-295) and specific UV absorbance at 254 nm (SUVA₂₅₄), suggested that high molecular weight DOM with a substantial aromatic component predominated in the WGIS regions. Conversely, the DIS regions showed low CDOM values, low SUVA₂₅₄ values, and elevated S_275-295 values, indicative of low molecular weight CDOM with lower aromaticity. Furthermore, we observed significant negative correlations between the biomass of Phaeocystis antarctica (P. antarctica) and phosphate (PO₄) in the WGIS regions. However, no such relationship was found in the DIS region. These findings imply that the high concentration and molecular weight of a₃₅₀ in the WGIS regions, spanning from the surface layer to deeper depths, are predominantly driven by autochthonous sources, notably the colony-forming blooms of P. antarctica. The results of this study highlight the crucial role of bloom conditions in shaping both the quantity and quality of DOM in the Amundsen Sea.

How to cite: Jung, J., Son, J., Lee, Y., Kim, T.-W., Park, J., and Jeon, M. H.: Distinct optical properties of dissolved organic matter near Getz and Dotson ice shelves in the Amundsen Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1761, https://doi.org/10.5194/egusphere-egu25-1761, 2025.

The Ross Sea is a critical region for understanding phytoplankton dynamics and their role in polar marine ecosystems in the west Antarctica, particularly in the context of rapid environmental change. Investigating the spatial variability of phytoplankton biomass and community structure is essential for assessing ecosystem productivity and biogeochemical cycling in this vulnerable area. From January to February 2023, a field survey of the Ross Sea was conducted onboard the icebreaker research vessel Araon, utilizing high-resolution, continuous observations of surface seawater with the Algae Online Analyzer and Imaging FlowCytoBot. These observations provided precise spatial distribution data on phytoplankton biomass and species abundance. The results revealed intricate spatial variability in phytoplankton community structure and biomass across the eastern and western Ross Sea, spanning coastal to offshore gradients, and in the dynamic waters near polynyas and ice shelves. The phytoplankton community was predominantly composed of diatoms, especially Fragilariopsis spp., and/or Phaeocystis spp., with their dominance varying across spatial gradients. The observed patterns suggest that multiple interacting environmental factors, including sea ice concentrations, water masses, and ocean currents, influence on phytoplankton distribution in the region. These findings highlight the critical role of physical and biological interactions in phytoplankton distributions and offer valuable insights into their potential responses to environmental changes in the Ross Sea ecosystem.

How to cite: Lee, Y., Jung, J., Park, J., and Moon, J. K.: Spatial Variability of Phytoplankton Communities in the Ross Sea: Insights from High-Resolution Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1764, https://doi.org/10.5194/egusphere-egu25-1764, 2025.

Walter Munk’s seminal work, Abyssal Recipes, has established a foundational framework for comprehending abyssal water upwelling for almost 60 years. While it has profoundly influenced theoretical, laboratory, and observational studies of deep-ocean processes, discrepancies arise when compared with long-term observational data. One prominent paradox is known as the interior downwelling conundrum: When Munk's framework is applied to conditions of bottom-intensified mixing, it predicts downwelling rather than upwelling, which contradicts the mass balance in the abyssal ocean. This study revisits this challenge by investigating the unsteady dynamics of abyssal isopycnals. We demonstrate that under a cooling regime in the abyssal ocean, which is linked with the formation of the Antarctic Bottom Water (AABW) in the last little Ice Age more than 1000 years ago, rising water parcels can co-exist with downward diapycnal velocities. This reconciliation aligns Munk’s theory with the observed mass balance and resolves the longstanding paradox. These findings provide fresh insights into the conundrum and contribute to advancing our understanding of the closure of the global thermohaline and overturning circulations.

How to cite: Han, L.: Revisiting the Conundrum of Interior Downwelling in Munk’s Abyssal Recipes: An Unsteady Perspective Linked to a Cooling AABW, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2364, https://doi.org/10.5194/egusphere-egu25-2364, 2025.

The ongoing increase of global mean temperature, caused by anthropogenic CO₂ emissions, will most likely lead to enhanced melting and calving of Antarctic ice shelves in the coming decades. As a consequence, the freshwater input into the Southern Ocean is expected to increase as well. The resulting change in ocean salinity could have significant consequences for ocean circulation, water column stratification, and water mass formation in the Southern Ocean, which are all expected to affect the capacity of the surface ocean to remove CO₂ from the atmosphere, and the sequestration of carbon in the deep ocean. However, the magnitude and spatio-temporal patterns of these changes and their links to freshwater forcing are not yet well understood. To reduce these uncertainties, increase our understanding, and better quantify the feedbacks on the climate system, the international SOFIA initiative (Swart et al., 2023) defines freshwater input protocols for consistent use in various Earth System Models. Here we study the impact of additional freshwater around Antarctica on circulation and carbon fluxes in a steady preindustrial climate state using four Earth System Models. Most of the models show a decrease in the uptake of CO₂ by the surface of the Southern Ocean, caused by a strengthened outgassing of natural CO₂ between 50°S and 60°S. The stronger outgassing can be attributed to an increase in sub-surface dissolved inorganic carbon concentration south of the Antarctic Circumpolar Current that is associated with a redistribution of water masses in the Southern Ocean. Furthermore the reduction of the production and downward flow of Antarctic Bottom Water is leading to a decrease of its volume, and the expansion of carbon-rich Circumpolar Deep Water, which increases the carbon content at depth and thus weakens the overall CO₂ uptake. However, the models disagree in terms of the intensity of the weakened Southern Ocean CO₂ uptake. This difference seems to be mainly linked to the model resolution and the representation of the ocean mean state, e.g. the strength of the stratification, which is a determining factor for the redistribution of the additional freshwater to depth. To pursue this work, experiments with additional freshwater forcing in various climate states are conducted to analyse the ocean carbon cycle’s response and quantify potential climate feedbacks.

How to cite: Jouet, O., Hauck, J., Danek, C., Haumann, A., Hattermann, T., Muilwijk, M., Pauling, A. G., Swart, N. C., and Völker, C.: Impact of additional freshwater around Antarctica on the Southern Ocean carbon cycle : an inter-model comparison, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2563, https://doi.org/10.5194/egusphere-egu25-2563, 2025.

The physical and biogeochemical processes governing the air-sea CO2 flux in the Southern Ocean are still widely debated. This presentation focus on an anomalously large sink of CO2 observed north of the Weddell Sea in Summer 2022. The processes behind this anomalous situation are analyzed based on the combination of in situ observations, various satellite parameters (altimetric currents, Chl-a, ice concentration and sea surface salinity) and ocean model reanalysis.

The “Southern Ocean Carbon and Heat Impact on Climate” cruise in Summer 2022 aimed at studying physical and biogeochemical processes in the Weddell Sea and in its vicinity. A “CARbon Interface OCean Atmosphere” (CARIOCA) drifting buoy was deployed in January 2022 in the subpolar Southern Ocean, providing hourly surface ocean observations of fCO2 (fugacity of CO2), dissolved oxygen, salinity, temperature and chlorophyll-a fluorescence for 17 months. An underwater glider was piloted with the buoy for the first 6 weeks of the deployment to provide vertical ocean profiles of hydrography and biogeochemistry. These datasets reveal an anomalously strong ocean carbon sink for over 2 months occuring in the region of Bouvet Island and associated with large plumes of chlorophyll-a (Chl-a). Based on Lagrangian backward trajectories reconstructed using various surface currents fields, we identified that the water mass reaching the Bouvet Island region originated from the south-west, from the vicinity of sea ice edge in Spring 2021. We suggest that a strong phytoplankton bloom developed there in November 2021 favoured by early sea ice melt in 2021 in the Weddell Sea. These waters, depleted in carbon, then travelled to the position of the CARIOCA buoy. The very low values of ocean fCO2, measured by the buoy (down to 310 μatm), are consistent with net community production previously observed during blooms occurring near the sea ice edge, partly compensated by air-sea CO2 flux along the water mass trajectory. Early sea ice retreat might therefore have caused a large CO2 sink farther north than usual in Summer 2022, in the Atlantic sector of the subpolar Southern Ocean. Such events might become more frequent in the future as a result of climate change.

How to cite: Boutin, J., Naëck, K., Swart, S., Du Plessis, M., Merlivat, L., Beaumont, L., Lourenço, A., D'Ovidio, F., Rousselet, L., Ward, B., and Sallée, J.-B.: Anomalous Summertime CO2 sink in the subpolar SouthernOcean promoted by early 2021 sea ice retreat, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2577, https://doi.org/10.5194/egusphere-egu25-2577, 2025.

Dense water formation on the Antarctic continental shelf drives the abyssal overturning circulation, being the main process by which Antarctic Bottom Waters form. Despite its importance, most ocean models cannot simulate dense water formation at the Antarctic coast and flow down the continental slope (i.e., overflow) due to the fine resolution required by these processes. While many studies have looked at the impact of horizontal and vertical resolution in the deep ocean on the overflows, no studies have investigated whether surface vertical resolution impacts dense water formation. In this work, we varied the surface ocean cell of two dense water-forming models from 1m to 5m thickness as a simple vertical resolution sensitivity test. We used the ACCESS-OM2 and the Pan-Antarctic ocean and sea ice models, each employing a different boundary layer parameterization. Thickening the surface cell to 5m in ACCESS-OM2 decreased the dense water formation at the Antarctic continental shelf by 45% (1.5 Sv) and ceased its overflow through the continental slope after 10 years of simulation. In the Pan-Antarctic, thickening the surface cell reduced the Antarctic dense water formation by 34% (1.5 Sv) and its overflow by 67% (2.5 Sv) after 10 simulation years. The dense water formation reduction in 5m experiments is explained by a southward shift in the surface Ekman transports, bringing light offshore waters to the coast and prohibiting dense water formation at the Antarctic continental shelf. This response is independent of the boundary layer scheme employed.

How to cite: Aguiar, W., K. Morrison, A., McC. Hogg, A., Huneke, W., Hutchinson, D., Spence, P., Colombo, P., and D Stewart, K.: Sensitivity of Antarctic dense water formation to surface vertical resolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3356, https://doi.org/10.5194/egusphere-egu25-3356, 2025.

While the Southern Ocean is freshening, the sources of this freshening and their variability remain uncertain. Freshwater enters the ocean as Meteoric Water (MW; precipitation, river runoff, glacial discharge) and Sea Ice Meltwater (SIM). These inputs can be quantified using seawater salinity and stable oxygen isotopes in seawater, δ¹⁸O; however it involves the challenging task of determining the isotopic signature of MW (δ¹⁸O_MW). Here, we apply Self-Organising Map (SOM), a machine learning technique, to water mass properties to estimate the global distribution of the isotopic signature of MW (δ¹⁸O_MW) by characterizing distinct salinity-δ¹⁸O relationships from two comprehensive datasets. The inferred δ¹⁸O_MW is then used in a 3-endmember mixing model to estimate MW and SIM contributions to global ocean freshwater content. Our results show the large scale distribution of MW and SIM, as well as giving insights into their role in mass transformation and interannual variability. We highlight the MW content in Ice Shelf Water and Antarctic Bottom Water linked to glacial melt, which is concurrent with brine content derived from sea ice formation. Our results also show that AABW has freshened since the 1990’s due to a reduction of sea ice formation (less brine production) rather than an increase in glacial melt, and suggest the emergence of anthropogenic forced signals in seawater δ¹⁸O.

How to cite: Davila, X., L. McDonagh, E., Jebri, F., Gebbie, G., and P. Meredith, M.: Freshwater Sources and their Variability through Salinity-δ18O Relationships: A Machine Learning Solution to a Water Mass Problem, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4254, https://doi.org/10.5194/egusphere-egu25-4254, 2025.

The Southern Ocean plays a crucial role in the global carbon cycle, uptaking about 43% of the ocean’s CO₂ uptake. However, significant uncertainties remain regarding the processes governing CO₂ fluxes in this region. Mesoscale eddies have been identified as a potential source of these uncertainties. In this study, we analyze 30 years of daily, high-resolution (10 km) global simulations using the ICON-O ocean model coupled with the HAMOCC biogeochemistry model. The Okubo-Weiss parameter and vorticity is used to classify four flow regimes—anticyclonic eddy cores, cyclonic eddy cores, eddy core peripheries, and quiescent background—allowing us to generate composites for each. Our results show that CO₂ flux is directed into the ocean across all four regimes, with the magnitude of CO₂ uptake varying by regime. Anticyclones have a greater capacity for CO₂ uptake compared to cyclones. In certain regions (i.e. Agulhas retroflection and the Brazil-Malvinas confluence) anticyclones exhibit the highest CO₂ uptake capacity among the four regimes, a pattern linked to the greater eddy intensity of these areas. An analysis of the CO₂ flux terms shows that wind speed and ∆pCO₂ are the primary contributors to flux magnitud and variability. As atmospheric pCO₂ is prescribed, the main changes in ∆pCO₂ are related to changes in oceanic pCO₂. These variations are primarily driven by dissolved inorganic carbon (DIC) and sea surface temperature, which tend to compensate for each other, with DIC having a stronger influence. To explore DIC changes, we analyzed DIC budgets in the first 300 m. In the surface layer, the total DIC tendency is predominantly driven by vertical diffusion, which causes a net loss of DIC to deeper layers and induces atmospheric CO₂ flux into the ocean. Vertical diffusion is particularly stronger in anticyclonic eddies, explaining their enhanced ability to absorb CO₂. In deeper layers, the total DIC tendency is primarily controlled by the divergence of advective fluxes, while changes in DIC from sources and sinks (i.e. biogeochemical processes) are almost entirely balanced by vertical diffusion. These findings highlight the dominant role of mesoscale eddies in oceanic carbon uptake and underscore the need for more refined models to accurately represent their impact on the global carbon cycle.

How to cite: Salinas Matus, M., Serra, N., Chegini, F., and Ilyina, T.: Impact of Mesoscale Eddies on CO₂ Fluxes in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5183, https://doi.org/10.5194/egusphere-egu25-5183, 2025.

The Southern Ocean plays a critical role in global climate regulation, acting as a key pathway for heat, momentum, and biochemical transport. Using expanded hydrographic observations from the WOCE/CLIVAR SR03 section (1991–2018), a choke point between Tasmania and Antarctica, along with high-resolution GLORYS12 reanalysis data (1993–2018), we investigate long-term changes in the Antarctic Circumpolar Current (ACC) and Tasman Outflow (TO) and their relationship with the southward migration of the Subtropical Front (STF). Our results reveal a significant southward migration of the STF, while the Subantarctic Front remains meridionally stable. This STF migration is associated with an intensified TO and strengthened Ekman convergence, leading to enhanced subduction of cold, low-salinity intermediate water in the Subantarctic Zone and increased subsurface warming (0.4–0.8°C per decade) north of the ACC. The TO has strengthened significantly (+3.1 Sv per decade), while the ACC’s geostrophic transport shows an increasing trend (+3.2 Sv per decade), indicating the poleward expansion of subtropical circulation towards Antarctica. Additionally, the correlation between TO transport and the Southern Annular Mode (SAM) index (r = 0.48, p < 0.05) suggests that westerly wind variability plays a key role in modulating ocean transport in this region. Our findings emphasize the importance of STF migration and TO expansion in shaping Southern Ocean circulation under climate change. The poleward shift of westerlies and subtropical systems underscores the ongoing sensitivity of transport processes to global warming. Further research with extended observations and high-resolution climate models is needed to refine projections of circulation changes and their impact on oceanic heat transport in the Southern Ocean.

How to cite: Park, T., Kim, Y. S., and Park, J.: Poleward Migration of the Subtropical Front and Its Implications for the Tasman Outflow and Antarctic Circumpolar Current in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7897, https://doi.org/10.5194/egusphere-egu25-7897, 2025.

The Southern Ocean (SO) plays a crucial role in regulating the Earth’s climate by absorbing atmospheric carbon and storing it in the deep ocean. (Brovkin et al., 2012). Past studies reported a reduction in this uptake capacity over the last few decades. However, the temporal and spatial variability of the Southern Ocean’s carbon pump appears to be more complex  (Landschützer et al., 2015), with the Indian sector arguably being the least explored and understood region. A key objective of this PhD is to use radiocarbon signatures of dissolved inorganic carbon (DI¹⁴C), combined with stable carbon isotopes to quantify exchange processes between the ocean, atmosphere and sediments along the East Antarctic margin. A comprehensive sample set of sea water, sediment pore water and sediment surface samples was collected during three expeditions (2022 - 2024) on R/V Polarstern from more than 50 stations. Several sites from key locations will be selected to study ocean ventilation, water mass alterations and exchanges, and bottom water formation in the region. In addition, we will perform analyses in high spatial resolution to target more specific research questions, in particular ocean – ice shelf interactions. The East Antarctic coast from Prydz Bay to Vincennes Bay lends itself as case study area for tracing the inflow of Circumpolar Deep Water onto the continental shelf and its interaction with the marine-based portions of the Amery and Shackleton Ice Shelves, as well as Totten Glacier. Sections sampled along meridional transects further offshore and to the west allow to differentiate between regional varieties of Antarctic Bottom Water formation. Comparing water column data with ¹⁴C signals recorded in surface sediment benthic foraminifera will contribute to an improved understanding of the use of radiocarbon as a ventilation age proxy. Initial analyses are being conducted at the radiocarbon laboratory at Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, equipped with an accelerator mass spectrometer (AMS) Mini Carbon Dating System (MICADAS). Next steps involve method development, focusing on optimizing the graphitization process of DIC samples prior to radiocarbon analysis.

Brovkin, V., Ganopolski, A., Archer, D., & Munhoven, G. (2012). Glacial CO 2 cycle as a succession of key physical and biogeochemical processes. Climate of the Past, 8(1), 251-264.

Landschützer, P., Gruber, N., Haumann, F. A., Rödenbeck, C., Bakker, D. C., Van Heuven, S., ... & Wanninkhof, R. (2015). The reinvigoration of the Southern Ocean carbon sink. Science, 349(6253), 1221-1224.

How to cite: Rosmann, M. L., Grotheer, H., Lembke-Jene, L., Haumann, A., and Mollenhauer, G.: Carbon Exchange and Ocean Ventilation along the East Antarctic Margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8065, https://doi.org/10.5194/egusphere-egu25-8065, 2025.

The Southern Ocean plays a crucial role in absorbing carbon dioxide from the atmosphere, accounting for 20% of the ocean's carbon sink despite covering only 10% of the global ocean area. Antarctic Krill (Euphausia superba) are central to this process, as their faeces help remove carbon from the upper ocean by sinking to deeper layers. However, microplastics are increasingly polluting the Southern Ocean, and have been found in zooplankton, especially krill. These buoyant microplastics may slow the sinking of krill faeces, potentially reducing the amount of carbon that is trapped in the deep ocean. Whether, and to what extent, microplastics impact faeces sinking is still an open question. To address this gap, we developed a theoretical model to study how microplastics affect the density and fragmentation of krill FP which in turn will impact their vertical sinking to the oceanic depths. Our findings suggest that in environmentally relevant concentrations, microplastics could slow down the sinking of these pellets. Larger microplastics have the most impact, causing greater fragmentation of the faeces as they settle in the water column. While the buoyancy effect of microplastics is currently marginal due to the density change, under a business-as-usual scenario. Our results highlight that future increases in microplastics will likely have a significant negative impact on the ability of krill to promote the storage of carbon in the deep ocean.

How to cite: Wu, N., Hunter, A., and Manno, C.: Impact of Microplastics on Antarctic Krill Faeces Carbon Sequestration in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8079, https://doi.org/10.5194/egusphere-egu25-8079, 2025.

Ocean export production is the backbone of marine ecosystems, and it is crucial to carefully track their changes under the global warming. This study analyzes the irreversibility of ocean export production and the major factors affecting it by performing CO2 reduction experiments with the LOVECLIM, a medium complexity model. After a 1,500-year spin-up to the present-day CO2 level of 367 ppm, it was increased by 1% per year for 140 years and then decreased again for the same period. The present-day CO2 level was then held for 5,000 years. Ocean export production decreased at low latitudes and increased at mid- and high-latitudes, with the largest changes occurring in the equatorial and Antarctic Circumpolar Current regions. Nutrient concentrations in the euphotic zone decreased as the global ocean circulation weakened and ocean stratification intensified. Nevertheless, ocean export production has increased at high latitudes because of deepen mixed layer depth due to strong westerlies of both hemisphere and the creation of sea ice melting, which has led to the widen euphotic zones, high nutrient concentrations despite decreased nutrient concentrations, and increased ocean temperatures. The irreversibility of ocean export production with CO2 concentration is seen in 84.3% of the world's oceans. It will take more than 1,300 years for global ocean export production to return to its initial concentration after a CO2 reduction experiment, so it will need to be carefully monitored over a long period of time.

Acknowledgements: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Government of Korea (MSIT) (No. 2022R1A2C1008858) and Global - Learning & Academic research institution for Master’s·PhD students, and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2024-00443714).

How to cite: Wie, J. and Moon, B.-K.: Analyzing Factors that Influence the Irreversibility of Ocean Export Production, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8145, https://doi.org/10.5194/egusphere-egu25-8145, 2025.

Assessing and understanding the factors that control the biological carbon pump (BCP), i.e. the transfer of organic carbon biologically fixed by primary production (PP) from the euphotic zone to the deep ocean, remains a major challenge in marine biogeochemistry. Among these factors, the intensity of PP and the structure of phytoplankton community play key roles in the biogeochemical fluxes of the BCP and depend on the physico-chemical conditions of the ocean. Although the BCP has received significant attention in the last decades, the magnitude of this process remains poorly quantified, notably for under-sampled areas such as the Indian sector of the Southern Ocean (ISSO). The latter hosts contrasting biogeochemical provinces, from low productive systems with High Nutrient Low Chlorophyll areas, to high productive regimes in the vicinity of the Subantarctic Islands as a consequence of natural iron fertilization.

This study aims to assess the links between the PP and phytoplankton community structure in the ISSO. We present results from the SOCARB (South Indian Ocean CARBon fluxes from the surface to the mesopelagic twilight zone) cruise conducted during the late austral summer of 2023. This includes: (i) PP from ¹³C method and on-deck incubations, (ii) relative phytoplankton chemotaxonomic groups from pigments data and total chlorophyll a (TChla) and (iii) phytoplankton size classes abundances from on board cytometry flow analyses. PP and pigments were size-fractioned (< 3 µm; 3-20 µm; > 20 µm) following the three considered phytoplankton classes (pico-; nano-; microphytoplankton) to quantify their impact on organic carbon production and to address the size structure of the phytoplankton community.

At the Antarctic zone (AZ) and the Polar Frontal zone (PFZ), integrated TChla – over the 0.01 % euphotic layer depth – was structured by TChla_NANO (46 ± 12 %) and TChla_MICRO (40 ± 14 %) and featured a community dominated by diatoms and haptophytes (68 ± 8 %). The Subantarctic zone (SAZ) differs from the rest of the Southern Ocean (i.e. south of the subtropical front) with a distinct community and a TChla structured in pico- (42 %) and nano- (36 %). In the South Indian Ocean, the Subtropical zone (STZ) exhibited a TChla structured by TChla_PICO (43 ± 8 %) and TChla_NANO (39 ± 9 %) with a diversified community. From linear correlations and relative contribution of phytoplankton groups to TChla, we show that PP in the AZ and PFZ is conditioned by diatoms and haptophytes algal biomass in both nano- and micro- size classes. In the STZ, PP is mainly conditioned by the algal biomass of cyanobacteria in the pico- and by haptophytes, chlorophytes and dinoflagellates in the nano- size classes. Our results also underline the intra-zonal variability of PP and TChla through bottom-up processes, such as cyclonic eddy in the STZ or water mass intrusion in the PFZ. This study paves the way for a better comprehension of phytoplankton productivity and community size structure, which could contribute to a more detailed knowledge on their role in the BCP.

How to cite: Deteix, V., Ridame, C., Thyssen, M., Dimier, C., Lo Monaco, C., Metzl, N., Tribollet, A., and Planchon, F.: New insights into the primary production and the structure of the phytoplankton community in the Southern Indian Ocean from the subtropical to the Antarctic zones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8253, https://doi.org/10.5194/egusphere-egu25-8253, 2025.

There is general agreement that iron is the main nutrient limiting primary productivity in the Southern Ocean. In contrast to the average low biological productivity elsewhere in the Southern Ocean, the Kerguelen region is home to massive blooms that extend hundreds of kilometers offshore, and serve as a backbone to rich ecosystems. The blooms have been shown to be sustained by continental iron inputs, in particular by the resuspension of iron-enriched sediments over the plateau, transported eastward by the Antarctic Circumpolar Current. However, iron inputs from glacial erosion and ice melt may be another iron source. In particular, two of the outlet glaciers of Kerguelen's Cook Ice Cap transport iron-enriched lithogenic material downstream to the coastal waters of the Golfe des Baleiniers. Whether the circulation is able to connect the glacier outlets to the pelagic area, and how much of the pelagic bloom can be influenced by glaciogenic iron, are two open questions that we address here. Using in situ and satellite data, we show the persistence, on an interannual basis, of a chlorophyll-enriched plume connected to the Golfe des Baleiniers and driven by a horizontal advection of iron. Using a Lagrangian methodology,  we reconstruct the horizontal advection of iron and show that glaciogenic iron supply influences up to a third of the spatial extent of the open ocean bloom onset. We find that the new high resolution SWOT observations allow a significant reduction in altimetry biases attributable compared to previous products, allowing a better representation of fine scale biogeochemical structures. Our results are particularly relevant in the context of the negative mass balance of ice caps and glacial retreat evidenced both on Kerguelen and other Southern Ocean islands in the context of climate change.

How to cite: Nalivaev, A., D'Ovidio, F., Bopp, L., Berta, M., Azarian, C., Rousselet, L., and Blain, S.: Lagrangian reconstruction of a glaciogenic iron delivery to the Kerguelen blooms, Southern Ocean: comparison of SWOT-merged products with conventional altimetry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8611, https://doi.org/10.5194/egusphere-egu25-8611, 2025.

The recent decline in Antarctic sea-ice, notably the extreme low winter cover in 2023 and 2024 is a major source of concern. Some progress has been made towards determining the drivers of ice loss but uncertainty remains regarding its impacts, particularly for ocean-atmosphere interaction. Resolution of this uncertainty is important as ice decline can significantly modify surface heat loss, and thus the ocean and atmosphere. We show that the substantial failure of ice regrowth in winter 2023 provided a major new source of turbulent ocean heat loss to the atmosphere. Ice concentration in the Weddell, Bellingshausen and Ross Seas is reduced by up to 80% and is accompanied by an unprecedented doubling of mid-winter ocean heat loss. Furthermore, peak heat loss shifts from late April to mid-June with weaker than normal heat loss in austral autumn. The strengthening of winter surface heat loss is accompanied by changes on both sides of the ocean-atmosphere interface. These include a rise in frequency of atmospheric storms and greater surface heat loss driven dense water formation. The findings reveal that the record low winter 2023 Antarctic sea-ice cover substantially modified Southern Ocean-atmosphere interaction and motivate in-depth analysis of the wider climate system impacts. The subsequent evolution of low ice conditions together with their ocean-atmosphere impacts through to 2025 will also be considered.

How to cite: Josey, S., Meijers, A., Blaker, A., Grist, J., Mecking, J., and Ayres, H.: Record Low Winter 2023 Antarctic Sea-Ice Increased Ocean Heat Loss, Dense Water Formation and Storms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9075, https://doi.org/10.5194/egusphere-egu25-9075, 2025.

Dense waters formed in the North Atlantic (NADW) propagate southward into the Southern Ocean. There, a portion upwells to the surface in a south-eastward spiral (Tamsitt et al. 2017), a portion is entrained into the northward flowing Antarctic Bottom water (AABW), and a portion propagates northward at mid-depths of the Indian and Pacific Oceans. Combined, these processes result in a gradual eastward dilution of NADW at its core density around the Southern Ocean. However, no consensus has been reached on the dilution rate of these waters and how it impacts global ocean ventilation. Here, we use historical  observations to track the dilution of NADW around the Southern Ocean. We find a persistent  maximum at mid-densities, corresponding to the NADW core, which erodes eastward from the Atlantic sector. Furthermore, we evaluate the dilution of an artificial North Atlantic dye in three models of ocean transports: a 1° global configuration of the Nucleus for European Modelling of the Ocean (NEMO), version 2 of the Ocean Circulation Inverse Model (OCIM), and the Total Matrix Intercomparison (TMI). The erosion of the North Atlantic dye maximum around the Southern Ocean varies markedly across models, and is largest in TMI. The available  data point to an overly rapid erosion in TMI, but remain too scarce in the Pacific sector to place strong constraints on the dilution rate of the NADW core. A zonal circumpolar transect of salinity and  measurements, together with characterisation of inter-laboratory offsets, would greatly help to constrain the fate of NADW.

How to cite: Millet, B., Gray, W., de Lavergne, C., Waelbroeck, C., Reverdin, G., Pöppelmeier, F., and Roche, D.: On the fate of North Atlantic deep waters in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9664, https://doi.org/10.5194/egusphere-egu25-9664, 2025.

Under future climate forcing, in both strong emission (SSP585) and strong mitigation (SSP126) scenarios, the deep overturning circulation of the Southern Ocean collapses by 2050 in almost all CMIP6 simulations.  The SSP scenario applied ultimately has a relatively small impact on this shutdown, which appears to have commenced in the last few decades of the historical scenario runs – the present day.  The ensemble mean drops to 50% (10±4 Sv) strength, from a pre Industrial strength of 20±9 Sv, with individual models decreasing by over 65%.    This ‘collapse’ occurs over 30-50 years, with most of the temporal variation explained by internal variability within models. 

Associated with the change in overturning strength is a reduction in Antarctic Bottom Water (AABW) volume south of 30S by >2x10¹⁶ m³, driven by a reduction in formation rates around Antarctica.  Here surface warming reduces formation by approximately 4 Sv, whilst freshwater effects (due to sea ice reduction for example) are relatively weak.  Walin analysis shows that subsequent entrainment of Circumpolar Deep Water (CDW) by convecting AABW also significantly decreases, further reducing AABW volume and export.

The reduction in AABW formation results in an expansion of CDW volumes.  CDW upwelling reduced due to the lower cell shutdown is largely compensated by a wind driven increase in upper cell overturning of approximately 25%.  The increased upper cell overturning enhances SAMW export by up to 5 Sv, with a significant boost to net heat export due to both enhanced volumes and temperatures of this water mass.

We explore the wider impact of these Southern Ocean shifts in overturning on the net warming and carbon storage of the global ocean, and the wider global climate.  We contextualize climate model representations of bottom water processes against observations and other model studies to suggest that a deep overturning tipping point may have already been reached.

How to cite: Meijers, A. and Rosser, J.: Collapse of deep overturning under future climate forcing and impacts on ocean heat and carbon uptake in CMIP6 models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9932, https://doi.org/10.5194/egusphere-egu25-9932, 2025.

Dissolved organic carbon and nitrogen (DOC and DON, respectively) are important components of the carbon and nutrient cycle in marine systems. However, there are still significant gaps in understanding the role of these compounds in the biogeochemical cycles of polar environments, due to the limitations of spatiotemporal sampling. Here, we present an overview of surface distributions of DOC and DON along the Northern Antarctic Peninsula (NAP) and Atlantic Southern Ocean (ASO), using the dataset available between 1992 and 2022, collected during different seasons. We used DOC data collected from RV Almirante Maximiano cruises by the Brazilian High Latitude Oceanography Group (GOAL), along the NAP, DOC and DON data collected from RV Polarstern cruises (Alfred Wegener Institute, Germany), along the ASO, as well as DOC and DON datasets from international repositories. DOC and total dissolved nitrogen were analyzed primarily with a Shimadzu TOC-L® Series. DON was calculated by subtracting dissolved inorganic nitrogen concentrations from total dissolved nitrogen. Excess DOC and DON were calculated by subtracting the respective deep concentrations. Surface DOC concentrations ranged from 42.0 to 127.0 μmol kg^–1, while surface DON concentrations ranged from 1.0 μmol kg^–1 to 11.3 μmol kg^–1. At the surface, the highest concentrations of DOC and DON were observed mainly in the western sector of the Southern Atlantic (longitudes > 20° W), due to the proximity of coastal areas such as in the Gerlache Strait in the NAP and South Georgia and the Falklands Islands in the ASO. Increase in the surface concentrations of both DOC and DON were also associated to frontal systems. The accumulation of DOC and DON along the NAP and western sector of the southern Atlantic seem to confirm the link between the production of organic matter and the proximity of iron-supplying land-masses leading to enhanced primary production and plankton biomass (chlorophyll-a concentrations and particulate organic carbon). The production of nitrogen-rich organic matter by zooplankton seemed to be the main factor determining DON distributions. The wide spatial coverage of DOC and DON made it possible to identify significant differences in DOC and DON distributions between different regions, as well as interannual and seasonal differences from data collected in the same regions. DOC and DON can be considered important indicators for evaluating the coupling between physical, biogeochemical, and climate processes over time.

How to cite: Avelina, R., Klaas, C., Hamacher, C., O. Farias, C., Ludwichowski, K.-U., Burau, C., Kerr, R., P. Koch, B., M. Mata, M., and C. da Cunha, L.: Drivers of surface distribution of dissolved organic carbon and nitrogen along the northern Antarctic Peninsula and the Atlantic Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10829, https://doi.org/10.5194/egusphere-egu25-10829, 2025.

The dense waters formed on the broad continental shelves of the western Weddell Sea are the source of Weddell Sea Bottom Water (WSBW) found along the slope and at the bottom of the basin. WSBW is considered to be an effective conduit for carbon sequestration, eventually contributing to the redistribution of carbon around the global oceans. To quantify the efficiency of this carbon sequestration, it is necessary to have a good understanding of the processes that transform carbon within the ocean surface layer, as well as those occurring along the slope. Lack of biogeochemical data in the southern and western Weddell Sea is hindering progress. In situ observations are particularly necessary in the wintertime, when dense shelf waters and WSBW are formed and thus CO₂ uptake is constrained. We present a method that reconstructs the wintertime fugacity (i.e., the adjusted partial pressure) of CO₂ (fCO₂) on the southwestern continental shelves from the dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements made in the WSBW, but collected in the summertime, when most expeditions take place. The method relies on relationships between the contributions of different water masses to WSBW, and potential temperatures, as found in previous work. Results for reconstructed surface wintertime fCO₂ are comparable to the very few other wintertime observations elsewhere in the Weddell Sea. Without in situ wintertime observations, validation of the results is challenging. The results suggest a negligible role for biogeochemical processes transforming DIC between wintertime shelf water and WSBW. Assumptions in the methodology need to be tested against in situ biogeochemical measurements on the shelves and along the slope. We applied our method to recent data and relate findings to other biogeochemical variables to narrow down the uncertainty in our assumptions. 

How to cite: Hoppema, M., Droste, E. S., Bakker, D. C. E., and Huhn, O.: Reconstructing wintertime surface layer fCO2 in the western Weddell Sea using summertime observations of Weddell Sea Bottom Water, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10954, https://doi.org/10.5194/egusphere-egu25-10954, 2025.

Temperature reconstructions from Greenland and Antarctica during glacial times show anti-phased oscillations which are assumed to be connected to changes in ocean circulation patterns. However, the research focus of most studies is set to the Northern Hemisphere, connecting Dansgaard-Oeschger (DO) events to changes of the Atlantic Meridional Overturning Circulation (AMOC). Meanwhile, in the Southern Ocean (SO), millennial-scale oscillations, driven by changes in the formation of deep water, have been found in different climate model simulations, yet the exact mechanism leading to these changes is still not fully understood. These oscillations, diagnosed by the strength of Antarctic Bottom Water (AABW) formation, have been simulated under warmer climate conditions and also in experiments with additional freshwater input to the North Atlantic. Here we present results of multi-millennial experiments with the fast Earth system model CLIMBER-X and the coarse resolution General Circulation Model (GCM) CM2Mc in which the AMOC is collapsed by freshwater forcing in the north Atlantic and convection is eventually triggered in the SO. We aim to find the drivers of convection onset in the SO and the subsequent strengthening of AABW by analysing the changes of temperature and salinities in both models. Between the two models, we compare how the dynamics of features such as sea-ice, wind stress and thermodynamic ocean variables contribute to changes of SO convection and AABW formation. We also analyze additional sensitivity experiments with CLIMBER-X to explore which conditions lead to oscillations.

How to cite: Höse, A., Willeit, M., Feulner, G., and Robinson, A.: Understanding multi-millennial variability in the Southern Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11096, https://doi.org/10.5194/egusphere-egu25-11096, 2025.

Under a warming climate ice sheets are releasing freshwater to the ocean and affecting the global ocean circulation. One important region where freshwater is released and impacts the global ocean is the Antarctic continental shelf, where Antarctic Bottom Water (AABW) is formed. The response of AABW formation and circulation to increased glacial melt is uncertain, because global ocean models struggle to capture AABW formation, sinking and spreading across the abyss. Here, we present an equilibrated 1º degree global ocean-ice configuration with AABW formation on the shelves and realistic abyssal ventilation. This configuration also includes dye and age tracers that track ventilation pathways and timescales. Using this model, we perform an idealized ‘antwater’ hosing experiment, releasing exactly 0.1 Sv of freshwater uniformly around the Antarctic coast for a century (without offset from surface salinity restoring). The response of Southern Ocean circulation and ventilation is analysed and discussed in the context of previous model studies.

How to cite: Gülk, B., de Lavergne, C., Sallée, J.-B., Madec, G., and Rousset, C.: The response of the Southern Ocean to freshwater hosing in an equilibrated 1º NEMO configuration with realistic ventilation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11251, https://doi.org/10.5194/egusphere-egu25-11251, 2025.

The interaction between ocean eddies and the large-scale ocean circulation results in a pronounced multidecadal variability in the Southern Ocean, with a dominant periodicity of 40-50 years, referred to as the Southern Ocean Mode (SOM) (Le Bars et al., GRL, 2016). The SOM plays a critical role in modifying the ocean heat content, thereby influencing both sea-ice extent and basal melt around Antarctica. Additionally, this multidecadal variability propagates northward into the Atlantic Ocean, modulating the Atlantic Meridional Overturning Circulation (AMOC) strength. The AMOC is one of the most prominent climate tipping elements on Earth and can potentially collapse as a consequence of surface freshwater input in the North Atlantic. Here, we investigate the impacts of an AMOC collapse on multidecadal variability in the Southern Ocean using the results of the first modeled AMOC collapse in a high-resolution and strongly eddying (0.1° horizontal resolution) ocean-only model, the Parallel Ocean Program (POP). Our findings indicate that the magnitude of the SOM reduces significantly following an AMOC collapse. An analysis of the SOM variability before and after an AMOC collapse allows us to study the role of background stratification, baroclinic instability, and convection in shaping the SOM.

How to cite: Smolders, E., van Westen, R., and Dijkstra, H.: The Impacts of an AMOC Collapse on Southern Ocean Multidecadal Variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12345, https://doi.org/10.5194/egusphere-egu25-12345, 2025.

How to cite: Poumaëre, N., MacGilchrist, G., and Khatri, H.: Changes in ideal age distribution in response to coupled wind/sea ice perturbations in a Southern Ocean channel model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13255, https://doi.org/10.5194/egusphere-egu25-13255, 2025.

How to cite: Tranchant, Y.-T., Legresy, B., Foppert, A., Pena-Molino, B., and Phillips, H.: SWOT reveals fine-scale balanced motions and dispersion properties in an energetic meander of the Antarctic Circumpolar Current, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15046, https://doi.org/10.5194/egusphere-egu25-15046, 2025.

Antarctic Intermediate Water (AAIW), occupying a vast area in the intermediate depths of the southern hemisphere oceans, is important for heat and freshwater redistribution in the global oceans. However, questions remain regarding its sources at the surface and pathways to the intermediate depths. To answer these questions, a recently-defined distance metric, which can distinguish similar water parcels, is applied to Argo data. Results show that the Pacific and Atlantic AAIW originates from only limited regions at the surface, specifically near the Subantarctic Front in the southeast Pacific and the Falkland Plateau in the southwest Atlantic. Further investigation indicates that through subduction in the deep mixed layer in these regions, low-salinity water penetrates into the intermediate depths of the Pacific and Atlantic Ocean. In the Indian Ocean, such a fast pathway for low-salinity to reach the intermediate depths is absent. Instead, the AAIW here, characterized by lower oxygen concentrations, consists of a mixture of locally slow downward spreading low-salinity water and AAIW inflow carried by the Antarctic Circumpolar Current from the Atlantic Ocean. The AAIW in the three southern hemisphere oceans exhibits slightly different physical properties, reflecting their different origins. Additionally, results indicate that mixing with surrounding saline waters is crucial in the transformation of surface waters into the AAIW cores. Our findings confirm the reliability of the distance metric and emphasize the importance of localized physical processes in the penetration of AAIW into the ocean interior.

How to cite: Hong, Y.: Origin of the Antarctic Intermediate Water in the Southern Ocean identified by a distance metric, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15079, https://doi.org/10.5194/egusphere-egu25-15079, 2025.

Monitoring changes in the marine environment is important for both oceanography and ecology study as it helps us understand the process by which oceanic conditions influence the entire ecosystem. In Antarctica, marine mammals encounter substantial changes due to seasonal variation of water mass composition and the complicated submarine topography. However, unbroken observations on ocean conditions are highly restricted to the summer season only when research vessels are available. In this study, we explored how seasonal variations in oceanic conditions affect the foraging behaviors of Weddell seals (Leptonychotes weddellii) in the Ross Sea, Antarctica using miniaturized CTD tags. Over the course of three consecutive years, from 2021 to 2023, 64 adult individuals were instrumented to collect data on water temperature and salinity as well as head acceleration. From the head movement, we found that seals foraged more often in modified shelf water and ice shelf water compared to Antarctic surface water. Additionally, as the lower boundary of Antarctic surface water descends from March to July, the seal dived to greater depths. Additionally, CTD profiles from CTD tags were classified using machine learning methods. Temperature and salinity from each profile were linearly interpolated across depths from 1 to 600 meters. Principal Component Analysis was then applied to extract three principal components for each profile. These components were subsequently used as input for a Gaussian Mixture Model, which classified the profiles into four distinct clusters. Each cluster had distinct temperature and salinity profiles and showed spatial and temporal separation (Warm surface Cluster: Distributed near Terra Nova Bay, predominant in summer (February); Intermediate surface Cluster: Distributed near Terra Nova Bay, predominant in fall (March); Cold Cluster: Distributed near Terra Nova Bay, predominant in winter (May, June, and July); Warm subsurface Cluster: Distributed near the shelf break, predominant in April and May). Prey capture attempts of dives were highest in Warm surface Cluster (3.16) and lowest in Intermediate surface Cluster (2.91). This study reveals the spatial and temporal shifts of foraging behavior with the surrounding oceanographic conditions, and it further emphasize that these oceanic factors should be considered for estimating their foraging activities.

How to cite: Chung, H., Park, J., Park, M., Kim, Y., Chun, U., Yun, S., Lee, W. S., Choi, H. A., Yoon, S.-T., Na, J. S., and Lee, W. Y.: Oceanographic and behavioral monitoring inferred from seal CTD tagging in the Ross Sea, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15611, https://doi.org/10.5194/egusphere-egu25-15611, 2025.

How to cite: Chidhambaranathan, B., Gayen, B., and Vreugdenhil, C.: Exploring Southern Ocean’s hidden drivers with direct numerical simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16813, https://doi.org/10.5194/egusphere-egu25-16813, 2025.

The sea ice extent (SIE) in the Southern Ocean experienced a drop in the late 1970s, though less pronounced compared to the one post-2016. Though several studies explain the drop since 2016, the 1970s decline is critical in understanding the long-term variability of SIE. To investigate the underlying mechanisms for this decline, we conducted wind perturbation experiments using the general circulation model EC-Earth3. We perturb the model by adding wind stress anomalies derived from ERA5 to the model winds. Using this technique, we simulate the evolution of SIE over the period 1958-2023. Our analyses show that the perturbation induces a drop in the 1970s despite the control (no-perturbation) run showing no such trend. This strongly indicates the important role of winds in driving the drop, though ocean processes and other feedback mechanisms may also contribute to the decline. The SIE shows spatial heterogeneity in the variations driven by the wind. Presently, we are conducting multiple ensemble simulations to evaluate the influence of initial conditions on the results.

How to cite: Francis, F., Goosse, H., and Barriat, P.-Y.: Wind perturbation experiments to simulate the 1970s drop in the sea ice extent in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17472, https://doi.org/10.5194/egusphere-egu25-17472, 2025.

The global ocean has taken up an estimated 89% of excess heat and 26% of annual CO₂ emissions resulting from anthropogenic activity in recent decades. Despite the crucial role of the ocean in the climate system, there remains significant uncertainty around ocean heat and carbon uptake. Processes of ocean carbon and heat uptake and redistribution are among the leading processes limiting scientists’ ability to predict the rate and magnitude of global climate change. Furthermore, despite being responsible for as much as 40–50% of global annual oceanic CO₂ uptake, the Southern Ocean remains the most controversial ocean basin, with large differences in both the magnitude and variability of CO₂ fluxes across various estimation methods. The disagreement regarding the magnitude and variability of Southern Ocean CO₂ fluxes largely arises due to the sparsity and uneven distribution of in-situ observations of carbon in the ocean, a problem which also constrains estimates of ocean heat uptake. However, machine learning has been extensively demonstrated in recent years to be an effective tool for overcoming such data limitations through gap-filling methods.

We leverage existing expertise in machine learning methods from generating the SOM-FFN and MOBO-DIC ocean carbon data products and apply this expertise to the development of a novel machine learning generated ocean heat uptake data product for the Southern Ocean. We integrate new Earth Observation data from Copernicus satellites with in-situ observation data from the Southern Ocean as input for our new data products. Here, we present a beta-version of our new machine-learning based products of carbon and heat uptake in the Southern Ocean and a first analysis of the variability and trends of the carbon and heat uptake of these data products. This work presents our first steps towards data products which will allow us to identify transport pathways of carbon and heat within the ocean interior. The novel machine learning-based products will also be used to support the advancement of Earth System Models and climate change predictions.

How to cite: Burt, D. and Landschützer, P.: Filling the gaps in Southern Ocean carbon and heat: Machine learning-based products from sparse observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18124, https://doi.org/10.5194/egusphere-egu25-18124, 2025.

Wintering periods are vital for the survival and reproductive success of Antarctic animals, yet their winter behaviors remain poorly understood. This study investigates the winter movements and foraging behavior of Weddell seals (Leptonychotes weddellii), an indicator species for the Commission for the Conservation of Antarctic Marine Living Resources, at Terra Nova Bay in the Ross Sea. Using CTD-Satellite Relay Data Loggers, which record head movement and oceanographic data (temperature and salinity), we tracked 48 individuals for three consecutive years (2021–2023) over the migratory period. Of these, 23 seals migrated an average of 339 km (up to 911 km) northeast or southeast between March and early April, while 25 seals remained within 200 km from the summering site restricted at near Terra Nova Bay. Migratory seals exhibited higher prey capture attempts (4.88 ± 1.23 attempts per hour) compared to resident seals (3.86 ± 0.96 attempts per hour), suggesting that long-distance travel provides foraging benefits despite associated energetic costs and risks. Regions frequented by migrants, particularly near continental shelf edges, exhibited warmer water (-1.44 ± 0.37°C) intake at 200–350 m depth, indicative of nutrient-rich conditions. These findings reveal divergent wintering strategies in Weddell seals, highlighting a trade-off between migratory risks and feeding advantages. Long-term integrated monitoring of seal behavior and environmental changes is essential to advancing our understanding of their ecological adaptations and the Antarctic marine ecosystem.

How to cite: Lee, W. Y., Park, J., Park, M., Kim, Y., Chun, U., Chung, H., Choi, H. A., Yoon, S.-T., Na, J. S., Yoon, S., and Lee, W. S.: Trade-offs Between Migration and Foraging Success: Winter Behavior of Weddell Seals in the Ross Sea, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19210, https://doi.org/10.5194/egusphere-egu25-19210, 2025.

The Southern Ocean, a region highly vulnerable to climate change, plays a critical role in regulating global nutrient cycles and atmospheric CO₂ via the biological carbon pump. Diatoms, a group of photosynthetically active plankton with dense opal skeletons, are central to this process, as their exoskeletons are thought to enhance the transfer of particulate organic carbon to depth, making them main vectors of carbon storage. However, conflicting observations obscure the mechanistic link between diatoms, opal, and particulate organic carbon fluxes, especially in the twilight zone where the greatest flux losses occur.

Here we present direct springtime flux measurements from different sectors of the subpolar Southern Ocean, demonstrating that across large areas of the subpolar twilight zone, carbon is efficiently transferred to depth: however, not by diatoms. Instead, opal is retained near the surface ocean, indicating that processes such as diatom buoyancy regulation and grazer repackaging can negate the ballast effects of diatoms’ skeletons. Using image data, we further reveal species-specific differences in diatom flux dynamics, highlighting the complexity of their role in the carbon cycle.

Our findings challenge the assumption that diatom-rich surface waters are necessarily associated with effective carbon export and transport in the Southern Ocean. They suggest that shifts in phytoplankton community composition driven by climate change may have a smaller impact on biological carbon storage than current models predict.

How to cite: Giering, S. L. C., Williams, J. R., Baker, C., Pabortsava, K., Blackbird, S., Martin, A., Poulton, A., Briggs, N., Carvalho, F., Le Moigne, F., Villa-Alfageme, M., Henson, S., Espinola, B., Iversen, M., Liu, Z., Moore, M., Passow, U., Romanelli, E., Thevar, T., and Sanders, R. and the The following authors are missing: The Diatom Disconnect: Retention near surface waters limits carbon transfer to the deep ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19855, https://doi.org/10.5194/egusphere-egu25-19855, 2025.

Antarctic Bottom Water (AABW) plays a crucial role in the intensity and variability of the Global Overturning Circulation (GOC). AABW formation sustains the lower cell of the GOC, fundamentally regulating the storage and transport of heat and carbon, key properties influencing Earth’s climate. Changes in AABW have far-reaching implications for the stability of the GOC. However, the mechanisms governing the variability and potential changes in the strengthening of the AABW cell remains uncertain. Understanding the variability of AABW is crucial for projecting changes in ocean circulation and assessing associated climate impacts. This study aims to use the outputs of the high-resolution Community Earth System Model (CESM-HR) pre-industrial run to explore AABW natural variability and water mass transformation processes governing its formation. By analyzing the pre-industrial run, we aim to characterize baseline dynamics in the absence of anthropogenic forcing, focusing on how atmospheric variability drives changes in AABW properties and transport. Additionally, we investigate the role of surface cooling and salinization in shaping AABW characteristics through a water mass transformation framework. While small-scale processes are not fully resolved, CESM-HR offers an improved perspective on large-scale patterns and variability compared to coarser-resolution models. This study aims to provide insights into the baseline state of AABW dynamics under pre-industrial conditions. The results are expected to provide an enhanced understanding of AABW variability, offering insights into anthropogenic impacts and underscoring the need for complementary observational and modeling efforts to refine our understanding of AABW formation.

How to cite: Noro, M., Wainer, I., Dotto, T., and Marcello, F.: Natural Variability of Antarctic Bottom Water in the Pre-Industrial CESM-HR simulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20050, https://doi.org/10.5194/egusphere-egu25-20050, 2025.

In winter 2015, Antarctic sea ice underwent a drastic change, transitioning from a period of record high sea ice coverage to a period of record low sea ice coverage. While both an intensified atmospheric circulation and warmer ocean temperatures have been invoked as possible causes for this sea ice regime shift, a detailed process understanding is still missing. Using ~110,000 hydrographic profiles from the seasonal ice zone of the Southern Ocean and atmospheric reanalysis, we reconcile how storm-driven mixing interacted with subsurface warming to change the sea ice state. We observe a gradual thinning of Antarctic Winter Water that acts as a barrier between the warmer deep water and the surface over the period 2005 to 2015 (~2 m per year). This thinning is likely induced by an increased near-surface density stratification in this period, hampering Winter Water formation. As a result, the reservoir of warmer deep water moved closer to the surface and the sea ice. In winter 2015, anomalously strong winds enhanced mixing across the thin Winter Water layer, which broke down stratification over the upper ocean and enhanced connectivity between the ocean mixed layer and deeper interior. Consequently, this reduced stratification allows warmer deep waters to melt sea ice. Our findings thus show that an oceanic preconditioning was a prerequisite for the potential sea ice regime shift that was ultimately triggered by strong storm-driven mixing in 2015.

How to cite: Spira, T., du Plessis, M., Haumann, A., Giddy, I., Silvano, A., Narayanan, A., and Swart, S.: Thinning of Antarctic Winter Water preconditions recent storm-triggered Antarctic sea ice decline, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-426, https://doi.org/10.5194/egusphere-egu25-426, 2025.

Antarctic Bottom Water (AABW) is formed in select locations around the Antarctic margin, filling the bottom 40% of the world’s ocean and circulating heat, carbon and nutrients throughout all basins. Recent observations suggest that almost half of AABW is formed in the western Weddell Sea and that since 1992, its formation has decreased by around 40%. A combination of anthropogenic warming, through the addition of freshwater from melting glaciers and ice shelves, and natural climate variability, is thought to have led to this drastic decrease. The Interdecadal Pacific Oscillation (IPO), known to exhibit teleconnections to the Weddell Sea, has been proposed as responsible for some of this decrease. However, it remains unclear to what extent recent shifts in the IPO have influenced AABW formation because limited observations include the impact of all natural and anthropogenic climate drivers and not just the IPO. Here we use the 1/10° global ocean-sea ice model ACCESS-OM2 to simulate the isolated impact of the IPO on Weddell Sea dense shelf and bottom water formation. We find indications that southward wind anomalies associated with a negative IPO phase push sea ice towards the coastline, prevent polynyas from opening and have reduce dense shelf and bottom water formation. In the Ross Sea we see the opposite, with more dense shelf water formation during negative IPO phases compared to positive phases, especially for the highest density ranges. This indicates that during IPO phase shifts, Weddell Sea AABW changes might be compensated by changes in the Ross Sea. These findings have implications for interpreting decadal-scale variability in dense shelf and AABW production and its impacts on the global ocean circulation under a rapidly warming climate.

How to cite: Huguenin, M., Ryan, S., Ummenhofer, C., and England, M.: Linking the recent decrease in Weddell Sea dense shelf water formation to shifts in the Interdecadal Pacific Oscillation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-462, https://doi.org/10.5194/egusphere-egu25-462, 2025.

The Southern Ocean plays a crucial role in global climate regulation through its influence on the oceanic and atmospheric circulation. Due to the very cold temperatures in the polar Southern Ocean, salinity exerts a fundamental control on vertical mixing, water mass renewal, and the global overturning circulation. Thus, understanding the causes and consequences of salinity changes in the Southern Ocean is not only essential for understanding changes in regional climate dynamics but also implications the changes have on the global climate system. We here investigate spatial and temporal changes in the salinity of Antarctic Bottom Water (AABW) from historic hydrographic data to understand the coupled system of freshwater inputs and salinity changes in the Southern Ocean. Using a novel database of seawater isotopes and noble gases as tracers, combined with in-situ salinity measurements, we identify freshwater sources in AABW within the Weddell Sea, and their changes over time. We find that AABW has freshened and become enriched in δ¹⁸O from 1995 to 2016, suggesting a coupled relationship between reduced sea ice export from the Weddell Sea continental shelf, increased sea ice melting, and reduced basal melting of the adjacent Filchner-Ronne Ice Shelf. The observed decline in meteoric water contributions, including basal melt, further supports this inference, suggesting that declines in sea ice export from the continental shelf significantly impact AABW formation and export.

How to cite: Robertson, E., Haumann, A., and Meredith, M.: Identifying Changes in Ice-Ocean-Atmosphere Fluxes in Antarctic Bottom Water, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1904, https://doi.org/10.5194/egusphere-egu25-1904, 2025.

Observations beneath ice-covered oceans and within ice-shelf cavities are central to understanding the ocean-ice interactions that influence ice-shelf stability, contribute to global sea-level change, and shape large-scale ocean circulation patterns. However, direct observations in these regions, particularly those capturing wintertime conditions, remain scarce due to the logistical challenges posed by persistent ice cover. Emerging autonomous technologies and new applications now offer opportunities to address these observational gaps.
Since 2020, we have deployed 20 unconventionally programmed Argo floats in key regions of the Ross Sea, including the Terra Nova Bay and Ross Ice Shelf polynyas, the critically under-sampled Eastern Gate, and along the Ross Ice Shelf front. These floats provide year-round thermohaline and biochemical measurements, which, among other capabilities, allow for the quantification of water mass transformations, sub-ice dynamics, and key processes such as the production of High Salinity Shelf Water, a precursor to Antarctic Bottom Water. This represents a significant advancement, as previous studies have largely relied on summer ship-based or satellite-derived observations, which fail to capture the full seasonal cycle.
Futhermore, with measurements from three Argo floats operating for several months beneath the Ross Ice Shelf, we directly observed and quantified processes that had previously only been hypothesized. These include the intrusion of seasonally warmed Antarctic Surface Water, identified as a primary driver of frontal and basal melting, along with its associated effects on ocean heat content and basal melt rates, as well as the outflow of Ice Shelf Water, the coldest ocean water in the world.

Building on the insights gained over the past 4 years, we argue that broadening the deployment of grounded-mode Argo floats across Antarctica can provide a unique understanding of ocean-ice interactions. By enabling continuous, autonomous measurements — even in winter and under ice — this approach can improve our capacity to quantify key processes, such as the lateral and vertical extent of shelf water production and the mechanisms driving basal melt. Our results demonstrate that Argo floats offer direct evidence of how heat absorbed at the surface is transported into ice-shelf cavities, contributing to basal melting and reshaping our understanding of water mass formation processes in coastal polynyas. Expanding the float network would enhance our ability to detect interannual variability, characterize longer-term trends, and reduce uncertainties in ice-shelf and sea-level projections, ultimately supporting more accurate climate model predictions for these critical polar environments.

How to cite: Falco, P., Krauzig, N., Castagno, P., Garzia, A., Martellucci, R., Cotroneo, Y., Flocco, D., Menna, M., Pirro, A., Mauri, E., Memmola, F., Solidoro, C., Pacciaroni, M., Notarstefano, G., Budillon, G., and Zambianchi, E.: Winter thermohaline evolution along andbelow the Ross Ice Shelf, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2291, https://doi.org/10.5194/egusphere-egu25-2291, 2025.

The Antarctic sea ice extent has displayed two large drops over the past 65 years, a small one at the end of the 1970s and a more substantial one after 2016. The atmospheric forcing provided a dominant contribution to those drops. Wind changes strongly control the spatial pattern of sea ice reduction, especially in the Pacific sector of the Southern Ocean and in the western Weddell Sea. The relationship between the winds and sea ice seems less direct in the east Antarctic sector (i.e., mainly in the eastern Weddell Sea and in the Indian sectors), where oceanic processes are expected to play a larger role. This contribution of oceanic processes in the sea ice reduction in the east Antarctic sector is estimated here using simulations performed with the ocean-sea-ice  model NEMO, substantiated by observations. The simulations cover the period 1958-2023, driven by both the ERA5 reanalysis and a forcing derived from a recent reconstruction that displays more homogenous time series than ERA5 over the whole period. Observations are used at first to evaluate the model behaviour over the past decades when the data network is denser. The simulations, then, allow the analysis to be extended over the past 65 years, to estimate the changes in oceanic heat transport, heat content and oceanic heat flux towards the sea ice. Several simulations with NEMO are compared, using different initial conditions and parameters influencing mixing to estimate how they influence the sea ice extent variations and thus to disentangle the role of different oceanic processes in the observed changes.

How to cite: Goosse, H., Francis, F., Mezzina, B., Richaud, B., and Dalaiden, Q.: Contribution of ocean processes to the drops in Antarctic sea ice extent at the end of the 1970s and after 2016, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2487, https://doi.org/10.5194/egusphere-egu25-2487, 2025.

Ice shelves in the Amundsen Sea, West Antarctica, are being melted rapidly from below by warm ocean waters, causing sea-level rise. Amundsen Sea oceanography and ice-shelf melting are both subject to long-term (centennial) trends and natural decadal variability. We study the atmospheric drivers of the decadal variability using perturbation experiments in which the mechanical (winds) and thermodynamic atmospheric forcings are applied individually in an ice-ocean model of the Amundsen Sea. We find that the decadal variability is predominantly driven by mechanical forcing of the winds, through impacts on the melting and formation of sea ice. This variability in the sea ice drives variability in the Amundsen Sea undercurrent and the heat fluxes towards the ice shelves, which in turn leads to decadal variability in the melting of the ice shelves. While winds are the primary driver of this variability, it is also found that a significant part of the variability is due to nonlinear effects, and cannot be explained by the individual impacts of either winds or thermodynamics. Our results also highlight how the processes that drive variability differ depending on the timescale (e.g., annual, decadal, centennial) of interest.

How to cite: Haigh, M. and Holland, P.: Decadal variability of ice-shelf melting in the Amundsen Sea driven by winds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2551, https://doi.org/10.5194/egusphere-egu25-2551, 2025.

The ice shelves in Amundsen Sea are experiencing a rapid melting phase, largely due to the intrusion of warm Circumpolar Deep Water (CDW) from off the continental shelf. At intrusion sites on the continental slope, the CDW, along with an eastward undercurrent, breaks the Taylor–Proudman theory, causing southward cross-isopycnal intrusion. To explore the mechanisms of the intrusion, we developed a coupled ocean-sea ice-ice shelf numerical model for the Ross Sea-Amundsen Sea system, reconstructing the circulations and simulating the CDW intrusion. The vorticity budget along the continental shelf break of Amundsen Sea is examined using the depth-averaged vorticity budget equation based on the model’s outputs. Results show that the advection of planetary vorticity (APV) and the joint effect of baroclinicity and relief (JEBAR) dominate the vorticity balance at the CDW intrusion sites on the shelf break. The intensity and vertical structure of the eastward undercurrent upstream significantly affect the density structure in the downstream intrusion area, promoting the JEBAR effect. The velocity of the eastward undercurrent is linked to the local wind field. We find that stronger eastward undercurrent speeds are associated with stronger westerly winds and weaker wind stress curl. Westerly winds can drive undercurrents via modifying the meridional sea surface altitude gradient, while wind stress curl reduces the undercurrent by weakening the strength of the continental slope front, which represents the wind field's own internal constraints on the undercurrent. Stronger negative wind stress curl in the Amundsen Sea could drive a stronger geostrophic component of Sverdrup transport under the Ekman layer, which may compress local isopycnals to alter the undercurrent on a seasonal timescale.

How to cite: Li, Z., Wang, C., and Zhou, M.: Mechanisms of wind-field regulation of eastward undercurrents in the Amundsen Sea, West Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4674, https://doi.org/10.5194/egusphere-egu25-4674, 2025.

Freshwater hosing experiments are a widely used tool for understanding the impacts of Antarctic ice shelf basal melting on the Southern Ocean and global climate. Most existing coupled climate models lack the necessary physics to explicitly simulate basal melting. Therefore, freshwater anomalies have to be imposed as a proxy. Previous studies have employed diverse freshwater scenarios and application methods. In this study, we explore variations in the application and representation of meltwater anomalies around Antarctica. We compare simulations where freshwater anomalies are introduced at the ocean surface over different spatial extents and at specified depths along the continental slope for a more realistic representation of plume dynamics resulting from basal melting. Additionally, we investigate ocean heat fluxes variability when accounting for the latent heat of fusion required to melt the ice. It is possible to observe similarities and differences in ocean responses depending on the methodology used to impose the freshwater anomaly. Surface application scenarios tend to exhibit more diffuse impacts on ocean stratification and circulation, while depth-specific applications lead to localized but more intense changes in water mass transformation. Accounting for latent heat can introduce further complexity, altering the thermal structure and influencing buoyancy-driven dynamics. By comparing these approaches, we want to highlight the sensitivity of simulated ocean dynamics to the spatial and physical representation of meltwater inputs. Accurately parameterizing ice-ocean interactions in models is necessary to improve the reliability of projections regarding Antarctic contributions to sea level rise and global climate variability.

How to cite: Zapponini, M., Sidorenko, D., Scholz, P., Semmler, T., Streffing, J., and Jung, T.: The role of Antarctic basal meltwater , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4705, https://doi.org/10.5194/egusphere-egu25-4705, 2025.

Coupled with the Antarctic Slope Front (ASF), the Antarctic Slope Current (ASC) encircles Antarctica and has variable structures. Two types of the ASC/ASF have been identified in the Ross Sea. Yet, the spatial characteristics of the ASC/ASF have not been depicted in detail, and the transition zone between the two different types is still unclear. Using an eddy-permitting coupled regional ocean-sea ice-ice shelf model, we aim to investigate the spatial characteristics and energy sources of the ASC/ASF in the Ross Sea. Based on the simulated results, three distinct structures of the ASC/ASF have been identified in three regimes from east to west: (i) in Regime I, the ASC is characterized by a westward flow in the upper layer and an eastward countercurrent in the lower layer; (ii) in Regime II, the undercurrent of the ASC reverses to the west and features a bottom intensification; (iii) in Regime III, the ASC in the upper layer is eastward, and the westward undercurrent still occupies the lower layer. By analyzing the momentum budget of the ASC, we quantified the respective contributions of barotropic and baroclinic pressure terms in determining the structure of the ASC/ASF. Furthermore, by analyzing the Mean Kinetic Energy budget of the ASC, we found that the Mean Available Potential Energy plays an important role in converting energy to the Mean Kinetic Energy, indicating that the maintenance of the ASC is closely associated with the available potential energy released from the ASF.

How to cite: Liu, Y., Liu, C., Wang, Z., Yan, L., Han, X., Liu, K., Haigh, M., Xia, Y., Zeng, J., Li, X., and Liang, X.: Spatial Characteristics and Dynamic Mechanisms of the Antarctic Slope Current in the Ross Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4882, https://doi.org/10.5194/egusphere-egu25-4882, 2025.

Importance of the Weddell Sea for the global ocean circulation is irrefutable as it is a hotspot of dense water formation, a precursor of the Antarctic Bottom Water (AABW), which dominates the abyssal ocean ventilation. Furthermore, the region hosts the largest by volume floating ice shelf, Filchner-Ronne Ice Shelf (FRIS), which is vulnerable to episodic Warm Deep Water (WDW) inflows that induce basal melting at the ice shelf front and within its cavities. The resulting meltwater enters the system as freshwater forcing and alters local density gradients and water mass distribution, ultimately contributing to global sea level rise and potentially disrupting the global overturning circulation. Hence, knowledge about the seasonality and magnitude of the WDW inflows carries significance both locally and globally.

In 2017 and 2018, the anomalously warm inflows were observed yet there is still no agreement on the mechanism for their trigger. The on-shore propagation of WDW inherently depends on several factors, such as e.g circulation at the continental margins, the seasonal cycle of sea ice, the thermocline depth, position and magnitude of the Antarctic Slope Front (ASF). The intricate interplay between these factors yields a fairly complex system, which is difficult to unravel with the scarce available observations. Albeit the Weddell sea remains fairly undersampled up to this day, significant efforts are being undertaken to supplement data gaps with numerical modeling.  

The aim of my research is to shed light on seasonal and interannual variability of the coastal circulation upstream of the FRIS and its sensitivity to external forcings, using existing in-situ observations, reanalyses and the high-resolution eddy-permitting Finite-Element/Volume Sea Ice-Ocean Model (FESOM), in order to investigate the impact of wind and buoyancy forcing. Preliminary results indicate that the maximum thermocline depth exhibits a distinct seasonal cycle which is not consistent with sea ice formation, as previously speculated. Apparently, it reaches its deepest position in the Dronning Maud Land region in austral summer, then downstream at Kapp Norvegia in austral autumn and further downstream next to the FRIS – in austral winter, which rather implies generation and along-coast propagation of a large-scale baroclinic signal and further accentuates the importance of along-coast connectivity. Furthermore, the character of thermocline also needs to be considered. For instance, in 2017-2018, when the anomalous warm inflows were observed at the FRIS, thermocline thickness was increased rather than its depth, which raises the question of necessary and sufficient conditions for said inflows to occur.

How to cite: Volkova, V., Janout, M., and Kanzow, T.: Upstream governors of the Warm Deep Water inflows towards the Filchner-Ronne Ice Shelf, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5829, https://doi.org/10.5194/egusphere-egu25-5829, 2025.

The melting of Antarctic ice shelves is driven by heat fluxes from the underlying ocean to the ice. The relationship between basal heat fluxes and ocean conditions is an active topic of research, as current state-of-the-art parameterizations perform relatively poorly in all but the fully-turbulent well-mixed regime^a. Indeed, discrepancies between observed and predicted melt rates under certain ice-shelves have been detected^b. Here, we  perform laboratory experiments of tabular ice cuboids melting in salty water. We aim to improve our understanding of the basal melting of ice shelves in the diffusive-convective regime, for which there is currently no parameterization^c. To this end, we investigate the melting rate and underlying fluid dynamics over a broad range of water salinity and temperature, without any external forcing. Our work uniquely complements field observations, which are difficult and sparse, and simulations, which most often approximate the dynamics for computational expediency.

We use a meter-scale tank, which we fill with saltwater and place a freshwater tabular ice cuboid to melt on top. A bottom heating plate is used to maintain the bottom saltwater temperature at a prescribed value and the setup is placed in a cold room. The depth-dependent seawater temperature, salinity, currents, and the melt rate are explored for different bottom water temperatures and initial salinity values. We use a moving temperature and salinity sensor, PIV data and shadowgraphy images of the retreating ice-water front to provide a relatively comprehensive data set from which we derive a mapping between the average melting rate and the flow statistics (kinetic energy density, dissipation rate, temperature gradient) of interest to polar oceanography.

We find that temperature and salinity vertical gradients in the system can create a layered system, depending on the conditions. In particular, we observe the formation of a freshwater layer insulating the ice plate, and slowing the melting, at relatively low temperature. When the bottom temperature is relatively large, the two-layer organisation disappears as convection becomes vigorous enough to penetrate and mix the freshwater layer with the ambient. 

References :

        a - Malyarenko, A., Wells, A. J., Langhorne, P. J., Robinson, N. J., Williams, M. J., \& Nicholls, K. W. (2020). A synthesis of thermodynamic ablation at ice–ocean interfaces from theory, observations and models. Ocean Modelling, 154, 101692.

        b - Kimura, S., Nicholls, K. W., \& Venables, E. (2015). Estimation of ice shelf melt rate in the presence of a thermohaline staircase. Journal of Physical Oceanography, 45(1), 133-148.

        c - Rosevear, M. G., Gayen, B., \& Galton-Fenzi, B. K. (2022). Regimes and transitions in the basal melting of Antarctic ice shelves. Journal of Physical Oceanography, 52(10), 2589-2608.

How to cite: Collin, B., Couston, L.-A., Joubaud, S., and Volk, R.: Ice melting in saltwater: laboratory experiments in the diffusive-convective regime, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6361, https://doi.org/10.5194/egusphere-egu25-6361, 2025.

How to cite: Lemaire, E., Massonnet, F., Fichefet, T., Pirlet, N., Mathiot, P., Marini Marson, J., and Olivé Abelló, A.: Investigating Iceberg–Sea Ice Interactions in the Southern Ocean Using NEMO-ICB, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6705, https://doi.org/10.5194/egusphere-egu25-6705, 2025.

Transport of warm Circumpolar Deep Water (CDW) into ice shelf cavities is known to be a primary source of heat driving Antarctic Ice Sheet mass loss. This CDW originates in the open ocean, and thus, basal melt rate variability is often linked to wind-driven fluctuations in cross-shelf CDW transport. While cross-shelf heat fluxes are certainly an important part of the story, less focus has been placed on the offshore processes that bring CDW from the open ocean to the shelf break. Here, we use in situ data from profiling floats in combination with Lagrangian particle release experiments in an ocean model to investigate the pathways by which CDW moves toward the continental slope, which is a necessary precursor to the cross-shelf exchange that has been studied in more depth. Observations and models suggest that CDW transport exhibits considerable spatial heterogeneity in the form of concentrated pathways linked to bathymetric features, both on- and off- shore of the continental slope. This suggests that pathways to the shelf break are characterized by distinct timescales and varying degrees of water mass transformation across different sectors. In addition, temporal variability on mesoscale, seasonal, and interannual timescales is present. This is potentially important context through which to understand regional and long-term variations in continental shelf heat content and ice shelf basal melt, which in turn, has implications for future sea level rise.

How to cite: Prend, C., Girton, J., MacGilchrist, G., and Thompson, A.: Remote pathways of ocean heat transport toward the Antarctic Ice Sheet, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7499, https://doi.org/10.5194/egusphere-egu25-7499, 2025.

Despite the ongoing decline in Arctic sea ice extent over the past 30 years, an increase had been observed in Antarctic sea ice until 2016. That year, however, the sea ice melt season started unusually early, leading to a decrease in sea ice extent. Since then, this decline has continued, culminating in a record low sea ice extent in 2023. This reduction in sea ice is primarily linked to changes in atmospheric patterns, along with the impact of rising ocean surface temperatures. Additionally, it has been identified that the decrease in Antarctic sea ice extent significantly affects the atmosphere by increasing the surface heat loss from the Southern Ocean. Simultaneously, a continuous thinning of the sea ice has been observed. This decline in ice thickness is expected to lower the drag coefficient between the sea ice and the ocean, which would, in turn, enhance the influence of wind on the flow of sea ice. Notably, increased sea ice flow during the melting season could accelerate the melting process and increase seasonal sea ice variability. We analyzed the changes in the sea ice-ocean drag coefficient using satellite-observed sea ice concentration and thickness, along with reanalysis wind data over the past 20 years. Although regional differences exist, the overall trend indicates a clear decline in the sea ice-ocean drag coefficient.

How to cite: Kim, T.-W., Yang, H., Kim, Y., and Park, J.: Changes in the Sea Ice-Ocean Drag Coefficient Due to the Decrease in Antarctic Sea Ice, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7568, https://doi.org/10.5194/egusphere-egu25-7568, 2025.

The formation of dense water in the Southern Ocean plays a key role in the global overturning circulation of the ocean, and thus affects the distributions of heat, carbon, oxygen and nutrients across the World Ocean. However, the simulation of dense water properties by climate models remains problematic. These models often generate dense water in incorrect locations and for wrong reasons, primarily through deep convection in the center of the Weddell and Ross Seas. We hypothesize that this inability to simulate the formation and fate of dense water stems partly from the erroneous or absent representation of coastal polynyas and their key drivers, particularly landfast ice. A recent study presented a restoring method that accurately represents Antarctic landfast ice and demonstrated its essential role in shaping coastal polynyas and enhancing sea ice production in the NEMO4-SI3 model. Here, building on this study, we investigate the impact of this landfast ice representation on water mass properties simulated by the model over the Antarctic continental shelf. We perform two simulations: one with the landfast ice scheme activated and one with this scheme turned off. A comparison of the simulation results confirms the expected densification of water masses within polynyas when landfast ice is represented. However, the results also reveal unexpected regions of fresher water beneath landfast ice, which influence the polynya dynamics downstream. On a circumpolar scale, incorporating landfast ice enhances the model's agreement with observations, particularly in terms of bottom salinity, temperature and mixed layer depth. Notably, the mixed layer depth undergoes significant changes, which in turn affect the Southern Ocean's coastal dynamics and lead to enhanced ice shelf melting. Overall, representing landfast ice improves the simulation of dense water formation and shelf ocean dynamics, thereby advancing our understanding of key physical processes in these critical regions.

How to cite: Pirlet, N., Fichefet, T., Vancoppenolle, M., and de Lavergne, C.: Impact of a representation of Antarctic landfast ice on the shelf water properties simulated by NEMO4-SI³, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8172, https://doi.org/10.5194/egusphere-egu25-8172, 2025.

The increasing release of meltwater from Antarctica represents one of the most profound yet uncertain consequences of global climate change. The lack of interactive ice sheets in state-of-the-art climate models, including those participating in the Coupled Model Intercomparison Project (CMIP6), combined with the inadequate representation of key processes driving ice shelf basal melting, prevents the direct calculation of ice-ocean feedbacks and leaves a high uncertainty on the magnitude and impacts of meltwater discharge. Previous studies that explored meltwater impacts produced partially contradictory findings, largely relied on experiments with single models, had inconsistent experimental designs, and imposed varying freshwater forcing rates. To address these shortcomings, this study employs results from the new  "Southern Ocean Freshwater Input from Antarctica” (SOFIA) initiative to assess the effect of meltwater-induced ocean warming on basal melting and potential future Antarctic mass loss. We evaluate the ocean response to meltwater across a suite of 10 CMIP6 models and compare it to future scenarios simulations without additional meltwater (SSP5-8.5), assessing model bias and both meltwater- and global warming-induced anomalies in the Southern Ocean. Applying these anomalies to a regional basal melting parameterization, constrained by a new observational hydrographic climatology, our findings reveal that meltwater feedbacks amplify warming on the continental shelf and enhance ice loss in many sectors around Antarctica. However, in the West Antarctic regions where the greatest ice mass loss was observed in recent years, most models show either cooling or reduced warming on the shelf, hence indicating a negative feedback due to the meltwater input. Consistent with previous studies, we confirm that regional disparities are driven by advection and acceleration of the Antarctic Slope Current. Our results suggest that mass loss from East Antarctica will become increasingly important under future global warming. The meltwater-induced feedback causes an additional 750 Gt/year of ice loss in the multi-model median response to our perturbation experiments. For comparison, observations estimate current anomalous ice shelf loss at approximately 1,000 Gt/year, while SSP5-8.5 simulations, which account for global warming without additional Antarctic meltwater, project an anomalous 3,400 Gt/year of ice loss by the end of the century.

How to cite: Muilwijk, M., Hattermann, T., and Beadling, R. and the SOFIA team: Uncertainty in Future Southern Ocean Warming and Antarctic Ice Shelf Melting Due to Meltwater-Driven Climate Feedbacks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11288, https://doi.org/10.5194/egusphere-egu25-11288, 2025.

Subsurface warming has been identified as a likely causal factor for the sustained low Southern Ocean sea ice extent in recent years. Subsurface-to-surface heat transport is impacted by the water mass structure of the water column and the depth of vertical mixing, which can in turn be altered by sea ice processes. These interactions create potential feedback effects that remain insufficiently explored in the context of the recent low Southern Ocean sea ice extent. In this study, we investigate interaction mechanisms and feedbacks between Southern Ocean sea ice and the underlying water column through a simple one-column box model, focusing on water mass structure and properties. The model represents the surface mixed layer, subsurface Winter Water, and the Upper Circumpolar Deep Water as distinct ocean boxes. A fourth box represents sea ice when present, interacting with the mixed layer through heat and salt exchange. The evolving mixed layer depth is calculated using a mixed layer model, with subsurface Winter Water formed when the mixed layer shoals and re-entrained when the mixed layer deepens. In this contribution, we present the box model framework and discuss preliminary insights, as well as challenges encountered during the model development process.

How to cite: Schramm, M. and Haumann, F. A.: Southern Ocean Sea Ice-Ocean Interactions in a Simple Box Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11729, https://doi.org/10.5194/egusphere-egu25-11729, 2025.

Hydrographic surveys in the Dotson Trough in the Amundsen Sea in January-February 2022 using a fleet of ocean gliders reveal deep (~400 m) isolated lenses of cold, dense water. The water contained within these lenses is colder, saltier, deeper and denser than the typical regional Winter Water (temperature-minimum) layer that appears above the lenses at about 200 m in summer.  The lenses occur in the stratified layer influenced by the warm, salty and dense modified Circumpolar Deep Water below, that provides heat to the underside of the vulnerable ice shelves in this region. We do not have evidence of the lenses travelling beneath the ice shelves, but they are at about the right depth to do so. If they did, they would insulate the base of the ice shelf from the warmer water below, helping to prevent basal melting that is prominent in this region.

The lenses are colder (close to the local freezing point of seawater) than the surrounding waters at the same depths and densities, and fresher than the surrounding water at the same densities. Their dissolved oxygen concentration is similar to that of Winter Water and their pH is lower than Winter Water, but both properties are increased compared with surrounding water at the same depth. Thus, they provide a mechanism to sequester carbon and oxygen deeper than typical Winter Water formation can achieve.

We explore possible formation mechanisms for the lenses and the water mass they contain, using wintertime profiles of temperature and salinity obtained from tags on seals. One possibility is that local chimneys of deep convection succeed in penetrating sporadically to 400 m, and are subsequently capped by other water masses.  We do not find convincing evidence to support this.  Our favoured hypothesis is that the shallower regions (less than 500 m water depth) surrounding the Dotson Trough (e.g. Bear and Martin Peninsulas) host enhanced surface heat loss and subsequently intense brine rejection during sea ice formation, leading to very cold, dense water in winter. The seal tag profiles do indeed show wintertime water masses in these shallower regions of the same temperature, salinity and density as the lenses. We speculate that this water spills off into the deeper water sporadically (perhaps akin to the formation mechanism for Meddies). They would then populate the layer at which they are neutrally buoyant, beneath the typical Winter Water and invading the uppermost layers of modified Circumpolar Deep Water.

How to cite: Heywood, K. J., Pickup, D., Bakker, D., Glassup, F., and Webber, B.: Cold deep lenses in the Dotson Trough, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12192, https://doi.org/10.5194/egusphere-egu25-12192, 2025.

Subglacial discharge beneath ice shelves is a source of freshwater, and therefore buoyancy, at the grounding line. Being released at depth, it accelerates an ascending plume along the ice-shelf base, enhancing entrainment of ambient waters, and increasing melt rates. By now it is understood that subglacial discharge is a key control on melt rate variability at the majority of Greenland's glaciers. However, its importance in present-day and future Antarctic melt rates is less clear. To address this point, we use the Energy Exascale Earth System Model (E3SM) and investigate the effects of subglacial discharge addition in both idealized setups and realistic, global, sea-ice ocean coupled configurations. For realistic Antarctic configurations, we use the subglacial hydrology model from the MALI ice-sheet model run at 4-20 km resolution to calculate steady state subglacial discharge across the grounding line under historical ice-sheet conditions.  This subglacial meltwater discharge is implemented as a grounding line freshwater flux in MPAS-Ocean, the ocean component of E3SM.

Results from idealized, rotating ice-shelf configurations show a stronger melt rate dependence on discharge than in previously studied non-rotating Greenland-like fjord scenarios. We also find that the melt-rate response is strongly sensitive to the location of the discharge along the grounding line; the efficiency of subglacial discharge, in terms of total melt-rate increase, grows with distance from the area where meltwater accumulates due to rotational effects. The analysis of subglacial discharge effects in realistic, global configurations focuses on ice-shelf melt rates, cavity circulation, continental shelf properties, and sea-ice conditions around Antarctica. Results from realistic, global configurations indicate that, although some regions are more affected than others, overall the present-day levels of subglacial discharge result only in relatively minor changes in ice-shelf melt rates and continental shelf properties. Significant oceanic changes would require at least an order of magnitude stronger subglacial discharge than present-day estimates.

How to cite: Vaňková, I., Asay-Davis, X., Branecky Begeman, C., Comeau, D., Hager, A., Hoffman, M., Price, S., and Wolfe, J.: Subglacial discharge effects on ice-shelf basal melting in Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14740, https://doi.org/10.5194/egusphere-egu25-14740, 2025.

Despite the increase in global mean temperature and massive sea ice loss in the Arctic, the Antarctic sea ice extent has not changed significantly throughout reliable satellite records starting in 1979. Long-term trends rather show an increase in the Antarctic sea ice area, resulting in record high anomalies in 2014 and 2015. However, after the moderate expansion in the sea ice extent, a sharp decline occurred in 2016 and has remained low since then. The record Antarctic sea ice loss in recent years may be a sign the region has entered a new regime of low sea ice coverage in a warming world. Meanwhile, Antarctic Bottom Water (AABW), driving the lower limb of the global meridional overturning circulation and ventilating the abyssal ocean interior has warmed and freshened in recent decades, leading to a decrease in AABW formation. Ross Sea shelf water which is responsible for 20–40% of the total AABW production, has experienced the largest freshening. However, repeat hydrographic data have shown that since the mid-2010s the salinity of Ross Sea shelf water has sharply rebounded from the multidecadal freshening trend. Here, it is interesting that the abrupt transition from a high to low state of Antarctic sea ice since the mid-2010s coincides with the onset of the salinity rebound of dense shelf water on the Antarctic continental shelf.

As the planet warms global sea ice has continued to get a lot of attention due to the substantial implications for planetary albedo, ice sheet and ice shelf stability, atmosphere-ocean interactions, cryosphere ecosystems, biogeochemical cycle, and the Southern Ocean freshwater cycle. Particularly, sea ice’s growth and melting play an important role in water mass transformations. Here, we investigate how the rapid decline in the Antarctic sea ice in recent years has contributed to the rebound of shelf water salinity in the Ross Sea, using satellite observations of sea ice, as well as oceanic and atmospheric reanalysis data. Our result shows that despite the rapid decrease in the Antarctic sea ice in recent years, the sea ice formation rate in the Ross Sea continental shelf has increased. During the salinification period since the mid-2010s, local anomalous winds and surface heat flux associated with the remote and large-scale forcing that drive the recent change in the Antarctic sea ice, induced the reduced sea ice cover and larger polynya area on the Ross Sea continental shelf, increasing sea ice formation rate. Furthermore, data-based sea ice budget analysis indicates that due to the anomalous wind forcing, the sea ice has moved to the outer shelf through dynamic processes such as advection and divergence, creating a sustained favorable environment for sea ice formation and brine rejection.

How to cite: Kim, T., Choo, S.-H., Moon, J.-H., Jin, E. K., Kim, D., and Cha, H.: Reversal of freshening trend of Ross Sea shelf water links to an abrupt transition from high to low state of Antarctic sea ice since the mid-2010s, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14844, https://doi.org/10.5194/egusphere-egu25-14844, 2025.

Sea ice extent around the Antarctic exhibits a high level of variability on interannual and longer timescales, characterized by a positive trend since the satellite era and interruptions due to e.g., the emergence of the Maud Rise Polynya in 2016. Given the relatively short period of observational data and the high level of natural variability it has remained challenging to unequivocally identify the anthropogenic fingerprint in Antarctic sea ice. Moreover, to properly study the Antarctic sea-ice and its response to future warming, it is necessary to capture important dynamics, such as polynyas, the Antarctic slope current and coastal leads. Many models within the CMIP6 model portfolio do not even have the spatial resolution to adequately resolve these features. This implies that their Antarctic projections may not be as trustworthy and robust as those for the Arctic Ocean.

In this study, we employ the high-resolution OpenIFS-FESOM (AWI-CM3) coupled general circulation (nominally 31 km atmosphere and 4-25 km ocean resolutions) to investigate the Antarctic sea ice response to greenhouse warming, following a SSP5-8.5 greenhouse gas emission scenario. Our simulation exhibits a sudden decline of Antarctic sea ice in the Weddell Sea (WS) which can be explained by a combination of physical processes that involve continued strengthening of westerlies, increasing of horizontal density and pressure gradients, intensifying of atmosphere-ocean momentum transfer due to sea ice decline, a spin-up of the cyclonic gyre and westward current and corresponding vertical and horizontal supply of heat into the Weddell Sea. The resulting decrease of sea ice further leads to heat accumulation in austral summer due to the absorption of short-wave radiation, which can further weaken winter sea ice extent and intensify the momentum transfer and associated heat transport into the Weddell Sea. 

Our study highlights the relevance of positive atmosphere-sea ice-ocean feedback in triggering the abrupt decline in Antarctic sea ice.  

How to cite: Kim, D.-W., Zapponini, M., Sharma, S., Jung, T., Kim, M.-H., Koldunov, N., Puthiyaveettil, N., Sidorenko, D., Streffing, J., Timmermann, A., Semmler, T., and Park, W.: Positive low cloud feedback accelerates abrupt Southern Ocean sea-ice decline in high-resolution global climate model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14845, https://doi.org/10.5194/egusphere-egu25-14845, 2025.

The cavity underneath the second largest ice shelf in Antarctica, the Filchner-Ronne Ice Shelf, could flip under strong climate warming from its current 'cold' state into a 'warm' state (Hellmer et al., 2012). Numerical models show that this regime shift occurs relatively abrupt, within a decade, with sub-shelf melt rates increasing 21-fold (Naughten et al., 2021). The increase in melting will reduce the ice shelfs buttressing capacity, thereby driving grounded ice loss, and a contribution to sea-level rise. Moreover, changes in sub-shelf melting, and the cavity geometry, in turn, can influence the ocean circulation, creating feedbacks that only emerge when considering the ice and ocean systems together. It is unclear, how these feedbacks influence the regime shift and subsequent evolution of the system, as well as a potential reversibility of the cavity. Here we run regional, numerical simulations of the coupled ice sheet and ocean system to investigate the role of ice-ocean feedbacks on the ocean regime shift, its reversibility, and the impact on ice sheet dynamics. We find that while sub-shelf melt rates increase only half as much as in the coupled system due to the geometric changes, the feedbacks do not influence a reversibility of the regime shift that we find in our simulations. Importantly, the reversal occurs more gradual than the 'cold' to 'warm' flip, and meanwhile the ice sheet continues losing ice and retreating. Our results imply that melt rate projections are ideally conducted in a coupled system, however, the regime shift and reversal of Filchner-Ronne cavity appears to be controlled by local atmospheric conditions, and is not qualitatively influenced by ice-ocean feedbacks. 

How to cite: Reese, R., De Rydt, J., and Naughten, K.: Regime shift of Filchner-Ronne Ice Shelf cavity remains reversible , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17058, https://doi.org/10.5194/egusphere-egu25-17058, 2025.

The Southern Ocean is the source of the Antarctic Bottom Water (AABW), a dense water mass that occupies 60 % of the global ocean. AABW is formed in different places around Antarctica as a mixture of dense shelf waters (DSW) and Circumpolar Deep Water (CDW).

CDW occupies the deep Southern Ocean at depths between 200-1500 m, and is considerably warmer (temperature higher than 0°C) than the water masses found at similar depths over the continental shelf. CDW is carried eastward around the continent by the Antarctic Circumpolar Current (ACC) but close to the shelf break it flows westward within the Antarctic Slope Current (ASC).

The ASC is a quasi-circumpolar feature that starts in the Bellingshausen Sea and vanishes next to the Antarctic Peninsula in the Weddell Sea. The ASC is connected to the flooding of the continental shelf with CDW modified by the interaction with local surface waters (modified CDW – mCDW) and the presence of mCDW over the continental shelf can affect the sea ice formation and represents a risk for the ice shelves fringing the Antarctic continent. 

The presence of warmer mCDW and freshwater resulting from the ice shelf melt reduce the sea ice production rates, a crucial part of DSW formation. When the ocean freezes salt (brine) is rejected into the water increasing local density which creates an instability and can trigger convection and produce DSW, when less sea ice is formed the deep convection potential is reduced; the DSW formed this way is termed HSSW.

HSSW can flow offshore and slide down the continental slope mixing with ambient waters along its path until it reaches the equilibrium depth as AABW. HSSW can also flow underneath the ice shelf cavity and reach the grounding line where the freezing temperature is lower than at the surface due to the pressure effect. There, it causes melting and the mixture with glacial melt water generates the supercooled (colder than surface freezing temperature) and slightly fresher Ice Shelf Water (ISW). ISW can mix with ambient waters forming a dense water type that sinks when it reaches the continental slope producing another type of AABW.

When the DSW leaves the continental shelf they are carried westward by the ASC together with CDW and lighter waters influenced by the ice shelf melt water. Part of the waters on the continental shelf are advected in the same direction by the Antarctic Coastal Current (ACoC), a westward flow observed in various sites around Antarctica. The waters transported by the ACoC and ASC can have an impact on the neighbouring basins. 

Starting from a Finite volumE Sea Ice-Ocean Model (FESOM2) that is able to reproduce the above-mentioned key characteristics around the Antarctic continent  we prepared 3 experiments to investigate the effects of the ice shelves upstream from 3 key DSW formation sites: the Filchner-Ronne Ice Shelf, Amery Ice Shelf and Ross Ice Shelf. We will present the experiment design and preliminary results.

How to cite: van Caspel, M., Timmermann, R., and Janout, M.: How upstream ice shelves affect Dense Water formation: Insights from FESOM2 Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18340, https://doi.org/10.5194/egusphere-egu25-18340, 2025.

To quantify the ocean-driven Antarctic ice mass loss and the subsequent sea level rise, the geophysical modeling community is pushing towards frameworks that fully couple increasingly complex models of atmosphere, ocean, sea ice and ice sheets & shelves. We will present results from an 8km ocean – ice shelf – ice sheet - bathymetry coupled model of Antarctica, based on the frameworks of the Regional Ocean Modelling System and the Parallel Ice Sheet Model.

Our projections for two different global warming trajectories (rcp2.6 & 8.5) suggest that warming of the Antarctic shelf seas diverge between scenarios from the 2050s onward. Our preliminary analysis focuses on the two largest ice shelves, the Ross and the Ronne-Filchner, both currently hosting cold ocean cavities.

Under the rcp8.5 trajectory, episodic warm water hosing over the eastern shelf in the Weddell Sea becomes a permanent feature at mid century, leading to a 1.5 degree increase of water temperature within a decade over the central and the eastern shelf. Basal melt rates of the entire Filchner-Ice-Shelf and the southern Ronne-Ice-Shelf exceed 2 m/yr, which in some areas is a magnitude larger than current rates.
In contrast the Ross-Ice-Shelf appears to remain stable under both climate trajectories. In the rcp8.5 scenario the shelf sea over the north-south stretching banks warms moderately by 0.25-0.5 degrees but this increased heat has no access to the cavity according to our model results.
We will present insight to the mechanisms that drive the sudden warming in the Weddell Sea and construct a hypothesis of why the Ross Ice Shelf appears more protected from Southern Ocean heat.

How to cite: Jendersie, S., Alevropoulos-Borrill, A., Lowry, D., and Golledge, N.: Indications of the Ronne-Filchner-Ice-Shelf becoming host to a warm cavity within the second half of the 21st century under high emission scenario, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20716, https://doi.org/10.5194/egusphere-egu25-20716, 2025.

The Arctic Ocean is projected to warm twice more than the global mean in a warming 21st century, contributed by an increased solar heat input due to sea ice decrease. Here we find more solar heat input into the Arctic Ocean in a higher-resolution climate model. This is due to the impacts of Arctic marine heatwaves (MHWs), known as episodes of extreme ocean warming. The explicit consideration of MHWs, which are stronger and more realistic in higher-resolution models, increases melting of sea ice and thus solar heat input, thereby reinforcing the long-term Arctic Ocean warming. A positive feedback is identified between stronger MHWs and larger Arctic Ocean warming. We emphasize that Arctic Ocean warming is underestimated by the current generation of climate models, which generally have a too low spatial resolution to resolve Arctic MHWs. We conclude that future eddy- and storm-resolving models will provide a new perspective on the Earth system's response to past and future climate and environmental extremes.

How to cite: Gou, R., Deng, Y., Cui, Y., Qi, S., Wang, S., Wu, L., and Lohmann, G.: Underestimated future Arctic Ocean warming due to unresolved marine heatwaves at low resolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1748, https://doi.org/10.5194/egusphere-egu25-1748, 2025.

We analyzed ostracode and foraminifera assemblages, silicoflagellates, biogenic silica, and sediment grain size from two high-resolution box cores collected in the Alaska and Canadian Beaufort Sea during the 2022 Arctic Challenge for Sustainability (ArCSII) expedition. These cores provide biennial-scale, multi-proxy records of ecological change over the past ~70 years. At BC2, located east of Barrow Canyon, faunal assemblages over the last 41 years showed three distinct ecological periods: (1) the 1980s-1990s, dominated by species indicative of stable, ice-covered conditions; (2) a shift post-2000 with warmer temperatures, longer ice-free seasons, and increased sandy sediments; and (3) a recent period (2018–2022) characterized by subarctic and Pacific-affiliated species, reflecting productive, summer ice-free waters. Similarly, at MT1 near the Mackenzie Trough, three periods were identified: (1) cold, stable conditions with high sea-ice cover (1950-1980); (2) a transition in the 1990s marked by increased productivity and longer ice-free periods; and (3) a shift (2002–2022) toward more dynamic, productive conditions, with reduced sea-ice extent and increasing Mackenzie River discharge. The faunal transitions among ostracodes, foraminifera, and silicoflagellates correspond closely with changes in ocean conditions, providing key insights into the timing of ecological responses to anthropogenic climate change. By integrating instrumental data—such as temperature, sea-ice extent, and river discharge—with the biological proxy records, we constrained the timing of when these environmental shifts began affecting biological organisms. This analysis revealed that changes in faunal composition are tightly linked to warming, sea-ice loss, and altered freshwater inputs, and underscores the complex, cascading impacts of climate change on Arctic ecosystems. These ecological shifts are also influenced by large-scale ocean-atmosphere dynamics, such as the Pacific Decadal Oscillation, which further modulate the timing and magnitude of ecological responses in the Beaufort Sea ecosystem.

How to cite: Gemery, L., Szarek, R., Suzuki, K., Addison, J., Caissie, B., Joe, Y. J., Seike, K., Yamada, K., Onodera, J., Itoh, M., and Yamamoto, M.: Ecological response to anthropogenic climate change in the Beaufort Sea: Biennial-scale evidence from proxy and instrumental records during the last ~70 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2347, https://doi.org/10.5194/egusphere-egu25-2347, 2025.

Atlantification is a major phenomenon associated with rapid changes in the Arctic Ocean, including anomalous sea-ice loss, warming and salinification of the near-surface, enhanced mixing and changes in the ecosystem structure. Despite anomalous transport of Atlantic water in the Barents Sea/Fram Strait region is among the recognized causes of Atlantification, this phenomenon remains poorly characterized in the context of the historical (1850-present) period, hence far from being fully understood.

In this contribution, we illustrate recent progress of the Italian funded project “ATTRACTION: Atlantification dRiven by polAr-subpolar ConnecTIONs in a changing climate” that aims at providing a historical perspective on Atlantification by integrating observational evidence over the last decades, paleo-reconstructions and numerical climate simulations. We show results from a multi-model ensemble of historical climate simulations contributing to CMIP6 and depict robust traits of the simulated Atlantification across models and realizations toward fingerprinting the phenomenon at the gateway of the Arctic Ocean and toward a robust definition of an index for its quantitative characterization.

How to cite: Zanchettin, D., De Rovere, F., and Rubino, A.: Atlantification in a multi-model ensemble of historical climate simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3623, https://doi.org/10.5194/egusphere-egu25-3623, 2025.

The polar oceans, the high-latitude Earth systems, and the Earth climate system as a whole are strongly affected by the Antarctic and Greenland ice sheets. The recent developments of global climate models have allowed to accounting for the effects of the ice sheets either indirectly via parameterizations of freshwater fluxes, or via infrequent coupling between stand-alone ice sheet models and other climate models' components. The latter approach typically does not conserve mass across the model comonents.  In order to address these issues, we have developed a global ocean-cryosphere model iOM that includes synchronously coupled Antarctic and Greenland ice sheets in addition to sea ice and icebergs. The results of global simulations forced by the EAR-Interim reanalysis show strong seasonal and subseasonal variability in the ice-sheet/ocean interactons, demonstrating the importance of a tight synchronous coupling between the ice sheet and the ocean model components. iOM will allow us to explore interactions and feedbacks between the polar oceans and cryosphere on the subseasonal to decadal timescales.

How to cite: Sergienko, O., Huth, A., Harrison, M., and Schlegel, N.: Impacts of synchronously coupled dynamic ice sheets in the GFDL Global Ocean Cryosphere Model iOM, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3788, https://doi.org/10.5194/egusphere-egu25-3788, 2025.

Differences in salt content of North Atlantic surface waters drive variations in Nordic Seas' overturning circulation. These form a switchboard for changes in the oceanic heat transport to North European high latitudes, the 'Nordic Heat Pump', and for Atlantic Meridional Overturning Circulation (AMOC). We deduced changes in the Nordic Seas' overturning circulation during peak last glacial and early deglacial times (22-15 cal. ka) from two marine sediment cores with centennial-scale age resolution synchronized via radiocarbon (¹⁴C) plateau tuning. Sediment data suggest that the salinity of surface waters, advected through the Denmark Strait from the northwesternmost Atlantic, started to drop significantly near 18.4 cal. ka. This accompanied precisely an abrupt rise in bottom water temperature by up to 3.5°C and a drop in both ventilation and ¹⁴C ventilation ages of Denmark Strait overflow waters feeding the AMOC. Moreover, it paralleled a change in (detrital) Pb and Nd radiogenic isotopes suggesting that overflow waters then started to have their dominant source in the North Iceland Jet of upper North Atlantic Intermediate Water that overflows the shallow basaltic Iceland-Scotland Ridge east of Iceland. Off Norway, the salinity reduction north of Iceland went along with a fast rise in the ¹⁴C reservoir age of surface waters from ~600–1200 years up to ~2000 years and an abrupt breakdown of Nordic Seas' convection of young deep waters. Accordingly, warm Atlantic waters were replaced by slightly cooler Arctic polar waters aged like those of the East Greenland Current, inducing a breakdown of the 'Nordic Heat Pump' and start of 'Heinrich Stadial 1' as reflected by a precisely coeval cooling documented on top of the Greenland ice sheet, lasting until ~15 cal. ka. The outlined circulation changes starting near 18.4 cal. ka remind us of potential implications of the meltwater flow from West Greenland strongly enhanced today. 

How to cite: Sarnthein, M. and Blaser, P.:  Meltwater-induced salinity drop in Greenland Sea induced changes in AMOC and the onset of Heinrich-1 stadial 18 400 years ago – Potential analog to modern trends, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3821, https://doi.org/10.5194/egusphere-egu25-3821, 2025.

We present planktic foraminiferal and planktic stable isotope records from central and eastern Arctic Ocean sediment cores with a particular attention on the development of the structure of upper water masses during two past interglacials (here termed IG3 and IG2), in comparison to the present Holocene (IG1). The age of interglacials IG2 and IG3 is currently under discussion. While the "classic" age model based on Jakobsson et al. (2000, Geology) would relate them to marine isotope (sub)stages (MIS) 5e and 5a, latest work (e.g., Song et al., 2023, Earth Sci. Rev.; Razmjooei et al., 2023, Quat. Sci. Rev.) would assign ages of MIS 11, 9, 7 or 5.

Stable oxygen and carbon isotopes from polar planktic foraminifers Neogloboquadrina pachyderma give clues on their habitat within the upper water column which today is characterized by an ice-covered low-saline cold surface layer, underlain by high-saline warm Atlantic Water. Sediments from IG2 and IG3 containing also subpolar planktic foraminifers Turborotalita quinqueloba show oxygen isotope values of close to modern ones, indicating a similar water mass structure as today, with a transition level between freshwater-rich and Atlantic Water. Carbon isotope values are lower and may point at a higher bioproductivity due to less sea ice and a decomposition of carbon in the upper waters. Interestingly, in the sediments underneath, which are barren in T. quinqueloba but contain abundant N. pachyderma, both oxygen and carbon isotopes are significantly higher. These data can be interpreted as evidence of a strongly stratified water column, a deeper habitat of the foraminifers, a strong subsurface advection of Atlantic Water and more sea ice during the early phases of IG2 and IG3. In cases, due to a lack of carbonate microfossils this interval is not represented in all analyzed cores. We assume that near-surface salinities were below the tolerance limit of planktic foraminifers in the very early parts of IG2 and IG3, probably due to a strong influence of meltwater from disintegrating ice sheets on northern Eurasia in the preceding glacial stages. Our results reveal a two-step development of conditions in the central Arctic during previous warm intervals. In the first part, the uppermost water column (including the habitat depth of T. quinqueloba) always had very low salinties due to freshwater discharge from ice sheets on the continents. Only in the second part Atlantic Water was shoaling and allowed the occupation by shallow-dwelling T. quinqueloba. Data from the Kara Sea continental margin suggest that upper water conditions in the eastern Arctic remained under strong freshwater influence, at least throughout IG2.

How to cite: Spielhagen, R. F., Bauch, H. A., and Mackensen, A.: Atlantic Water distribution in the central and eastern Arctic Ocean during past interglacials, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6159, https://doi.org/10.5194/egusphere-egu25-6159, 2025.

Paleoceanographic records demonstrate linkages between the increasing Pacific water flux accompanying the postglacial submergence of the shallow (~ 50 m deep) Bering Strait and the progressive warming of the Western Arctic until ca. 4000 years BP (cf. de Vernal et al. Sci. Adv. 2024). The Pacific water flux also impacts the freshwater budget of the Arctic Ocean, which ultimately plays a role in the Arctic sea ice and freshwater export rate to the northern North Atlantic. Sea-level changes thus deserve special attention from an Arctic perspective as they can considerably modify the exchanges between the Pacific and Arctic oceans. Furthermore, sea level determines the status of the Arctic shelves, submerged or not, which in turn plays a role in sea-ice production, as well as in the latent heat from the Atlantic water mass flowing northward through Fram Strait and the Barents Sea. We hypothesize that the increased freshwater inflow from the Pacific into the Arctic and the enhanced sea ice formation rates resulting from the sea level rise have played a role in the large scale cooling trend of the eastern Arctic and subarctic North Atlantic that has marked the late Holocene.

How to cite: de Vernal, A. and Hillaire-Marcel, C.: Arctic gateways, sea level and climate changes in the subpolar North Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6192, https://doi.org/10.5194/egusphere-egu25-6192, 2025.

Modeling results and proxy data both suggest that sea-ice conditions in the Arctic Ocean were less severe during the last interglacial (MIS 5e) compared to the present interglacial (MIS 1), but spatial variability of the sea-ice cover is still poorly constrained. In this study, variations in the intensity and composition of biogenic sedimentary structures (bioturbation and trace fossils) are used to address spatial differences in sea-ice distribution between the two interglacials. The presence or absence of trace fossils and bioturbated sediment have long been used to separate interglacial and glacial and intervals in central Arctic Ocean sediment cores based on the premise that interglacial conditions with less sea ice and more open waters led to higher food flux to the benthic communities, and vice versa. However, spatial differences in sea-ice cover during the individual interglacials also led to differences in primary productivity and consequently to spatial variations in the intensity of bioturbation and the composition of trace fossils. Areas characterized by perennial sea ice and few open leads or polynyas have a lower food flux than areas close to the sea-ice margin or with abundant polynyas. Consequently, the areas with more severe sea-ice conditions display fewer trace fossils and less intensely bioturbated sediments than areas characterized by open leads, polynyas, or areas situated close to the ice margin where primary productivity is higher. The spatial pattern shows a clear decrease in bioturbation intensity and trace fossil diversity from areas today characterized by relatively open waters, towards areas characterized by thick perennial sea ice. There is also a general pattern of more diverse trace fossil communities and more intense bioturbation observed from MIS 5e sediments compared to MIS 1, suggesting that sea-ice conditions during MIS 5e were generally less severe than during the present interglacial. The application of trace fossils and bioturbation for the reconstruction of sea ice conditions is particularly viable because of the large number of cores with X-ray radiographs available from data repositories such as www.pangaea.de. The main limitation of the method comes from the generally poor age control of Arctic sediments beyond the range of radiocarbon dating.

How to cite: Löwemark, L.: A comparison of Arctic Ocean sea-ice conditions during interglacials MIS 5e and MIS 1 based on biogenic sedimentary structures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6455, https://doi.org/10.5194/egusphere-egu25-6455, 2025.