The Atlantic Meridional Overturning Circulation (AMOC) plays a major role in shaping the Northern Hemisphere climate, and assessing the risk of a future slowdown has become a key challenge in ocean research. Over the past two decades, advances in observations and modeling have substantially refined our understanding of where and how deep waters are formed.

In this ‎Award Lecture of the OS‎ Division, I will review these developments to examine the drivers of North Atlantic dynamics and their representation in coupled climate models. First, I will focus on observation-based estimates of water mass transformation in the subpolar gyre, highlighting the dominant role of local buoyancy forcing in the Irminger and Iceland basins. Second, I will examine how deep water formation is simulated in coupled climate models, identifying key biases that lead to excessive formation in the Labrador Sea and assessing their implications for the AMOC at subpolar latitudes. Finally, I will discuss the southward propagation of deep waters and the coherence of AMOC variability across the North Atlantic, placing these results in the broader context of AMOC change at different timescale.

How to cite: Petit, T.: On the Role of Atmospheric Forcing on the North Atlantic Dynamic – Insights from Observations and Climate Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10992, https://doi.org/10.5194/egusphere-egu26-10992, 2026.

Induced diffusion (ID), an important mechanism of spectral energy transfer due to interacting internal gravity waves (IGWs), plays a significant role in driving turbulent dissipation in the ocean interior. In this study, we revisit the ID mechanism to elucidate its directionality and role in ocean mixing under varying IGW spectral forms, with particular attention to deviations from the standard Garrett-Munk spectrum. The original interpretation of ID as an action diffusion process, as proposed by McComas et al., suggests that ID is inherently bidirectional, with its direction governed by the vertical-wavenumber spectral slope σ of the IGW action spectrum, n ~ m^σ. However, through the direct evaluation of the wave kinetic equation, we reveal a more complete depiction of ID, comprising both a diffusive and a scale-separated transfer rooted in the energy conservation within wave triads. Although the action diffusion may reverse direction depending on the sign of σ (i.e., red or blue spectra), the net transfer consistently leads to a forward energy cascade at the dissipation scale, contributing positively to turbulent dissipation. This supports the viewpoint of ID as a dissipative mechanism in physical oceanography. This study presents a physically grounded overview of ID and offers insights into the specific types of wave-wave interactions responsible for turbulent dissipation.

How to cite: Wu, Y. C. and Pan, Y.: Induced Diffusion of Interacting Internal Gravity Waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-221, https://doi.org/10.5194/egusphere-egu26-221, 2026.

A permafrost probability index (PPI) based on rock glacier inventory and machine learning models, including random forest, support vector machine, artificial neural network, and logistic regression, was generated for Kinnaur district, Himachal Pradesh, India. Intact rock glaciers were considered the dependent variable, and elevation, slope, aspect, and potential incoming solar radiation were used as independent variables to generate a spatially distributed, high-resolution permafrost probability index. Daily weather station data and daily multitemporal MODIS satellite data were used to train a linear regression model to predict the annual 0℃ isotherm in the region for the period of 2023-24, aiming to understand potential degradation by overlaying the isotherm on permafrost distribution. The random forest technique produced the best results with an overall accuracy of 89.43%. Seven glacial lakes were identified as located in potentially permafrost-degraded slopes, and the Kashang glacial lake was selected for detailed downstream glacial lake outburst flood process chain modeling based on its size, moraine-dammed proglacial setting, and potential downstream impact. The volume of the lake was estimated to be 8.6 × 10⁶  m³ by extrapolating the contours from overdeepening of the main glacier. Three sources of avalanches were identified based on permafrost degradation and slopes greater than 30 degrees. Subsequently, three scenario-based process chains for glacial lake outburst floods were modeled. We simulate avalanche initialization, displacement wave generation, overtopping, moraine erosion, and downstream flooding. The modelling results revealed that the potential GLOF can cause a peak discharge of 16,167 ms⁻¹, and floodwater can reach the Kashang, where a hydropower is located, within 16 minutes  in the high-magnitude scenario. The findings can give important insights into GLOF hazard mitigation in the valley and can aid as preliminary data for various stakeholders working towards mitigating glacier-related hazards.

Keywords: Permafrost, GLOF, machine learning, r.avaflow, Himalaya

How to cite: Alangadan, A. and Sattar, A.: Glacial lakes in permafrost terrain and downstream hazards, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1083, https://doi.org/10.5194/egusphere-egu26-1083, 2026.

The boreal early-spring of 2024 witnessed unprecedented marine heatwaves across the tropical Atlantic, setting a satellite-era record for basin-averaged marine heatwave intensity. Based on observational and reanalysis datasets and a mixed layer heat budget analysis, we identify three region-specific drivers. In the north (20°N–3°N), the event began in fall 2023 and was maintained by sustained positive shortwave radiation anomalies due to reduced cloudiness. Equatorial warming (3°N–3°S) was primarily driven by wind-driven ocean wave processes, amplified by a shallower mixed layer. In the south (3°S–20°S), the key mechanism was wind-driven mixed layer shoaling. The reduced cloudiness over the northern tropical Atlantic is linked to remote El Niño forcing, and the wind anomalies over the equatorial and southern tropical Atlantic are partly attributable to the concurrent South Atlantic Subtropical Dipole. Our findings clarify the multifaceted origins of such extreme marine heatwaves, offering crucial insights for improving their seasonal prediction.

How to cite: Yang, J.-C., Li, S., Richter, I., Liu, Y., Zhang, Y., Li, Z., and Lin, X.: Primary Factors Driving Extreme 2024 Early-spring Marine Heatwaves in the Tropical Atlantic: Shortwave Radiation and Mixed Layer Depth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3225, https://doi.org/10.5194/egusphere-egu26-3225, 2026.

Litter pollution has grown to be the most prominent threat to the coastal ecosystems, affecting both the environment and the local communities. An important step towards the mitigation of coastal pollution is the effective monitoring of the issue. The rapid evolution of Remote Sensing has offered many new techniques for the detection of beach litter, and Unmanned Aerial Vehicles (UAVs), especially, have proven to be invaluable tools. In this study, different approaches of beach litter detection are evaluated in order to determine which ones yield the most promising results. The data used were collected in the area of Palio Faliro, Greece and included RGB and Multi-spectral images. For the detection of the litter from the UAV images, two Deep Learning (DL) models were utilized, namely the Mask R-CNN and the YOLOv3. The accuracy of these two DL models in beach litter detection and also explore the potential challenges that may arise while trying to monitor the coastal environment with UAV methods. Our study findings suggest that the combined use of DL methods and UAV imagery can provide a cost-effective and scalable solution in litter detection and can assist relevant decision-making actions. Future work will focus on evaluating different DL methods under other experimental settings as well which will help towards assessing the wider applicability of the combined use of drone imagery and DL approaches in litter detection in coastal areas.

KEYWORDS: Remote Sensing, coastal little, UAVs, drones, deep learning, ACCELERATE project

Acknowledgements 

This study is financially supported by the ACCELERATE MSCA SE program of the European Union’s Horizon research and innovation program under grant agreement No. 101182930

How to cite: Mitsopoulou, C., Petropoulos, G. P., Detsikas, S. E., Lekka, C., Grigoriadis, K., Polychronos, V., Mamagiannou, E.-M., Gkotsikas, C., Chardavellas, K., and Katsou, E.: Litter detection and mapping from the combined use of multispectral UAV imagery and Deep Learning: A case study from Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3528, https://doi.org/10.5194/egusphere-egu26-3528, 2026.

Estuaries represent complex morphodynamic systems where interactions between tides, waves, and sediment processes control coastal stability and its ecological resilience. One such estuary, located along the bank of the Purna River in Navsari District, Gujarat, India, is currently experiencing severe erosion, with nearly two-thirds of the estuarine coastline affected.  Understanding spatio-temporal evolution of key coastal features is essential, including tidal flats, salt marshes, mangrove cover, and anthropogenic infrastructures within the study region. In this study, the coastal features segmentation is performed using the Random Forest on derived Landsat satellite imagery spectral indices spanning 2005–2024. The results indicate that over the past two decades, mangrove cover has increased by more than twofold, particularly near the estuary mouth. In contrast, tidal flat areas exhibited significant spatial variability, while salt marshes showed a considerable decline.

Shoreline change analysis shows extensive coastal erosion with the Net Shoreline Movement (NSM) exceeding 150 m in certain stretches, while the End Point Rate (EPR) ranged from 1.5 to 17 m/year (mean: 9.5 m/year). The analysis further indicates significant accretion in the estuaryward region and pronounced erosion along the seaward coast near its mouth. Further the coupled tide-wave numerical modelling was carried to attribute the observed changes. Overall, the findings highlight the complex interplay between natural coastal processes and anthropogenic pressures in this dynamic estuarine coastal system and provide valuable baseline information for coastal zone management and conservation planning.

Keywords: Estuary Dynamics, Random Forest, Shoreline changes, Tide Modelling, Wave Modelling, Remote Sensing.

How to cite: Makhan, P. K., Goud Lakku, N. K., Behera, M. R., and Vijaya Kumar, S.: Coastal Features Segmentation and Assessing their dynamics Using Machine Learning: Random Forest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3596, https://doi.org/10.5194/egusphere-egu26-3596, 2026.

The Gulf of Panama is a semi-enclosed tropical basin where coastal processes are driven by a multimodal wave climate with pronounced interannual-to-decadal variability (Vallarino-Castillo, 2026). Offshore wave conditions were characterized at three spectral locations near the Gulf entrance using GLOSWAC-5 spectral data (Portilla-Yandún and Bidlot, 2025), revealing dominant wave systems with distinct directional origins and seasonal variability. A persistent Southern Ocean swell dominates year-round from the south–southwest, while northerly wind-seas associated with the Panama Low-Level Jet prevail during the dry season (December–April). Their opposing directions lead to frequent crossing-sea conditions, particularly along the western Gulf entrance, where partial blocking by the Azuero Peninsula enhances directional spreading. In contrast, more exposed central-eastern locations exhibit consistently multimodal spectra, whereas sheltered eastern areas show reduced northerly wind-sea influence and narrower directional ranges. During the wet season (May–November), additional southerly swell components linked to subtropical trade winds and the Chocó Low-Level Jet reinforce low-frequency energy, while episodic North Pacific swell incursions further increase spectral complexity. Building on these offshore patterns, we analyze how wave systems transform as they propagate across the Gulf’s complex basin geometry.

To resolve coastal wave conditions efficiently, we applied a hybrid spectral downscaling framework across the Gulf. Remote swell was reconstructed using BinWaves (Cagigal et al., 2024), which disaggregates each offshore spectrum into frequency–direction bins and propagates them individually with SWAN, assuming linear wave superposition over the nearshore of the Gulf of Panama, such that nonlinear wave–wave interactions are neglected during propagation. Nearshore spectra are then reassembled using precomputed propagation coefficients that account for coastal geometry. Locally generated seas were reconstructed with HyXSeaSpec, which extracts dominant atmospheric modes via multivariate dimensionality reduction, projects SWAN spectra onto a reduced EOF/PCA space and learns the nonlinear mapping between atmospheric modes and spectral coefficients using radial basis functions (RBFs). During prediction, new wind fields are projected into the reduced space to recover full directional spectra through inverse transforms. The hybrid workflow generates a 3-hourly directional wave spectrum hindcast (1969–2023) that combines remote swell and locally generated wind-sea contributions throughout the basin.

The ongoing nearshore analysis uses the reconstructed spectra to identify dominant variability patterns and coherent wave regimes, assessing how energy is redistributed within the gulf and how nearshore conditions respond to seasonal and interannual atmospheric forcing.

References:

Vallarino-Castillo R, Antolínez JAA, Negro-Valdecantos V, Portilla-Yandún J (2026). “Beyond understanding the role of far-field climate in the Gulf of Panama coastal dynamics: an analysis of long-term and seasonal variability of wave systems”. Climate Dynamics. https://doi.org/10.1007/s00382-025-08007-w

Portilla-Yandún J, Bidlot J-R (2025). “A global ocean spectral wave climate based on ERA-5 data: GLOSWAC-5”. Journal of Geophysical Research: Oceans. https://doi.org/10.1029/2025JC022629

Cagigal, L., Méndez, F.J., Ricondo, A., Gutiérrez-Barceló, D. & Bosserelle, C. (2024). “BinWaves: An additive hybrid method to downscale directional wave spectra to near-shore areas” en Ocean Modelling. 84, 102346.

How to cite: Vallarino-Castillo, R., Bellido, G., Cagigal, L., Negro-Valdecantos, V., Portilla-Yandún, J., Méndez, F., and A. A. Antolínez, J.: Hybrid spectral downscaling and climate-driven variability of multimodal wave systems in the Gulf of Panama, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3616, https://doi.org/10.5194/egusphere-egu26-3616, 2026.

             The variability in the circulation of the Northern Ionian Gyre (NIG) during 1988-2020 is assessed via dynamic-height fields in the upper layer (0-120 m and 0-398 m) derived from the monthly-averaged temperature and salinity fields of the Copernicus reanalysis data. The yearly-averaged dynamic-height fields agree with the corresponding fields of altimetric sea-surface topography used in previous studies that found, at the area of the NIG, a maximum-variability mode in the sea-surface topography of the Ionian Sea. In the present results, the NIG coincides with the area of a) the variability maxima of the dynamic-heights, existing on the standard-deviation (std) maps of the yearly-averaged dynamic heights during 1988-2020, b) the std maxima of the averaged density in the upper layer and c) the std maxima of the averaged salinity in the upper layer; the density-salinity correlation coefficients in the upper-layer within the NIG range from 0.87 to 0.74.

            Moreover, the std maxima of the precipitation fluxes, which have the dominant role on the evaporation-minus-precipitation (E-P) budget, are also located on the NIG area.   The 5-year running-averaged values of yearly E-P and salinity in the upper-layer of the NIG, which filter out the variability in less that ~5-6 years while they preserve the dominant variability in the periodicities (~8-10 years) of the NIG-circulation, have statistically significant correlations ranging from 0.53 for the period 1990-2018 to 0.73 for the period 1997-2018. After ~2005, the two timeseries resemble to each other even more.  In the upper layer, the area to the east-southeast of the NIG has statistically significant correlations in salinity (correlation coefficients: ~0.68-0.8) with the NIG area. This area can feed its higher-salinity signal to the NIG via northward transfer during the cyclonic circulation mode of the NIG.

How to cite: Kontoyiannis, H., Tsiaras, K., Iona, A., and Ballas, D.: The role of the air-sea water fluxes and the lateral influence on salinity in the bimodal circulation variability of the Northern Ionian Gyre in the period 1988-2020, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5203, https://doi.org/10.5194/egusphere-egu26-5203, 2026.

The Karakoram is known for its numerous surge glaciers and associated hazards from ice-dammed lake outburst floods. However, significant discrepancies persist in our understanding of surge trends and flood frequency. Therefore, this study aims to clarify the surge behaviour and related glacial lake outburst flood (GLOF) history for the Kumdan group of glaciers (Chong Kumdan, Kichik Kumdan, and Aktash). The study analysed historical archives, high-resolution satellite imagery, elevation changes derived from digital elevation models (DEMs), and glacier surface velocity from the ITS_LIVE dataset. Based on an in-depth review of historical records and cross-verified with multi-temporal satellite imagery, 16 GLOFs have been documented from this group since 1835, primarily originating from Chong Kumdan and Kichik Kumdan. The Aktash Glacier has surged several times but has not formed any ice-dammed lake due to efficient subglacial drainage, which prevents river blockages. Chong Kumdan and Aktash glaciers exhibit longer active phases (~7-10 years), whereas Kichik Kumdan Glacier shows shorter phases (~2 years). Out of all three Kumdan glaciers, the Chong Kumdan has produced the most devastating floods in 1835, 1926 and 1929. This glacier comprises two tributaries (a and b) and main trunk. Tributary ‘a’ follows a ~77-year surge cycle, and tributary ‘b’ and the main trunk exhibit asynchronous surge records. The surge cycle duration of Kichik Kumdan Glacier decreased from 33 years (1833–1866) to 27 years (1970–1997) due to climate warming. The last GLOFs from Chong Kumdan and Kichik Kumdan occurred in 1934 and 1903, respectively. DEM analysis from 2015 to 2022 reveals thickening in the reservoir areas of Chong Kumdan (~22 m) and Kichik Kumdan (~20 m), suggesting potential future surge but with a low probability of GLOF events. Overall, our study observed a decline in surge-generated GLOFs due to climate warming, reduced mass accumulation and weakening of ice dams. These insights will help downstream communities and risk management authorities better understand future risks and develop effective mitigation strategies.

How to cite: Halder, S. and Bhambri, R.: Impact of climatic warming on glacier surges and associated ice-dammed lake outburst floods in the Eastern Karakoram, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5669, https://doi.org/10.5194/egusphere-egu26-5669, 2026.

How to cite: Bouzaiene, M. and Menna, M.: Development of a Fine-Scale (1/648°) Nested Ocean Forecasting Model for the Tunisian Shelf, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8164, https://doi.org/10.5194/egusphere-egu26-8164, 2026.

Ocean salinity serves as a key indicator of the global water cycle and exerts important controls on oceanic circulation, sea level, and stratification, thereby playing a critical role in marine thermodynamic and dynamic processes. In recent years, salinity variability in the tropical Indian Ocean, particularly its dynamic mechanisms and climatic effects, has attracted growing scientific interest. Using 31 years of satellite observations, in-situ data sets, and model reanalysis data, this study investigates the decadal variability and formation mechanisms of the low salinity tongue in the South Indian Ocean between the equator and 20°S. The results indicate that both the volume and mean salinity of the low-salinity tongue exhibit a quasi-12-year oscillation, which is primarily associated with the Interdecadal Pacific Oscillation (IPO). Further analysis reveals that on decadal timescales, variability in the volume of the upper 50 m low-salinity tongue is mainly driven by local precipitation. Through anomalous atmospheric circulation, sea surface temperature anomalies in the tropical Pacific lead to multi-year precipitation anomalies in the southeastern Indian Ocean, which subsequently alter the westward extension of the surface low-salinity tongue and ultimately govern its volume variability in the upper 50 m. However, in the subsurface layer (50 to 200 m), variability in the volume and average salinity of the low salinity tongue is dominated by freshwater transport associated with the Indonesian Throughflow (ITF). During negative IPO phases, wind anomalies over the tropical Pacific trigger oceanic wave adjustments, which enhance the ITF salinity transport. This process subsequently leads to an expansion of the low salinity tongue and a decrease in its average salinity in the southeastern Indian Ocean. Based on the three-dimensional variability of the low salinity tongue, this study reveals the relationships between the volume and average salinity of the tongue at different depths and local freshwater forcing, as well as salinity transport by the ITF, thereby contributing to an improved understanding of how regional water mass changes respond to long-term climate variability.

How to cite: yanran, P., sun, Q., zhang, Y., zhang, Y., chi, J., and du, Y.: Analysis of the mechanisms underlying the low-frequency variability of the low-salinity tongue in the southeastern Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10227, https://doi.org/10.5194/egusphere-egu26-10227, 2026.

The rapid expansion of offshore wind energy infrastructure represents a major anthropogenic modification of coastal and marginal seas, yet the physical interactions between monopile foundations, hydrodynamics, and sediment transport remain insufficiently quantified. This study investigates the impact of monopile foundations at the Meerwind offshore wind farm (German Bight, North Sea) on local and regional coastal dynamics. Using a high-resolution coupled wave-current-sediment transport model, we analyze hydrodynamic and sediment processes with mesh refinement of ~2 m near the structures to capture turbulent wake effects.

Our results demonstrate that monopile arrays act as significant sinks for wave energy: monthly mean significant wave heights (Hs) and mid-depth velocities decrease by ~5%, while turbulent kinetic energy increases by up to 70% near the foundations. Dominant westerly wind-driven waves modulate tidal asymmetry on the leeward (eastern) side of the piles, generating asymmetric turbulent wakes and altering bottom shear stress patterns.

Reduced wave-induced bottom stress enhances localized sediment deposition, increasing surface suspended sediment concentration (SSC) while reducing near-bottom loads. On a regional scale, wave attenuation leads to a ~1% decrease in depth-averaged SSC over a 20 km east of the piles. In consequence, the presence of the wind farm reduces the net inflowing sediment flux by ~25% within a 5 km radius during March 2020, linked to a ~2 cm attenuation of Hs.

These findings highlight how large-scale offshore energy infrastructure can reorganize sediment budgets and coastal morphodynamics under changing human activities, providing critical insights for the sustainable management of multi-use ocean spaces. Further work, including additional wind farms and extended simulation periods, is planned to substantiate these initial findings and better quantify cumulative impacts, particularly in light of ongoing erosion challenges in the Wadden Sea under sea-level rise.

How to cite: Hosseini, S. T., Pein, J., Staneva, J., Stanev, E., and Zhang, Y. J.: Influence of Offshore Wind Farm Monopiles on Multi-Scale Hydrodynamics and Sediment Transport in a Wave-Current Environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10444, https://doi.org/10.5194/egusphere-egu26-10444, 2026.

Effective coastal monitoring and forecasting systems rely on the availability and timeliness of interoperable, standardized, and accessible marine data across observational, modelling and service layers. Fragmented data formats, legacy infrastructures, and non-standardized access mechanisms remain significant barriers to the seamless integration of ocean observations into operational monitoring and forecasting systems and downstream applications.

This study presents the development of a standards-based data workflow designed to enhance interoperability, scalability, and facilitate marine data integration, through the adoption of international standards and best practices. The proposed approach focuses on establishing robust data flows that transform, validate, and harmonize heterogeneous datasets (e.g., in situ near-real-time observations and numerical model outputs) into NetCDF format. Standardized and programmatic access to these datasets is enabled though the OGC API Environmental Data Retrieval protocol, implemented using the pygeoapi platform. By adopting open standards and service-oriented architectures, this framework enables efficient spatio-temporal querying of ocean variables, facilitating their assimilation into forecasting systems, decision-support tools, and customized applications. In parallel, geoportal interfaces were updated to integrate the new OGC API EDR services, ensuring that interoperable data access is available both through machine-to-machine interfaces and user-friendly graphical tools, supporting a broad range of user profiles and promoting citizen involvement and ocean literacy.

By addressing interoperability at the data, service, and user-interface levels, this work demonstrates how standardized data infrastructures are key enablers for improved, scalable, and sustainable coastal monitoring and forecasting capabilities.

How to cite: Dias, T., Videira, C., Lobo, V., Costa, A. C., and Lourenço Baptista, M.: Improving coastal monitoring and forecasting systems through interoperable OGC API EDR-based data services, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11166, https://doi.org/10.5194/egusphere-egu26-11166, 2026.

The South Lhonak Lake (SLL) Glacial Lake Outburst Flood (GLOF) cascade event of 3-4 October 2023 triggered widespread devastation across Sikkim and the downstream region of Bangladesh, causing significant loss of lives and property. The post-disaster research shows that the GLOF event was triggered by a moraine failure, creating tsunami waves in the lake, eventually leading to the breach of the frontal moraine. Despite partial drainage of the lake in the 2023 event, the hazard potential of the lake needs further investigation. This makes it extremely important to continuously monitor the surrounding regions to identify unstable slopes that can potentially fail and impact the lake. The present study utilises a Sentinel-1 Small Baseline Subset (SBAS) workflow performed in the ASF OpenSARLab environment to analyse the condition of the moraines post-SLL disaster. Post-disaster analysis spanning October 2023 to September 2025 reveals continued moraine instability, characterised by an actively deforming zone along the right flank of the failed zone. This region shows a maximum LOS displacement rate of approximately -4 cm yr^-1, with a maximum cumulative LOS displacement reaching around -6 cm in the ascending track and -5 cm in the descending track. The results indicate persistent post-failure deformation and ongoing slope instability in the moraines of South Lhonak. The study provides a critical insight into the temporal behaviour of moraine slopes. This study aimed at strengthening the disaster management strategies by integrating satellite-based deformation monitoring for early warning and risk reduction measures.

How to cite: Verma, U. and Sattar, A.: Monitoring post-GLOF moraine dynamics at South Lhonak lake using satellite radars, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11635, https://doi.org/10.5194/egusphere-egu26-11635, 2026.

Coastal ecosystems are highly vulnerable to nutrient-driven eutrophication from anthropogenic sources such as urbanization, wastewater discharge, and industrial development, among others, which alters their ecosystem services. In order to determine nitrogen sources, the nitrogen isotopic composition (δ¹⁵N) was analyzed in the macroalgae Boodleopsis verticillata and Bostrychia spp., collected between 2014 and 2016 at four localities with different degrees of anthropogenic disturbance: Valencia – Very Low Intervention (MBI-VA), Chucheros – Low Intervention (BAI-CHU), San Pedro – Moderate Intervention (MOI-SP), and Piangüita – High Intervention (ALI-PI) in the Colombian Pacific. The δ¹⁵N values ranged between 0.3 and 2.4‰ in MBI-VA, 1.8 and 3.4‰ in BAI-CHU, 2.3 and 5.5‰ in MOI-SP, and 2.3 and 10.16‰ in ALI-PI. Since the assumptions of normality and homogeneity of variances were not met (p < 0.05), a non-parametric Kruskal–Wallis test was applied, revealing significant differences in δ¹⁵N among localities (p < 0.0001). Dunn’s test indicated that MBI-VA and BAI-CHU differed significantly from MOI-SP and ALI-PI (p < 0.05). Three nitrogen sources were defined: atmospheric deposition, oceanic waters, and wastewater. Both species (B. verticillata andBostrychia spp.) showed a decreasing gradient of atmospheric deposition (87% ± 3% to 52% ± 7% and 82% ± 6% to 21% ± 11%, respectively) from MBI to ALI, in contrast to an increase in oceanic waters (8% ± 4% to 37% ± 13% and 12% ± 7% to 38% ± 21%) and wastewater contributions (5% ± 2% to 12% ± 6% and 7% ± 3% to 41% ± 12%). This pattern was more evident in Bostrychia spp., suggesting greater sensitivity to variations in nitrogen sources. Linear regression between δ¹⁵N and nitrate concentration yielded coefficients of determination of R² = 0.71 for B. verticillata and R² = 0.89for Bostrychia spp., indicating that isotopic variability was explained by nitrate. The potential of macroalgae as bioindicators of anthropogenic intervention in coastal ecosystems of the Colombian Pacific is suggested.

How to cite: Arce-Sánchez, R. S., Medina-Contreras, D., and Sánchez-González, A.:  Use of δ15N and macroalgae as indicators of the level of anthropogenic intervention in the Colombian Pacific., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13664, https://doi.org/10.5194/egusphere-egu26-13664, 2026.

Arctic sea ice plays a critical role in regulating global climate and marine primary production, yet long-term records documenting its natural variability remain sparse in the Pacific sector of the Arctic Ocean. This limitation hampers our ability to establish a regionally coherent understanding of how sea ice responds to climatic and oceanographic forcing on centennial to millennial timescales. Here, we present a new biomarker-based reconstruction of Holocene sea ice and environmental change from the southern Chukchi Sea, north of the Bering Strait.

A 519-cm sediment core (SKQ-VC29) was recovered using a vibracorer and spans the last ~8.6 kyr, based on 17 AMS radiocarbon dates from shells and terrestrial macrofossils. Downcore concentrations of highly branched isoprenoids (HBIs) and sterols were quantified to reconstruct sea-ice conditions, marine productivity, and terrestrial organic matter (OM) inputs. Seasonal sea ice presence is inferred from IP25, a mono-unsaturated HBI produced by sea-ice diatoms, while open-water conditions and phytoplankton productivity are tracked using HBI III, brassicasterol, and dinosterol. These proxies are combined using the PIP25 index to provide a semi-quantitative reconstruction of sea-ice cover. Terrestrial inputs are assessed using vascular-plant sterols (campesterol and β-sitosterol), alongside bulk δ¹³C and C:N ratios.

The record indicates predominantly open-water conditions during the early to mid-Holocene, followed by the reappearance of seasonal sea ice at ~2.5 kyr BP—substantially later than in more northerly Arctic records. This delayed signal suggests that Neoglacial sea-ice expansion in the Pacific Arctic was spatially heterogeneous. Bulk OM proxies and declining β-sitosterol concentrations indicate a progressive reduction in terrestrial OM delivery through the Holocene, while marine productivity remains relatively stable. A pronounced shift at ~4 ka BP marks reduced organic carbon accumulation and broader environmental reorganization.

Together, these results improve spatial coverage of Holocene sea-ice reconstructions in the Pacific Arctic and highlight the complex, regionally variable nature of sea-ice evolution in a climatically sensitive gateway region.

How to cite: Jin, K., de Vernal, A., Pickart, R. S., Chen, M., Otiniano, G., and Porter, T.: Holocene Sea Ice and Organic Matter Dynamics in the Southern Chukchi Sea Revealed by Lipid Biomarkers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15143, https://doi.org/10.5194/egusphere-egu26-15143, 2026.

Sea ice classification is often a crucial step to predict climatic insights and ensure safe marine navigation. In the last few decades, satellite information has been widely used to classify sea ice in broad areas for practical applications. However, common problems are:

1) Low resolution of satellite images to provide precise classification,

2) High computational need, and

3) Scarcity of general models to discover unknown patterns in the data, especially those that enable free selection of satellite sensors to fit the application at hand.

We propose an explainable unsupervised model to integrate ice-experts’ inputs to models so that the problem of having low-resolution data can be overcome. In other words, the results of the models, given as semantic maps, can be further refined using inputs from ice-experts.

Model explainability and visual interpretation of models serve as tools to talk to’ domain experts. The use of Explainable AI in such vital activities ensures trust and easy detection of error. We present an example from a sea ice classification with Sentinel-1 time-series in the scope of the Horizon 2020 project ExtremeEarth.

A further example from the Horizon Europe project dAIEdge demonstrates the use of these explainable models for ‘on-the-edge’ inference.

How to cite: Dumitru, C. O., Karmakar, C., and Wiehle, S.: Explainable Expert-in-the-loop sea-ice classification with statistical models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16248, https://doi.org/10.5194/egusphere-egu26-16248, 2026.

Dust in the Arctic is an emerging topic related to climate and environmental impacts. The United Nations (UN) General Assembles and the UN Coalition to Combat Desertification (UNCCD) have reiterated that the global frequency, intensity, and duration of Sand and Dust Storms (SDS) have increased in the last decade and that SDS have natural and human causes that can be exacerbated by desertification, land degradation, drought, biodiversity loss, and climate change. UNCCD and FAO have also highlighted that emerging SDS source areas have been associated with the warming of the Arctic and high latitude regions, the seasonal or permanent drying of inland waters and river deltas, or are following large-scale deforestation and wildfires, or even the ploughing of a single field. Loss of snow cover, retreat of glaciers, and increase in drought intensity due to climate change can lead to surface conditions that increase the likelihood of creation, continuation and expansion of SDS source areas.

Climatic feedback mechanisms and ecosystem impacts related to dust in the Arctic include direct radiative forcing (absorption and scattering), indirect radiative forcing (via clouds and cryosphere), semi-direct effects of dust on meteorological parameters, effects on atmospheric chemistry, as well as impacts on terrestrial, marine, freshwater, and cryosphere ecosystems. Here we give an overview of our recent understanding on dust emissions and their long-range transport routes, deposition, and ecosystem effects in the Arctic as presented in Meinander et al. (2025), part of the series of review papers of the Arctic Council Working Group AMAP (Arctic Monitoring and Assessment Program) and CAFF (Conservation of Arctic Flora and Fauna), where the target audience is the scientific community focusing on the Arctic. Additional audiences include policy advisers and other staff in environmental-related ministries.

We conclude that the multiple mechanisms related to dust emissions, transport and deposition both cool and warm the climate system, with an uncertain net effect. Dust plays a significant role in terrestrial and aquatic ecosystems, e.g., by providing nutrients, and with impacts on the availability of light and water. Due to Arctic warming, HLD dust emissions can be expected to increase. The contributions of LLD and HLD complicates the interpretation of how much different sources contribute to the dust loadings and corresponding temporal and spatial deposition patterns. Another challenge is that low latitude dust source emissions of road and agricultural dust is barely characterized.

Reference:

Meinander O, Uppstu A, Dagsson-Waldhauserova P, Groot Zwaaftink C, Juncher Jørgensen C, Baklanov A, Kristensson A, Massling A and Sofiev M (2025). Dust in the arctic: a brief review of feedbacks and interactions between climate change, aeolian dust and ecosystems. Front. Environ. Sci. Sec. Interdisciplinary Climate Studies, Volume 13 – 2025. doi: 10.3389/fenvs.2025.1536395. CAFF-special issue.

How to cite: Meinander, O., Uppstu, A., Dagsson-Waldhauserova, P., Groot-Zwaaftink, C., Juncher Jørgensen, C., Baklanov, A., Christenson, A., Massling, A., and Sofiev, M.: Dust in the Arctic: feedbacks and interactions between climate change, aeolian dust and ecosystems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16935, https://doi.org/10.5194/egusphere-egu26-16935, 2026.

Accelerated glacier retreat in the Western Himalaya has led to rapid expansion of glacial lakes and increasing concern over Glacial Lake Outburst Flood (GLOF) hazards. This study presents a basin-scale assessment of glacial lake evolution, potential future lake formation, and GLOF susceptibility in the Chenab Basin, integrating multi-temporal remote sensing, terrain analysis, and probabilistic exposure modelling. A decadal inventory of glacial lakes was developed for five time periods (1990, 2000, 2010, 2020 and 2025) using Landsat and Sentinel-2 imagery, combined with semi-automated extraction and geomorphological classification. Results reveal a consistent increase in both lake number and total area over the last three decades. Potential future glacial lakes were identified using various ice-thickness modeling approach applied to current glacier extents. This analysis presents an inventory of the future glacial lake in the entire basin giving special emphasis to determining the characteristics of the future lake including maximum extent of the future lakes and volume of the future glacial lakes. GLOF susceptibility of existing lakes was evaluated using a multi-criteria framework to identify critical lakes requiring priority monitoring. Downstream exposure was further assessed using the Monte Carlo Least Cost path approach, explicitly accounting for uncertainty in breach location and flood routing parameters to delineate probable impact corridors. The framework provides new insights into evolving cryospheric hazards in the Chenab Basin and demonstrates the utility of combining lake dynamics, future lake potential, susceptibility assessment, and probabilistic exposure analysis for improved GLOF risk prioritization in the Western Himalayas.

How to cite: Das, D. R. and Sattar, A.: Evolution of present and potential future glacial lakes and implications for GLOF hazard in the Chenab Basin, Western Himalaya, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17889, https://doi.org/10.5194/egusphere-egu26-17889, 2026.

Machine learning (ML) remains one of the best approaches for long-term seasonal streamflow forecasting in cold regions owing to its capacity to capture nonlinearity between inputs and outputs, as well as its scalability across hydroclimatic regimes. ML’s main advantage lies in the generalizability of these models when applied to heavily glacierized catchments. In this data-driven study, we mainly utilize the Extreme Gradient Boosting (XGBoost) regression to train and test seasonal streamflow predictions using the LArge-SaMple DAta for Hydrology and Environmental Sciences for Iceland (LamaH-Ice). This new dataset for Iceland, published in 2024 consists of topographic, hydroclimatic, land cover, vegetation, soil, geological, and glaciological attributes that are essential for understanding cryosphere–hydrology processes in cold regions. For more than 100 basins, time series information on meteorological forcings and variables relevant to cold-region hydrology, such as MODIS (Moderate Resolution Imaging Spectroradiometer) snow cover, glacier albedo are also available. The majority of gauged rivers in LamaH-Ice are reported to have minimal human disturbances, making the dataset particularly unique. The XGBoost model demonstrates strong predictive skill across the study basins, as indicated by Kling-Gupta Efficiency (KGE) and Nash-Sutcliffe Efficiency (NSE) metrics exceeding 0.98. Ultimately high-precision streamflow forecasting is needed to track hydrometeorological hazards and to aid our ability to manage water resources in cold regions, which are a source for irrigation and hydropower.

References

Helgason, Hordur Bragi, and Bart Nijssen. “LamaH-Ice: LArge-SaMple DAta for Hydrology and Environmental Sciences for Iceland.” Earth System Science Data, vol. 16, no. 6, 13 June 2024, pp. 2741–2771, doi:10.5194/essd-16-2741-2024. 

Strahlendorff, Mikko, et al. “Forestry Climate Adaptation with HarvesterSeasons Service—a Gradient Boosting Model to Forecast Soil Water Index SWI from a Comprehensive Set of Predictors in Destination Earth.” Frontiers in Remote Sensing, vol. 5, 20 Dec. 2024, doi:10.3389/frsen.2024.1360572.

How to cite: Prakasam, G., Strahlendorff, M., Kröger, A., and Gunnarsson, A.: Machine Learning based Seasonal Streamflow Forecasting in Cold-Region Catchments: Insights from LamaH-Ice dataset, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20743, https://doi.org/10.5194/egusphere-egu26-20743, 2026.

Near-surface water ice in Phlegra Montes, Mars, could support human exploration and settlement. Orbital sounding radar provides strong evidence for the existence of this ice, as does morphology consistent with debris-covered glaciers. Impact excavation of these glacier-like features has exposed ice, visible in HiRISE images, but the distribution and quantity of this ice is uncertain, necessitating further evaluation for its potential to support human exploration. We are developing the Prototype Radar Sounding Experiment for Unveiling the Subsurface (PERSEUS) instrument to study debris-covered glaciers on Earth and Mars, and we propose D-PERSEUS, a mission to study a debris-covered glacier in Phlegra Montes using a drone-based Ground Penetrating Radar like this one. This mission could verify the presence of water ice in-situ and improve characterization of water ice resources, which could serve as exploratory work ahead of a potential Mars Life Explorer mission.

How to cite: Spurling, R., Nerozzi, S., and Aguilar, R.: D-PERSEUS: A Drone Radar Mission to Study a Debris-Covered Glacier on Mars, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21809, https://doi.org/10.5194/egusphere-egu26-21809, 2026.

The dataset with the most spatial coverage for data assimilation of biogeochemical models in operational systems is the satellite-derived data. Nevertheless, variables derived from Remote Sensing Reflectance (RSR), like the sea surface chlorophyll concentration, for regions like coastal areas, can reach big errors if compared with in situ measurements. For this reason, a suggestion with the aim of improving the assimilated results comes from the direct assimilation of Remote Sensing Reflectance, removing the error derived from inferring the biogeochemical variable before assimilating. In this work, we focus on a case study, using the Biogeochemical Flux Model (BFM) merged with a hydrological model, we study the effects of the direct and indirect assimilation of RSR in a region located in the Ligurian Basin of the northwestern Mediterranean Sea.  For both assimilation experiments, the algorithm used was an Error Subspace Kalman Filter. To assess the results, we compared them with climatologies computed with in situ measurements, highlighting the advantages and disadvantages of both approaches. 

How to cite: Soto Lopez, C. E., Lazzari, P., Anselmi, F., and Teruzzi, A.: Indirect assimilation of remote sensing reflectance: case study in the Liguria Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22059, https://doi.org/10.5194/egusphere-egu26-22059, 2026.

Coupled Model Intercomparison Project (CMIP) and Ocean Model Intercomparison Project phase 2 (OMIP2) models from the 6^th phase of the CMIP group were used in the current study to represent the annual mean biases of hydrographic features. The OMIP2 models are ocean-only simulations, while the CMIP models are coupled ocean-atmosphere-land-sea ice simulations. These models are assessed against the observations in the Tropical Indian Ocean (TIO). This study identified that many of the models from both CMIP and OMIP2 exhibited cold and warm temperature biases at the surface (0-100m) and subsurface (100-300m) on an annual scale, respectively. Overall, the CMIP models were observed to have larger biases than the OMIP2 models. Also, strong positive biases of salinity were identified in the south-eastern Arabian Sea (AS) and the western Bay of Bengal than in other regions of TIO. In addition, a deeper thermocline was identified in the northern AS and Seychelles-Chagos Thermocline Ridge region in CMIP and OMIP2 models compared to observations, which was predominant in the CMIP models than in the OMIP2 models. This deeper thermocline is associated with subsurface warm temperature biases. Brunt-Väisälä frequency revealed weaker stratification from surface to 100m with a peak at 80m. Further, vertical shear currents revealed strong shear bias at the top 40m, that can result in vertical mixing, which is chiefly accountable for the biases of temperatures and salinities. The heat and salt transport analysis at different straits in the TIO suggested positive northward and negative southward transport. Positive transport occurred during the post-monsoon season, while negative transport occurred during other seasons. SST-based upwelling index analysis revealed strong upwelling signals during summer months in all individual models for all regions. However, strengthened upwelling has been identified in the CMIP models than in OMIP2 models due to strong winds over the upwelling regions. A strong negative correlation has been identified between surface temperature and windspeed in CMIP models over most of the TIO, suggesting that strong surface wind speeds lead to vertical mixing, which in turn causes further surface cooling.

How to cite: Madhu, B., Vissa, N. K., and Venkata Sai Udaya Bhaskar, T.: Hydrographic features in the Tropical Indian Ocean: Insights from coupled and uncoupled models from the CMIP6 group, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-51, https://doi.org/10.5194/egusphere-egu26-51, 2026.

Western boundary current (WBC) regions play a critical role in air–sea heat exchange, influencing weather patterns and regulating climate. Despite their importance, how the coupled ocean–atmosphere seasonal variability in these regions responds to global warming remains unclear. Using observations (ERA5 and OAFlux) and Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations, we examine long-term trends in the seasonal amplitudes of sea surface temperature (SST) and latent heat flux (LHF) across major WBC systems. Over the past six decades, SST amplitude has significantly decreased, whereas LHF amplitude has increased. This contrast stems from an enhanced seasonal amplitude of air–sea specific humidity difference, driven by a stronger reduction in near-surface air temperature seasonality relative to SST. Future projections suggest that this thermodynamic mechanism will persist over the next four decades, with strong inter-model agreement confirming the robustness of the trend. Unlike previous studies that mainly focused on the climatological modulation of the SST annual cycle by ocean heat advection in WBC regions, our analysis highlights long-term changes in the coupled SST–LHF seasonal coevolution under global warming. These findings reveal that warming climate to some extent alters the seasonal air–sea coupling in WBC regions, with potential consequences for regional climate variability, extreme weather events, and the global surface energy budget.

How to cite: Tong, J., Song, X., Oltmanns, M., and Xie, S.-P.: Enhanced seasonal cycle of air‒sea latent heat flux in western boundary current regions due to global warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-157, https://doi.org/10.5194/egusphere-egu26-157, 2026.

Diagnostics from a 1/12 degree resolution ocean model simulation have confirmed that depth-integrated upper-ocean boundary pressure anomalies can be predicted from a simple theory involving wind stress and net meridional flows through the southern boundaries of the Atlantic and Indo-Pacific basins. In particular, the difference between eastern Atlantic and eastern Pacific boundary pressures is mainly determined by wind stress in this model, with the Indo-Pacific overturning playing a significant secondary role. We apply this framework to the analysis of CMIP-6 simulations and find that, for centennial changes, the dominant factor becomes the changing Indo-Pacific overturning (itself related to AMOC changes), and that the resulting boundary pressure changes predict an important proportion of the change in Pacific-Atlantic sea level difference. We also find an amplification mechanism, whereby small changes in deep ocean pressures result in larger sea level changes than would be expected from a simple hydrostatic balance argument.

How to cite: Hughes, C., Gururaj, S., Bingham, R., Blaker, A., Styles, A., Boland, E., and Jones, D.: Ocean boundary pressures link Atlantic-Pacific sea level difference to Indo-Pacific Overturning., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2571, https://doi.org/10.5194/egusphere-egu26-2571, 2026.

We conducted high-resolution (1/8°) dynamical downscaling over the North Pacific to produce ocean climate simulations for 1982–2100. The Regional Ocean Modeling System (ROMS) was forced by seven top-ranked CMIP6 global climate models (GCMs) under four SSP scenarios (SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5), which provided the required forcing fields. Evaluation of sea surface temperature (SST) over the recent 20 years (1995–2014) shows that the ROMS ensemble mean (EM) substantially reduces the warm bias present in the GCM EM, improving the RMSE by 10.1%, with particularly strong improvement in the subpolar region (17.5%). These SST improvements primarily result from a more realistic representation of the Kuroshio, which alleviates the unrealistic overshooting in coarse-resolution GCM simulations, and are accompanied by improved wintertime net surface heat flux (NHF) near the Kuroshio path. Future projections (2081-2100) reveal pronounced differences between the GCM EM and ROMS EM in the subpolar region. Although both EMs project a strengthened and northward-shifted Kuroshio under higher-emission scenarios, the GCM EM exhibits an excessively large poleward shift. As a result, the GCM EM projects exaggerated, scenario-dependent wintertime changes in SST and NHF, which are substantially mitigated in the ROMS EM. These results highlight the importance of high-resolution regional ocean modeling for reducing biases in western boundary current systems and improving the reliability of future ocean climate projections.

How to cite: Lim, K.-G., Oh, S.-G., Lee, S.-T., Jung, J., Kim, B.-G., and Cho, Y.-K.: High-resolution North Pacific climate changes using dynamical downscaling: impact of improved Kuroshio, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3215, https://doi.org/10.5194/egusphere-egu26-3215, 2026.

The ocean is a vast store of anthropogenic and natural carbon, the former from manmade CO₂ emissions absorbed at the surface and the latter produced in the interior from biological regeneration. However, for how long this carbon will remain sequestered in the ocean under a warming climate remains poorly constrained. Here, I quantify the impact of climate change on the sequestration efficiency of the ocean by computing the distribution of times and spatial locations at which carbon currently stored in the ocean is exposed to and exchanges with the atmosphere. These novel calculations fully take into account the time-evolving circulation and buffer chemistry of the ocean under a range of emission scenarios. I show that a projected increase in stratification and concomitant slowdown in the global overturning circulation due to global warming lengthens by centuries to thousands of years the time for which carbon remains sequestered. Moreover, this increase in storage time is evident even under low emission, high mitigation scenarios, and is accompanied by a shift in circulation pathways that further enhances the dominance of the Southern Ocean as the location at which the accumulated carbon remerges at the surface. These results highlight the potential long-term impact of global warming-induced changes in the marine carbon cycle on climate.

How to cite: Khatiwala, S.: Climate change increases the sequestration efficiency of the ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4878, https://doi.org/10.5194/egusphere-egu26-4878, 2026.

Relative roles of wind stress and buoyancy forcing in shaping Pacific circulation and sea level remain unclear. Using large-ensemble simulations from Community Earth System Model version 2, we disentangle the contributions of wind and buoyancy fluxes during 1960–2014. Wind stress accounts for 81% of circulation changes and explains 54% of regional sea-level trend, while buoyancy forcing contributes 19% of circulation changes but 46% of regional sea-level trend. Circulation changes diagnosed from Barotropic Stream Function match estimations from Sverdrup Stream Function, underscoring the reliability of wind-driven frameworks. Wind stress drives ocean heat redistribution through meridional transport and subduction, inducing sea-level rise along poleward flanks of subtropical gyres. Buoyancy forcing partly compensates for wind-driven changes in the North Pacific and exerts a weaker, synergistic influence in the South Pacific. These findings highlight the dominant yet regionally modulated role of wind stress in shaping Pacific circulation and sea level under climate changes.

How to cite: huang, R.: Disentangling wind- and buoyancy-driven changes in Pacific circulation and regional sea level during 1960–2014, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4915, https://doi.org/10.5194/egusphere-egu26-4915, 2026.

A robust basin-wide “fresh-gets-fresher” response in tropical sea surface salinity (SSS) emerges in CMIP projections, with freshening in the Indo-Pacific and salinification in the Atlantic Ocean. Yet large uncertainties persist due to long-standing mean-state biases and inter-model spread in CMIP simulations, limiting confidence in future SSS projections and their underlying mechanisms. By correcting ocean mean-state biases, we show that CMIP models with a strong equatorial cold tongue bias substantially underestimate future western Pacific freshening. Using a bias-corrected ocean model forced by air–sea flux anomalies from multiple CMIP6 models, we disentangle the respective roles of surface freshwater forcing and ocean dynamics. Freshwater flux changes advected by the climatological circulation dominate the basin-scale Pacific–Atlantic salinity contrast, while changes in wind-driven circulation strongly modulate regional SSS anomalies, particularly in the equatorial Indo-Pacific. The balance between these processes varies markedly across CMIP6 forcing sets. Our results demonstrate that improving the representation of the tropical mean state, equatorial winds, and the Walker circulation—together with their projected changes—is essential for reducing uncertainty in CMIP-based projections of future ocean salinity. More broadly, this work highlights how targeted bias correction and process-based analysis can help bridge CMIP limitations and advance robust projections of the future ocean.

How to cite: Pang, S., Lengaigne, M., and Vialard, J.: The role of freshwater flux and wind-driven circulation in shaping future tropical salinity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5648, https://doi.org/10.5194/egusphere-egu26-5648, 2026.

A key question for the future ocean is what will happen to the Atlantic Meridional Overturning Circulation (AMOC). In climate models that are run beyond 2100 for a high emission scenario it shuts down in the majority, where the shutdown is preceded by a ceasing of convection in the North Atlantic subpolar gyre (SPG). As some climate models already show a collapse of SPG convection around 2040, it is key to know what this means for the AMOC. What is the interaction between SPG and AMOC? If convection in the SPG stops, is the AMOC bound to shut down as well? Or will other regions or processes take over?

For deep water formation both deep convection in the gyre centre, as well as densification in the boundary current play a role. Climate models do not resolve the SPG boundary current and eddies due to their coarse resolution, meaning key processes for deep water formation are parametrised. I will discuss the relative role of densification in the boundary current and deep convection in the SPG gyre centre for the AMOC in both CMIP6 models and ocean reanalyses. Using causal inference the importance of the two processes for the AMOC is investigated, distinguishing the respective roles of the Labrador and Irminger seas.

Differences between CMIP6 models and reanalyses are discussed, and put in the context of the recent OSNAP results on the relative importance of the eastern and western SPG for the AMOC. This sheds light on the representation of the process of deep water formation that are relevant for the AMOC in climate models, and aids in understanding the impact a collapse of SPG convection would have on the AMOC.

How to cite: Falkena, S. and von der Heydt, A.: The role of the Subpolar Gyre in the future of the AMOC, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8218, https://doi.org/10.5194/egusphere-egu26-8218, 2026.

The AMOC is crucial and sensitive part of the global climate system. It transports huge amounts of heat north across the equator into the northern Atlantic. It is the main reason why the Northern Hemisphere is 1 – 2 °C warmer than the Southern Hemisphere (Feulner et al. 2013) and makes Europe’s climate unusually mild for its latitude.

Due to AMOC instabilities, the northern Atlantic region has been the major hotspot of drastic climate changes in Earth’s history, as seen in data from Greenland ice cores and many other sources of paleoclimatic proxy data (Rahmstorf 2002).

The AMOC is expected to weaken strongly in response to human-caused global warming (IPCC 2021). Since Stommel (1961) and Broecker (1987) the risk of the AMOC being destabilised at a tipping point has been much discussed, especially since the emergence of a remarkable cooling patch in the subpolar gyre to the west of the British Isles (Drijfhout et al. 2012, Rahmstorf et al. 2015).

Until recently this has been considered a ‘low probability high impact risk’, to be taken seriously mainly because of the devastating impacts it would have. New research over the past years has changed this viewpoint. Neither a full AMOC shutdown nor a subpolar gyre convection collapse (also with major impacts on society) can be considered ‘low probability’ any more (e.g. Swingedouw et al. 2021, Drijfhout et al. 2025).

This talk will discuss recent scientific developments regarding the risk of AMOC instability.

References

Broecker, W. (1987). Unpleasant surprises in the greenhouse? Nature 328: 123.

Drijfhout, S., J. R. Angevaare, J. Mecking, R. M. van Westen and S. Rahmstorf (2025). Shutdown of northern Atlantic overturning after 2100 following deep mixing collapse in CMIP6 projections. Environmental Research Letters 20(9) doi: 10.1088/1748-9326/adfa3b.

Drijfhout, S., G. J. van Oldenborgh and A. Cimatoribus (2012). Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns? Journal of Climate 25(24): 8373-8379 doi: 10.1175/jcli-d-12-00490.1.

Feulner, G., S. Rahmstorf, A. Levermann and S. Volkwardt (2013). On the origin of the surface air temperature difference between the hemispheres in Earth's present-day climate. Journal of Climate 26(18): 7136-7150 doi: doi:10.1175/JCLI-D-12-00636.1

IPCC (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva, IPCC. 2391 pages.

Rahmstorf, S. (2002). Ocean circulation and climate during the past 120,000 years. Nature 419(6903): 207-214 doi: 10.1038/nature01090.

Rahmstorf, S., J. E. Box, G. Feulner, M. E. Mann, A. Robinson, S. Rutherford and E. J. Schaffernicht (2015). Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change 5(5): 475-480 doi: 10.1038/nclimate2554.

Stommel, H. (1961). Thermohaline convection with two stable regimes of flow. Tellus 13: 224-230.

Swingedouw, D., A. Bily, C. Esquerdo, L. F. Borchert, G. Sgubin, J. Mignot and M. Menary (2021). On the risk of abrupt changes in the North Atlantic subpolar gyre in CMIP6 models. Ann N Y Acad Sci 1504(1): 187-201 doi: 10.1111/nyas.14659.

How to cite: Rahmstorf, S.: AMOC tipping risk reconsidered, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10486, https://doi.org/10.5194/egusphere-egu26-10486, 2026.

Reliable projections of future wave climate are vital for coastal adaptation, infrastructure planning, and marine renewable energy development. This study introduces a high-resolution spectral wave dataset for the European Atlantic coast, produced through regional downscaling of CMIP6-based climate projections. The dataset was developed using the WRF atmospheric model in combination with two spectral wave models, WAVEWATCH III (WW3) and Simulating WAves Nearshore (SWAN), to generate 3-hourly directional wave spectra at 1,031 offshore locations, spaced at 10 km intervals and situated approximately 50 km from the coastline. The simulations encompass three 30-year periods: a historical baseline (1985–2014) and two future time slices (2030–2059) under the SSP2-4.5 and SSP5-8.5 scenarios. Two datasets are provided: spectral energy densities and integrated wave parameters, both validated and formatted in CF-1.8-compliant NetCDF-4 files. The spectral dataset enables the initialization of new SWAN simulations, facilitating efficient site-specific wave modeling, while the integrated parameters support regional-scale analyses of climate change impacts on wave conditions. The dataset is publicly accessible via the Centre for Environmental Data Analysis (CEDA) repository and constitutes a valuable resource for research, engineering applications, and policy-making in coastal and marine environments.

How to cite: Arguilé-Pérez, B., Costoya, X., Ribeiro, A. S., deCastro, M., Carracedo, P., and Gómez-Gesteira, M.: High-Resolution Wave Climate Projections along the European Atlantic Coast Based on Downscaled CMIP6 Wind Forcing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11072, https://doi.org/10.5194/egusphere-egu26-11072, 2026.

Dynamic sea-level change (ΔDSL) is a key process in shaping the pattern of future sea-level rise. CMIP6 models predict a range of ΔDSL under 1% increase of CO2 per year. We analyse this CMIP6 spread into contributions from: 1) surface flux change (dF) and 2) model sensitivity to it (Φ). Specifically, we perturb the pre-industrial simulation of an ocean model with space- and time-varying dF diagnosed from different CMIP6 models (one at a time). The CMIP6 spread is thus decomposed into a flux-driven spread and a residual; the latter is linked to model spread of Φ. We improve upon previous studies by: (a) deriving the perturbed forcing ensemble using an ocean-only setup and (b) comparing it with the CMIP6 ensemble for both variance and correlation. This reveals distinct roles of surface forcing in driving the CMIP6 spread in different regions. In the North Pacific, differences in windstress forcing primarily explain the CMIP6 spread, while in the North Atlantic, differences in model sensitivity are more important. For the latter region, although buoyancy forcing drives a ΔDSL spread there, it correlates poorly with the CMIP6 spread. In the Southern Ocean, differences in forcing and sensitivity are both important for explaining the CMIP6 spread. The surface forcing affects the spread along 40°S via windstress and the spread around the Antarctic via buoyancy flux. In addition to ΔDSL analysed here, the perturbed forcing ensemble can be used to analyse future changes in other ocean variables, such as temperature, salinity and the Atlantic meridional overturning circulation. The full ensemble data is openly available online and can be freely used for future studies.

How to cite: Wu, Q. and Gregory, J.: Surface flux contributions to CMIP6 spread of dynamic sea-level change vary across regions: insights from an ocean-only perturbed forcing ensemble, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11315, https://doi.org/10.5194/egusphere-egu26-11315, 2026.

Based on simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), projections of marine heatwave (MHW) annual accumulated days (AAC_day) and intensity (AAC_intensity) remain highly uncertain, even in regions where anthropogenic signals are expected to emerge. Total projection uncertainty is decomposed into contributions from intermodel differences, internal variability, and emission scenarios. In the near term, internal variability dominates uncertainty in climate-mode-influenced oceans, while intermodel uncertainty prevails elsewhere. From the mid- to long-term, uncertainties associated with both internal variability and intermodel differences decrease nearly globally. Scenario uncertainty remains negligible until becoming evident over tropical oceans in the long term. Anthropogenic signals in AAC_day (AAC_intensity) emerge over only 2.2% (1.9%), 16.5% (1.8%), and 43.1% (2.0%) of the global ocean in the near-, mid-, and long-term, respectively, but expand to 32.4% (11.2%), 63.5% (18.4%), and 79.9% (20.7%) when intermodel differences are removed. These results demonstrate that internal variability and model uncertainty substantially delay the detectability of MHW changes, highlighting the importance of reducing model spread to improve future projections of MHW risks.

How to cite: Li, Y. and Jin, C.: Uncertainty Dominance Delays the Emergence of Marine Heatwave Signals in CMIP6 Projections, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11421, https://doi.org/10.5194/egusphere-egu26-11421, 2026.

The Antarctic slope current (ASC) is a westward flow around the Antarctic continental shelf. The ASC plays a key role in regulating the heat transport onto the shelf and thereby affects the ice-shelf melt. However, estimating changes in the ASC in response to greenhouse warming and attributing their potential drivers remain uncertain due to a lack of observations and the limitations of high-resolution coupled climate modeling. Previous equilibrium simulations using the ultra-high-resolution Community Earth System Model, comparing present-day (PD) and quadruple CO₂ (4×CO₂) simulations, showed the strengthening of the ASC in 4×CO₂ relative to the PD simulation mainly due to a decrease in salinity. This enhanced freshening was primarily driven by reduced brine rejection associated with sea-ice formation in mainly austral winter-spring and by enhanced precipitation minus evaporation in year-round. To examine transient changes in the ASC and associated freshwater forcings that could not be captured in the equilibrium experiments, we also used Alfred Wegener Institute Climate Model, version 3 (AWI-CM3) coupled climate model with SSP5-8.5 greenhouse gas emission scenario. We analyzed transient experiment with 31 km and 10 km horizontal resolution for atmosphere and ocean, respectively (TCo319). Since the 2020s, the ASC has rapidly strengthened and expanded meridionally. At the same time, salinity and sea ice concentration began to decrease abruptly, and the freshened region also expanded similarly to the ASC. Compared to the historical period (1981–2010), the future period (2071–2100) showed a strengthened ASC, with increased mean temperatures from the surface to 200 m depth confined to the continental slope. Precipitation also increased along the Antarctic coast region and over the continental slope. By using both equilibrium and transient simulations, we better understand future changes in ASC and the mechanisms linking freshwater factors to the ASC change. Our study has important implications for mesoscale ocean circulation, ocean heat exchanges, and marine ecosystems around Antarctica.

How to cite: Kim, M.-H. and Lee, J.-Y.: Effects of freshwater forcing on the Antarctic slope current in a warmer climate using coupled climate model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11779, https://doi.org/10.5194/egusphere-egu26-11779, 2026.

The Ross Sea is a critical source region for Antarctic Bottom Water (AABW), driven by the export of Dense Shelf Water (DSW) through major submarine troughs. Under a warming climate, reduced sea-ice production and enhanced surface freshening are projected to weaken buoyancy loss on Antarctic shelves. However, how the Ross Sea shelf–slope circulation reorganizes under such significantly altered surface forcing remains poorly understood. Using high-resolution (0.1°) CESM simulations, we examine the response of this circulation to progressive warming and freshening under present-day, doubled (2xCO₂), and quadrupled (4xCO₂) CO₂ conditions. Our results show that as DSW formation declines, shelf waters become increasingly buoyant, with the most pronounced changes occurring on the western shelf. This asymmetric freshening reshapes the cross-shelf density structure and eventually reverses the horizontal density gradient. In the Joides Trough, the traditional two-layer overturning pattern disappears under 4xCO₂ forcing; instead, warm Circumpolar Deep Water (CDW) enters along the bottom, establishing a new deep pathway from the slope onto the shelf. Oceanic instability metrics indicate that strengthened lateral density gradients become comparable to, and locally exceed, the stabilizing effect of vertical stratification along the continental slope. We suggest that conditions favorable to symmetric instability may facilitate vertical exchange and support the emergence of this deep inflow, even as the Antarctic Slope Current intensifies. Rather than providing a single deterministic outcome, these findings illustrate a physically consistent scenario for the regime shift in Antarctic shelf-slope exchange, with profound implications for future abyssal ventilation and global ocean heat uptake.

How to cite: Ju, J., Nam, S., Park, T., and Park, J.: A regime shift in Ross Sea shelf-slope circulation and abyssal ventilation under future extreme CO2 forcing conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16402, https://doi.org/10.5194/egusphere-egu26-16402, 2026.

How to cite: Zhang, Y., Chen, C., Hu, S., Wang, G., McMonigal, K., and Larson, S.: Summer Westerly Wind Intensification Weakens Southern Ocean Seasonal Cycle Under Global Warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17945, https://doi.org/10.5194/egusphere-egu26-17945, 2026.

Climate change and climate variability have significant effects on atmospheric and oceanic processes, with semi-enclosed basins such as the Black Sea being particularly vulnerable due to their unique physical and chemical structure. In recent decades, the basin has experienced pronounced changes in temperature, salinity, and circulation, with important consequences for its biogeochemical and ecological functioning. Understanding the mechanisms driving these changes and their future evolution is therefore essential. This study investigates the historical and projected variability of key physical processes in the Black Sea over the period 1950-2100 using a high-resolution regional ocean model (NEMO). Temperature, salinity, mixed layer depth, and Cold Intermediate Layer (CIL) dynamics are analyzed, using atmospheric forcings from reanalysis data (ERA5) and a regional climate model (MAR) forced by a global climate model (EC-Earth). Future projections are conducted under two IPCC Shared Socioeconomic Pathways (SSP1-2.6 and SSP5-8.5). The historical simulations (1950-2020) are validated against in situ CTD observations and satellite-derived sea surface temperature and sea surface height, demonstrating good skill in reproducing the observed thermal and haline structure of the basin. Results from the historical simulations show a progressive weakening of the CIL and a shift toward stronger upper sea stratification. Future simulations aim to quantify how different climate change pathways will modify temperature and salinity dynamics. Together, the results provide new insight into the atmospheric drivers controlling Black Sea hydrodynamics and offer projections of regional climate change impacts on this highly sensitive system.

How to cite: Belen, B., Dişa, D., Acar, A. O., Arkın, S., Yücel, M., Fach, B., and Salihoğlu, B.: Variability of Black Sea Physical Processes from 1950 to 2100, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21419, https://doi.org/10.5194/egusphere-egu26-21419, 2026.

Extreme swell events along the Peruvian coast pose recurrent risks to coastal communities, infrastructure, and maritime activities. These events originate far offshore, with their sources varying seasonally: during the austral winter they primarily develop in the South Pacific, while in summer they are typically generated in the western North Pacific. This study investigates the atmospheric circulation patterns associated with extreme wave events along the Peruvian coast generated in both hemispheres, with particular emphasis on the characteristics of the upper-level jet. Furthermore, the potential influence of climate change on the intensity of these events is assessed using an analogue-based methodology.

Events classified by the Peruvian Directorate of Hydrography and Navigation as very strong were selected for those originating in the Southern Hemisphere (SH), whereas strong events were selected for those originating in the Northern Hemisphere (NH). This difference is because events originating further away experience greater dissipation and therefore tend to be weaker. Using ERA5 reanalysis data, a composite analysis of atmospheric circulation revealed characteristic patterns in each hemisphere. SH events were associated with a dipolar cyclonic–anticyclonic pattern, producing strong pressure gradients, intense southwesterly surface winds, and an almost barotropic vertical structure. In contrast, events originating in the western North Pacific were linked to a deep cyclonic system, also exhibiting a barotropic structure. Complementing these results, analysis of the upper-level jet across multiple parameters indicates a more intense and latitudinally confined jet, generally exhibiting a positive tilt in both hemispheres. However, a key hemispheric difference emerges: in the SH, these features correspond to the polar front jet, whereas in the NH they reflect a strengthening of the subtropical jet.

Finally, to assess the anthropogenic influence on 10-m wind intensity between past and present periods, a flow-analogue approach was applied. In the SH, atmospheric circulation similar to those observed during the events is associated with stronger winds in the recent period. This intensification appears to be partly driven by the positive trend in the Southern Annular Mode, linked to anthropogenic ozone depletion and greenhouse gas forcing. In contrast, for events originating in the NH, the anthropogenic signal is less evident due to the pronounced interannual and interdecadal variability of the North Pacific, resulting in analogue-based reconstructions that show wind intensification in some events and weakening in others. Overall, these results highlight the distinct atmospheric dynamics governing swell generation in each hemisphere and provide insights that may inform early-warning systems, coastal risk assessments, and long-term adaptation strategies for Peru.

Acknowledgments: This work was supported by the SAFETE project, which has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 847635 (UNA4CAREER).

How to cite: Agurto Barragan, G., Collazo, S., and García-Herrera, R.: Atmospheric and Climate Drivers of Extreme Swells Along the Peruvian Coast, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1676, https://doi.org/10.5194/egusphere-egu26-1676, 2026.

Describing intricate concurrent wave processes frequently proves challenging and unwieldy. Although the influence of reflection rates on the development of extreme nonlinear waves remains poorly understood, controversy has emerged over whether elevated reflection rates amplify nonlinearity in the upper tail of the wave height distribution. Aided by fully nonlinear simulations, we present a theoretical framework that isolates the effects of shoaling length, bottom slope magnitude, and reflection rates. Comparing the simulation results with the theory for steep and reflective slopes, it is noticed that the theoretical excess kurtosis stabilizes for steep slopes with a high reflection rate, and that the simulated kurtosis remains in the confidence interval of our new theory. We therefore conclude that the high reflection rate is the main reason for anomalous wave statistics becoming stable.

How to cite: Mendes, S., Zhang, J., and Benoit, M.: Effect of wave reflection on submerged plane slopes on the evolution of extreme wave fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2792, https://doi.org/10.5194/egusphere-egu26-2792, 2026.

This study investigates wave-driven Langmuir turbulence (LT) in an idealized, depth-varying coastal channel representative of an estuarine bay or tidal river. In the open ocean, LT is a key turbulent process in the surface boundary layer, controlling the transport and mixing of momentum and density. LT arises from wave-current interactions that generate wind-aligned vortices, often visible as surface windrows of aggregated buoyant material such as plankton, bubbles, oil, and microplastics. To examine how LT influences the wind-, tide-, and density-driven circulation in a coastal channel, we develop a turbulence-resolving large eddy simulation (LES) framework with terrain-following coordinates representing a deeper central channel flanked by shallower margins. LT is generated through the Craik-Leibovich (CL) vortex force, which incorporates Stokes drift from wind-driven surface gravity waves. The simulations show that LT substantially enhances turbulent mixing, reducing vertical stratification and shear. Faster tidal currents in the deeper channel differentially advect salt, producing tidally varying lateral salinity gradients. These gradients generate baroclinically driven lateral and vertical tidal currents, whose development is both accelerated and intensified by LT. Conversely, vertical stratification and vertical shear of lateral currents can inhibit LT. Additionally, lateral shear of along‑channel currents associated with the channel bathymetry produces channel‑wide pairs of vertical vorticity that are tilted by Stokes‑drift shear, forming strong and persistent lateral circulations. Overall, the results reveal complex two‑way interactions between LT and the mean circulation, demonstrating that LT significantly modifies both tidally resolved and tidally averaged channel dynamics.

How to cite: Kukulka, T., Thoman, T., and Sullivan, P.: Langmuir turbulence in a depth-varying coastal channel: Insights from large eddy simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3198, https://doi.org/10.5194/egusphere-egu26-3198, 2026.

Accurate prediction of surface waves under tropical cyclones requires realistic representation of storm-induced ocean currents, which can strongly modulate wave growth and propagation. This study synthesizes results from a coupled modeling investigation and an observational analysis using drifting buoys deployed in four Gulf of Mexico hurricanes: Ian (2022), Idalia (2023), Helene (2024), and Milton (2024). The modeling system consists of the WAVEWATCH III wave model coupled to the Modular Ocean Model 6. The ocean model uses a mixing scheme that explicitly includes wave-induced Langmuir turbulence enhancement, resulting in reduced surface Eulerian currents that are more consistent with observations. The surface current introduced in the wave model combines the Eulerian current and the enhancement of the dominant wave group velocity arising from nonlinear interactions with coexisting waves. Idealized experiments show that omitting surface currents leads to systematic overestimation of maximum significant wave height by up to ~9%, with similar sensitivity to the specification of the upper-ocean mixing scheme. In real storms, drifter-based validation confirms that neglecting storm-induced currents results in consistent overestimation of significant wave height and peak period, particularly in regions of strong currents. These current-induced reductions in wave energy occur primarily because dominant wave packets propagate more rapidly and spend less time under intense winds. The effect is strongest in deep water but remains substantial in intermediate depths (20–70 m), where most observations were collected. Together, these results provide compelling evidence that storm-driven currents frequently reduce wave heights and periods under tropical cyclones. Incorporating realistic surface‐current effects into operational models is therefore essential for improving wave forecasts in tropical cyclones and enhancing coastal hazard assessments.

How to cite: Ginis, I., Papandreou, A., and Hara, T.: Wave Reduction by Storm-Driven Ocean Currents in Tropical Cyclones: Coupled Modeling and Drifting Buoy Observations , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4294, https://doi.org/10.5194/egusphere-egu26-4294, 2026.

A wave-ice interaction coupled model that resolves both ice-induced wave attenuation and wave-induced ice breakage was implemented within the Finite-Volume Community Ocean Model (FVCOM) framework and applied to the Bohai Sea, one of the lowest-latitude seasonally ice-covered seas in the Northern Hemisphere. Multi-source observations were used to validate the simulated wave and sea ice variables. We investigate wave–ice interactions under different ice conditions (mild, normal and severe ice years) and assess coupling effects by comparing a fully coupled (two-way) configuration with an uncoupled configuration and a one-way coupled configuration that accounts only for ice-induced wave attenuation. The presence of sea ice reduces wave energy and alters wave propagation. In turn, wave-driven processes exert complex influences on sea ice, potentially mediated by wave–current interactions, and wave activity can enhance melting along the ice fringe, highlighting the importance of explicitly representing two-way wave–ice interactions for accurately simulating ice-cover dynamics in the Bohai Sea.

How to cite: Qiu, S., Lettmann, K. A., Fujisaki-Manome, A., Wang, J., and Chen, X.: Fully Coupled Interactions between Sea Ice and Waves in the Bohai Sea under Different Ice Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4487, https://doi.org/10.5194/egusphere-egu26-4487, 2026.

The drift associated with the motion of inviscid, irrotational water waves was first derived by Stokes in the mid 19^th century, and is today called Stokes drift. In deep water this takes the form u_s=a²kωe^2kz₀, where a is the wave amplitude, k the wavenumber, ω the radian frequency and z₀ the initial particle depth. This formally second-order quantity is derived from linear theory, and is implemented in a wide variety of wave models to calculate the motion of marine contaminants and other passive tracers.

Adhering to linear wave theory, superposition allows for the immediate generalisation of the Stokes drift from a single wave to a wave spectrum. However, once more than one Fourier mode is included in the lowest order solution, nonlinear effects occurring at second and third order - chief among them the appearance of bound modes - should be considered when calculating Stokes drift.

We introduce a new, analytical correction to the Stokes drift

u_s= ∑_j a_j²ω_jk_je^2k_jz₀+∑_ki>kjω_ia_i²a_j²(k_i-k_j)²(ω_i-ω_j)^-1e^{2(k_i-k_j)z₀}

under assumptions of unidirectional waves and deep water for analytical simplicity - and test this using direct numerical integration of particle paths [1]. Velocity fields for numerical work up to third order are obtained from the reduced Hamiltonian formulation of the water-wave problem due to Zakharov [2], and allow for the inclusion or exclusion of bound harmonics, amplitude evolution and dispersion correction to distinguish among competing effects. In particular, on the typical scale of particle motion the amplitude evolution can be neglected, allowing us to use an algebraic expression for the velocity field in terms of the (initial) Fourier amplitude spectrum [1]. Such an approach has also been successfully employed for deterministic forecasts of the ocean surface [3].

To summarise: we show how higher order contributions to the Stokes drift have an effect throughout the water column. At the surface this is connected to the critical role of high frequencies in the Stokes drift, where dispersion corrections are most influential, as well as contributions from sum-harmonic terms. At greater depths difference harmonics can come to dominate the flow-field and therefore the Stokes drift, as previously demonstrated for wave groups. All of this points to a need to reconsider the common formulation stemming from linear wave theory.

References:

[1] R. Stuhlmeier, Wave-induced drift in third-order deep-water theory, arXiv:2507.15688 (2025).

[2] R. Stuhlmeier, An introduction to the Zakharov equation for modelling deep water waves, D. Henry (ed.) Nonlinear Dispersive Waves (Springer Lecture Notes in Mathematical Fluid Mechanics), Springer (2024), pp. 99-131.

[3] M. Galvagno, D. Eeltink, and R. Stuhlmeier, Spatial deterministic wave forecasting for nonlinear sea-states, Physics of Fluids, (2021) 33 102116

How to cite: Stuhlmeier, R.: On the higher-order wave-induced drift in deep water, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4780, https://doi.org/10.5194/egusphere-egu26-4780, 2026.

The interaction between wind and water waves is a complex process at the interface of two turbulent fluids. In this context, lakes provide conditions of intermediate complexity between the open ocean and wave tank experiments, allowing the investigation of fetch-limited wave responses under both directional and turbulent wind regimes, and in the absence of swells.

We developed an experimental setup installed on the LéXPLORE research platform on Lake Geneva (Switzerland) [1] to record the spatial and temporal variations of the water surface elevation using a pair of stereo cameras, as well as in situ wind profiles obtained with ultrasonic anemometers. To reconstruct the surface elevation and generate local directional spectra, we employ the optimised WASS algorithm [2], which has already proven effective during oceanic expeditions. The motion of the platform is tracked using an inertial measurement unit, which also helps refine wind-speed estimates. Moreover, the wave data are compared with in situ measurements acquired by buoys.

The LéXPLORE platform is ideally located for our study, as it simplifies the physical analysis and interpretation of the measurements. It lies far enough from the coast to ignore boundary effects, in deep water where bathymetry influences on wave propagation can be neglected, and at long fetch (for south-westerly winds) where wind forcing is maximised.

We will present the experimental setup and preliminary results on the reconstruction of directional spectra under different wind regimes during an experimental campaign in Spring 2026. 

[1] Wüest et al., WIREs Water 8, e1544 (2021)

[2] Bergamasco et al., Computers and Geosciences 107, 28 (2017)

How to cite: Kunz, B., Brunetti, M., Babanin, A., and Kasparian, J.: Experimental setup and first measurements of wind-wave interaction from the LéXPLORE platform on Lake Geneva, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6877, https://doi.org/10.5194/egusphere-egu26-6877, 2026.

Global information about ocean wind waves is crucial for understanding their role in the climate system, validating model outputs, and assessing risks for shipping and marine structures. Recent advances in marine radar technologies have enabled accurate, high-resolution measurements of surface wind waves and their spectral characteristics. Making these measurements available in real-time opens a wide new range of products for many user communities. Here we introduce SeaVision, a ship-based monitoring system that, once integrated into a standard shipborne X-band radar, considerably improves real-time observational networks along major shipping routes. SeaVision automatically measures significant wave height, peak period and directional wave spectra at temporal resolutions down to seconds. First developed for research purposes in 2020, SeaVision passed an extensive period of validation using Spotter wave buoys and satellite data. Validation onboard research vessels was conducted for a wide range of latitudes, from the Arctic to Antarctica. SeaVision is fully operational, cost‑effective, and capable of transmitting wave parameters continuously via satellite. Further developments of SeaVision allow for retrieving near surface wind speed, surface currents and ice parameters with the same resolution. Extensive installations of SeaVision (as well as similar systems) onboard commercial and research vessels allow for establishing a near-global observational network (as a part of GCOS and GOOS) largely exceeding capabilities of the present VOS network which over the last few decades are experiencing a dramatic decline and is also regionally complementing satellite missions. SeaVision will enhance coverage of the so far inadequately sampled global oceans.

How to cite: Gulev, S., Ezhova, E., Natalia, T., Gavrikov, A., Sharmar, V., Trofimov, B., Bargman, S., Koltermann, P., Grigorieva, V., and Suslov, A.: Real-time open ocean wind waves from navigation radars for a truly global wind wave operational observing system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7925, https://doi.org/10.5194/egusphere-egu26-7925, 2026.

The nearshore region provides the link between the land and the ocean. Waves play a crucial part in many nearshore processes including sediment transport, coastal erosion, dispersion of pollutants, rip currents, and many more. It is also the region where most people interact with the ocean. Nevertheless, the nearshore is not well presented in operational wave forecasts.

Here we test the merit of resolving the nearshore region in a regional WAVEWATCH III ® setup. We test this for two contrasting wave climates: the swell-dominated west coast of British Columbia with tidal ranges up to 4m, and the fetch-limited, non-tidal western Baltic Sea with storm surges reaching +-1.5m. Both models are on unstructured grids, and we test the feasibility of zoomed-in regions of very high grid resolution. The effect of currents and water level are evaluated as additional forcing fields.

The models are validated against in-situ wave buoy observations including an array that tracks the wave evolution along two 2km shoaling paths. Gradual wave height reductions of >25% per km are observed, but little change in the spectral shape or directional characteristics.

 These observations are challenging to replicate in the model. We find that the inclusion of currents and water level yield the strongest improvement on significant wave heights and directional spreading, whereas increased grid resolution is beneficial for resolving small-scale bathymetric features.

How to cite: Gemmrich, J., Brooks, B., and Holtermann, P.: Modelling and monitoring waves in the nearshore region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8446, https://doi.org/10.5194/egusphere-egu26-8446, 2026.

Wind-driven gravity–capillary waves play a key role in air–sea interactions and in small-scale energy dissipation across the surface microlayer (SML). Despite decades of studies, the transition from smooth gravity–capillary waves to intermittent microscale breaking under weak wind forcing is still not well understood. 
This study investigates gravity–capillary wave dynamics and micro-breaking in a 24 m long, 1 m wide wind–wave tank with a total height of 1.5 m and a mean water depth of 0.51 m. Measurements were performed using a newly developed Colour Imaging Slope Gauge (CISG), providing high-resolution spatio-temporal observations of surface slopes within a 33.2 cm × 26.8 cm field of view (FOV), at a spatial resolution of 0.024² cm² per pixel and a frame rate of 400 Hz. A total of 18 experiments were conducted over a range of low wind speeds (1.8 m s⁻¹– 4.0 m s⁻¹) with small increments. Wire-wave gauge measurements were used to support three-dimensional surface reconstructions. 
Spectral, wavelet, and band-pass filtering techniques were applied to isolate capillary-scale features associated with micro-breaking. Particular attention was given to surface curvature as a geometric indicator of micro-breaking. The wide FOV enables direct tracking of isolated events and reveals a clear increase in capillary activity and micro-breaking occurrence with increasing wind forcing. 
First results indicate a distinct transitional regime at wind speeds near 2.0 m s⁻¹, where the first clear capillary signatures associated with micro-breaking emerge in the frequency-wavenumber spectra. The CISG successfully captures the spatial onset of 
these micro-breaking induced capillaries with wavelengths between 0.4 cm and 3 cm. 
By applying wavelet and band-pass filtering, these features were isolated, allowing for the identification of the "birth" of micro-breaking induced capillaries within the FOV. 
This work establishes a methodological framework for detecting micro-breaking and provides new insights into the surface conditions governing small-scale dissipation processes in wind-driven wave systems.

How to cite: Morales Meabe, J. M., Gade, M., Tondu, C., and Buckley, M.: Detection of Microscale Breaking in Wind-Driven Waves using a Colour Imaging Slope Gauge (CISG) , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10831, https://doi.org/10.5194/egusphere-egu26-10831, 2026.

Results from a suite of simulations with a version of the Norwegian Earth-System Model which includes an ocean surface-waves (OSW) component, WW3, are presented.
OSW are forced by surface winds in the control integration and may be additionally coupled to atmosphere, ocean and sea-ice components through several parametrisations dependent on wave-supported stress, wave significant height, Stokes drift, and wave radiant stress.
Significant effects on the simulated model climatology are found for each of such additional couplings.
However, for the processes considered, the effects of two-way coupling between atmosphere and OSWs, or between sea-ice and OSWs, are highly dependent on the model background climatology -- and therefore also on model systematic biases.
By contrast, additional mixing caused by Langmuir turbulence systematically causes the ocean mixed layer to deepen, with a robust impact on sea-surface temperatures (SSTs), viz mid-latitude cooling in the summer hemisphere, and mid-latitude warming in the winter hemisphere.
Replacing the dynamic OSW model, WW3, with an analytical scheme predicated on a local equilibrium sea-state (Li et al., 2017) to drive Langmuir mixing gives similar results, with a slight exaggeration of the deepening especially in the tropics likely due to missing wind-wave misalignment in the analytical formulation.

How to cite: Ali, A., Bentsen, M., Breivik, Ø., Carrasco, A., Debernard, J. B., Ellevold, T., Michel, C., and Toniazzo, T.: Including dynamic ocean surface waves in NorESM climate simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12764, https://doi.org/10.5194/egusphere-egu26-12764, 2026.

Exchanges of momentum and energy across the sea surface microlayer (SML)  are controlled by turbulent dynamics within the first millimeters above/below the wavy water surface. Wind-generated waves, ubiquitous at the ocean surface, strongly influence turbulent processes in the air and water near the surface, especially as the waves grow and (microscale) break. Surface-active substances, commonly found in coastal waters, are known to dampen waves over a wide range of scales. However, the influence of these surfactants on the coupled air-water flow dynamics and associated fluxes remains unclear. Indeed, some of the phenomena involved take place at a sub-millimeter scale, which makes it challenging to investigate the complex mechanisms at stake.

A combination of two experimental techniques (PIV, particle image velocimetry, and LIF, Laser Induced Fluorescence) with a high resolution (33 µm/pixel for the PIV and 55 µm/pixel for the LIF) were used to determine flow motions on both sides of the SML. The complex set-up was installed at a fetch of 15.5m at the 24-m long, 1-m wide, 1-5m high wind wave tank of the University of Hamburg (Germany) which is specially designed for studies with surfactants (Oleyl Alcohol, OLA in this work). Here, we focus on conditions with a reference windspeed of 4.5m/s measured by an ultrasonic anemometer at 64 cm above the water surface.

The wide field of view (51cm) enables us to capture the evolution in time and space of turbulent shear stress above and below individual wind waves. As the waves move through the field of view, steepen and microbreak, high magnitude turbulent shear is produced in the airflow past the wave-crest and can sometimes spread over several wavelengths when intense air-flow separation events occur. A quadrant analysis shows that negative momentum flux (Q1 and Q3) events are usually encountered before wave-crests whereas positive momentum fluxes (Q2 and Q4) events are produced past them on average. In the water, positive turbulent shear stress mainly shows up below the windward side of the waves, while negative turbulent shear is present below their leeward sides. An estimation of the viscous and turbulent energy dissipation integrated over the first centimeter underneath the water surface shows that the production of bound capillary waves enhances the energy dissipation, which becomes more intense as the capillary train grows up.

When surfactants are present, a reduction of sweeps and ejections (Q2 and Q4) past the dampened wave crests is notable and can be associated with the reduced occurrence and intensity of air-flow separation events. In the water, the removal of most capillary waves leads to a reduction in energy dissipation, as well as in the (phase) averaged turbulent kinetic energy below crests.

How to cite: Tondu, C., Buckley, M., Gade, M., and Marcelo Morales Meabe, J.: Laboratory study of turbulent momentum and energy fluxes above/below microscale breaking wind waves, and influence of surfactants, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14205, https://doi.org/10.5194/egusphere-egu26-14205, 2026.

Storm surges are phenomena caused by wind conditions of greater magnitude than usual, whether local or remote. The ocean-atmosphere interaction is important in the development of these events, since wind is the main factor that increases wave heights, leading to an increase in their energetic potential. For this reason, the study focuses on characterizing the meteorological conditions that triggered swells categorized as M3, M4 and M5 of the Escala de Impactos de Marejadas developed by the MarejadasUV (MUV) of the University of Valparaíso in the northern, central and southern areas of the country.
Three representative points were selected on the coasts of Chile: in the north (-23°S,72°W), in the center (-32°S,75°W) and in the south (-44°S,78°S). Datasets were extracted every 3 [hrs] for significant wave height, mean period, mean direction and wave energy spectra modeled with WaveWatch III forced with surface wind and sea ice area fraction from the ERA5 reanalysis. With these data, thresholds related to 2, 5 and 10 years of return period were obtained to categorize the events into M3, M4 and M5, respectively, that occurred between May and October from 1979 to 2022, obtaining 29 cases in the north, 28 in the center and 21 in the south.
The northern area was characterized by more remote swell events (24) than local (5). The latter have a similar configuration where the south winds (more commonly known as Surazo) developed swells of the three categories, with different wind magnitude. The remote events were generated by low pressure (LP) formed at different points of the study area mainly located below the 40°S in deep water. In the center area, there were a greater number of local events (8), which in addition to being formed by south winds were also formed by LPs developed near the study point and the shore. This last configuration being similar for the remote events (20), but the distance which they were developed was greater. In the southern area, there were more local events (17) than remote events (4), mainly formed by a LP that were formed nearly the study point.
In conclusion, the categorization of these events depends on the wave climate. Most of the local events in the north and center were formed by winds from the south. The rest of the events are developed by LPs originated in different parts of the study area.

How to cite: Vasquez, M., Garreaud, R., and Aguirre, C.: Synoptic characterization of extreme wind-wave events in Chile, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14883, https://doi.org/10.5194/egusphere-egu26-14883, 2026.

We present a preliminary study on the one-way coupling between the spectral wave model Wavewatch III (WW3) and the hydrodynamic model SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) to investigate wave–current interactions in the Columbia River estuary (USA) and its adjacent coastal ocean. WW3 is implemented on an unstructured grid, enabling high-resolution representation of the spatially complex conditions at the estuary mouth and extending into the open ocean, and it is forced with time-varying currents and water levels from SCHISM simulations. Preliminary results are compared with buoy observations and satellite-derived sea surface heights from the Surface Water and Ocean Topography (SWOT) mission, exploring the potential of these data for model evaluation. The study combines model evaluation using satellite and buoy data with the coupling of wave and hydrodynamic models in an estuarine environment, while highlighting the relevance of unstructured grids for representing fine-scale coastal processes within a broader oceanic context.

How to cite: Fernández, L., Seaton, C., and Haller, M.: Wave–Hydrodynamic Modeling of the Columbia River Estuary Using One-Way Coupling Between SCHISM and WAVEWATCH III on Unstructured Grids, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15527, https://doi.org/10.5194/egusphere-egu26-15527, 2026.

Short wind-wave growth is central to estimating sea-surface drag and air-sea momentum transfer, as it increases surface roughness and facilitates directional wave breaking. Therefore, field observations that resolve the full wind-sea scale are essential for parameterizing air-sea fluxes and validating numerical weather prediction models.

In these efforts, we developed a measurement system with a polarimetric camera integrated into a UAV platform, leveraging RTK-enabled aircraft positioning and an inertial measurement unit for high-precision georeferencing. With varying altitudes, we resolve ocean waves ranging from centimeters to decameters, extending the polarimetric camera’s capabilities to those of wave buoys.

Field measurements were conducted from May to July 2025 on the coast of Rye, New Hampshire, under various conditions, including gusty winds, limited/unlimited fetch, and misaligned wind-swell and current. The observations yield 3D directional wave spectra, resolving wavelengths from 20 m to 6 cm and frequencies from 0.3 to 5 Hz. The directional spreading, current shear, and bimodal peaks are plotted against the mean current direction and wind speed, which were measured by a nearby buoy. With these measurements we aim to explore the dynamics of locally generated surface waves by linking the gravity capillary scales to larger wind-sea.

How to cite: Duvarci, G. and Laxague, N.: Observations of Locally Generated Wind Waves using a Novel Airborne Polarimeter, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16778, https://doi.org/10.5194/egusphere-egu26-16778, 2026.

Extreme wave events are recurring meteorological and oceanographic hazards that have a significant impact on coastal regions, leading to infrastructure damage, beach erosion, and adverse effects on fisheries and port operations, resulting in substantial economic losses in Chile. In recent decades, both the frequency and intensity of extreme wave events have increased, and this trend is projected to continue due to climate change, making Chile's extensive coastline particularly vulnerable. In this context, having access to accurate and high-resolution coastal wave forecasting is crucial for coastal users and stakeholders involved in assessing and managing the risks associated with extreme wave events. Here, we present a high-resolution coastal wave forecasting system, which is validated using in situ measurements in Valparaíso Bay. Additionally, an impact-based extreme wave intensity scale has been developed to improve risk communication, support the issuance of official early warnings, and enhance emergency response. A five-category scale, derived from a qualitative analysis of historical impacts on beaches and coastal infrastructure, is fully integrated into the forecasting system. Video cameras have been installed to provide real-time broadcasts of the coastline, facilitating continuous monitoring of wave conditions and their impacts during extreme wave events. Furthermore, the information is disseminated through a dedicated public website and various social media platforms to effectively communicate warnings and promote preventive actions. Key national public institutions responsible for issuing warnings and managing emergencies participate in the information flow, thereby strengthening risk governance and public decision-making, and increasing confidence in the reliability of the coastal wave intensity forecasts.

How to cite: Aguirre, C., Correa, S., Molina, M., and Bahamondez, S.: Impact‑based extreme‑wave intensity scale for high‑resolution coastal forecasting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19619, https://doi.org/10.5194/egusphere-egu26-19619, 2026.

Semi-analytical studies have demonstrated that water compressibility, seabed elasticity, and gravitational potential modify tsunami phase speed and can explain systematic arrival-time deviations observed in farfield measurements [1]. However, operational and research tsunami models remain based on incompressible formulations, preventing explicit simulation of acoustic modes and limiting investigation of gravity–acoustic coupling in large-scale free-surface flows.

We present the derivation and numerical implementation of a compressible set of water-wave evolution equations compatible with the widely used finite-volume tsunami modelling frameworks. Starting from the compressible Euler equations, the formulation retains weak compressibility and acoustic propagation while preserving the long-wave structure required for basin scale simulations. Particular attention is given to the pressure closure, dispersion relation, and numerical consistency with existing solvers.

The equations are being implemented within an open-source solver and validated against analytical limits and controlled numerical benchmarks. Preliminary results demonstrate stable coexistence of surface-gravity and acoustic modes, recovery of expected dispersion behaviour, and improved consistency of wavefront propagation speed relative to incompressible formulations. Synthetic impulsive source experiments of landslides illustrate the generation and radiation of coupled hydroacoustic–surface wave fields and their sensitivity to compressibility effects.

The proposed framework provides a physically consistent pathway for extending dispersion based corrections into fully time-dependent numerical models, which enables systematic investigation of gravity–acoustic coupling, compressibility effects, and wave–acoustic energy partitioning in long-wave ocean dynamics. The formulation also establishes a foundation for coupling numerical wave physics with hydroacoustic observations in future integrated modelling studies.

Reference

[1] A. Abdolali, U. Kadri, & J. Kirby, 2019. Effect of Water Compressibility, Sea-floor Elasticity, and Field Gravitational Potential on Tsunami Phase Speed. Scientific Reports, 9 (1), 1-8. 

How to cite: Kadri, U., Hunt, M., Abolali, A., Kim, J., Omira, R., and Ramalho, R. S.: Compressible water-wave evolution equations for coupled gravity–acoustic modelling of long ocean waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21071, https://doi.org/10.5194/egusphere-egu26-21071, 2026.

Wind waves are inherently irregular and random, making the goal of finding a fully deterministic description practically impossible. However, knowing their probabilistic properties is crucial for engineering applications and for understanding ocean dynamics. To deepen this understanding and build more efficient wind-wave models, machine-learning approaches are likely to become increasingly valuable.  Recent progress in physics-informed machine learning (PIML) has transformed fluid mechanics by combining data-driven approaches with physical fundamental equations, enabling more robust and generalizable models.

In our study, we apply PIML techniques to identify probabilistic characteristics of wind waves. Our research is based on learning probability distributions directly from data, which allows us to avoid restrictive assumptions or classical approximations.

We utilize field measurements collected in Skulte, Latvia, during August–September 2022. The dataset includes pressure time series and 3D velocity profiles, providing a detailed description of wave dynamics. Building upon existing PIML architectures, we developed a framework capable of inferring an accurate and efficient probabilistic model of wind waves. Preliminary results show promising agreement with theoretical expectations and previous studies.

The dataset was provided by Kevin Parnell and colleagues from Tallinn University of Technology (TalTech), together with the Latvian Institute of Aquatic Ecology. Our findings highlight the potential of PIML for improving probabilistic wave modelling and set the foundation for future applications in coastal engineering and environmental monitoring.

How to cite: Grzonka, L., Parnell, K., and Herman, A.: Data-Driven Study of the Probabilistic Characteristics of Wind Waves in Latvia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21442, https://doi.org/10.5194/egusphere-egu26-21442, 2026.

Since their inception in the 1990s, the third-generation spectral models, used both for the operational wave forecast and for research, reached significant advances in their performance. This success, however, depends on the criteria for this performance and on the aims of the model usage. In the presentation, we will discuss what is missing and what applications require attention, revision or further development of model physics.

We will argue that the main problems, as far as the traditional aim of spectral models is concerned – the wave forecast, is with predicting swell, wave-current interactions and directional spectra of wind-generated waves. Swell is poorly predicted in terms of the wave height, but arrival time is its particular problem - swell can be up to 20 hours early or late by comparison to its forecast. We will demonstrate that partially this can be connected to the issue of wave-current interactions.

The problem of directional wave spectra connects us to a new role of wave models – providing the air-sea fluxes into coupled models for large-scale environments such as Atmospheric Boundary Layer, including spray production, tropical cyclone intensity, for modelling the upper ocean, including ocean mixing, air-sea gras transfer, biogeochemistry, for marginal ice zone, among other application, for climate. In the presentation, we will discuss the new criteria for model performances and avenues of reaching the new aims for spectral models in these new applications.

How to cite: Babanin, A.: Wave foecast models: what is missing? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21550, https://doi.org/10.5194/egusphere-egu26-21550, 2026.

A comprehensive analysis of direct wind stress estimation is performed from a field campaign measurements carried out in the Gulf of Mexico. Air-sea interaction spar buoys were deployed and operated at three locations in order to study ocean-atmosphere interactions under a variety of meteorological conditions. Variability of atmosphere and ocean conditions is a very important issue that provide us with the best analysis of the influence of wave direction in the relative direction of wind stress upon the mean wind direction. Results of relatively simple cases with only one wave system show a gradual direction change of wind stress very much associated with the relative wave direction with respect to wind, specially under low to moderate wind conditions. These type of conditions are always more frequent in the ocean generally. When the calculation of the wind stress is performed in a reference frame aligned with wave propagation direction, a clearer evidence of the wave coherent stress component is observed. Main results of this work are obtained in such a coordinate system aligned with the waves claiming the paramount importance of the wave-coherent stress. The effect of multiple wave systems in the wind stress is addressed taken considering special conditions when atmospheric fronts were present in the region. The ultimate goal is to provide a proper parametrization of the momentum transfer to be used in the next generation of numerical models.

How to cite: Ocampo-Torres, F. J.: The influence of waves in wind stress direction as from the analysis of buoy direct measurements., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22146, https://doi.org/10.5194/egusphere-egu26-22146, 2026.

Accurate forecasting of ocean and climate can provide actionable information for decision-making and ocean governance, which is essential for transferring ocean science to sustainable development. However, huge common biases of ocean, typhoon and climate models hinder our forecasting ability. The programme of “Ocean to climate Seamless Forecasting system (OSF), approved by the UN Ocean Decade in 2022, aims to provide a solution. This presentation will introduce the OSF Programme and what it has achieved.

With huge heat content, ocean controls the evolution of TC and climate. In this regard, ocean is the key to improve forecasting ability. A key breakthrough of OSF is quantifying the dominant role of surface waves in upper-ocean mixing and air-sea fluxes, processes previously omitted in large-scale models. By integrating wave-induced physics into models, OSF has achieved fundamental improvements, reducing summer sea surface temperature bias in ocean models by ~80%, decreasing typhoon intensity forecast error by ~40%, and cutting climate model SST bias by ~60%. OSF further translates science into actions through its global network, innovative low-cost buoy observations, and operational systems such as OCEANUS and COAST, delivering actionable forecasts and tools for disaster risk reduction, ecosystem protection, and coastal resilience.

How to cite: Wang, S. and Qiao, F.: Towards Seamless Ocean-Climate Forecasting: Surface Wave Dynamics and the UN Ocean Decade OSF Programme, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22826, https://doi.org/10.5194/egusphere-egu26-22826, 2026.

Air-sea heat flux intensifies during cold airs and other strong weather events. However, due to the lack of long-term observations during such cold air processes, the quantitative enhancement of air-sea heat flux and its underlying mechanisms remain poorly understood. To address this issue, based on a tower-based platform in the southern Bohai Sea, a high-frequency turbulence measurement system was implemented to conduct a two-year air-sea flux measurement, collecting air-sea heat flux data covering 20 cold air outbreak events. This study quantitatively analyzes and reveals the pronounced variations in air-sea sensible heat flux of SHF and latent heat flux of LHF during cold air events, as well as the distinct roles of wind speed, air-sea temperature difference and specific humidity difference. The enhancement of SHFand LHF is further quantified. Our results show that the significant increases in wind speed and air-sea temperature difference are the primary drivers of the enhanced heat flux. Although LHF exhibits higher magnitude than SHF during cold air processes, LHF is predominantly controlled by increased wind speed, whereas SHF is mainly influenced by both wind speed and the air-sea temperature difference, with its enhancement being substantially greater than that of LHF. Compared to calm weather conditions, SHF and LHF under cold air conditions increased by an average of 12.8 and 1.6 times, respectively, while the total heat flux increased by 2.6 times on average. The increasement of heat flux can exceed 10 times during cold waves, even can reach the magnitude comparable to that observed during tropical cyclones.

How to cite: Wu, S. and Qiao, F.: Enhanced Air-Sea Heat Flux during Cold Air Events: Observations and Mechanism Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22828, https://doi.org/10.5194/egusphere-egu26-22828, 2026.

The Northwest Atlantic Ocean is a climatically sensitive region influenced by two major boundary currents—the warm Gulf Stream and the cold Labrador Current—which transport water masses of contrasting temperature and salinity, and it also hosts a crucial component of the Atlantic Meridional Overturning Circulation (AMOC). Recent studies indicate substantial changes in both currents, with potential implications for regional ocean dynamics. In this study, we investigate the evolution of stratification in the Northwest Atlantic over the period 1993–2023 using an eddy-permitting reanalysis dataset. Stratification is quantified through the Brunt–Väisälä frequency, and long-term trends are assessed. To diagnose the drivers of the observed stratification changes, we further examine the variability in current pathways using Lagrangian parcel tracking. Additionally, Optimum Multiparameter (OMP) analysis reveals that changes in circulation are redistributing water masses across the study domain, which likely contributes to the modulation of water-column stratification. Stratification in this region is a key regulator of ocean primary production, oxygen ventilation, vertical mixing, and deep convection, thereby influencing both ecosystem dynamics and large-scale ocean circulation. 

How to cite: John, A. B., Pant, V., and Lahiri, S. P.: Do Changes in the Western Boundary Circulation Cause Stratification Changes in the Northwest Atlantic Ocean?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1151, https://doi.org/10.5194/egusphere-egu26-1151, 2026.

The circulation of the North Atlantic Ocean plays a vital role in the Earth's climate system. Numerous studies, mainly through computer simulations, have examined the stability of the Atlantic Meridional Overturning Circulation (AMOC) in the context of a warming climate. Some of these studies predict a potential collapse of the AMOC in the foreseeable future, which would require a significant influx of freshwater into the subpolar North Atlantic (NA) and/or Nordic Seas. Paleoreconstructions of the NA circulation indicates a major shift in the position of the subpolar cold front either precedes or coincides with substantial changes in AMOC dynamics. These changes imply a significant alteration in circulation patterns, beginning with noticeable restructuring of the subtropical and subpolar gyres. This would lead to modifications in the Gulf Stream system and the North Atlantic Current (NAC), affecting the thermohaline fields as well as the position and strength of these two current systems. Although some models predict a significant slowdown or even collapse of the AMOC, recent observational studies offer a more cautious perspective. For instance, the Gulf Stream system exhibits high resilience to perturbations from ongoing sea-surface warming. In this study, we analyze the decadal variability of temperature and salinity from in situ observations, along with upper-ocean currents in the subpolar NA (SPNA). We find that the thermohaline pattern of the upper ocean layers in the SPNA and Nordic Seas has remained resilient for over 70 years. The deceleration of the AMOC is evident but relatively modest, with average velocities in the upper layers decreasing by less than 10-15% over 30 years. This deceleration is also not consistent throughout the NAC region. Furthermore, the subpolar front migration over 70 years is a maximum of 3° of latitude, with the spatial variability of the yearly 10°C isotherms substantially less than that. Overall, the conclusion about the resilience of the NAC aligns well with that of the Gulf Stream, with no substantial changes in the position or intensity of the subpolar gyre. We conclude that while the AMOC is susceptible to some deceleration due to ongoing surface warming and/or freshening at high latitudes, it may also be sufficiently resilient to withstand these changes. Although it cannot be entirely ruled out that the AMOC may reach its tipping point within this century, an analysis of data on decadal variability in the upper arm of the AMOC suggests that such a collapse is unlikely.

How to cite: Mishonov, A., Seidov, D., and Reagan, J.: Resilience of the North Atlantic Circulation on Decadal Timescales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1758, https://doi.org/10.5194/egusphere-egu26-1758, 2026.

The North Atlantic is a key region of the climate system, where ocean circulation redistributes heat across latitudes and drives pronounced variability on interannual to multidecadal time scales. Observational programs have provided valuable insights into Atlantic circulation and variability, but their spatial and temporal coverage remains limited. Global ocean reanalyses offer a complementary, spatially complete view of the ocean and therefore provide potentially valuable tools to investigate heat transports and their variability in the North Atlantic.

This contribution aims to provide a clear and quantitative assessment of the usefulness and reliability of ocean reanalyses for diagnosing ocean heat transport and its variability in the North Atlantic. It is carried out within the framework of the Marine Environment Reanalyses Evaluation Project (MER-EP), which aims to systematically evaluate and intercompare global marine reanalyses. By systematically comparing multiple reanalyses with observational and budget-based constraints, we aim to identify where and under which conditions reanalysis-derived transport estimates are robust and where important limitations remain. This assessment is essential for the appropriate use of ocean reanalyses in studies of North Atlantic variability and its role in climate change.

We evaluate ocean heat transport and related diagnostics in the North Atlantic using a large ensemble of global ocean reanalyses from different modelling centers. Transport calculations are performed using the newly developed StraitFlux (Winkelbauer et al. 2024) diagnostic framework, which enables consistent transport estimates across different model grids and vertical coordinate systems.

Our analysis focuses on the North Atlantic sector and its variability, with particular attention to major observing systems such as RAPID, SAMBA, OSNAP (Winkelbauer et al., preprint) and transports across the Greenland-Scotland Ridge, Fram Strait and the Barents Sea Opening. In addition to transports obtained from ocean reanalyses and insitu observations, we estimate meridional ocean heat transport indirectly from the ocean heat budget. These inferred transports are obtained by combining surface heat fluxes inferred from the atmospheric energy budget, ocean heat content tendencies and contributions from sea ice melt, and by imposing appropriate boundary conditions at basin chokepoints. This approach provides complementary ocean heat transport estimates that are largely independent of both the reanalysis circulation fields and the insitu observations. It allows to assess ocean reanalysis performance consistently across the entire North Atlantic, including regions and latitude bands where no insitu transport measurements are available.

Winkelbauer, S., Mayer, M., and Haimberger, L.: StraitFlux – precise computations of water strait fluxes on various modeling grids, Geosci. Model Dev., 17, 4603–4620, https://doi.org/10.5194/gmd-17-4603-2024, 2024.

Winkelbauer, S., Winterer, I., Mayer, M., Fu, Y., and Haimberger, L.: Subpolar Atlantic meridional heat transports from OSNAP and ocean reanalyses – a comparison, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-4093, 2025.

How to cite: Winkelbauer, S., Mayer, M., Forget, G., Song, Y., and Haimberger, L.: On the reliability of reanalysis-derived heat transport in the North Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3145, https://doi.org/10.5194/egusphere-egu26-3145, 2026.

Based on satellite altimetry, GRACE space gravimetry and ARGO-based steric data down to 2000m, recent studies have shown that the North Atlantic sea level budget of the past two decades is not closed, with strong regional residuals in the mid-latitudes. It was proposed that this results from salinity errors reported since 2015/2016 in some Argo float measurements. In this study,  we revisit the North Atlantic sea level budget using altimetry, GRACE, different Argo products and ocean reanalyses. The reanalyses are used to estimate the manometric contribution for further comparisons with GRACE data, as well as for estimating the deep ocean contribution to the sea level budget, not yet sampled by Argo. We first find that using the CIGAR ocean reanalysis-based manometric component instead of GRACE reduces the residuals of the sea level budget in the North Atlantic (ie, altimetry-based sea level minus sum of components). We also find that accounting for the deep ocean (below 2000m) thermal expansion (from the CIGAR reanalysis) allows for the quasi closure of the North Atlantic sea level budget. The North Atlantic halosteric component in the upper 2000 m displays a small decrease since the early 2010s, significantly larger after 2016. The 2010–2016 halosteric decrease may reflect a real salinity increase in the region, but salinity measurement errors may have impacted the halosteric component after that date. The main result of this study is that deep ocean warming plays a non-negligible role in the North Atlantic and has to be accounted for in the sea level budget assessment.

How to cite: Song, Z., Cazenave, A., Llovel, W., Storto, A., and Bouih, M.: North Atlantic sea level budget revisited, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3385, https://doi.org/10.5194/egusphere-egu26-3385, 2026.

Ocean tides play an important role in shaping circulation, stratification, and mixing in the North Atlantic subpolar region, yet their impacts at fine spatial scales remain insufficiently quantified. In this study, we investigate the effects of ocean tides in the North Atlantic subpolar area using a km-scale high-resolution ocean model. Two numerical experiments are conducted: a control simulation including full tidal forcing and a sensitivity experiment in which tidal forcing is suppressed. By comparing these experiments, we isolate the tidal contributions to sea surface elevation, currents, and vertical mixing.

The results show that tides substantially enhance barotropic and baroclinic variability, particularly over complex topography such as continental slopes and ridges. Tidal currents intensify near-bottom shear and promote vertical mixing, leading to modifications in stratification and water mass properties. In addition, tide–topography interactions generate internal tides that propagate into the interior basin, influencing submesoscale circulation and energy redistribution. These tidal effects further modulate the mean flow and variability in the subpolar gyre, with implications for regional heat and salt transport. Meanwhile, natural variability also plays a role when distinguishing between tidal effects and internally generated variability.

Our findings highlight the importance of explicitly resolving tidal processes in high-resolution ocean models for accurately representing circulation and mixing in the North Atlantic subpolar region. This study emphasizes that tides are a key component of subpolar ocean dynamics and should be considered in studies of climate-relevant processes in this region.

How to cite: Lin, L.: The effects of ocean tides in North Atlantic subpolar area studied by high-resolution ocean model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4458, https://doi.org/10.5194/egusphere-egu26-4458, 2026.

The North Atlantic subpolar gyre is a highly dynamic region where ocean–atmosphere interactions are shaped by variations in freshwater export from the Arctic, import of subtropical waters via the Gulf Stream, mixing, in addition to large-scale atmospheric circulation patterns such as the NAO and the strength and position of the North Atlantic jet. Building on our earlier analysis using the CANARI Large Ensemble (HadGEM3-GC3.1), we previously showed that winters with anomalously fresh surface waters systematically exhibit shallower mixed layers, cooler SSTs, and weaker surface heat loss. These conditions imply enhanced freshwater-driven stratification, reduced deep convection, and a tendency for heat to be trapped below the surface, features consistent with the structure and persistence of the North Atlantic Warming Hole (NAWH).

In this study, we extend that framework to examine how this wintertime surface cooling and associated changes in the surface heat fluxes interact with the overlying atmosphere across a range of background circulation states. Using ensemble member–specific sea level pressure anomaly patterns and a regime-classification approach, we identify multiple atmospheric response modes that differ in the strength and latitude of the North Atlantic pressure gradient. These regimes reveal that the atmospheric response to subpolar cooling is not uniform. The background fields play a decisive role in determining how the surface cooling interacts with the large-scale atmospheric circulation.

Together, our results highlight a dynamically consistent pathway linking freshwater import, ocean stratification changes, regional winter SST cooling, heat flux responses, and large-scale atmospheric circulation shifts. This work provides new insight into the range of possible atmosphere–ocean climate feedbacks associated with ongoing and future freshening of the subpolar North Atlantic.

How to cite: Ayres, H. and Oltmanns, M.: Freshwater-driven subpolar gyre cooling and atmospheric regime responses using a large ensemble climate model., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5455, https://doi.org/10.5194/egusphere-egu26-5455, 2026.

The subpolar North Atlantic Ocean and adjacent Arctic Seas experience strong winter heat loss and surface water densification which play a key role in the large-scale ocean circulation. The balance of processes driving this heat loss is expected to change as the climate system heats up and ice cover declines with impacts on both the ocean and the atmosphere. Here, we use a 40-member ensemble of runs with the HadGEM3-GC3.1 model (termed the CANARI Large Ensemble) to investigate these changes. The runs span 1950-2014 and 2015-2100 using CMIP6 historical and SSP3-7.0 forcings, and model resolution is ¼ degree ocean – N216 atmosphere. In the sub-polar Atlantic, the winter heat loss initially strengthens through to the mid-1980s before weakening by of order 50% by 2100 due primarily to variations in the sea-air temperature gradient. In the Arctic, the winter heat loss is initially dominated by ice decline before becoming dominated by atmospheric conditions from mid-century onwards. Regional variations in the impacts of these changes on both the ocean and the atmosphere will also be explored.

How to cite: Josey, S., Blaker, A., Grist, J., Mecking, J., and Sinha, B.: Future Evolution of Subpolar Atlantic and Arctic Ocean-Atmosphere Interaction in the CANARI Large Ensemble , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5497, https://doi.org/10.5194/egusphere-egu26-5497, 2026.

The Atlantic Multidecadal Variability (AMV) is a prominent mode of low-frequency climate variability, characterized by basin-scale sea surface temperature (SST) variations in the North Atlantic and strong global impacts. AMV can be forced externally by surface radiative fluxes or internally generated. The latter generation mechanism is commonly attributed to variations in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that a robust AMV can arise and be sustained by large-scale atmosphere–ocean interactions, even in the absence of a dominant role for AMOC variations, in a fully coupled model—the Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). The simulated AMV is driven primarily by surface shortwave and turbulent heat fluxes across the North Atlantic. Essentially, an initial warming over the Gulf Stream region strengthens and spreads by reducing low cloud cover and enhancing surface shortwave radiation. This mechanism is enabled by the relatively narrow width of the North Atlantic, compared to the North Pacific. Our results broaden the conceptual understanding of AMV physics and underscore the importance of atmosphere–ocean interactions in sustaining it.

How to cite: Hu, S., Li, X., Fedorov, A., and Van Roekel, L.: Atlantic Multidecadal Variability driven by the western current warming–eastern low cloud reduction mechanism, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6053, https://doi.org/10.5194/egusphere-egu26-6053, 2026.

The North Atlantic re-emergence phenomenon is an intermittent event in which winter sea surface temperature (SST) anomalies subduct under the seasonal thermocline and re-emerge the following winter when the mixed layer deepens. This means that the ocean acts as a `memory' for North Atlantic winter climate on interannual scales. Previous studies of the North Atlantic re-emergence phenomenon are limited by short observational records, forced-ocean models, or poor resolution. The CANARI Large Ensemble (65 years x 40 members of the UK Met Office Climate Model (HadGEM3) at N216 atmosphere and 1/4 degree ocean resolution) provides an opportunity to robustly analyse these events and their mechanisms. We have tested existing mechanistic theories, and are answering other pertinent questions, such as; does stratospheric preconditioning occur and can we harness predictability from it? Are there multi-year impacts and cascading effects? Can we quantify the relationship between Atlantic meridional overturning circulation (AMOC) and re-emergence?

How to cite: Collingwood, E., Sinha, B., Marsh, R., Blaker, A., Marshall, G., Scaife, A., and King, J.: The North Atlantic Re-emergence Phenomenon in a Coupled Large-ensemble Climate Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6755, https://doi.org/10.5194/egusphere-egu26-6755, 2026.

A simplistic view of the Atlantic Meridional Overturning Circulation (AMOC) is that it is composed of a northward flow of warm, salty water that sinks at high latitudes as a result of wintertime surface buoyancy losses, subsequently returning southward as the cold, fresh Deep Western Boundary Current. Its strength is often expressed as the maximum of the overturning streamfunction in a depth and latitude range, the latter normally centred at around 26°N. In reality, however, the AMOC is partly “carried” by the largely wind-forced and near-barotropic horizontal gyre circulation, and the production of upper NADW has also been shown to depend on a chain of surface densification around the subpolar gyre, in addition to the deep convection localised in the Labrador and Irminger Seas. The overturning streamfunction in depth coordinates is therefore far from being a complete description of the AMOC.

We present a range of AMOC metrics using a set of centennial simulations of the HadGEM3-GC5 coupled model with a ¼° NEMO ocean component. These include a gyre index based on the barotropic streamfunction, surface-forced indices, regional mixed-layer volumes, and transport indices evaluated against a range of vertical axes. We compare these indices with the traditional overturning metric at the RAPID section at 26°N, and discuss the causal links between them. This work is carried out under the EU HORIZON25 project Explaining and Predicting the Ocean Conveyor (EPOC).

How to cite: Megann, A., Blaker, A., Hirschi, J., and Aksenov, Y.: Disentangling the AMOC: influences of the gyre circulation, surface density transformations and overturning circulation on AMOC variability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6942, https://doi.org/10.5194/egusphere-egu26-6942, 2026.

The Atlantic Ocean exhibits a persistent northward heat transport at all latitudes, providing a key source of heat to the relatively northerly located European continent. Consequently, variability in the North Atlantic circulation plays a central role in modulating regional climate patterns in Europe. However, the widely reported lack of meridional coherence in the Atlantic basin on interannual to decadal timescales impedes the detection of large-scale circulation changes and their separation from internal climate variability. The Subpolar Gyre (SPG) is particularly important because variability in its circulation strength and hydrographic properties impacts both local dense water formation and heat transport towards the Arctic. In this study, we examine the structure and variability of the SPG circulation and quantify the recirculation within the gyre versus the throughput towards the Nordic Seas across the Greenland–Scotland Ridge. We employ the Lagrangian trajectory tool TRACMASS to identify the dominant pathways of recirculation and throughput, and quantify the associated volume and heat transports. We utilise a 1/12° ocean hindcast as Eulerian input fields for the period 1979–2021, and seed Lagrangian particles in the North Atlantic Current at 53°N. The Lagrangian trajectories allow us to quantify the spatio-temporal variability of the circulation, and to localise the depth- and density-dependent connectivity between the SPG and the Nordic Seas. The results lay the groundwork for a better understanding of the SPG as a potential modulator of heat transport towards the Arctic.

How to cite: Unegg, J., Asbjørnsen, H., and Svendsen, L.: Variability in the Subpolar Gyre circulation and throughput towards the Nordic Seas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7013, https://doi.org/10.5194/egusphere-egu26-7013, 2026.

The Rockall Trough (RT), located in the northeastern Atlantic Ocean, is a dynamically complex region characterized by a strong tidal forcing, flow–bathymetry interactions, intense mesoscale activity and deep winter mixing. These processes promote enhanced vertical and lateral mixing, making the RT a key region for the transformation of intermediate and deep water masses of southern origin that subsequently feed high-latitude convective sites. As such, modifications occurring in the RT may have important implications for deep convection and large-scale circulation in the subpolar North Atlantic (SPNA).

The Mediterranean Outflow Water (MOW), originating in the Gulf of Cádiz and spreading northward along the European continental margin, is a major contributor to the heat and salt budgets of the North Atlantic. Although previous studies have identified the presence of the MOW within the RT, its pathways, transformation processes and interaction with surrounding water masses in and beyond this region remain poorly understood. In particular, the extent to which the MOW properties are modified before entering the SPNA is still uncertain.

In this study, we combine an extensive dataset of more than 20 years of Argo float observations with a set of simulated Lagrangian trajectories to investigate the pathways of the MOW in the RT, the evolution of its properties, and the interactions of the MOW with the resident water masses. Argo data are used to identify the MOW signal at intermediate depths and to quantify changes in temperature and salinity along its pathways, while Lagrangian simulations provide insight into the paths, residence times, and connectivity within and beyond the RT.

In addition, Copernicus reanalysis data are employed to characterize the persistent features of the intermediate circulation in the RT, allowing us to assess how these structures influence the transport, spreading, and mixing of the MOW in this key transition region.

The long-term Argo record further allows us to examine the interannual variability of the MOW pathways and properties, providing new insights on the processes that regulate its spreading further north, into the SPNA.

How to cite: Calvo, E., Malanotte, P., Menna, M., Martellucci, R., and Zambianchi, E.: Pathways and transformation of the Mediterranean Outflow Water in the Rockall Trough, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9291, https://doi.org/10.5194/egusphere-egu26-9291, 2026.

How to cite: Ypma, S. L., van Westen, R. M., von der Heydt, A. S., and Dijkstra, H. A.: Deep convection variability across strong and weak AMOC states in an eddy-resolving ocean simulation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9655, https://doi.org/10.5194/egusphere-egu26-9655, 2026.

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of Earth’s climate system, regulating large-scale ocean heat and nutrient transport. Paleo reconstructions indicate that the AMOC varied substantial during late Quaternary climate transitions. Sedimentary ²³¹Pa/²³⁰Th has been widely used as a tracer for reconstructing past AMOC strength. However, recent studies have questioned its applicability, as ²³¹Pa/²³⁰Th is also sensitive to particle fluxes. In particular, variations in export productivity and metalliferous particles emitted from hydrothermal vents may overprint the circulation signal.

Here, we investigate the sensitivity of sedimentary ²³¹Pa/²³⁰Th to past AMOC variability by compiling new and revised down-core ²³¹Pa/²³⁰Th records spanning the last 30,000 years from a geographically confined sector of the mid–North Atlantic, covering water depths from 2,102 to 4,110 m. This justifies the assumption of very similar particles fluxes for all core locations in this pelagic environment. However, despite their close spatial proximity, the down-core ²³¹Pa/²³⁰Th records exhibit two clearly distinguishable trends, with increasing ²³¹Pa/²³⁰Th at shallower sites and decreasing trends at deeper sites. These trends are unlikely to be the result of changes in particle scavenging alone: biogenic opal concentrations reveal similar down-core trends throughout all sites, while the absolute concentrations remain consistently below 10 wt% and bulk sediment Fe/Ti and Cu/Ti ratios at most sites provide no evidence for significant local inputs of metalliferous particles associated with enhanced hydrothermal activity despite the region's proximity to the mid-ocean ridge. The only exception is one core closest to multiple active hydrothermal vents showing intermittent intervals of elevated Fe/Ti and Cu/Ti ratios, which are associated with elevated ²³¹Pa/²³⁰Th ratios.

By incorporating the ²³¹Pa/²³⁰Th records from this geographically confined study area into a basin-wide North Atlantic compilation, we show that the inverted ²³¹Pa/²³⁰Th trends observed over the last 30,000 years are coherent North Atlantic-wide features. To investigate the underlying mechanisms, we conducted a set of conceptual Holocene and LGM AMOC simulations using the ²³¹Pa/²³⁰Th-enabled Bern3D model. The simulations show that during the LGM a weaker AMOC, relative to the Holocene, can reproduce the observed depth-depend ²³¹Pa/²³⁰Th response. This pattern is most likely caused by the spatiotemporally variable balance between particle-mediated scavenging and lateral advection of ²³¹Pa. Importantly, changes in both processes are governed on basin scale by the AMOC. These findings indicate that shallow to intermediate-depth sediment cores capture signals of past circulation strength, even when their ²³¹Pa/²³⁰Th response is inverse to the conventional deep-ocean interpretation of higher ²³¹Pa/²³⁰Th reflecting a weaker AMOC and vice versa.

How to cite: Gerber, L., Lippold, J., Repschläger, J., Friedrich, O., Testorf, P., Ehnis, M., Blaser, P., Pöppelmeier, F., and Jaccard, S. L.: Processes driving 231Pa/230Th in the mid-North Atlantic Basin over the last 30,000 years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10057, https://doi.org/10.5194/egusphere-egu26-10057, 2026.

The Atlantic Meridional Overturning Circulation (AMOC) plays a major role in shaping the Northern Hemisphere climate, and assessing the risk of a future slowdown has become a key challenge in ocean research. Over the past two decades, advances in observations and modeling have substantially refined our understanding of where and how deep waters are formed.

In this ‎Award Lecture of the OS‎ Division, I will review these developments to examine the drivers of North Atlantic dynamics and their representation in coupled climate models. First, I will focus on observation-based estimates of water mass transformation in the subpolar gyre, highlighting the dominant role of local buoyancy forcing in the Irminger and Iceland basins. Second, I will examine how deep water formation is simulated in coupled climate models, identifying key biases that lead to excessive formation in the Labrador Sea and assessing their implications for the AMOC at subpolar latitudes. Finally, I will discuss the southward propagation of deep waters and the coherence of AMOC variability across the North Atlantic, placing these results in the broader context of AMOC change at different timescale.

How to cite: Petit, T.: On the Role of Atmospheric Forcing on the North Atlantic Dynamic – Insights from Observations and Climate Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10992, https://doi.org/10.5194/egusphere-egu26-10992, 2026.

Recent Arctic warming and melting sea ice are iconic features of global warming. Yet, it is unlikely that anthropogenic forcing is solely responsible for these changes. The Early-20th-Century Arctic Warming (ETCAW), comparable to the recent one, provides a benchmark for natural climate variability but remains poorly understood. Sparse sea-ice observations is a major issue—limiting also past modelling studies. Here, we use a new physically based sea-ice reconstruction and atmospheric model experiments to replicate, for the first time, the rapid ETCAW. We find that two-thirds of the strong winter warming is driven by increased ocean heat release, amplified further by the lapse-rate feedback. This response is linked to extensive sea-ice loss present in the reconstruction and to strengthened poleward Atlantic heat transport. These results clarify the role of sea-ice loss in the ETCAW and provide new insight into natural variability’s influence on future Arctic climate change.

How to cite: Li, F., Semenov, V., Keenlyside, N., Aldonina, T., Ha, K.-J., Chung, E.-S., and Yang, X.-Q.: Unveiling the Role of Sea-Ice Loss in Early-20th-Century Arctic Warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11351, https://doi.org/10.5194/egusphere-egu26-11351, 2026.

The sinking flux of particulate organic carbon, i.e., the downward limb of the biological carbon pump, is estimated to supply 8-10 petagrams of carbon to the ocean interior every year. The sinking of carbon co-regulates atmospheric CO₂ concentrations and provides energy for mesopelagic and deep ecosystems, including lucrative commercial fisheries. In this context, the Labrador Sea in the Northwest Atlantic plays a critical role as this region supports enormous phytoplankton spring blooms, important fisheries, and coastal communities that rely on marine resources. While there is concern that changes in ocean conditions due to climate warming will negatively impact both productivity and the biological carbon pump in the Northwest Atlantic, mechanistic understanding of the biological carbon pump remains poor, which is reflected in the lack of skill and predictive power of state-of-the-art numerical models.

Here, we present new data collected in the framework of the collaborative Biological Export in the Labrador Sea (BELAS) project, one of the most comprehensive examinations of the carbon pump in the Labrador Sea to date. Building on field campaigns during the spring of 2022, 2024, and 2025, we will discuss (a) fluxes of sinking organic carbon and opal based on the ²³⁴Th/²³⁸U disequilibrium method, (b) particle data from underwater vision profilers (UVP), and (c) net primary productivity estimates. These data sets enable us to compare sinking carbon and opal fluxes between years and between different phytoplankton communities (Phaeocystis versus diatoms) and provide observational constraints on the efficiency of the biological carbon pump in this region.

How to cite: Kienast, S., Healey, M., McBride, C., Roca Martí, M., Laget, M., Sipler, R., Devred, E., and Finkel, Z.: Quantifying organic carbon fluxes and the efficiency of the biological carbon pump in the Labrador Sea, Northwest Atlantic , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11731, https://doi.org/10.5194/egusphere-egu26-11731, 2026.

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 streamfunction of 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), where both overturning systems substantially contribute to tropical Atlantic variability. At 11°S, the AMOC has been monitored for more than 10 years by the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) observing system, which combines dedicated moorings and Pressure equipped inverted echo sounders (PIES) at the western and eastern boundaries with available observations across the basin.

Here, we investigate specifically the variability of the deep western boundary current (DWBC) and different estimates for the AMOC anomaly transport at 11°S based on different parts of the observing system. DWBC transport estimates are derived from full-depth alongshore velocity fields using regression-based techniques combining the moored observations with the shipboard velocity sections. The sensitivity and robustness of the AMOC estimate is evaluated through an observing system simulation experiment (OSSE) employing the high-resolution ocean model VIKING20X.

The results reveal that the Deep Western Boundary Current (DWBC) transport is dominated by strong intraseasonal variability associated with the passage of deep eddies, while longer-term variability becomes apparent as the observational record lengthens. The geostrophic upper AMOC transport at 11°S is primarily characterized by pronounced seasonal variability, with peak-to-trough amplitudes of approximately 20 Sv. Despite its comparatively sparse design, the TRACOS array is capable of capturing key aspects of AMOC variability, while also identifying opportunities for further improvements, particularly with respect to reducing uncertainties on longer time scales.

How to cite: Hummels, R., Hans, A. C., Dengler, M., and Brandt, P.: Variability of the Western Boundary Current System and AMOC at 11°S, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17048, https://doi.org/10.5194/egusphere-egu26-17048, 2026.

To understand the AMOC response to historical greenhouse gas (GHG) forcing including the role of resolution, new historical simulations with GHG forcing fixed at 1950s level (fixedGHG) are performed in EPOC project. We compare results from fixedGHG run with HighResMIP control run and historical run, to isolate the impact of GHG on historical changes in the AMOC. We assess AMOC variability and its associated dynamical sea level (SSH) fingerprint in two configurations of in MPI-ESM-1.2-HR (0.4º ocean) and MPI-ESM-1.2-ER (0.1º eddy-resolving). In MPI-ESM-1.2-ER, the historical run exhibits a significant negative trend of the AMOC, while fixedGHG run exhibits a significant positive trend of the AMOC. The results support the ideas that GHG forcing leads to slowdown of the AMOC and aerosol forcing lead to spin-up of the AMOC. The change of the AMOC is coherent across latitudes, with larger amplitudes of trend in the subpolar North Atlantic in MPI-ESM-1.2-ER. In MPI-ESM-1.2-HR, neither experiment shows a significant long-term trend, although a slight AMOC decline emerges after the mid-1990s in the historical run. We further evaluate the AMOC–SSH relationship at 26°N using AVISO altimetry and RAPID observations. Both observations and the ER historical run display a canonical Gulf Stream–related dipole: positive SSH anomalies south and negative anomalies north of the Gulf Stream SSH ridge, along with negative anomalies along the Labrador Current—consistent with a strengthened Gulf Stream and Deep Western Boundary Current during strong AMOC states. The HR configuration fails to reproduce this fingerprint, underscoring the importance of eddy-resolving simulations for capturing AMOC–SSH covariability. We are further analyzing SSH patterns in fixed-GHG simulations to isolate the effects of GHG forcing and to elucidate the underlying mechanisms.

How to cite: fan, H. and Barcelona Martín, J.: The AMOC Response to GHG Forcing and Its Fingerprint on Dynamical Sea Level, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17696, https://doi.org/10.5194/egusphere-egu26-17696, 2026.

The subpolar North Atlantic exhibits pronounced variability on decadal and longer time scales, which has implications for decadal predictability of climate and marine biogeochemical fields. Yet, its driving mechanisms are still under debate. It has been shown from both observations and modeling that this variability is associated with anomalous deep water formation in the Labrador Sea generated by the surface heat fluxes associated with the North Atlantic Oscillation (NAO) and the consequent adjustment of thermohaline circulation, most evident during the 1990s when the NAO was persistently positive. However, the direct observations of overturning circulation along the sections east and west of Greenland (Overturning in the Subpolar North Atlantic Program; OSNAP) do not support this view, as observed overturning in the Labrador Sea is very weak under positive NAO conditions during the observed period from 2014 onward. In this study, we use high-resolution (0.1°) forced ocean–sea-ice simulations, which reasonably reproduce the mean overturning in density coordinates observed at the OSNAP line, to elucidate this contrasting overturning between the two periods under similar positive NAO conditions. Simulated deep water formation in the Labrador Sea is indeed weak during the 2010s, while it is very active during the 1990s. These signals are meridionally coherent, suggesting coherent changes in Atlantic meridional overturning circulation (AMOC). We also find that this anomalous overturning in the Labrador Sea takes place over densities far heavier than the density where maximum overturning occurs, thus the maximum overturning time series cannot accurately capture these signals. We have conducted sensitivity experiments to identify whether the weak overturning during the 2010s is due to oceanic or atmospheric conditions. These experiments reveal that the weaker overturning is largely generated by weak surface heat release due to a warmer air temperature over the Labrador Sea. We have further performed composite analyses using the CESM2 pre-industrial and transient (historical plus SSP370) simulations to investigate how such warm air conditions come about over the Labrador Sea. The composite analyses suggest that the warm air temperature is likely due to a warm SST condition in the Labrador Sea, internally generated, rather than externally forced. Conversely, the strong overturning during the 1990s was likely because of cooler conditions in the Labrador Sea.

How to cite: Kim, W. M., Yeager, S., Rosbson, J., and Walsh, A.: Subpolar North Atlantic overturning: the 1990s versus the 2010s, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17732, https://doi.org/10.5194/egusphere-egu26-17732, 2026.

How to cite: Hermanson, L., Smith, D., and Seabrook, M.: Using climate forcings runs to attribute the decadal variability and predictability of the AMOC, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18744, https://doi.org/10.5194/egusphere-egu26-18744, 2026.

The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the climate system, transporting heat, freshwater, and momentum across the Atlantic basin and playing a critical role in regulating regional climate and global heat uptake. It is commonly portrayed as a conveyor belt, with warm saline waters travelling northward, losing heat to the atmosphere and freshening due to precipitation and ice melt. Sinking occurs at high latitudes once the water is sufficiently dense, and the newly formed dense waters travel southward. However, this picture is overly simplistic. The AMOC is not spatially uniform: transport anomalies at one latitude do not always map directly to anomalies elsewhere, because of internal recirculations, gyre-scale compensations, mixing, and local forcing.

Within the EU-funded EPOC (Explaining and Predicting the Ocean Conveyor) project we have examined transport variability and coherence on seasonal and longer timescales in observations and a range of numerical models. Starting from the Arctic gateways and progressing southward, transports and variability of volume, heat and freshwater are compared at key observational sections are compared. Meridional coherence of the AMOC is examined using latitude-correlation and EOF decomposition methods, and comparisons against recent Bayesian modelling heat and observation-based freshwater transports are made. In this poster we summarise the key analysis and work performed under WP1.

This work is funded by the UKRI (grant number 10038003) as part of the EPOC project (Explaining and Predicting the Ocean Conveyor; grant number: 101059547).

How to cite: Blaker, A., de Steur, L., Megann, A., Vallivattathillam, P., Aksenov, Y., Fredriksen, H.-B., and Hirschi, J. and the Contributors to EPOC WP1: AMOC transport variability and coherence from observations and models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19285, https://doi.org/10.5194/egusphere-egu26-19285, 2026.

The Southwest Iberian margin is a critical oceanographic region, forming the boundary of an eastern boundary upwelling system and serving as the primary pathway for Mediterranean Outflow Water (MOW) into the Atlantic Ocean. An analysis of year-long (November 2023–December 2024) observations from moored instruments at two sites on the continental slope was conducted: a mid-slope location (PA-II; 1488 m depth) and a lower-slope site (PA-I; 2606 m depth), the latter located on the so-called Shackleton site. We investigated the natural variability of temperature, salinity, turbidity, and current velocity and direction at both subsurface and deep levels at each mooring.

Analysis of Temperature-Salinity (TS) diagrams revealed three distinct water masses. At 2606 m depth in PA-I, the North East Atlantic Deep Water (NEADW) was present, while more diluted NEADW and MOW were occasionally identified at ~ 1488 m depth in PA-II. Subsurface waters (~353 m depth in PA-I and ~418 m depth in PA-II) were characterized by the presence of the Eastern North Atlantic Central Water (ENACW) of subtropical and subpolar origins, respectively. The TS time series reveals that short-term fluctuations were more prominent than clear seasonal signals.

Current speeds were higher in subsurface waters (≥0.3 ms⁻¹) than in deep waters (0-0.3 ms⁻¹). After tidal removal, the dominant directions in PA-I were eastward in the subsurface level and north/northwestward at 2606 m depth. At PA-II, subsurface currents flow north/northeast, while deep waters move north/-northwest. Notably, currents at ~ 1488 m depth in PA-II were highly influenced by tidal components as indicated by a directional change from northeast/ southwest to north/northwest and maximum speed reduction from 0.4 ms⁻¹ to 0.3 ms⁻¹, a pattern not observed at other depths, after removing the tidal influence.

Persistent values ranging from 0.1–0.5 FTU (Formazin Turbidity Units) over extended periods were interpreted as long-lived increases in turbidity associated with upwelling, background sedimentation, and resuspension cycles. Short-lived turbidity peaks (≥0.5 FTU), lasting hours to days, are also recorded. Turbidity amplitudes were generally lower in deep waters compared to subsurface waters.

Based on surface winds, surface temperature, chlorophyll concentration, and the upwelling index, we interpret the subsurface, low-moderate turbidity signals at PA-I as offshore transport of particles along isopycnals during the peak upwelling phase (July- September). During this period, ENACW was upwelled, consistent with the subsurface current flow directions at both sites. The low-to-moderate deep-water turbidity variations, indicative of near-bottom resuspension events, coincided with the timing of local bottom trawling activities. A prominent short-lived event recorded in subsurface waters at PA-II is linked to a regional earthquake in August 2024 (~ 57 km to epicentre), while other short-lived events coincided with increased riverine sediment discharge driven by rainstorms in the west part of the peninsula.

Overall, these integrated hydrographic, currents, and turbidity observations underscore the strong coupling between water-mass structure, upwelling dynamics, and lateral transport pathways. They emphasize how both physical oceanographic processes and episodic natural and human-induced forcing are pivotal in shaping subsurface and deep-water environments in this dynamic boundary region.

How to cite: Hewage, P. L., Arjona-Camas, M., Sanchez-Vidal, A., J. Sierro, F., and Ausín, B.: Subsurface and deep-water mass characteristics and variability in the Southwest Iberian margin from year-long observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20033, https://doi.org/10.5194/egusphere-egu26-20033, 2026.

Since the post-industrial era, sea surface temperature (SST) has shown a consistent warming trend at a global scale, while chlorophyll-α (Chl-α) concentrations have generally exhibited declining trends in the open ocean. Some recent studies suggest that intensified coastal upwelling, driven by increased alongshore winds, may locally counteract these negative trends by enhancing biological productivity. This study assesses the evolution of long-term trends in SST, Chl-α, meridional wind stress and Saharan dust over the Northeast Atlantic, focusing on both open-ocean oligotrophic regions and coastal upwelling systems. We applied the methodology developed by Siemer et al. (2021), using the same satellite and in situ datasets, updated to include the most recent years, and defining additional subregions to better resolve smaller open-ocean areas of interest. Our results reveal a significant acceleration of SST warming across the entire study area during the last six years. In open-ocean regions, this acceleration is accompanied by a strengthening of negative Chl-α trends, indicating a continued decline in phytoplankton biomass. In contrast, coastal upwelling regions, particularly the Northwest African upwelling system, exhibit a slowdown in the decline of Chl-α and productive area. However, trends in upwelling-favourable wind stress over the African coast are predominantly negative, suggesting a weakening of the atmospheric forcing traditionally associated with enhanced coastal productivity. The inclusion of Saharan dust variability allows us to assess the combined role of atmospheric forcing and aerosol deposition in modulating recent biophysical trends in the region.

How to cite: Pereira, C., Fraile-Nuez, E., González-Vega, A., Machín, F., Puig Montellà, E., Martín- Díaz, J. P., and Ayuso- Candal, S.: Multi-decadal trends in SST, chlorophyll- a, NPP and atmospheric forcing across oligotrophic and upwelling regions of the Northeast Atlantic , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20957, https://doi.org/10.5194/egusphere-egu26-20957, 2026.

There is substantial model uncertainty in how the AMOC evolves in CMIP6 historical simulations. A significant part of this uncertainty has been related to the substantial uncertainty in the strength of the historical Anthropogenic Aerosol forcing. However, there is also significant uncertainty in how models simulate the AMOC response to historical greenhouse gas forcing, which is not well understood. Therefore, this raises the question, how sensitive is the real-world AMOC to historical greenhouse gas forcing? Here, we use simulations from the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) to isolate the historically forced signal, and to understand the spread in AMOC response between models. By 2014, AMOC declined significantly in hist-ghg simulations as expected, but there is a very large spread in AMOC decline that is approximately equivalent to the uncertainty due to aerosol forcing. We find that the decline appears to be strongly related to changes in turbulent heat loss in the subpolar North Atlantic, which is itself related to the change in air-sea temperature and humidity contrasts (i.e., it is thermodynamically driven). The spread in hist-ghg simulations is also consistent with the spread in abrupt 4xCO2 simulations, and further analysis of those simulations supports a causal relationship between the spread in the initial forced changes in the air-sea temperature contrasts and the resultant AMOC decline.

How to cite: Madan, G., Robson, J., and Sutton, R.: Understanding the uncertainty in simulated AMOC changes to historical greenhouse gas emissions in CMIP6, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22473, https://doi.org/10.5194/egusphere-egu26-22473, 2026.

The Southern Ocean (SO) is of major importance in shaping climate transitions due to its substantial potential in storing carbon in the deep ocean and its release to the atmosphere most dominantly during glacial terminations. Through wind driven upwelling of deep waters and high latitude deep water formation, the SO acts as a gateway between the surface ocean and its interior. With the Antarctic Circumpolar Current (ACC), the world’s largest current system, the SO connects all three major basins of the global ocean and therefore integrates and responds to climate signals across the globe. Additionally, the SO exerts a major influence on the Antarctic Ice Sheet and partly controls its mass balance.

The evolution of deep-water formation and export, as well as its interplay with the ACC and the Antarctic Ice Sheet are important factors that are still poorly constrained. We present a largely isotope geochemical based high-resolution multi-proxy reconstruction of IODP Site U1537 to examine these interplays in the southern Scotia Sea. The Scotia Sea is a key area in the SO, where newly formed well ventilated Weddell Sea Deep Water (WSDW) is admixed into and entrained underneath the ACC.

Sedimentation in this area is mainly modulated by the strong ocean currents, as seen by extremely high sediment focusing throughout. Detrital neodymium (Nd) as well as authigenic and detrital lead (Pb) isotope compositions in Southern Ocean sediments provide insights into sediment sources, which can be clearly identified due to the distinct crustal ages of East and West Antarctica and its surrounding areas. Sediments at Site U1537 are dominantly sourced from the Antarctic Peninsula and the Weddell Sea region. The sediment provenance investigations are additionally complemented by K’-Ar analyses on the <63 µm fractions of the sediment samples, providing average age information. All of our obtained isotopic records reveal substantial variations during glacial-interglacial transitions.

Site U1537 provides evidence for low bottom water oxygenation (derived from authigenic uranium) and likely no WSDW export into the Scotia Sea during the Last Glacial Maximum. The data further suggests early deglacial pulses of WSDW export. We advocate that these pulses might be a considerable contributor to the reestablishment of interglacial-type deep ocean ventilation and AMOC conditions. A substantial increase in current-shelf interaction along the Antarctic margin in the Pacific sector is seen during MIS5e. Taken together, our multi-proxy approach highlights the complex sedimentation regime in the Scotia Sea and provides new paleoceanographic insights towards the circulation and frontal dynamics as a function of climatic boundary conditions at submillennial-scale resolution.

How to cite: Hallmaier, M., Gutjahr, M., Hemming, S. R., Lippold, J., Weber, M. E., and Eisenhauer, A.: Climate state dependent sedimentation dynamics in the southern Scotia Sea during the last four glacial cycles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-506, https://doi.org/10.5194/egusphere-egu26-506, 2026.

Phaeophyta (i.e., brown seaweeds) are significant primary producers in high-latitude environments, serving as a key nutritional source to fauna and carbon sink. Despite being the dominant biomass source in polar regions, they have largely been overlooked in carbon assessments, trophic ecology, and biogeochemical studies. Our understanding of how these ecosystems will respond to climate change is limited, based on a handful of studies that are primarily Arctic focussed. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis of macroalgae has often been used as a tool to assess nutrient sources, energy transfer and photosynthetic mechanism but has rarely been applied to polar macroalgae. The rapid environmental change in both poles has the potential to shift the isotopic baseline. Our current understanding is poor, to our knowledge only 28 studies have published biogeochemical assessments of Antarctic macroalgae, half of which are from the South Shetland Islands in the northern West Antarctic Peninsula. Yet biogeochemical data can provide a wealth of information regarding nutrient source changes, light dynamics, productivity and nutritional quality. Larger species, such as Himantothallus grandifolius (Antarctic) or Saccharina latissima (Arctic) can provide seasonal or even multi-annual data through incremental stable isotope analysis along macroalgal blades. Changing productivity rates can be tracked through δ¹³C values, fluctuating due to sea ice break out, carbon demand and growth requirements. Over 20 specimens of H. grandifolius and Arctic kelps have been collected over several field trips to the Antarctic Peninsula, East Greenland coastline and Svalbard for δ¹³C and δ¹⁵N analysis; forming the largest biogeochemical dataset for polar macroalgae to date. Large variations > 15 ‰ were recorded for the Antarctic species H. grandifolius from a single organism, a significant variation when considering trophic level shifts are on a scale of ~ 3–5 ‰. Cyclical trends in productivity were also identified in several specimens with wider implications for shifting isotopic baselines of primary producers in response to environmental change on seasonal and multi-year time scales. Strong seasonal responses in δ¹³C are linked to sea ice and fluctuating light conditions with increased run off through glacial melting. Nitrogen was found to vary between sub-tidal and inter-tidal species, as well as incrementally along blades of larger species. New nitrogen sources may be introduced to remote polar regions as increased tourism increases the risk of wastewater and pollutant inputs to these fragile ecosystems. Macroalgae could become an ideal tracer in coastal environments where nutrient sources can be assessed at varying time scales.

Our incremental approach provides high resolution isotopic data with the capacity to generate seasonal to multi-year records from an understudied ecosystem. Polar environments are set to change in unprecedented ways, the shifting isotopic baseline has repercussions for the wider food web, ecosystem structure and functioning that macroalgae play a key role in.

How to cite: Alldred, F. and Gröcke, D.: Stable isotope analysis of polar seaweeds: Assessing productivity and response to environmental change on seasonal and multi-annual time scales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-722, https://doi.org/10.5194/egusphere-egu26-722, 2026.

Hydrothermal venting along mid-oceanic ridges alters deep ocean water masses around the globe. These vent systems supply geothermal heat and various biogeochemical properties to water masses close to the bottom. In the Arctic, the influence of hydrothermal venting is largely unknown, and whether it should be considered in relation to other processes that modify the highly isolated deep water masses.

We present new results from the expedition PS 137 on water masses and the flux associated with the Aurora Vent Site at the Gakkel Ridge, 82.9° N, the only system in the Arctic Ocean to have been visited twice. Utilising CTD observations including physical and biogeochemical data, we assess the dimensions of the hydrothermal plume and estimate the heat flux. A spatially restricted plume core with a horizontal extent of less than 1000 m but a large rise height of 1200 m above the seafloor results in an estimated heat flux of approximately 180 MW.

A comparison with the observations made in 2014 reveals that the plume exhibits a comparable height, suggesting a constant heat flux over the period. Furthermore, the combined helium isotope measurements indicate that water masses on a larger scale at the Gakkel Ridge are also influenced by hydrothermal systems.

How to cite: Mette, J., Walter, M., Sültenfuß, J., and Mertens, C. and the R/V Polarstern PS137 Science Team: Hydrothermal Plumes in the Arctic Ocean -The Aurora Site at Gakkel Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1071, https://doi.org/10.5194/egusphere-egu26-1071, 2026.

Dissolved oxygen is a key indicator of ocean health and is being altered by climate-driven warming. The Arctic–subarctic system is warming at an exceptional pace due to Arctic amplification, but how this rapid warming translates into basin-wide oxygen change is still not well constrained. Using observations of Atlantic Water (AW) pathways, we find that the Atlantic inflow exerts a leading control on recent deoxygenation in the Arctic Ocean. Oxygen declines are detected in the upper eastern Arctic and in the intermediate layers of the western Arctic at rates of −0.41 ± 0.17 to −0.47 ± 0.07 μmol kg−1 yr−1, approximately six times the global mean. We identify amplified warming in Arctic gateway regions as the dominant driver, primarily through a strong reduction in oxygen solubility. The resulting low-oxygen signal is then propagated into the interior Arctic by rapid subduction and circulation of AW, extending the impact to deeper layers and increasing risks to Arctic marine ecosystems. These results emphasize that warming Atlantic inflow is central to shaping Arctic oxygen dynamics; continued temperature rise is therefore expected to sustain and potentially strengthen ongoing deoxygenation, calling for heightened attention and broader monitoring across the Arctic.

How to cite: Wu, Y.: Arctic amplification–driven warming of Atlantic inflow intensifies oxygen loss across the Arctic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2655, https://doi.org/10.5194/egusphere-egu26-2655, 2026.

Global climate change is concurrently and profoundly altering the ocean’s physical and biogeochemical environment. The intermediate water in the South Atlantic and Indian Oceans, which together constitute a critical component of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), lies at the heart of these changes. The intermediate layer of the South Atlantic is ventilated by two primary sources: relatively fresher and younger waters (characterised by low Apparent Oxygen Utilization, AOU) originating from the Pacific Ocean, and saltier and older waters advected from the Indian Ocean via the Agulhas System. However, the extent to which the intrusion of saline, older Indian Ocean waters via the Agulhas System modulates the stability of the AMOC’s upper limb remains poorly understood. Specifically, the temporal variability and long-term contribution of the Indian Ocean waters to the South Atlantic intermediate layer remains a knowledge gap.

Here, we focus on observational evidence of and investigate the underlying mechanisms driving the influence of Indian Ocean intermediate waters on the Atlantic Ocean in a warming climate. We examine AOU-salinity covariability across decadal to multi-decadal time scales within South Atlantic intermediate water. This analysis integrates high-quality observational databases of temperature, salinity, dissolved oxygen, and water age, as well as repeat hydrographic sections, allowing us to link their observed variability to changes in circulation and mixing, while considering oxygen disequilibrium effects and the influence of the biological carbon pump in changing AOU.

We find an increasing influence of Indian Ocean water in the South Atlantic at the intermediate layer over the past 30 years. The most strongly impacted regions are identified. In addition, we quantify the impact of Indian Ocean influence on the South Atlantic and show that this signal has become progressively detectable over the past 30 years, but has not yet exceeded the level of internal variability, indicating an ongoing ‘Indianization’ of the South Atlantic intermediate layer. We identify the underlying mechanisms related to the increasingly positive phase of the Southern Annular Mode (SAM) and an associated multidecadal increase in Agulhas leakage. Finally, we will discuss the potential implications of this phenomenon for the long-term stability of the AMOC.

How to cite: Tan, Z., McDonagh, E., Speich, S., Florindo Lopez, C., Davila Rodriguez, X., and Jeansson, E.: Growing Influence of Indian Ocean Waters in the South Atlantic Intermediate Layer over the last 30 years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6434, https://doi.org/10.5194/egusphere-egu26-6434, 2026.

Reconstructing past ocean-cryosphere interactions can provide crucial insight to the ongoing rapid climate change in the polar regions and beyond. However, there are large uncertainties in existing proxies commonly used in downcore reconstructions, including a lack of low-temperature (<9°C) culture-based Mg/Ca-temperature calibrations for planktic foraminifera. In the polar oceans the foraminiferal assemblage is not diverse and commonly dominated by Neogloboquadrina pachyderma, yet there is limited understanding of non-thermal influences on this proxy in this species. N. Pachyderma also precipitates a thick low-Mg/Ca crust over its inner higher Mg/Ca lamellar calcite, that contributes to uncertainties and inaccuracies in high-latitude palaeotemperature reconstructions.

To address this, we cultured N. pachyderma across a 2 to 9°C temperature range, at a range of salinities (29.8–36.6), and carbonate chemistry conditions with both co-varying and decoupled pH (7.65–8.4) and [CO₃^2-] (64–243 µmol/kg) and analysed their trace element composition using Laser Ablation Inductively Coupled Plasma Mass Spectrometry.

We present a new method that distinguishes the crust and lamellar calcite using trace element profiles from both cultured and fossil shells. This allowed us to show distinct geochemical signals in the crust and lamellar calcite of laboratory-grown N. pachyderma, including lower Mg/Ca, Na/Ca, and B/Ca in the crust compared to the lamellar calcite. We present new Mg/Ca-relationships, with independent calibrations for the crust and lamellar calcite. The temperature calibrations extend the lower range of culture-based Mg/Ca-calibrations down to 2°C. Furthermore, we show significant and opposing pH and [CO₃^2-] influences on Mg/Ca when these variables are decoupled and no statistically significant influence of salinity on Mg/Ca. Crust and lamellar calcite element/Ca are found to have different sensitivities to changing environmental conditions. Our results also show that environmental conditions control the crust-lamellar proportions and shell thickness which has implications for both downcore reconstructions and ongoing ocean acidification and warming.

Overall, our findings suggests that the crust and lamellar calcite precipitate via contrasting biomineralisation strategies and/or varying precipitation rates, leading to distinct geochemical compositions and different sensitivities to changing environmental conditions. We propose that distinguishing the two components and applying Mg/Ca-environmental relationships with separate calibrations for the crust and lamellar calcite will substantially reduce uncertainties in high-latitude palaeoceanographic reconstructions. We are now in the process of applying these methods and relationships to Quaternary sediment records from the central Arctic Ocean and the Nordic Seas.

How to cite: Westgård, A., Ezat, M. M., Sykes, F. E., Meilland, J., Chalk, T. B., Milton, J. A., Chierici, M., Knies, J., and Foster, G. L.: Improving polar ocean temperature reconstructions with crust-lamellae specific Mg/Ca-temperature calibrations and improved understanding of its non-thermal forcers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6726, https://doi.org/10.5194/egusphere-egu26-6726, 2026.

Antarctic krill populations exhibit substantial interannual fluctuations with highly variable success in the survival of larvae to the juvenile stage. Understanding connectivity between spawning hotspots and the areas where larvae successfully develop into juvenile populations is essential for predicting population dynamics and informing fishery management, yet the drivers of krill connectivity variability across the Southern Ocean remain poorly quantified.

This project uses Lagrangian particle tracking simulations to investigate krill larval connectivity patterns over 30 years (1993-present) based on high-resolution ocean circulation model outputs. We release over a billion virtual larvae throughout the spawning season across known spawning grounds and track their drift to quantify: (1) the variability of natural connectivity  among populations, (2) how spawning timing influences dispersal success, and (3) which large-scale climate patterns (SAM, ENSO, ACC variability) drive strong versus weak connectivity years.

Network analysis identifies critical source populations that supply multiple recruitment areas and vulnerable sink populations dependent on external larval input. This connectivity baseline is essential for distinguishing natural fluctuations from climate-driven changes in future projections.

Results will inform the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) by revealing which populations require protection and identifying critical hubs that sustain networks of connected krill populations. The Lagrangian model framework and open-source outputs will provide a foundation for subsequent climate change projections examining how changes in Southern Ocean circulation may alter connectivity patterns by 2050-2100.

How to cite: Gourgue, O., Barbut, L., Barthélémy, A., Dulière, V., Fichefet, T., Hanert, E., Lacroix, G., Massonnet, F., Richaud, B., Schön, I., Sylvester, Z., and Van de Putte, A.: Antarctic krill connectivity: a Lagrangian modeling framework to understand Southern Ocean population dynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8075, https://doi.org/10.5194/egusphere-egu26-8075, 2026.

Deep-water formation plays a crucial role in global climate, yet it is absent in the modern North Pacific. However, the existence of North Pacific Deep Water (NPDW) formation during the Pliocene and its impact on the marine carbon cycle remain controversial. Here, we present high-resolution sedimentary records of authigenic neodymium isotope composition(ε_Nd), barite accumulation rates (BAR), and barite barium (Ba) isotope compositions (δ¹³⁸Ba_barite) from ODP Site 882 in the subarctic Northwest Pacific.

Our results reveal pronounced shifts in ε_Nd and Ba-proxy records synchronous with the intensification of Northern Hemisphere Glaciation. Specifically, across the ~2.73 Ma interval, seawater ε_Nd values decrease from +0.2 to -1.4, marking a shift from northern-sourced NPDW to southern-sourced Pacific Deep Water (PDW). This circulation collapse was accompanied by elevated BAR and δ¹³⁸Ba_barite values, indicating a transient peak in export production. Intriguingly, this productivity pulse is decoupled from biogenic opal accumulation, which declines during the same interval. We propose that the cessation of NPDW formation allowed the upwelling of nutrient-rich PDW. This process fueled a transient increase in export production but partly drove the ecosystem from a silicate-replete to a silicate-limited regime, or reduced nutrient burial efficiency.

In contrast to the dynamic Late Pliocene, our new data from the Early Pliocene (~4.3–3.6 Ma) show relatively stable ε_Nd and Ba-proxy records. These findings challenge previous hypotheses of an Early Pliocene circulation transition derived from Japan Sea records, suggesting that open ocean circulation in the subarctic Pacific remained stable prior to the onset of major glaciations. Our study highlights the critical role of physical circulation thresholds in regulating the efficiency of the biological carbon pump and nutrient inventory in the North Pacific.

How to cite: Chu, Y., Zhang, R., Li, X., Xie, R., Yao, W., and Xu, A.: Evidence for Pliocene North Pacific Deep Water Formation and Its Paleoproductivity Imprints, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8271, https://doi.org/10.5194/egusphere-egu26-8271, 2026.

The Southern Ocean (SO) plays a crucial role in connecting all the world's oceans through the Antarctic Circumpolar Current (ACC). Global ocean circulation is also affected by processes in the Southern Ocean that change the density and subduction rate of water. Argo has provided consistent and basin-wide coverage of this region over the past two decades, enabling analyses not possible with the sparse and episodic ship-based observations available earlier. This study uses the Argo float  dataset collected between 2004 and 2025  along the ACC. The data was gridded and processed using the pseudoeulerian approach; the full dataset and decadal differences were obtained for 18 sections of the SO. We will present the spatial variability of water masses in the Southern Ocean, highlighting their decadal and interannual variability and the associated large-scale spatial gradients relevant to Southern Ocean dynamics. The use of Argo float observations provides unprecedented details for examining the spatial and temporal evolution of density patterns resulting from salinity and temperature changes, with important implications for global ocean circulation and climate.

How to cite: Amaral Wasielesky, A., Menna, M., Mauri, E., Rubino, A., Martellucci, R., and Bowen, M.: Changes in the Southern Ocean over the last two decades from Argo float measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8433, https://doi.org/10.5194/egusphere-egu26-8433, 2026.

The Southern Ocean plays a central role in the global climate system by regulating large-scale circulation, facilitating interbasin exchange, absorbing large amounts of anthropogenic heat and carbon and influencing Antarctic Ice Sheet stability. Yet, observational data, including rare earth element measurements such as neodymium (Nd) isotopes and samarium (Sm), have so far remained sparse in the Indian sector and along the East Antarctic continental margin, thereby limiting our understanding of circulation, water mass transformation, and sediment-ocean interactions in a changing climate.

Water masses in the Southern Ocean are traditionally characterized using hydrographic parameters (e.g., potential temperature, salinity, neutral density). During physico-chemical weathering along the East Antarctic margin, old continental crust supplies a distinctly unradiogenic neodymium isotope signature (low εNd) to regional shelf waters that interact with more radiogenic Antarctic Circumpolar Current waters (higher εNd) further north. This isotopic difference makes neodymium isotopes an especially powerful tracer of regional circulation and mixing along the East Antarctic continental margin. We present the first high-resolution dataset of dissolved εNd, together with Nd and Sm concentrations, from the Indian sector of the Southern Ocean. Gridded water column samples in conjunction with bottom water samples extracted from multicorer sediment supernatants, collected during expedition EASI-2 onboard RV Polarstern (Dec 2023–Feb 2024), provide a meridional transect from the Denman Glacier front (~66°S) to ~45°S along 100°E. Combined with hydrographic observations, this dataset provides a detailed framework for examining water mass structure, mixing, and regional boundary fluxes along the transect.

Away from direct Antarctic continental influences, the εNd distributions show largely conservative behavior in intermediate to deep waters and allow clear identification of major Southern Ocean water masses. A striking feature is the persistence of a remnant North Atlantic Deep Water εNd signal within lower Circumpolar Deep Water, highlighting long-range interbasin connectivity. Near the Denman Glacier, warm and radiogenic modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf, evident in both physical properties and εNd signatures below ~400 meters water depth. As seen in earlier studies, a pronounced mCDW tongue was observed to reach close to the Denman Glacier front, with associated high basal melt rates evident from potential temperature and salinity in sampled local East Antarctic shelf waters.

Nd and Sm concentrations increase linearly with depth north of the Polar Front, but exhibit substantial enrichment south of the front, reflecting deep-water upwelling, biogenic scavenging, and a latitudinally gradual boundary exchange. Pronounced variations in εNd and rare earth element concentrations in bottom waters point to substantial benthic additions in the southern reaches of the transect driven by weathering inputs from ambient terrigenous sediments, whereas particle-related scavenging appears to dominate offshore.

This study closes a critical observational gap in the Indian sector of the Southern Ocean and provides new constraints on the present-day circulation, water mass structure, and the influence of Antarctic crustal and benthic Nd additions, while demonstrating the value of εNd as a tracer in modern and paleoceanographic contexts.

How to cite: Ehnis, M., Gutjahr, M., Menzel, D., Huang, H., Creac'h, L., Oetjens, A., Rieke, O., Herraiz Borreguero, L., Janout, M., Rickli, J., Frank, M., Tippenhauer, S., and Lippold, J.: Neodymium Cycling and Water Mass Structure in the Indian Sector of the Southern Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10619, https://doi.org/10.5194/egusphere-egu26-10619, 2026.

The Fram Strait is a unique deep-sea gateway connecting the Arctic Ocean to the North Atlantic. Under modern conditions, the eastern Fram Strait remains largely ice-free due to the influx of warm Atlantic water via the West Spitsbergen Current (WSC). Conversely, the western Fram Strait is characterized by the export of cold, fresh Arctic water masses and sea ice from the central Arctic Ocean via the East Greenland Current (EGC), thereby impacting the North Atlantic thermohaline circulation. Paleoceanographic records, however, suggest radical departures from this oceanic regime during glacial periods (Geibert et al., 2021; Nørgaard-Pedersen et al., 2003).

Here, we present a high-resolution record of oceanographic conditions during Marine Isotope Stage (MIS) 6 using a sediment core KH14-GPC02, recovered from the eastern Fram Strait (77° 31' 22.7994" N, 8° 24' 4.9674" E) during Expedition CAGE19-3 in 2019.  Core KH14-GPC02 was analyzed for biomarker lipids, e.g., highly branched isoprenoids, sterols, and glycerol dialkyl glycerol tetraether lipids, providing the first complete, high-resolution records of sea-ice conditions and ocean temperature during the penultimate glacial maximum from this climatically critical region.

Our results reveal a two-phased evolution of sea-ice conditions in the eastern Fram Strait. Early MIS 6 was dominated by a marginal sea-ice cover with significant seasonal variability, as evidenced by high concentrations of the sea-ice biomarker IP₂₅ and open-ocean biomarkers. From ~165 ka onward, however, a sharp decline in IP₂₅ and open-ocean biomarkers signals a shift to perennial ice cover. To investigate the climatic drivers of this environmental transition, we conducted simulations with the complex Earth system model AWI-ESM. While lowered summer insolation and the closure of the Canadian Arctic Archipelago gateways contribute to regional cooling during glacial climates in the Fram Strait and Nordic Seas (e.g., Lofverstrom et al., 2022), our simulations demonstrate that these factors alone are insufficient to explain the perennial ice cover in the eastern Fram Strait during late MIS 6. Instead, we propose that the closure of the Denmark Strait, driven by ice‑sheet expansion, acted as a critical threshold to explain the heavy sea-ice cover in eastern Fram Strait during late MIS 6. This geographic blockage not only halted sea‑ice export through the Denmark Strait but also diminished the inflow of warm Atlantic water, fundamentally altering sea‑ice dynamics in the Arctic-Atlantic gateway. Our findings highlight the crucial – but often underestimated – role of oceanic gateways in regulating Arctic sea-ice extent during extreme glacial climates.

References

Geibert, W. et al., 2021. Nature 590, 97-102.

Lofverstrom, M. et al., 2022. Nature Geoscience 15, 482-488.

Nørgaard-Pedersen, N. et al., 2003. Paleoceanography 18, 1063.

How to cite: Mikler, M., Song, P., Knies, J., Lohmann, G., Ahn, Y., Nam, S.-I., and Müller, J.: A trapped East Greenland Current: Sea ice expansion during late MIS 6 in the eastern Fram Strait, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11521, https://doi.org/10.5194/egusphere-egu26-11521, 2026.

Anthropogenic climate change is accentuated at high latitudes due to Polar Amplification, a process driven by interactions and feedback mechanisms among terrestrial, atmospheric, and oceanic systems. Arctic sea ice is in rapid decline while Antarctic sea ice has experienced recent extreme lows after relative stability, with global climatic and ecological responses and impacts. Multiple proxies have been developed for past sea ice reconstructions to assess modern trends and aid future forecasting. These include microfossil assemblages (dinocysts, foraminifera, ostracodes) and biomarker concentrations derived from ice-edge and open-water diatoms. Two sea ice biomarkers, IP25 (Ice Proxy with 25 carbon atoms) and IPSO25 (Ice Proxy for the Southern Ocean with 25 carbon atoms), can be used to produce semi-quantitative reconstructions of past sea ice extent when combined with phytoplankton derived biomarkers (e.g. phytosterols brassicasterol and dinosterol). However, there gaps remain in understanding past sea ice states and the critical processes that drive change, including at critical transitions that may provide insight into current and predicted future warming. We present an overview of synthesised sea-ice proxy records spanning key periods characterised by lower and higher than pre-industrial CO₂ background states: the Mid-Holocene (6ka; 8.2–4.2 ka BP), the Last Glacial Maximum (LGM; 21ka; ~ 19–23 ka), the Last Interglacial (LIG; 127ka; ~130–115 ka BP), and the Mid Pliocene Warm Period (mPWP: 3.264–3.025 Ma). These syntheses are feeding into model-data integration as a key component of the EU Horizon project Past-to-Future (P2F), which aims to radically advance our knowledge of past climatic conditions to better understand Earth’s climate response to different kinds of forcing. A better understanding of past sea ice states and stronger data–model integration are essential for improving our ability to anticipate the future trajectory of sea ice and its cascading effects on global climate.

How to cite: Hole, G. M., Kjær, H. A., Andreasen, N., and McClymont, E.: Defining the States and Variability of Sea Ice via Marine Proxy Data Synthesis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13232, https://doi.org/10.5194/egusphere-egu26-13232, 2026.

Polar ocean ecosystems are a key source of nutrients such as nitrogen (N) and phosphorus (P) to the rest of the world’s oceans. Climate change is already altering many processes affecting elemental cycling at the poles, but interruption of polar nutrient export could suppress global primary productivity and fisheries by around a quarter over multi-century timescales. It is thus essential to constrain both the mechanisms and variability of these fluxes, and their global implications.

The Arctic specifically exports an excess of P relative to N, equivalent to ~90% of the net phosphate flux to the Atlantic at 47°N, and supporting a significant fraction of North Atlantic N-fixation. Of the gateways into the Atlantic, Davis Strait has the strongest net southwards transport. A mooring array has been tracking volume and freshwater transports there since 2004, yet biogeochemical transports remain poorly quantified. To move towards addressing this gap, two autonomous water samplers were deployed at the western boundary of Davis Strait; targeting the P*-rich core of the Baffin Bay outflow (~100db) enabled the monitoring of nutrient transport of waters being exported into the North Atlantic, and the variability in their N:P relationship. 

Deployed in Ocober 2022, samples were collected at ~2 week intervals and analysed for inorganic and organic nutrients, oxygen isotopes and pH, forming the first two years of a dedicated biogeochemical time series of the western boundary outflow.  

Initial results show substantial chemical variability across all measured parameters, with a clear seasonal cycle in salinity-normalised nutrients and oxygen isotopes. When combined with velocity fields then concentration difference drive varialbity in the transports. While temperature and salinity also vary strongly on seasonal (and shorter) timescales, their cycles showed some temporal offsets, suggesting different underlying forcing mechanisms. Differences between the slope and off slope sites (a stronger amplitude in both concentrations and transports closer to the shelf)also highlight spatial structure in the exported water masses. 

Across the Straits then preliminary P* transport estimates underscore the dominant role of the western core in total nutrient export through Davis Strait. Early indications are of longer-term changes in N:P ratios in the outflow. Ongoing work will further refine transport estimates and assess implications for Arctic–Atlantic nutrient connectivity.

How to cite: Brown, P., Mawji, E., Painter, S., Lenetsky, J., Gabriel, C.-E., Martin, A., Azetsu-Scott, K., and Lee, C.: Variability in biogeochemical Arctic outflow through Davis Strait, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14945, https://doi.org/10.5194/egusphere-egu26-14945, 2026.

The Arctic Ocean (AO) is changing very rapidly. Retreating sea-ice and the subsequent increase in light availability has significantly increased AO Net Primary Production (NPP). However, recent studies postulate that nutrients (and not light) now control NPP dynamics. We present observations from the Fram Strait (1998-2023) where this transition is revealed around 2009 as a sharp decline in fixed-nitrogen concentrations in the Polar Surface Water and an accompanying increase in Si:N ratios. We suggest that this represents a regime shift where fixed Nitrogen (N) has emerged as the main limiting factor for NPP in the contemporary AO. This reduction of N levels in the last decade may have resulted from increased benthic denitrification (BD) on the shelves. We investigate this by combining modelled BD rates and Lagrangian trajectories, which show a sharp increase in BD around 2009, with increasing contributions from the Chukchi and East Siberian shelves. We attribute this biogeochemical response to a drastic reduction in sea ice and circulation shift around this time. We suggest that Arctic climate change has led to a regime shift where low N levels resulting from increased loss of fixed N on the shelves now exert a tighter control on Arctic NPP. 

How to cite: Santos-Garcia, M., Ganeshram, R., Oziel, L., Dodd, P., de Steur, L., Tuerena, R., and Stedmon, C.: Sea ice loss drives a regime shift in Arctic Ocean nitrogen biogeochemistry , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17410, https://doi.org/10.5194/egusphere-egu26-17410, 2026.

The Northern Antarctic Peninsula (NAP) marine ecosystem is experiencing rapid environmental changes, yet the evolving relationships among atmospheric forcing, sea ice dynamics, and primary productivity remain poorly understood. This study investigates the interannual variability of summer chlorophyll-a (Chl-a) concentration and its physical drivers over the past two decades (2001–2024), utilizing multi-source satellite data and atmospheric reanalysis products. We identify a significant regime shift in climate-ecosystem interactions occurring around 2014.

Since 2014, major climate modes have shown concurrent trends: the Southern Annular Mode (SAM) accelerated towards a positive phase, while the Interdecadal Pacific Oscillation (IPO) shifted towards a negative phase. These combined trends led to a significant deepening of the Amundsen Sea Low (ASL), resulting in intensified regional winds (r=−0.79, p<0.01). This change in atmospheric circulation coincided with a rapid retreat of sea ice, marked by a significant increase in Ice-Free Days (IFD) in the NAP after 2014.

The significant change in climatic and physical conditions fundamentally altered the biological response patterns. Prior to 2014, the correlation between climate indices and summer Chl-a concentrations was weak, likely limited by the presence of sea ice cover. However, under low sea ice conditions after 2014, this association was notably strengthened. The correlation between the spring SAM index and summer Chl-a increased from 0.39 to 0.58 (p<0.1). The retreat of sea ice exposed the surface ocean directly to atmospheric forcing, enhancing the availability of irradiance and wind-driven vertical mixing. The enhanced mixing can facilitate the replenishment of limiting nutrients (e.g., iron) to the euphotic zone, thereby sustaining summer phytoplankton blooms. These findings suggest that the NAP ecosystem has entered a new state where productivity is tightly coupled with atmospheric dynamics.

How to cite: Ye, S., Zhang, Z., and Uotila, P.: Synergistic Climate Modes Drive a Regime Shift in Physical-Biological Coupling in the Northern Antarctic Peninsula Region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18273, https://doi.org/10.5194/egusphere-egu26-18273, 2026.

The Southern Ocean is a major sink for atmospheric carbon dioxide and critical to the current and future carbon cycle. This net annual CO₂ flux reflects the balance between strong seasonal variability characterized by opposing periods of winter outgassing and summer uptake. Using a simple framework, we evaluate how model biases in both the amplitude and timing of dissolved inorganic carbon (DIC) and total alkalinity (TA) and in the amplitude of sea surface temperature (SST) impact simulated pCO₂. We examine seasonal CO₂ fluxes and pCO₂ south of the Subantarctic Front in 42 Earth System Model and three state estimate simulations. Only 11 of the 45 simulations have a seasonal pCO₂ cycle with a correlation of ≥0.7 to observed pCO₂, while 26 have a correlation of <0. Four of the well-correlated models accurately represent the seasonality of SST, DIC, and TA, while TA biases compensate for DIC or SST biases in the other seven. DIC and SST amplitude biases are related to mixed layer (MLD) biases, with shallow MLDs, especially in the summer, correlated with larger amplitude DIC and SST cycles than observed. The amplitude of seasonal Net Primary Production is correlated to DIC and TA timing. We provide input on the main adjustments needed to correct the simulated pCO₂ seasonality in each of the evaluated models. These findings highlight the difficulty and importance of capturing the seasonal processes influencing the carbonate system to correctly model and predict the Southern Ocean carbon sink and its response to a changing climate. 

How to cite: Bushinsky, S., Arteaga, L., Fassbender, A., Hauck, J., Mazloff, M., Cerovečki, I., Landschützer, P., Rödenbeck, C., Danek, C., Romanou, A., Lerner, P., Gray, A., and Schlunegger, S.: Nonlinear Interactions of Timing and Amplitude Biases in Modeled Southern Ocean pCO2: The Roles of Dissolved Inorganic Carbon, Total Alkalinity, and Sea Surface Temperature, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19010, https://doi.org/10.5194/egusphere-egu26-19010, 2026.

Antarctic Intermediate Water (AAIW) is a fundamental component of the global overturning circulation and a key determinant of the pycnocline structure. It is produced in the high-latitude Southern Ocean, where interactions between the ocean, atmosphere, and sea ice strongly shape its physical characteristics. From the late twentieth century through 2015, Antarctic sea ice underwent a sustained expansion of roughly 3% per decade. This prolonged growth enhanced the seasonal meltwater supply, increased surface stratification, and contributed to a gradual freshening of AAIW, whose effects were observed in the subtropical basins. Beginning in 2016, this pattern shifted abruptly. A sequence of unprecedented annual sea-ice losses signalled a rapid transition away from the earlier expansion phase. By combining Argo float measurements with satellite observations and reanalysis data, we demonstrate that this regime change is already affecting the properties of intermediate-depth water masses throughout the Southern Hemisphere. Our analysis indicates that, since 2016, the contribution of sea-ice meltwater has declined at a rate of up to 36 mSv per decade, a stark contrast to the pre-2015 increase of about 14 mSv per decade. This reduction in freshwater input has driven a concurrent increase in the density and salinity of AAIW, with core salinity rising by approximately 6×10^-3 g kg^-1 per decade. Together, these trends point to an emerging, hemispheric-scale adjustment of Southern Ocean–sourced water masses.

How to cite: Mishra, K., Gayen, B., and Naveira Garabato, A. C.: Reversal of Antarctic Intermediate Water trends triggered by sea ice decline, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21652, https://doi.org/10.5194/egusphere-egu26-21652, 2026.

Sea Surface Temperatures (SST) in the Southeastern Tropical Atlantic Ocean off Angola and Namibia feature pronounced variability on interannual time scales with impacts on the marine ecosystem and rainfall over Southwest Africa. Extreme warm and cold events, so called Benguela Niños and Niñas, are typically remotely forced by wind changes in the western equatorial Atlantic and subsequent Kelvin and coastal trapped wave propagation. Local processes such as coastal wind variations, air-sea heat flux anomalies and freshwater anomalies can additionally drive or amplify the events. Freshwater and thereby salinity anomalies, which have only recently been discussed as a local forcing, are mostly related to anomalous discharge of the Congo river.

In this study, we use an extensive in-situ data set in an attempt to quantify the impact that these sea surface salinity variations have on the mixed-layer turbulent heat fluxes and consequently on Benguela Niños and Niñas. We find that the impact occurs via the changes in stratification with fresh anomalies leading to stronger surface layer stratification, which reduces the mixing with cold waters from below, thus enhancing SSTs. Comparing the 1995 Benguela Niño that featured very low salinity values with the 1997 Benguela Niña that was accompanied by high surface salinity, the mixed layer turbulent heat loss was found to be three times lower in the former case than in the latter. In general, interannual variations in surface salinity, dominated by salt advection, strongly impact the heat exchange between the ocean surface and subsurface layer off Angola in early boreal spring when the Congo river discharge is at its seasonal maximum.

How to cite: Lübbecke, J., Aroucha, L. C., and Hummels, R.:  How sea surface salinity variability contributes to ocean turbulent heat fluxes during Benguela Niños and Niñas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1850, https://doi.org/10.5194/egusphere-egu26-1850, 2026.

The western equatorial Atlantic Ocean features a variety of dynamical processes. The upper-ocean circulation is characterized by both western boundary currents and the equatorial current system, which are important pathways of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). On subseasonal to seasonal timescales, equatorial waves further influence local ocean dynamics. Yanai waves (sometimes referred to as mixed Rossby-gravity waves) are observed in all tropical ocean basins on timescales of 10 to 30 days. They are associated with meridional velocities at the equator, setting them apart from other equatorial waves. While Yanai waves have been thoroughly analyzed regarding their energy dissipation, generation mechanisms, and propagation characteristics, little observational evidence has been provided regarding their surface trajectories. This study investigates the trajectories of Lagrangian surface drifters with respect to the presence of Yanai waves. Only few surface drifters remain long enough at or close to the equator to offer insights into equatorial phenomena since the prevailing poleward Ekman flow near the equator typically drives drifters to higher latitudes fairly quickly which makes measurements difficult, but valuable. During a research cruise in May 2023, 8 surface drifters were deployed into the western boundary current system off Brazil along 35°W between the equator and 2.25°S. Three of these drifters became trapped within circling surface velocities centered around the equator which can be attributed to a Yanai wave. One commonly accepted generation mechanism of Yanai waves in the ocean is cross-equatorial fluctuation of the meridional velocity component of the wind. Evidence for fluctuations at the same period as the drifters oscillations was detected in current velocities driven by wind fluctuations. By conducting a series of numerical experiments with artificial drifters, combining the mean background flow of the area with theoretical Yanai wave-induced surface velocities, the observed trajectories can be reproduced. The Yanai wave is characterized by a 14-day period and velocity amplitudes of approximately 0.6 to 0.7 m/s.

How to cite: Andrae, A., Brandt, P., Hummels, R., Tuchen, F. P., and Lübbecke, J.: Observed Yanai wave trajectories in the Equatorial Atlantic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1940, https://doi.org/10.5194/egusphere-egu26-1940, 2026.

The Pacific ENSO and the Atlantic Niño/Niña change oppositely in the 21^st century. Here, we find the weakened Atlantic Meridional Overturning Circulation (AMOC) plays a key role. Via reducing the equatorial Pacific trades and the Atlantic poleward heat transport, the weakened AMOC contributes to, at surface, a similar Niño-like sea surface temperature (SST) warming and a strengthened atmospheric stratification in both basins, while, at subsurface, a western Pacific cooling in comparison to an intense Atlantic warming. The distinct subsurface changes induce strengthened Pacific oceanic stratification to enhance Bjerknes feedback, in contrast to an insignificant change in the Atlantic. Moreover, the similar surface changes exert different impacts, with a strengthened atmospheric stratification suppressing the Atlantic Bjerknes feedback, an influence offset in the Pacific by an eastward shift of deep convection due to Niño-like SST warming. Such offset is absent in the Atlantic owing to the northern-hemisphere-located deep convection.

How to cite: Yang, Y., Wu, L., Cai, W., Cheng, X., Mei, X., Jia, F., Li, S., Geng, T., Chen, Y., and Wang, H.: Weakened Atlantic Meridional Overturning Circulation Contributes to Opposite Responses of ENSO and the Atlantic Niño/Niña to Greenhouse Warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2395, https://doi.org/10.5194/egusphere-egu26-2395, 2026.

Tropical north Atlantic sea surface temperature anomalies (TNA SSTAs) have far-reaching climate impacts both locally and remotely. For the first time, this study reveals pronounced multidecadal variations in the seasonal persistence of spring TNA SSTAs, which is relatively higher at the early and late 20^th century but significantly lower during the middle 20^th century. Contrast to the nonstationary El Niño-Southern Oscillation (ENSO)-TNA connection, these fluctuations in TNA SSTA seasonal persistence are mainly linked to multidecadal shifts in the North Atlantic Oscillation (NAO)-TNA connection. Specifically, the asymmetric impacts of extreme NAO events drive both multidecadal fluctuations in the TNA SSTA seasonal persistence and shifts of NAO-TNA connection. The asymmetric impacts of extreme NAO events are enhanced in historical periods by external forcings and is projected to amplify further under further climate conditions.

How to cite: Yan, X.: Multidecadal variations of the persistence of Tropical North Atlantic sea surface temperature, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4717, https://doi.org/10.5194/egusphere-egu26-4717, 2026.

The tropical Atlantic Ocean is characterized by energetic mean currents, pronounced equatorial wave activity, and enhanced seasonal upper-ocean instability. Variability and extreme events in this region exert a strong influence on weather and climate over adjacent continents and contribute to atmospheric teleconnections with the other tropical ocean basins. Despite this importance, sustained observations of equatorial Atlantic Ocean dynamics remain limited.

To address this gap, a moored observatory has been operated at 0°, 23°W since December 2001, with evolving configurations. In its current design the observatory consists of a subsurface mooring equipped with Acoustic Doppler Current Profilers measuring velocities in the upper 900 m and a McLane Moored Profiler sampling the 900-3,300 m depth range. Additional single-point current meters are deployed at variable depths. These time series are combined with current meter measurements from the nearby PIRATA moored surface buoy and with full-depth shipboard velocity profiles collected during regular French and U.S. PIRATA mooring service cruises.

In this presentation, we provide updated perspectives and new results on the structure, variability and trends of the zonal and meridional velocity components of the equatorial Atlantic circulation. For instance, in the upper ocean, the eastward Equatorial Undercurrent (EUC) exhibits a marked decline in volume transport since 2018, following a decade of strengthening transport and enhanced oxygen ventilation of the eastern equatorial Atlantic between 2008 and 2018. Consistent with this decadal variability, the mean core depth of the EUC has shoaled by approximately 10 m relative to 2018.

Variability in the meridional velocity component is dominated by intraseasonal time scales (~10-50 days), reflecting the presence of tropical instability waves (TIWs) in the upper 100 m, and equatorial Yanai waves at greater depths. While observations suggest an apparent increase in TIW activity, which are on average most pronounced from July to September, a detailed seasonal analysis indicates that the inferred long-term trends largely arise from a systematic shift toward an earlier onset of TIW activity rather than from a sustained intensification.

Since 2022, a combined full-depth velocity product has been released shortly after each mooring recovery (Tuchen et al., 2022). Maintaining long-term observations such as the equatorial moored observatory at 23°W is logistically and financially demanding, but remains essential for detecting and interpreting decadal variability and long-term trends in the tropical Atlantic.

Tuchen, F. P., Brandt, P., Hahn, J., Hummels, R., Krahmann, G., Bourlès, B., Provost, C., McPhaden, M. J., and Toole, J. M. (2022): Two Decades of Full-Depth Current Velocity Observations From a Moored Observatory in the Central Equatorial Atlantic at 0°N, 23°W. Front. Mar. Sci. 9:910979. https://doi.org/10.3389/fmars.2022.910979 

How to cite: Tuchen, F. P., Brandt, P., and Hummels, R.: Equatorial Atlantic Ocean dynamics across time scales from sustained velocity observations between 2001-2025, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5468, https://doi.org/10.5194/egusphere-egu26-5468, 2026.

Upwelling processes bring nutrient-rich waters from the deep ocean to the surface. Areas of upwelling are often associated with high productivity, offering great economic value in terms of fisheries. Thus, predictive skill for regional oceanographic conditions is highly desirable. On this aspect, we analyzed the seasonal prediction of the marine ecosystem drivers such as the net primary production and phytoplankton biomass along the Senegalo-Mauritanian and Moroccan upwelling systems, using the latest version of the German Climate Forecast System GCFS2.2. Our results generally show that the Senegalo-Mauritanian upwelling system is predictable 1 to 4 months in advance during boreal winter, consistent with the sea surface temperature and wind forcing (physical variables). On the other hand, in the Morocco system the effective predictability horizon for the marine ecosystem drivers extends up to 4 months, whereas that for the physical variables hardly reaches one month. Our results highlight the different mechanisms and properties impacting the different predictability horizon one can expect in the Senegalo-Mauritanian and Moroccan upwelling systems.

How to cite: Sylla, A., Brune, S., Capet, X., Mignot, J., and Baehr, J.: Mechanisms and seasonal marine biochemical prediction over the Canary upwelling system using the German Climate Forecast System GCFS2.2, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7349, https://doi.org/10.5194/egusphere-egu26-7349, 2026.

The North Atlantic exhibits pronounced variability on decadal to multidecadal timescales, commonly referred to as Atlantic Multidecadal Variability (AMV). AMV has been linked to climate variability in many regions across the globe. Modelling studies indicate that the global teleconnections of AMV are sensitive to how the tropical branch is represented, though understanding the processes governing its development has received little attention. Moreover, coupled climate models show substantial diversity in simulating the tropical arm of AMV, yet the mechanisms responsible for this inter-model spread remain poorly understood. Here we show that tropical AMV exhibits a seasonal cycle in observations, with growth during boreal summer and decay during boreal winter. Coupled models differ markedly in their ability to capture this observed seasonality. Models that reproduce the observed seasonal evolution of tropical AMV provide insight into its underlying dynamics, revealing that variations in latent heat flux, shortwave radiation, and mixed-layer depth driven by changes in the trade winds are central to the growth and decay of tropical AMV. In contrast, models that fail to represent trade wind weakening and associated ocean-atmosphere interaction processes exhibit substantial deficiencies in simulating tropical AMV. Consequently, models that correctly capture the observed seasonality of tropical AMV also reproduce its associated climate impacts, including variability in Sahel rainfall and the Indian summer monsoon, whereas models that do not capture this seasonality fail to simulate these impacts properly. Given the sensitivity of global climate to tropical AMV, these results highlight the importance of accurately representing the processes linking the extratropical North Atlantic and tropical ocean-atmosphere interactions in coupled climate models.

How to cite: Senapati, B., O’Reilly, C. H., and Robson, J.: Mechanisms of Internal Tropical Atlantic Multidecadal Variability: Seasonality, Model Diversity, and Climate Impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8406, https://doi.org/10.5194/egusphere-egu26-8406, 2026.

The seasonal evolution of the M2 internal tide (IT) in the Senegalo–Mauritanian Upwelling Region is investigated using three years (2014–2016) of ocean simulations. The results reveal pronounced seasonal variability in M2 IT dynamics north and south of Dakar, primarily driven by seasonal stratification and remotely generated ITs propagating from the Cape Verde area (CVA). Seasonal stratification strongly modulates local tide-topography interactions, with stratification during the downwelling season (months 9-12) beneficial to IT generation. In the South Dakar area (SDA), local IT generation and dissipation co-vary seasonally, featuring an IT energy source. However, the seasonal dissipation is not directly linked to local generation in the North Dakar area (NDA). This contrasting seasonality suggests a strong influence of remotely generated IT from the CVA, which can seasonally penetrate onshore into the NDA, leading to enhanced dissipation during the upwelling season (months 1-4), and reduced dissipation during the relaxation season (months 5-8). Besides, interannual IT variabilities, mesoscale eddies, and seasonal circulation can further complicate the interpretation of coastal seasonal variability. These results highlight the combined effects of seasonal stratification, circulation, and remote IT propagation, playing a crucial role in modulating coastal IT dissipation and mixing across the Senegalo-Mauritania Upwelling Region.

How to cite: Huang, H., Brandt, P., J. Greatbatch, R., Zeng, Z., and Chen, X.: Seasonality of Internal Tide Dynamics in the Senegalo-Mauritanian Upwelling Regions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9571, https://doi.org/10.5194/egusphere-egu26-9571, 2026.

Boreal spring and summer tropical Atlantic variability is governed by the Atlantic Meridional Mode (AMM) and Atlantic Zonal Mode (AZM) at interannual timescales. Previous studies have identified a connection between AMM and AZM through ocean wave propagation and local wind forcing (Foltz and McPhaden 2010; Burmeister et al. 2016; Martín-Rey and Lazar 2019; Martín-Rey et 2023). For a positive phase of the AMM, anomalous negative wind curl north of the equator triggers a downwelling Rossby wave that propagates westward and is boundary-reflected into an equatorial Kelvin wave (KW). This dKW travels along the equator during summer months, activating the oceanic processes responsible to warm the sea surface, thus favouring the development of a positive AZM. However, the existence of anomalous local zonal winds could modulate the impact of the dKW, and consequently, the phase of the AZM following the AMM (Martín-Rey and Lazar 2019; Martín-Rey et al. 2023).

Here, we use the Maximum-Covariance based Python statistical tool Spy4Cast to explore the existence of a AMM-AZM connection, as well as, the relative role of each precursor (local wind vs oceanic waves). Spy4cast (Durán-Fonseca and Rodríguez-Fonseca 2025)  allows for identifying coupled modes of variability as well as to produce statistical predictions. In this way the AZM predictability will be assessed together with the stability of the connection. Thus, the non-stationary behaviour of this connection will be evaluated, as well as the favourable background conditions for each type (positive or negative) of AMM-AZM interaction. Observational datasets, and long-simulations from PIcontrol and historical CMIP6 simulations will be used.

How to cite: Martín-Rey, M., Rodríguez-Fonseca, B., Losada, T., and Polo, I.: Precursor mechanisms and multidecadal modulation of the Atlantic Meridional Mode-Atlantic Zonal Mode connection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10610, https://doi.org/10.5194/egusphere-egu26-10610, 2026.

El Niño–Southern Oscillation (ENSO) is a major driver of climate variability in the North Atlantic through atmospheric teleconnections that influence circulation patterns and climate conditions across the Euro-Atlantic region. These teleconnections constitute an important source of interannual climate variability and predictability. Previous studies suggest that, under greenhouse-warming scenarios, ENSO teleconnections to the North Atlantic are likely to intensify, largely as a result of changes in the mean climate state and projected modifications in ENSO characteristics.

In this study, we show that changes in ENSO–North Atlantic teleconnections under global warming have significant implications for seasonal climate predictability over Europe. Our results show that a strengthening and reorganization of these teleconnections alters the robustness and spatial coherence of ENSO-related climate signals, thereby directly influencing predictability in the Euro-Atlantic sector.

In addition, we explore the potential role of the Tropical North Atlantic as a modulator of ENSO teleconnections to Europe. Previous studies suggest that variability in Tropical North Atlantic sea surface temperatures can influence the atmospheric response to ENSO, potentially modifying its impact on the North Atlantic and European climate. Our results support the idea that the Tropical North Atlantic may play an important role in shaping ENSO-related climate signals and their predictability. 

How to cite: Santuy Muñoz, D., Polo Sánchez, I., and Rodríguez Fonseca, B.: ENSO teleconnection to the North Atlantic under greenhouse warming and its modulation by the Tropical North Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12276, https://doi.org/10.5194/egusphere-egu26-12276, 2026.

Atmospheric cold pools are transient sub-mesoscale to mesoscale weather phenomena generated by convective precipitation. They are characterized by cool, dense air spreading laterally, which strongly alters near-surface atmospheric conditions and the thermohaline properties of the ocean skin layer (< 1 mm) and near-surface layer (NSL: < 1 m). The spatial extent and thermodynamic intensity of atmospheric cold pools are moderated by mixing within the moist marine boundary layer (MBL), the lowest ~0.5–1 km of air above the ocean. Despite this moderation, accompanying wind surges and intense precipitation drive short-lived intensifications of atmosphere–ocean fluxes that can abruptly restructure upper-ocean thermohaline structure, an aspect that remains poorly constrained by in situ measurements and is underrepresented in coupled atmosphere–ocean models.

Here we present observations of atmospheric cold pool passages over the tropical Atlantic Ocean, combining ship-based meteorological measurements, aerial drones, weather balloon profiles, and high-resolution observations from the autonomous surface vehicle HALOBATES. Equipped with meteorological and oceanographic sensors sampling at high-resolution (1 Hz), HALOBATES resolves the ocean skin layer and the NSL dynamics, allowing us to identify and characterize cold pools as they passed over the study area during the R/V METEOR M211 field campaign (14 June–27 July 2025).

Between 13 and 18 July 2025, cold pool passages were identified by rapid air-temperature drops of 0.6–1.8 °C and, during rainy events, rainfall rates of up to 37 mm h⁻¹. All cold pools induced transient ocean surface cooling, leading to sea surface temperature anomalies (T_skin-T_NSL) of 0.02 – 0.35 °C, and amplified cooling in the skin layer relative to the NSL. To quantify the cooling and recovery of the ocean skin layer following the cold pool passage, we consider two different skin-layer depths: the infrared (IR) skin layer observed by thermal cameras and the skin layer measured in situ by HALOBATES. This comparison shows that skin-temperature responses evolve on timescales of only a few minutes and therefore require high temporal resolution in situ measurements to be adequately resolved.

Salinity responses depended critically on precipitation: cold pools that passed the study area without measurable rainfall produced negligible changes, whereas intense-rain events freshened the skin by up to 1.3 g kg^-1 and the NSL by 0.4 g kg^-1, forming shallow freshwater lenses that re-stratified the upper meter within approximately 15 minutes. Pronounced cold pool passages were accompanied by enhanced latent and sensible heat fluxes, with maximum increases of −140 W m⁻² and −30 W m⁻², respectively, driven primarily by increased wind speeds and indicating intensified ocean-to-atmosphere heat exchange. These observations demonstrate that cold pools strongly affect short-term variability in upper-ocean thermohaline structure through short-lived intense peaks in atmosphere–ocean fluxes, emphasizing the need to include these transient events in future coupled atmosphere–ocean models.

How to cite: Bibi, R., Jaeger, L., Rauch, C., Mintah Ayim, S., Gassen, L., Ribas-Ribas, M., W. Freeman, S., Britton, K., D. Grant, L., M. Falk, N., A. Neumaier, C., van den Heever, S., L. Monroy, D., Härter, J., and Wurl, O.: Cold Pools Drive Short-Term Thermohaline Variability in the Ocean Skin Layer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14270, https://doi.org/10.5194/egusphere-egu26-14270, 2026.

Using seasonal predictions from the ECMWF SEAS-5 System, we analyze the relationship between ENSO and upwelling indices in the regions of Benguela.

We find that reanalysis show correlations between El Niño and the Southern Oscillation phenomenon (ENSO) and upwelling indices in February-March-April. The system performs a very accurate prediction in the Tropics of the Sea Surface Temperature (SST) and wind stress variables leading to a good simulation of El Niño, but to a less realistic simulation of Benguela upwelling.

The simulation of the connection between ENSO and Benguela depends on the latitude studied: Northern Benguela Eastern Boundary Upwelling System (EBUS) is more related with ENSO than Southern Benguela EBUS.

There are also changes in the predictability depending on the period of study. We focus in two periods: 1981-1998 (P1) and 1999-2016.  In SEAS-5 there is a clear relationship between Northern Benguela Upwelling System and ENSO that appears to be consistent and similar for the two periods, while in observations the relation appears to be robust only after the year 2000’s.  This result highlights the importance of taking into account the impact of changes in the background state on predictability.

How to cite: Losada, T., Martín-Gómez, V., Rodríguez-Fonseca, B., Polo, I., and Martín-Rey, M.: SEAS5 skill of ENSO impact on Northern Benguela Coastal Upwelling Systems , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14892, https://doi.org/10.5194/egusphere-egu26-14892, 2026.

El Niño-Southern Oscillation (ENSO) affects weather and climate around the world but its impact on the equatorial Atlantic has been surprisingly inconsistent, with some major El Niño events followed by cooling in the equatorial Atlantic, while others were followed by warming. Here we use climate change projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to examine how ENSO’s relation with the equatorial Atlantic may change under global warming.

Similar to ENSO, variability in the equatorial Atlantic, also known as the Atlantic Zonal Mode (AZM), is influenced by the Bjerknes feedback, in which sea-surface temperature (SST) anomalies drive deep convection and a surface wind response that amplifies the original SST anomalies. Due to the stabilization of the atmosphere under global warming, this Atlantic feedback weakens in the CMIP6 projections. Instead, many models suggest a scenario in which the AZM is dominantly driven by ENSO’s thermodynamic forcing, namely the tropics-wide tropospheric warming (cooling) that follows El Niño (La Niña) events. As a result, ocean dynamics and coupled air-sea feedbacks play a much weaker role, while ENSO’s influence on the AZM becomes more consistent. Analysis with a simple linear prediction scheme suggests that this can also increase the predictability of the AZM, due to the high predictability of ENSO.

While many models envision an ENSO-dominated future, some continue to simulate an independent, dynamically driven AZM until the end of the 21st century. The potential reasons for this disparate behavior will be discussed.

How to cite: Richter, I., Chang, P., Kataoka, T., Kido, S., Noel, K., Kosaka, Y., Okumura, Y., Tokinaga, H., Tozuka, T., and Vilela, I.: Changes in the ENSO-AZM connection under global warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15698, https://doi.org/10.5194/egusphere-egu26-15698, 2026.

Historical Coupled Model Intercomparison Project Phase 6 (CMIP6) model outputs are analyzed to evaluate models' ability in simulating the seasonal and interannual variability in the South-Eastern Atlantic Ocean. The study focuses on SST in February-March-April, the main season of occurrence of the extreme interannual warm and cold Benguela Niño-Niña events. 

In the Angola-Benguela-Area (ABA), the CMIP6 SST ensemble-mean resembles the seasonality of the observations but with a strong bias. Unlike the seasonal cycle, the SST interannual variability in CMIP6 ensemble-mean is underestimated and occurs 3-4 months after the peak of maximum variability in the observations (June/July). Among the model ensemble, 2 groups of models emerge: a group featuring a maximum interannual variability in the right location (ABA) but delayed by about 2/3 months compared to the observation (~60% of the models); a group featuring the maximum variability in the right season (FMA) but in a location shifted southward in the South-Benguela (~30% of models). For the first group, results suggest that the time-shift of the peak in the SST variability is induced by the time-shift of the equatorial zonal wind stress. For the second group results show that the latitudinal-shift of the peak in SST variability is controlled by intense coastal wind activity in the south Benguela rather than by model bias and a southward shift in the position of Angola-Benguela-Frontal Zone. 

Finally, we examined the models’ ability in simulating extreme interannual Benguela Niño-Niña events. Very few individual models correctly simulate the phenology of the Benguela events, including the modulation of the equatorial zonal and coastal winds that drive development in the preceding months. Interestingly most of the best models have in common a fairly good representation of the South-Atlantic High-pressure system.

How to cite: Bachélery, M. L., Keenlyside, N., and Koseki, S.: Evaluation of Benguela Niño/Niña events in the CMIP6 historical simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18312, https://doi.org/10.5194/egusphere-egu26-18312, 2026.

The Benguela Niño/Niña events manifest the anomalous signals of sea surface temperature (SST) in the upwelling region off the west coast of Africa. These events are triggered by the interannual modulation of either equatorial waves or local atmospheric forcing. In the present study, the mechanism that equatorial waves induce the coastal SST anomaly is investigated in terms of the transfer episodes of wave energy by both numerical experiments and reanalysis data. The result of numerical experiments suggests that most of the coastal events can be reproduced by subseasonal wind forcing with an interannual modulation that excites oceanic waves of the first three baroclinic modes. The transfer routes of wave energy illustrate the role of wave dynamics that explains how the interannual variability of SST in the equatorial Atlantic is connected with that in coastal regions. The linearly superposed sign-indefinite potential energy flux owing to waves manifests its capability of sufficiently displacing the thermocline so as to trigger the coastal events. The diagnosis of wave energy for reanalysis data further confirms that there are clear wave energy routes from the equatorial Atlantic to the coastal region, along which different source regions for waves in the first-four modes are found, jointly contributing to the 2019 Niño event.

How to cite: Song, Q., Tang, Y., and Aiki, H.: The Role of Equatorially Forced Waves in Triggering Benguela Niño/Niña as Investigated by an Energy Flux Diagnosis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18445, https://doi.org/10.5194/egusphere-egu26-18445, 2026.

How to cite: Bellacanzone, F. and Bordoni, S.: South Tropical Atlantic and South Atlantic Convergence Zone Actively Shape ENSO Diversity: Physical Pathways, Subsurface Modulation, and Long-Lead Prediction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19537, https://doi.org/10.5194/egusphere-egu26-19537, 2026.

How to cite: Cavucci, V.: How Do Source Water Depths Respond to Wind and Stratification Variability in the Northwest African Canary Upwelling System? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20006, https://doi.org/10.5194/egusphere-egu26-20006, 2026.

Medicane Daniel, which formed during 4 - 12 September 2023, has stood out as the deadliest recorded storm in Mediterranean history. In this study, we investigate the role of oceanic features in contributing to the intensification of Medicane Daniel. Our findings reveal the presence of a warm core eddy (WCE), high ocean heat content (OHC), and a marine heatwave (MHW) at the location where Medicane Daniel intensified. These features were situated near the coastal region, facilitating the Medicane’s intensification close to the coast. Consequently, the Medicane did not weaken significantly after landfall, leading to severe damage along the coast of Libya. These conditions favoured the intensification of the Medicane and, owing to high moisture convergence, contributed to significant precipitation over the eddy and MHW regions. Observations further indicate that the total water column, low-level vorticity, and humidity at 850 hPa were elevated at the intensification location, reinforcing the Medicane’s intensification and associated heavy precipitation over the eddy and MHW region. Importantly, observations from the high-resolution SWOT satellite captured the WCE more accurately and in finer detail, enabling attribution of changes in biogeochemical properties, namely chlorophyll, phytoplankton, nutrients, and dissolved oxygen concentrations, to eddy-induced vertical mixing and upwelling. The biogeochemical properties tend to increase over the WCE and MHW locations due to enhanced mixing and upwelling associated with these oceanic features. Our case-study analysis suggests that under cyclone conditions, upwelling driven by Ekman pumping may play a more prominent role within WCEs in driving chlorophyll enhancement.

Key Words: Medicanes, Chlorophyll Concentration, Marine Heat Wave, SWOT observations, warm core eddy, Ekman pumping 

How to cite: Jangir, B. and Strobach, E.: Interaction Between a Medicane and the Mediterranean Sea: Sea Surface Temperature Anomalies Along the Path of Medicane Daniel, the Deadliest Mediterranean Cyclone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20748, https://doi.org/10.5194/egusphere-egu26-20748, 2026.

The influence of equatorial Atlantic SST on the El Niño Southern Oscillation (ENSO) has remained robust over the 40 years, despite the strong weakening of Atlantic Niño variability post 2000. To investigate the nature of these interactions through pacemaker experiments with the Norwegian Earth System Model (NorESM). In these experiments tropical Atlantic sea surface temperatures (SST) are restored to observations over the period 1980 to 2020. We perform two experiments, one with long-term warming included and one with it removed linearly. Each experiment consists of 20 ensemble members, sampling internal variability, model uncertainty (NorESM1/NorESM2), and nudging approach (anomaly vs full field restoring). Our results show first that equatorial Atlantic SST variations in the west determine the impact on the ENSO, rather than those in the east. And second, that the long-term warming of the tropical Atlantic SST has opposed this interaction. While the first effect has maintained the robust connection during the last 40 years, we expect the second effect to dominate in the long-term, leading to weaker Atlantic Niño impacts on the Pacific.

How to cite: Chiu, P.-G. and Keenlyside, N.: Disentangling equatorial Atlantic’s influence on ENSO since 1980, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22469, https://doi.org/10.5194/egusphere-egu26-22469, 2026.

The NASA–ISRO SAR (NISAR) mission will provide the first spaceborne dual-frequency (L- and S-band) radar observations of polar regions, which is capable of acquiring fully-polarimetric acquisitions. While L-band polarimetric capabilities are relatively well understood from previous missions and airborne campaigns, the S-band component represents a novel observational opportunity whose potential for sea ice characterization remains largely unexplored. Because of the fast changing sea ice regime in the Arctic, there is a growing need to understand what observational capabilities emerging synthetic aperture radar (SAR) system can offer for sea ice monitoring.

This study leverages very high resolution airborne UAVSAR imagery at fully-polarimetric mode from winter and summer seasons in the Beaufort Sea to investigate how dual-frequency SAR can provide separability of thinner sea ice classes over the annual thermodynamic cycle. We examine SAR backscatter and a range of parameters derivable from fully-polarimetric data to assess their utility in ice type discrimination.

Given S-band's intermediate wavelength between L-band and C-band, we anticipate distinct scattering behavior arising from its sensitivity to ice properties at scales different from those of established frequencies. This study aims to characterize how L- and S-band respond to varying ice conditions across seasons and to explore whether the two frequencies offer complementary information for ice classification. Also, we would like to develop a ranking for most efficient parameters from separability matrices. These findings will inform the development of sea ice monitoring frameworks for the imminent NISAR era.

How to cite: Mahboob, M. and Mahmud, M.: Fully polarimetric dual-frequency radar monitoring of Arctic sea ice, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-458, https://doi.org/10.5194/egusphere-egu26-458, 2026.

Lake ice cover is a key component of the Earth’s cryosphere system. The absence of lake ice can have important implications for local energy budgets and influence the occurrence of extreme weather events, such as lake-effect snow systems. Additionally, ice formation is critical in the establishment of ice road transportation networks in areas such as Northern Canada, which allow for the transportation of goods and people during winter months. Within the WMO’s Global Climate Observing System (GCOS) Lakes Essential Climate Variable (ECV), lake ice has been identified as a key indicator for monitoring climate change. While long-term ground-based records of lake ice exist, the quantity of these measurements has declined, and over the last two decades, remote sensing has become increasingly relied on to provide information about global lake ice conditions. This is reflected in the multitude of operational lake ice products now available, including: the Multisensor Snow and Ice Mapping System (IMS), the MODIS Snow and Ice Cover, ESA Lakes CCI+ Lake Ice, and the Copernicus Land Service Lake Ice Extent products. These products exhibit different advantages and disadvantages related to the quality of retrievals, number of lakes/spatial resolution, and temporal coverage, which can limit their application for real-time monitoring or understanding of changes in lake ice conditions for smaller lakes.

This presentation will showcase a new operational product, specifically focused on providing daily lake ice coverage for lakes in Canada larger than 2.25 km². The product is adapted from the processing chain utilized for the generation of the ESA CCI+ Lake Ice Cover Product but includes data for more than 36,000 lakes (500 m grid). The product is derived from over 1.5 PB of MODIS optical data and captures variation in the ice coverage during the two most recent decades (2000 – 2023). This presentation will describe and discuss the general trends and spatial patterns in lake ice cover across Canada, with connections to recent temperature trends. Additionally, an application of the product for monitoring ice roads will be highlighted by showcasing how the resolution of the product can be used to evaluate the timing of ice cover for lakes along key identified ice routes, such as the Tibbit to Contwoyto, Wekweèti, and Gamèti winter roads.

How to cite: Murfitt, J. and Duguay, C.: Canadian Lake Ice Cover in the Early 21st-Century, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2593, https://doi.org/10.5194/egusphere-egu26-2593, 2026.

Since the previous Japanese Arctic projects (ArCS and ArCS II projects), we have accumulated and evaluated scientific data, providing knowledge to the Joint Program for Scientific Research and Monitoring (JPSRM) established under the Central Arctic Ocean Fisheries Agreement (CAOFA). This knowledge could form the basis for conserving and utilizing marine resources. Here, contributions of the Japanese Arctic studies to the CAOFA JPSRM Implementation Plan are presented. During the 2020 R/V Mirai cruise in collaboration with the Synoptic Arctic Survey (SAS), unusually low oxygen and acidified water was found on the Chukchi Borderland (CBL), a high-seas fishable area of the Pacific Arctic. The integrated SAS data suggested that Beaufort Gyre shrinkage and Atlantification triggered a frontal northward flow along the CBL that transported the low oxygen and acidified water from the shelf-slope north of the East Siberian Sea. Therefore, the CBL area should be monitored as a bellwether of ecosystem degradation caused by ocean acidification and deoxygenation in the Central Arctic Ocean. This finding was cited in the CAOFA JPSRM Implementation Plan, and is proposing a most urgent monitoring site, which is at risk of ocean acidification and deoxygenation, in the Agreement Area. Future contributions to the CAOFA JPSRM are expected through the scientific surveys that will be conducted by the Japanese new icebreaker, Arctic Research Vessel (ARV) Mirai II.

How to cite: Nishino, S.: Japanese Arctic projects’ contributions to the Central Arctic Ocean Fisheries Agreement, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2619, https://doi.org/10.5194/egusphere-egu26-2619, 2026.

The Arctic Ocean experienced severe acidification in the subsurface layer owing to the increased invasion of Pacific Winter Water. However, the recent development and its control mechanisms remain unclear. Here we show that the subsurface acidifying waters (aragnite undersaturation) have further expanded northward, while it has been significantly thinned and shallowed in the western Canada Basin since 2015, in contrast to the thickening and deepening of the acidifying waters during 1994-2015. In the Northern Canada Basin, the more acidic waters in subsurface were also expanded substantially, which is mainly enhanced by the local enhancement of primary production in surface layer due to rapid sea ice loss.

How to cite: Li, C.: The impact of climate change on Arctic acidification, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2981, https://doi.org/10.5194/egusphere-egu26-2981, 2026.

The presence of sea ice at the coast prevents coastal erosion of permafrost in the Arctic most of the year. Decreasing sea ice due to climate change will extend the erosion season. We examine coastal ice presence in a single model large ensemble future projection under the SSP3.70 scenario. We compare sea ice presence against locations with favourable geological conditions for erosion. The Arctic Ocean circulation pattern determines whether nutrients from erosion will be retained in the Arctic or end up the North Atlantic. Both sea ice and circulation conditions depend on the large scale atmospheric pattern, therefore we examine the correlation between high erosion conditions and enhanced outflow conditions. The contributions of Rynders and Aksenov were supported by the National Capability Multicentre Round 2 funding from the Natural Environment Research Council (BIOPOLE grant no. NE/W004933/1 and CANARI grant no. NE/W004984/1).

How to cite: Rynders, S. and Aksenov, Y.: Potential for future erosion in the Arctic in the CANARI HadGEM3 large ensemble, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3566, https://doi.org/10.5194/egusphere-egu26-3566, 2026.

Observational evidence indicates enhanced ocean convection near the sea ice edge in the Nordic Sea. However, whether sea ice retreat in a warming climate would further strengthen convection remains uncertain. Using a high-resolution climate model and comparing it with a low-resolution version, we find a more pronounced future increase in convection in the Arctic Ocean. A key reason for this difference is that at higher resolution, resolved boundary currents transport high-density Atlantic water more efficiently toward high latitudes along the ocean boundary. As sea ice retreats and low-density freshwater input diminishes, the high-density water can no longer subduct. Meanwhile, surface currents strengthen more than deeper currents, and the resulting shear weakens stratification before heat loss occurs. Our study suggests that future ocean convection and ventilation in the Arctic Ocean may be stronger than present projections from low-resolution models indicate.

How to cite: Gou, R., Guo, H., Liu, Y., Lohmann, G., and Wu, L.: Resolved boundary currents at high resolution contribute to stronger future ocean convection in the Arctic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4435, https://doi.org/10.5194/egusphere-egu26-4435, 2026.

The Beaufort Sea has experienced significant sea ice retreat in recent decades, driven by both thermodynamic and dynamic processes. This study investigates the drivers and predictability of summer sea ice retreat in the Beaufort Sea by integrating an ocean–sea ice model with satellite-derived sea ice concentration data and atmospheric reanalysis products. Model diagnostics from 1994 to 2019 reveal that thermodynamic processes dominate annual sea ice loss (approximately 90%), with vertical heat flux accounting for roughly 85% of total oceanic heat input. The summer sea ice minimum area and the day of opening, derived from either model results and satellite observations, have a strong correlation with R² = 0.60 and R² = 0.77, respectively, enabling regression equations based solely on remote sensing data. Further multiple linear regression incorporating preceding winter (January to April) accumulated temperature and easterly wind yields moderately robust forecasts of minimum sea ice area (R² = 0.49) during 1998–2020. Additionally, analysis of reanalysis wind data shows that the timing of minimum sea ice area is significantly influenced by the frequency and intensity of sub-seasonal easterly wind events during melt season. These results demonstrate the critical importance of remote sensing in monitoring Arctic sea ice variability and enhancing seasonal prediction capability under a rapidly changing climate.

How to cite: Zheng, Z., Nie, H., Wei, S., Zhao, W., and Luo, X.: Study on the Mechanisms and Predictability of Beaufort Sea Ice Retreat: Insights from Ocean-Ice Model and Remote Sensing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4497, https://doi.org/10.5194/egusphere-egu26-4497, 2026.

Under ongoing global warming, Arctic sea-ice extent and thickness have declined markedly in recent decades, while open-water area has expanded. One immediate consequence is an intensification of wave activity due to enlarged fetches, which in turn strengthens wave–ice interactions. Compared with the understanding of ice impacts on waves, the response of sea-ice evolution to wave effects remains less well constrained—particularly from a basin-scale, climatological perspective. Here, using the Los Alamos sea-ice model (CICE), we incorporate four wave-related processes — Stokes drift, wave radiation stress, wave-induced mixing, and wave-induced fracture — and conduct a 10-yr simulation to quantify both the individual and combined impacts of these processes on Arctic sea-ice states. The results show that there are seasonal variations in the influence of ocean waves on sea ice concentration. However, the variations in modeled sea ice volume are not always the same for each year both qualitatively and quantitatively. In terms of the degree of influence, wave-induced ice fracture has the strongest influence on summer, autumn, and annual average sea ice concentration. But for specific periods and sea areas, the mechanism with the greatest influence may vary.

How to cite: Li, J. and Wang, Y.: The Influences of Surface Waves on the Modeling of Arctic Sea Ice, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4781, https://doi.org/10.5194/egusphere-egu26-4781, 2026.

The climate system is approaching dangerous tipping points, with the potential collapse within decades of critical components such as the Greenland Ice Sheet and the North Atlantic Subpolar Gyre posing severe risks to European weather and global climate stability. Changes in Arctic freshwater, driven by changes in ice-sheet and sea-ice melt and atmosphere-ocean-ice interactions, play a central role in these risks by influencing ocean stratification, deep water formation and air-sea fluxes. Despite the urgent need for early warnings, major gaps remain between existing observations and the data required to constrain predictive models, limiting confidence in future projections.

Earth-orbiting satellites and in situ observations provide essential information on large-scale ocean, cryosphere, and atmosphere change, but they struggle to capture fast processes at kilometre and sub-kilometre scales in complex regions such as marginal ice zones. A different type of observations is needed to quantify the role of these processes in exchanges of freshwater, heat, and momentum that the Arctic and the Greenland Ice Sheet to the North Atlantic Subpolar Gyre.

This paper will introduce AEROSTATS (Aerial Experimental Remote sensing of Ocean Salinity, heaT, Advection, and Thermohaline Shifts), a UK-led international project designed to demonstrate a new approach to long-term, low-cost, low-carbon monitoring of Arctic freshwater processes in Greenland’s dynamic ocean–ice margins. AEROSTATS focuses on innovative airborne platforms capable of remote, high-resolution imaging of total surface current vectors, near-surface winds, sea surface salinity, ocean colour, and sea surface temperature at 1-10km and sub-daily scales.

Funded as a high-risk, forward-looking project, AEROSTATS seeks to collect and integrate data from new airborne instruments, in situ surface and subsurface platforms, spaceborne sensors, and high-resolution reanalyses and models. A core element is a 2028 year-round field campaign in the Greenland/Subpolar Gyre region deploying airborne systems to observe freshwater-driven processes across seasons. By combining multi-platform observations with models and reanalyses using digital tools such as machine learning and digital twins, AEROSTATS aims to establish new long-term monitoring capability to substantially improve early warning for freshwater-related tipping points.

How to cite: Gommenginger, C., Martin, A. C. H., McCann, D., Marquez Martinez, J., Lavender, S., Lichtman, D., Buckingham, C., Marzocchi, A., Prime, T., Clément, L., Josey, S., and Grist, J.: Towards an Early Warning System for Arctic Freshwater-Driven Tipping Points in the Greenland Ice Sheet and North Atlantic Subpolar Gyre with AEROSTATS, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5839, https://doi.org/10.5194/egusphere-egu26-5839, 2026.

The Arctic Ocean has changed rapidly over recent decades as sea ice has declined, freshwater inputs have increased, and atmosphere–ocean coupling has evolved. Yet the resulting basin-scale variability in ocean mass remains poorly constrained because in situ observations are sparse and satellite altimetry is limited at high latitudes. Here we use satellite gravimetry from GRACE and GRACE-FO to quantify Arctic Ocean bottom pressure (OBP) variability from 2002 to 2024, providing a direct measure of mass redistribution that is independent of steric effects. To isolate dynamic ocean signals, we remove land leakage and eustatic contributions using a forward-modeling framework that accounts for self-attraction and loading. Empirical Orthogonal Functions (EOF) analysis of the residual OBP field reveals a leading-mode dipole pattern, with increasing ocean mass in the Beaufort Gyre region within the Canadian Basin and decreasing mass in the Kara Sea and Barents Sea. The corresponding principal component shows a robust strengthening over the two-decade record. While the underlying physical mechanisms warrant further investigation, the overall dipole structure is consistent with recent modeling studies suggesting intensified surface circulation under continued sea-ice loss. Overall, GRACE-derived ocean mass captures coherent, multi-decadal Arctic circulation change in a warming climate.

How to cite: Chae, Y., Seo, K.-W., and Nam, S.: A strengthening dipole pattern in Arctic Ocean dynamics observed by GRACE/GRACE-FO, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6365, https://doi.org/10.5194/egusphere-egu26-6365, 2026.

The Arctic region is undergoing a rapid and intense transformation driven by global climate change. Paradoxically, this coincides with a generalized decline in the density of hydrometric stations, resulting in fragmented spatiotemporal river discharge time series data that are insufficient to capture the complexity of ongoing dynamics. Under these challenging context, Arctic basins play a crucial role in regulating the freshwater budget, influencing ocean circulation and sea ice formation, and acting as a "litmus test" for the hydrological cycle's response to warming.

To address the scarcity of in-situ data, our work aims to provide continuous river discharge and runoff estimates at a daily scale and 0.25° spatial resolution for the entire continental Pan-Arctic region over the period 2003–2022. We employ STREAM model (SaTellite based Runoff Evaluation And Mapping; Camici et al., 2022), a semi-distributed conceptual hydrological model forced exclusively by temperature data and satellite observations, including precipitation, soil moisture, snow cover fraction, and Terrestrial Water Storage (TWS) anomalies from GRACE (Gravity Recovery and Climate Experiment) and its Follow-On mission (GRACE-FO). The integration of gravimetry data represents a key innovation—particularly relevant in the context of the future NGGM-MAGIC (Next-Generation Gravity Mission / Mass-change And Geophysics International Constellation) mission—as it enhances the characterization of hydrological processes in cold regions where TWS changes significantly drive river discharge and runoff variability. The model was first calibrated on 15 "donor" Arctic basins, achieving a median Kling-Gupta Efficiency index (KGE) of 0.80. To cover ungauged areas, we developed a regionalization framework based on aridity-index clustering, extending estimates to the entire Pan-Arctic domain. The resulting dataset was independently validated against 26 gauging stations and benchmarked against existing reanalysis products.

Results demonstrate that the regionalized model faithfully reproduces discharge seasonality and interannual variability over 70% of the Pan-Arctic area. Furthermore, trend analysis reveals statistically significant runoff trends in 18% of the domain, highlighting that the Pan-Arctic does not exhibit a uniform response to climate change, but rather diverse, localized reactions.

This work provides a consistent hydrological baseline based solely on satellite data, filling the gaps left by fragmented in-situ river discharge monitoring networks and offering a robust tool to investigate the interactions between climate change and hydrological extremes in the Pan-Arctic region, a critical climate hotspot.

How to cite: Leopardi, F., Saltalippi, C., Dari, J., Brocca, L., Saemian, P., Sneeuw, N., Tourian, M., and Camici, S.: Quantifying Pan-Arctic Freshwater Fluxes: A 20-Year Satellite-Based Daily River Discharge and Runoff Dataset, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6981, https://doi.org/10.5194/egusphere-egu26-6981, 2026.

Mesoscale eddies are widespread in the Arctic Ocean affecting circulation, stratification, the transport of heat and salt, and consequently sea ice melt. We detect and track coherent eddies in a 1-km resolution Arctic Ocean simulation using the unstructured-mesh Finite volumE Sea ice-Ocean Model (FESOM2). Their spatial and seasonal distributions are analyzed, and quasi-3D eddy composites are used to quantify their influence on the water column, surface heat fluxes, and sea ice.

Eddy formation is highest along topographic features and the boundary current, with eddy sizes roughly corresponding to the local Rossby radius. Anticyclonic eddies are larger and more energetic than cyclonic eddies and can lift warm, saline Atlantic Water toward the surface, which increases the vertical heat flux and can cause localized basal sea ice melt. Cyclonic eddies, by contrast, mainly transport cold surface water downward and have little impact on the surface heat budget or sea ice. Edd-induced anomalies are strongest in Fram Strait, weaken downstream, and are larger beneath pack ice than in the marginal ice zone. These results are consistent with an eddy-ice pumping mechanism, where ocean-sea ice stress enhances vertical transport and contributes to eddy decay. Overall, the analysis shows that mesoscale eddies play an important role in the vertical exchange of heat in the Eurasian Arctic making them an important factor in the ongoing Atlantification of the Arctic Ocean. Since the role of eddies is expected to become even more important in the future, adequately representing them in model simulations will be necessary, despite the high resolution and computational cost required to resolve them.

How to cite: Müller, V., Danilov, S., Jung, T., Koldunov, N., and Wang, Q.: Properties and Effects of Mesoscale Eddies in the Eurasian Basin of the Arctic from a Model Simulation at 1-km Resolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7485, https://doi.org/10.5194/egusphere-egu26-7485, 2026.

With much focus on the local and regional changes in the ocean and marine biogeochemistry, high-resolution ocean-sea ice models became a widely desired tool for ocean research.

We present analysis of the analysis of the kilometric scale resolution regional ocean model for the Arctic and North Atlantic developed at the National Oceanography Centre and aimed at resolving mesoscale circulation. The model ARC36 features NEMO ocean model coupled to the SI3 sea ice model [1].

To assess model performance and demonstrate its applicability to the regional assessments, the two case studies has been chosen: (1) the Western Fram Strait and the East Greenland Shelf and (2) the Svalbard. For these areas eddy dynamics has been analysed and compared to the mooring data and satellite imagery. We have also assessed exchanges between the fjords, the shelf and the open ocean.  The simulations are evident of fine structure in ocean currents and eddies, more detailed than in coarser resolution models. The model also simulates sea ice break-up at spatial scales from a few kilometres to several hundreds of kilometres, suggesting the usability of continuum sea ice models at high-resolution.

This work is funded by the Natural Environment Research Council (NERC) LTS-M Programmes CANARI (NE/W004984/1) and BIOPOLE (NE/W004933/1), by UKRI/NERC HighLight Topic Projects “Interacting ice Sheet and Ocean Tipping – Indicators, Processes, Impacts and Challenges (ISOTIPIC)”, by the grant NE/Y503320/1, and by the Met Office Advancing Arctic meteorological and oceanographic capabilities & services project, which is supported by the Department for Science, Innovation & Technology (DSIT), and uses the ARCHER2 UK National Supercomputing Service (https://www.archer2.ac.uk).

References

[1] 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.

How to cite: Aksenov, Y., Rynders, S., de Steur, L., James, H., Barton, B., Coward, A., Polton, J., and Blockley, E.: Ultra-high-resolution ocean-sea ice model: a new capability to simulate shelf-ocean processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7957, https://doi.org/10.5194/egusphere-egu26-7957, 2026.

The launch of the altimetric satellite SWOT (Surface Water and Ocean Topography) was a revolution in oceanography and hydrology. With its120 km swath width, a spatial resolution of 500m² (in Low Resolution acquisition mode) and an instrumental random error significantly lower than the one from nadir altimetry, the Ka-Band Radar Interferometer (KaRIn) onboard SWOT mission also present a huge potential to develop applications in the polar regions. Indeed, the SWOT product  enables the observation of leads, icebergs and polynyas (Dibarboure and al 2024) through the measures of surface topography and backscatter coefficient.

The surface discrimination between leads and floes is the first step toward polar ocean monitoring, ice thickness and snow depth estimations. However because of the complexity of the surface (different surface roughness properties in the leads, presence of melt pounds) added to residual sensing errors (residual KaRIn random error, residual systematic errors, …) this first required achieved is not straightforward. Therefore several classification approaches were developed : one based on a statistical method (Markov Random Field), one based on an unsupervised machine learning method (Kmeans) and another one based on a supervised machine learning method (XGBoost). The objective of this paper is thus to present the results of these methods (their robustness, strengths and weaknesses) through local comparisons with respect to optical, SAR images and global comparison with existing state of the art products (OSISAF ice concentration products).

How to cite: Treboutte, A., Jestin, G., Boy, F., Raynal, M., Fleury, S., and Dibarboure, G.: Sea Ice classification in SWOT L3 products , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9359, https://doi.org/10.5194/egusphere-egu26-9359, 2026.

The generation of internal wave generation might increase in the warming Arctic, especially at the sea surface, where wind power input into the upper ocean is substantially stronger as sea ice disappears. More internal wave energy implies stronger mixing that might lead to larger upward heat fluxes from the warm Atlantic Water, contributing to the ongoing sea-ice melt. To comprehensively test this hypothesis, a physics-based mixing parameterization building on an internal wave model that accounts for internal wave generation, propagation, and breaking, is required. To develop such a model, however, knowledge of the internal wave spectral characteristics and their spatio-temporal variability is indispensable. This study therefore investigates the vertical wavenumber spectra of internal wave energy and how their shape varies across the Arctic Ocean based on finescale hydrographic profiles collected by a variety of instrument platforms. It focuses on internal wave energy levels, vertical wavenumber spectral slope and bandwidth, and wave-driven mixing to elucidate their geographic and temporal variation and shed light on the environmental factors determining their variability.

How to cite: Pollmann, F.: Internal wave spectra and mixing in the warming Arctic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9495, https://doi.org/10.5194/egusphere-egu26-9495, 2026.

The Arctic Ocean (AO) is a critical sink for anthropogenic carbon (C_ant), sequestering emissions via intermediate and deep water formation, storing it for long periods of time. Understanding the timescales of these ventilation processes is essential for calculating the current inventory of C_ant as well as predicting the AO’s capacity to store CO₂ in a warming climate. However, observational constraints remain limited; while standard transient tracers (SF₆, CFC-12) and other radionuclides successfully resolve surface and intermediate layers, they often fail to capture the older waters of the deep basins. Atom Trap Trace Analysis (ATTA) has opened a new way to measure deep ocean water residence times by using the radioactive noble gas ³⁹Ar. Here we show the first fully resolved vertical profiles of Arctic ventilation and C_ant using Transit Time Distributions (TTDs) derived from a novel combination of short-lived tracers and long-lived radioisotopes (³⁹Ar, ¹⁴C). Their vertical distribution brings key information on ocean ventilation and hence the storage of anthropogenic carbon. We apply a Bayesian Inference framework to fit the TTD parameters from the different tracer data constraints.

We find that the mixing regime across the Nansen, Amundsen, and Makarov Basins is more advection-dominated than previously assumed in the deep basins. The profiles reveal that the Arctic stores up to 33% of its total C_ant inventory below 1,500 m—vastly exceeding the global ocean average of ∼7%. While the deep Makarov Basin holds roughly half the carbon content of the Eurasian Basin, both reservoirs play a disproportionate role in deep sequestration. Conversely, we demonstrate that for the Atlantic Water Layer, which contains the bulk of the carbon, adding long-lived radioisotopes offers negligible improvement over standard tracers. These findings refine the Arctic carbon budget and highlight the necessity of adding long-lived radionuclides for constraining the deep ocean sink.

How to cite: Arnedo, I., Scott, S., Negele, S., Arck, Y., Meienburg, F., Mandarić, N., Junkermann, A., Wachs, D., Casacuberta Arola, N., Tanhua, T., Oberthaler, M., and Aeschbach, W.: A Multi-Tracer Study of Ventilation and Anthropogenic Carbon Storage in the Arctic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12700, https://doi.org/10.5194/egusphere-egu26-12700, 2026.

The Beaufort Gyre is a prominent feature of Arctic Ocean circulation and a focal point for studies of Ekman dynamics. Midlatitude gyres are primarily forced by atmospheric winds, and Ekman pumping can be directly estimated from the atmosphere–ocean stress. However, in polar regions, the presence of sea ice modifies and mediates momentum transfer from the atmosphere to the ocean. In this context, the surface stress can be expressed as a weighted sum of atmosphere–ocean stress and ice–ocean stress, with the weighting determined by sea ice concentration. In regions of open water, the surface stress is dominated by direct atmospheric forcing, whereas in areas of high ice concentration it is largely controlled by the ice–ocean stress. Under these conditions, internal rheological stresses within the ice pack also play a role in redistributing the stress applied by the atmosphere before it is transmitted to the ocean. The combined action of surface and internal stresses determines the effective forcing felt by the ocean and has direct implications for Ekman pumping and the resulting circulation. To investigate the roles and influence of these stresses, we use output from the MIT general circulation model (MITgcm).

We begin with the free drift regime, in which internal rheological stresses are neglected, and assess the ability for this regime to produce sea ice drift. Observational data in the Arctic are limited, so we attempt to recover sea ice drift using readily available measurements, such as wind speed and altimeter derived sea surface height. Sea ice drift is first inferred from the balance between atmospheric and oceanic stresses, which captures the large scale features of motion reasonably well. Next, an iterative solver is applied to include the effects of Coriolis and sea surface tilt. Finally, comparison with the full rheology case shows that internal ice stress is necessary to reproduce the small scale features of ice motion. In regions of high ice concentration and during winter, rheological stresses become essential, and the free drift approximation no longer captures the observed motion.

Motivated by the limitations of the free drift approximation, the second part of this project examines how the presence of sea ice modifies the atmospheric stress transmitted to the ocean. In open water, wind stress acts directly on the ocean surface, whereas in ice covered regions the stress is applied to the ice and redistributed internally through rheological processes before reaching the ocean. Consequently, the stress experienced by the ocean differs from that applied at the surface. We analyze how internal ice stresses transform and redistribute atmospheric work across the ice pack, altering the effective surface stress and modulating Ekman pumping and ocean circulation within the Beaufort Gyre.

How to cite: Webb, E., Straub, D., and Tremblay, B.: Role of Ice Rheology in Modulating Surface Stress and Sea-Ice Drift in the Beaufort Gyre, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16244, https://doi.org/10.5194/egusphere-egu26-16244, 2026.

The Arctic system is transitioning into a new regime whose properties are yet to be determined, as several feedback processes are undergoing unprecedented changes. Accelerated loss of sea ice and glaciers, enhanced discharge from major pan‑Arctic rivers, widespread permafrost degradation, and a strengthening of the global hydrological cycle are collectively reshaping the upper ocean, making it warmer and increasingly fresh. Sea Surface Salinity (SSS), recognized as an Essential Ocean Variable, provides an integrated measure of atmosphere-ice-ocean coupling. This work investigates the spatial and temporal patterns of SSS and their short-term evolution across nine pan-Arctic regions of the over the satellite period 2011–2022, with a particular focus on how these changes relate to key drivers of surface freshening. To achieve this, we use three Arctic-dedicated satellite products –two from ESA’s Soil Moisture and Ocean Salinity (SMOS) mission, developed by the Barcelona Expert Centre (BEC) and the Laboratory of Ocean and Climatology Expertise Center (LOCEAN), along with the Climate Change Initiative Salinity (CCI) dataset– and GLORYS12v1 model reanalysis. The consistent agreement between satellite observations and model outputs in September –when Arctic coverage is at its annual maximum (r > 0.54) –highlights recent advances in salinity retrievals and their ability to capture key oceanographic processes. Throughout this month, the spatial SSS trend revealed a statistically significant freshening in the northern Barents Sea, with particularly low anomalies in 2019 and 2022. On the other hand, a basin‑wide freshening is evident in all regions except the Kara Sea, with the largest declines (~0.2 yr⁻¹) found near major Arctic river mouths, where a concurrent SST increase further highlights the influence of continental freshwater inputs. The seasonal analysis over the year‑round ice‑free regions (Nordic and Barents Seas) revealed pronounced winter discrepancies among all products –including against in situ data– and most notably in the Norwegian Sea, showing that the drivers of these differences are not yet fully understood. A significant summer freshening emerged along southeastern Greenland, largely shaped by the pronounced anomalies of 2017 and 2021. These shifts reflect the combined influence of variability in sea‑ice export, the timing of melt onset, and atmospheric circulation patterns that govern the delivery and redistribution of freshwater. Meanwhile, the highest summer SSS anomaly in the Barents Sea occurred near the ice edge in 2015, following a winter with exceptionally large sea‑ice volume anomalies. The northward winter transport of sea ice (> 0.18 km³ month⁻¹), enhanced by a positive Arctic Oscillation phase, displaced the ice edge northward, leaving the meltwater signature above 77.5º N. These results highlight the crucial role of remotely sensed SSS in providing insights into the Arctic Ocean's changing conditions and their global implications.

How to cite: Sánchez-Urrea, M., Umbert, M., Galí, M., McPherson, R., De Andrés, E., and Gabarró, C.: A Decade of Arctic SSS Variability from Satellite and Reanalysis Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17370, https://doi.org/10.5194/egusphere-egu26-17370, 2026.

Rapid Arctic change is altering not only local climate conditions but also the teleconnections linking the Arctic to the midlatitudes. In the emerging “New Arctic,” characterized by strong summer sea-ice loss, expanded first-year ice, and deeper tropospheric warming, traditional Arctic–midlatitude linkages are being reshaped in both structure and strength. This study examines how these teleconnections are evolving using a combination of satellite observations, reanalysis data, and climate-model simulations. A key background change is the expansion of newly formed winter sea ice since the mid-1990s, increasing at about 0.6 million km² per decade. This growth is driven by enhanced autumn refreezing following intensified summer melt and is spatially concentrated over the central and eastern Arctic Ocean north of Siberia. Seasonally, the increase is dominated by November ice formation, highlighting the growing importance of late-autumn processes in the New Arctic.

Under this new background, several Arctic–midlatitude teleconnections show distinct changes. First, since the late 1990s, the relationship between December Bering Sea ice extent and January Siberian cold extremes has strengthened, supported by model experiments showing enhanced ridge–trough wave propagation into Eurasia. Second, targeted simulations demonstrate that November Arctic sea ice plays a critical role in modulating troposphere–stratosphere coupling, with a markedly weaker atmospheric response under late-autumn ice-free conditions. Third, large-ensemble simulations reveal that under strong CO₂ forcing, the historically robust “warmer Arctic–colder Eurasia” linkage weakens and becomes less coherent.

These results show that teleconnections in the New Arctic are increasingly season-dependent and state-dependent, with important implications for midlatitude climate variability and predictability.

How to cite: He, S., Fan, K., Zhao, J., Xu, X., and Jiang, J.: Teleconnections in the ‘new Arctic’, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17581, https://doi.org/10.5194/egusphere-egu26-17581, 2026.

Internal waves are a key driver of diapycnal mixing in the global ocean and play an essential role in setting the large-scale overturning circulation. In the Arctic Ocean, however, internal-wave-driven mixing is assumed to be weak due to strong stratification and the presence of sea ice, which limits wind forcing and surface wave activity. The scenario of ongoing sea-ice decline raises the possibility of enhanced internal wave activity and associated mixing, potentially increasing upward oceanic heat fluxes and further accelerating ice loss.

In this study, we investigate the role of tidal forcing as a source of internal waves in the Arctic Ocean using the ocean model FESOM. We perform simulations with tidal forcing at unprecedented horizontal resolution (around 1 km). The simulations are conducted for different seasons and sea-ice conditions to examine how variations in sea-ice modulate tidal currents and internal wave generation. By comparing simulations with and without tidal forcing, we assess the impact of tides on sea-ice dynamics, providing initial insight into coupling between tides, internal waves, and sea ice in the Arctic Ocean. The diagnosed internal tide generation will serve to force the internal wave model IDEMIX, which we will couple to FESOM to provide a consistent mixing parameterization for the simulation of the warming Arctic.

How to cite: Bagaeva, E., Pollmann, F., Wang, Q., Scholz, P., and Danilov, S.: Tidal Forcing and Internal Wave Generation in the Arctic Ocean: High-Resolution FESOM Simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17796, https://doi.org/10.5194/egusphere-egu26-17796, 2026.

Earth system modelling is an important instrument to investigate climate change in an integrated way, taking into account the interactions between the different compartments of the Earth system. It is also an important tool to perform climate projections for different climate scenarios in order to take appropriate mitigation and adaptation measures. Such climate simulations are coordinated internationally as part of the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project Phase 7 (CMIP7).
The Alfred Wegener Institute (AWI) will participate in the CMIP7 project with the Earth system model AWI-ESM3. This is being done as part of the German contribution to the Coupled Model Intercomparison Project (CAP7).
One focus of our work is the impact of anthropogenic aerosol forcing during the historical CMIP7 period. Earlier versions of the AWI climate model setup used a fixed aerosol climatology and thus clearly overestimated the temperature increase for the historical period due to the lack of changing direct and indirect aerosol effects. The implementation of transient aerosols brings the simulated historical period closer to observed trends.
In this presentation we will show first results of the CMIP7 historical experiment performed by the AWI-ESM3 high resolution model including the impact of transient aerosol forcing. We will discuss the results with respect to the Arctic regions and the comparison to observations and climate performance indices.

How to cite: Wieters, N., Streffing, J., Hajdu, L. H., Goessling, H. F., and Jung, T.: AWI-ESM3 high resolution model contribution to CMIP7: First results of the model response in the Arctic regions during the historical period, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17934, https://doi.org/10.5194/egusphere-egu26-17934, 2026.

The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the climate system by transporting and redistributing heat to depth, thereby regulating the effective heat capacity of the ocean under global warming. Observations and projections indicate a potential decline of the AMOC in response to climate change, with far-reaching climate consequences. The Nordic Seas are a key region for the overturning circulation, as dense water formation north of the Greenland–Scotland Ridge feeds the lower limb of the AMOC.

Within this context, the ARCTIC-FLOW project aims to improve our understanding of water mass transformation and overturning processes in the Nordic Seas. The project focuses on identifying the main regions of surface water transformation, quantifying water mass transformation rates, characterizing the temporal and spatial scales of dense water formation, and assessing the impact of extreme freshening events across different subregions of the Nordic Seas.

To support these objectives, we have developed a novel 11-year satellite-based time series of freshwater and density fluxes for the Arctic and sub-Arctic regions. This dataset is derived from the combination of satellite sea surface salinity, sea surface temperature, and surface velocity fields, together with information on mixed layer depth. The satellite products are evaluated and complemented using an extensive set of in situ observations and results from numerical model experiments.

In this contribution, we will present preliminary results on the variability of the newly developed satellite-derived density flux product, highlighting its relevance for studying variability of water-mass transformation processes in the Nordic Seas.

How to cite: Gonzalez Gambau, V., Arias, M., Bergas-Ques, J., Beszczynska-Möller, A., Gabarro, C., García-Espriu, A., Goszczko, I., Karcher, M., Karlsson, N. B., Kauker, F., Olmedo, E., Piracha, A., Ruiz-Sebastián, A., Sabia, R., Sagués, A., Turiel, A., Umbert, M., Vrettou, A., and Wearing, M.: Exploring density flux variability in the Nordic Seas through new satellite products: Insights from ESA’s Polar Science Cluster ARCTIC-FLOW project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18611, https://doi.org/10.5194/egusphere-egu26-18611, 2026.

The Arctic Ocean is changing rapidly, and Atlantic Water circulation plays a key role in the warming, sea-ice decline, and ecosystem changes observed in the Arctic. Still, we have limited understanding of the pathways and circulation times of Atlantic-derived water both at surface and mid-depth layers in the Arctic Ocean, and their evolution over time.

Here, we present the water mass composition and circulation in the central Arctic Ocean in 2021 and assess temporal changes thereof between 2011 and 2021 by using the long-lived anthropogenic radionuclides I-129 and U-236 in the Transit Time Distribution model. Key findings for 2021 include a decline in surface radionuclide concentrations between the Amundsen and Makarov Basins, pointing to substantial fractions of Pacific Water reaching the Lomonosov Ridge from the Amerasian side. Similar radionuclide concentrations in halocline waters on both sides of the Lomonosov Ridge suggest a common formation region of these waters with a clear Atlantic Water signal. North of Greenland, a mixture of waters from the Canada and Amundsen Basins is observed at both surface and mid-depth. Between 2011 and 2021, we observe a shift of the Atlantic-Pacific Water front from the Makarov Basin towards the Lomonosov Ridge and an increase in circulation times in the mid-depth Atlantic layer. Overall, our findings provide a baseline of the circulation of Atlantic-derived waters in 2021 and provide evidence of circulation changes both in the surface and intermediate waters between 2011 and 2021.

How to cite: Wefing, A.-M., Payne, A., Scheiwiller, M., Vockenhuber, C., Christl, M., Tanhua, T., and Casacuberta, N.: Changes in water mass composition and circulation in the central Arctic Ocean between 2011 and 2021 inferred from tracer observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19573, https://doi.org/10.5194/egusphere-egu26-19573, 2026.

Mercury (Hg) concentrations in Arctic biota are elevated relative to lower latitudes, posing an increased risk of adverse health effects for Arctic populations that rely on them as an important food source. Hg readily cycles through different environmental compartments such as air–soil–river before reaching sea waters where it becomes available for methylation to methylmercury and is readily taken up and magnified in the marine food web.

Warming climate is expected to further enhance air-soil-river exchange, increase river discharge, mobilize additional Hg loads from thawing permafrost, erosion melting glaciers and sea ice. A recent Arctic Ocean (AO) Hg mass budget indicates that Hg inputs exceed outputs, indicating either a missing sink or an imbalance due to ongoing changes. 

We determined Hg stable isotope endmember signatures of Hg sources, including western Siberian organic-rich permafrost and mineral soils, and compile those with available literature data on endmembers. Central AO surface seawater samples were collected under trace metal clean conditions in Summer 2025 aboard RV ODEN and zooplankton samples in Summer 2015 aboard RV Polarstern. Solid samples were pre-concentrated using a double tube furnace set-up, while 40 L of sea water were pre-concentrated using a two-step purge and trap method. Hg stable isotopic composition was measured via online cold-vapor generation multicollector ICP-MS analysis.

We use the new Hg stable isotopes measurements together with available literature data to better constrain the Arctic Hg cycle by disentangling the relative importance of different Hg sources to AO surface waters, the entry point of Hg into the marine food web.

How to cite: Kleindienst, A., Barale, I., Lattaud, J., Kohler, S. G., Heimbürger-Boavida, L.-E., Pokrovsky, O. S., Sonke, J., and Jonsson, S.: Constraining Mercury Sources to the Arctic Ocean Using Mercury Stable Isotopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21848, https://doi.org/10.5194/egusphere-egu26-21848, 2026.

Thermohaline staircase layers have been consistently observed in the Arctic Ocean for over 50 years. Previous studies demonstrate that these structures exhibit large-scale spatial coherence.  However, on time scales beyond a few years, both the coherence and evolution of the layers are unknown. Using Ice-Tethered Profiler data from 2005--2022 in the Beaufort Gyre Region, we track staircase layers across time and space with an unsupervised clustering method. Individual layers are found to be coherent across the entire 17-year time period, with properties that appear to evolve on 40--50 year timescales or longer. This establishes, for the first time, the decadal-scale coherence of thermohaline staircases in the Arctic Ocean. Moreover, we find that the observed changes are not consistent with the staircase being in a state of equilibrium, but rather support the hypothesis that it is decaying slowly from an initial or on-going perturbation.

How to cite: Rosenblum, E., Schee, M., Lilly, J., and Grisouard, N.: Decadal coherence of Arctic thermohaline staircases, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22061, https://doi.org/10.5194/egusphere-egu26-22061, 2026.

Arctic areas like the Beaufort Sea are commonly characterized by the formation of sub-surface acoustic ducts. Conversely, the eastern Arctic is known for its upward refracting propagation environment, which creates surface ducts. Using moored passive acoustic recorders in the Fram Strait in the eastern Arctic Ocean, we measured the average distribution of sound energy in the water column. The moorings were deployed by the CMRE Environmental Knowledge and Operational Effectiveness program and instrumented with several oceanographic sensors and acoustic recorders. Even though some acoustic recorders failed to record for the entire experiment, we characterized the vertical distribution of the ambient noise field. We measured the formation of temporary sound energy duct-type areas in the thermocline. Using Copernicus Marine service data, we investigated the effects of the sea ice concentrations, sea ice drift and distance to the sea ice edge on the vertical distribution of ambient noise. The distance from the ice edge had a negative correlation with sounds levels, while ice drift and concentration were not correlated to the overall sound levels. Simultaneous sound speed measurements revealed the presence of potential sound channels. We investigated the possible origin of the sound energy, and the formation of potential sub-surface ducts, applying range-dependent sound propagation modelling coupled with high-resolution output of the double nested CMRE’s Pan-Arctic ocean-sea ice model.

How to cite: Giorli, G., Falchetti, S., Russo, A., and Real, G.: Observations of ducting of acoustic energy in the Fram Strait., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22753, https://doi.org/10.5194/egusphere-egu26-22753, 2026.

Recent and rapid environmental changes in the Arctic Ocean lead scientists to re-evaluate the way they operate in this area. NATO STO Centre for Maritime Research and Experimentation (CMRE) leads the Nordic Recognised Environmental Picture (NREP) trial series in order to understand how a better characterization of the Arctic environment is possible and how it can help to build more accurate underwater acoustic modelling capabilities. NREP25 was conducted in the Greenland Sea from July 16th to July 27th 2025. On-board NATO Research Vessel (NRV) Alliance, underwater acoustic propagation experiments were performed in various Arctic environments (packed ice, marginal ice, open waters, brash ice), deploying in-house built receivers alongside other innovative solutions. NRV Alliance acted as the acoustic source, transmitting pre-defined sequences of known waveforms that will be used for probing the Arctic environment, in particular the interactions of sound waves with the space and time-dependent ice cover. The latter was estimated using a combination of remote sensing capabilities. First, a new prototype of ship-borne X-band RADAR provided a continuous estimation of the positions of the ice floes. Second, remote sensing imagery from diverse satellites (Sentinel, COSMO-SkyMed, SWOT and RADARSAT) provided high-resolution images of the sea ice cover, obtained from SAR processing, several times a day. NRV Alliance also served as a “floating ground control” for drone activities. Namely, high resolution photogrammetry and point-cloud LiDAR data were obtained from aerial drone surveys. 

Oceanographic characterization was carried out through extensive CTD casts, as well as glider missions (with acoustic payloads as well). This characterization was used to design experimental configurations that were more likely to generate interactions of acoustic paths with the sea ice at the surface.

In addition, CMRE conducted specific characterization of the water ice interface by ROV inspection (with video, acoustic camera and altimetry data), and also ice coring (to be analysed at the centre).

An overall description of the experiment is presented, as well as an analysis of the data collected focusing on the contribution of remote sensing to the understanding of the underwater acoustic observations.

How to cite: Real, G., Pennucci, G., Akins, F. H., and Fabbri, T.: Multi-domain environmental sensing for underwater acoustics in the Arctic marginal ice zone., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22763, https://doi.org/10.5194/egusphere-egu26-22763, 2026.

 The Arctic, spanning over eight sovereign countries and the Arctic Ocean, is warming three tofour times faster than the global average, driving profound transformations of the cryosphereand ocean systems. It harbours four critical climate tipping points: the accelerated melt of theGreenland Ice Sheet, the thawing of boreal permafrost, the collapse of winter Arctic sea ice,and the weakening of the Labrador–Irminger Seas convection. These processes are tightlyinterconnected and play a key role in regulating the global climate, yet their combined impactsremain insufficiently constrained by observations.Within this context, the CASCADES Expedition is an international and interdisciplinary polarprogramme designed to investigate the coupled interactions between glaciers, sea ice, and theocean around Baffin Bay and Northwest Greenland. CASCADES is coordinated by the InstitutNordique du Québec, the Swiss Polar Institute, and the French Polar Institute, in collaborationwith Greenlandic institutions. It brings together more than 50 Canadian, French, Greenlandic,and Swiss researchers around 16 scientific projects from 13 research institutions.CASCADES will be conducted aboard the Canadian research icebreaker CCGS Amundsen andis structured into two complementary legs in 2026, aligned with critical seasonal phases of theArctic system. A first leg during the summer targets peak glacier melt and freshwater input,while a second leg during autumn focuses on the transition toward sea-ice freeze-up. This dual-season strategy enables the investigation of how physical, chemical, and biological processesevolve from melt to freeze-up, and how these transitions affect carbon cycling, productivity,and ecosystem structure.By providing coordinated, multidisciplinary observations across key Arctic seasons,CASCADES aims to improve understanding of cryosphere–ocean–ecosystem coupling and itsimplications for the North Atlantic and the global climate system. Beyond its core scientificobjectives, the expedition serves as a platform for international collaboration, sciencediplomacy, education, and engagement with Arctic communities, contributing to sharedobservation efforts and to the anticipation of climate-driven changes in polar oceans and beyond.

How to cite: Ruols, B., Tremblay, J.-É., Jaccard, S., and Dumont, D.: The CASCADES Expedition: multidisciplinary observations of a rapidly changing Arctic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22970, https://doi.org/10.5194/egusphere-egu26-22970, 2026.

Mesoscale eddies in the southeastern tropical Indian Ocean (SETIO) are crucial for regional circulation, heat transport, and ecosystem dynamics. Their interannual variability is closely associated with ENSO and IOD. Eddy activity is enhanced during pure La Niña and positive IOD years, but suppressed during pure El Niño and negative IOD years. When ENSO and IOD co-occur, their influences tend to counteract each other: the IOD dominates during the ENSO developing phase, whereas ENSO exerts a stronger influence during the decay phase. This variability is linked to changes in the Indonesian Throughflow and wind-driven upwelling associated with ENSO and IOD events. Numerical experiments further indicate that the interannual variability of SETIO eddies is primarily wind-driven, with winds over the equatorial Pacific, equatorial Indian Ocean, and SETIO all contributing significantly. Oceanic channel effects induced by equatorial Indo-Pacific winds are stronger than those arising from purely atmospheric processes.

How to cite: Zhou, Y., Cheng, X., Duan, W., Yang, C., and Chen, J.: Impacts of ENSO and IOD on Mesoscale Eddy Activity in the Southeastern Tropical Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3044, https://doi.org/10.5194/egusphere-egu26-3044, 2026.

The Indo-Pacific region is vulnerable to changes in the Indian summer monsoon and its onset. The associated monsoon rainfall strongly affects agriculture, water resources, and human security across the densely populated regions of South Asia. The Indian Ocean summer monsoon often develops in association with the southeast Arabian Sea warm pool and a monsoon onset air pressure vortex, giving rise to a complex air-sea coupled system, though their precise interactions and impacts remain unclear. In this study, our analysis covering 1992-2017 demonstrates that the vortex triggers an earlier onset due to vortex-induced rainfall when the monsoon system has not yet developed to its climatological intensity. The weaker monsoon at the onset time means the large-scale moisture transport is expected to be lower over the Indian subcontinent, and rainfall analysis confirms a drier central India, where agriculture is mostly rain-fed, at the monsoon onset time in the vortex years. These results help to understand the transition from localized synoptic activity to the large-scale monsoonal system, highlighting the crucial role of the vortex. More importantly, when a vortex is pre-observed, rainfall available for agricultural irrigation is expected to be lower, providing guidance for agricultural irrigation.

How to cite: Zhao, Z., Sprintall, J., and Du, Y.: How the Vortex over the Arabian Sea Warm Pool Triggers an Early Indian Summer Monsoon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3220, https://doi.org/10.5194/egusphere-egu26-3220, 2026.

The tropical Indian Ocean (TIO) has experienced pronounced warming trends in recent decades, with dynamical processes recognized as key drivers. However, the role of thermal processes remains uncertain due to discrepancies in surface wind-induced heat flux across existing datasets. The present study introduces a random forest machine learning algorithm that synergistically integrates the advantages of in situ observations and satellite data, yielding a monthly surface wind (MLAWind) dataset and corresponding air-sea heat flux from 1950–2022 with a horizontal resolution of 1°×1°. MLAWind exhibits high accuracy and robust generalization capability based on evaluations using both satellite and buoy observations. Besides, it is capable of effectively representing spatial and temporal characteristics of surface wind. In contrast to the majority of existing reanalysis datasets, MLAWind reveals a decline in surface wind over the TIO since 1950, which is further supported by the west-to-east asymmetrical variations in sea surface height and thermocline depth. The attenuation of surface wind is more significant in the eastern TIO as compared to the western TIO, leading to a remarkable reduction in evaporative cooling within the eastern TIO. The thermal processes associated with surface wind-induced heat flux serve as the essential drivers of the warming in the eastern TIO, with a contribution accounting for approximately 45% of that of dynamical processes. The findings of our study challenge existing reanalysis results but are aligned with state-of-the-art models, highlighting that the significance of thermal processes is substantially underestimated in most existing reanalysis datasets.

How to cite: Guo, W.: Unveiling the drivers of tropical Indian Ocean warming through machine learning-assisted surface wind, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3733, https://doi.org/10.5194/egusphere-egu26-3733, 2026.

The Indian Ocean Dipole (IOD) is an intrinsic climate mode in the Indian Ocean that typically peaks during boreal fall and influences weather and climate across surrounding regions. It is influenced by both the El Niño–Southern Oscillation (ENSO) and internal variability within the Indian Ocean. However, the relative contributions of the two ENSO types—namely, Eastern Pacific (EP) and Central Pacific (CP) ENSO—and internal variability to the IOD remain poorly quantified. Here, we employ a binary combined linear regression approach to isolate and quantify the contributions of these three factors. The results show that internal variability is the dominant driver of IOD-related sea surface temperature (SST) anomalies, explaining over 60% of the variance. ENSO accounts for approximately one-third of the variance, primarily through the CP type, whereas the EP type tends to influence the IOD mainly during extreme events. Their underlying mechanisms differ. ENSO primarily modulates the Indian Ocean wind field through the Walker circulation, whose effectiveness depends on the longitudinal position of the equatorial Pacific warming—eastern for EP events and central for CP events. In contrast, internal variability generates SST anomalies through local ocean–atmosphere feedbacks that sustain the IOD. Because El Niño tends to persist longer, co-occurring positive IOD events are more likely to evolve into basin-wide Indian Ocean warming in the following spring, a transition to which El Niño contributes more than 70%. Although internal variability shows no significant statistical association with this transition, a strong positive IOD alone still has the potential to trigger the basin-wide warming in the subsequent spring. These findings enhance our understanding of climate modes and inter-basin interactions.

How to cite: Zhang, L., Du, Y., and Zhang, Y.: Quantifying impacts of ENSO and internal variability on the Indian Ocean Dipole, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4605, https://doi.org/10.5194/egusphere-egu26-4605, 2026.

Climate conditions over East Asia are significantly affected by coupled ocean–atmosphere interaction in the tropical Indo-Pacific Ocean. In April 2024, South China suffered two rounds of extreme rainfall, occurring from 30 March to 6 April and from 19 to 30 April, resulting in the earliest flood over the Pearl River basin since 1998. This study finds that the early Indo–western Pacific Ocean capacitor (IPOC) effect and the Madden–Julian oscillation (MJO) jointly contributed to the extreme rainfall. Co-occurrence of El Niño and positive Indian Ocean dipole events in 2023–24 led to strong sea surface temperature (SST) warming in the western tropical Indian Ocean via wind forcings and oceanic waves. Such SST warming induced persistent easterly wind anomalies and maintained the anomalous anticyclonic circulation (AAC) over the western North Pacific. The IPOC effect was hence activated in April, approximately 2 months earlier than expected, inducing stronger northward water vapor transport. Moreover, two MJO events were observed in April. With the eastward propagation into the eastern Indian Ocean (phases 1–3), the MJO events facilitated the southwest flank of the AAC and enhanced the northward water vapor transport, leading to extreme rainfall along with strong convection in South China. This study emphasizes the synergistic contributions of climate modes on different time scales to extreme weather.

How to cite: Du, Y., Zhang, L., Zhang, Y., and Chen, Z.: Extreme Rainfall over South China in April 2024 Associated with Early IPOC and MJO Events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4649, https://doi.org/10.5194/egusphere-egu26-4649, 2026.

Tropical cyclones are among the most devastating natural hazards to occur in the South-West Indian Ocean basin (SWIO), posing considerable risk to vulnerable countries such as Madagascar and Mozambique. This study examines changes in tropical cyclone risk across the SWIO, the Main Tropical Cyclone Region, and the Madagascar Region over the last 45 cyclone seasons (1981–2025). Seasonal and monthly time-series of  key tropical cyclone metrics, namely frequency, maximum sustained wind (MSW), and accumulated cyclone energy (ACE) were computed and analysed for trends. The relationship these metrics have with major oceanic and atmospheric drivers, such as ocean temperature, the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden–Julian Oscillation (MJO), were examined.

Results indicate a significant decrease in tropical cyclone frequency in the SWIO and the Main Tropical Cyclone Region, while frequency remained relatively stable in the Madagascar Region. In contrast, MSW increased significantly across all regions (+3.91 Knots per decade), with the strongest intensification occurring within the Madagascar Region (+4.79 Knots per decade). This suggests risk has increased over time in the SWIO despite the occurrence of fewer storms.

Ocean temperatures exhibited significant warming at both surface and subsurface levels, with depths of 35 m and 45 m indicating the greatest warming trends and the strongest relationship with increased cyclone MSW. Cyclone frequency on the other hand was negatively correlated with ocean warming, suggesting warmer waters in the SWIO may create conditions less conducive to the formation of tropical cyclones.

ENSO was found to be a considerable driver of regional cyclone variability, with La Niña conditions associated with higher frequency and stronger cyclones. The MJO was also identified as a key modulator of cyclonic activity, particularly in the Madagascar Region, where active phases 3, 4, and 5 coincided with increased cyclone frequency and MSW. The IOD on the other hand showed little to no influence on cyclone metrics in the SWIO. The incorporation of this research into forecasting and intensity models has the potential to enhance early warning systems in the SWIO, thereby providing a valuable tool for the highly vulnerable region.

(Swan and Hallam, in prep, 2026)

How to cite: Swan, L. and Hallam, S.: The Changing Tropical Cyclone Risk in the South-West Indian Ocean over the last 45 tropical cyclone seasons (1981-2025), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5435, https://doi.org/10.5194/egusphere-egu26-5435, 2026.

This study investigates decadal changes in boreal summer Indian Ocean Basin Mode (IOBM) predictability (1948–2022) using the Model-based Analog Forecast (MAF) method, based on a library of 20 CMIP6 models. A pronounced decadal shift is identified, with forecast skill markedly increasing after 1980. This shift is primarily attributable to the decadal modulation of the ENSO–IOBM teleconnection. During the high-skill period, prolonged El Niño events induce significant southwestern Indian Ocean (SWIO) warming. This, in turn, activates a robust wind-evaporation-SST (WES) feedback, which maintains the basin-wide warming into summer, thereby providing an enhanced signal component for IOBM predictions. In contrast, during the low-skill period, weaker ENSO events fail to sustain this feedback, leading to premature termination of IOBM events and consequently lower forecast skill. These findings demonstrate that boreal summer IOBM predictability is nonstationary and reveal that accurately representing the ENSO–IOBM teleconnection is essential for advancing forecast skill.

How to cite: Xu, C. and Wu, Y.: Decadal change in seasonal prediction skills of the Indian Ocean Basin Mode during boreal summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6100, https://doi.org/10.5194/egusphere-egu26-6100, 2026.

Marine heatwaves (MHWs) are ocean temperature extremes that can occur at any ocean depth. Surface features and drivers of MHWs have been extensively explored based on satellite observations; however, their subsurface features and drivers remain unclear. This study investigates the characteristics and drivers of subsurface MHWs near the thermocline in the Bay of Bengal (BoB) from 1993 to 2024 using high-resolution ocean reanalysis datasets. The subsurface MHW days exhibit a dipole pattern in response to the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). During El Niño or positive IOD, the anticyclonic mesoscale eddies in the western BoB are favorable for MHW generation in this region, related to the anomalous anticyclonic winds and currents over the BoB. Meanwhile, the equatorial easterly anomalies drive upwelling Kelvin waves to propagate eastward into the eastern BoB, inhibiting MHW formation in that area. Thus, the subsurface MHW days in the BoB increase in the west but decrease in the east during El Niño or positive IOD events. Over the past decades, a significant increasing trend in the subsurface MHW days has been observed due to the rise in mean temperature over the BoB. This study highlights the inconsistent spatial responses of subsurface MHWs to distinct ocean dynamics induced by ENSO and IOD.

How to cite: Zhang, Y., Du, Y., Lin, X., and Qiu, Y.: Dipole variability of subsurface marine heatwaves in the Bay of Bengal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6646, https://doi.org/10.5194/egusphere-egu26-6646, 2026.

The acoustic travel time (τ) measured by an inverted echo sounder (IES) can be converted into vertical temperature profiles using the gravest empirical mode (GEM) technique based on the relationship between τ and temperature. In the Seychelles–Chagos Thermocline Ridge (SCTR) in the southwestern tropical Indian Ocean, where persistent subsurface upwelling occurs, continuous vertical temperature profiles are crucial for monitoring the upwelling variability. However, the conventional GEM method has large uncertainties at the SCTR due to temporal variability in upwelling strength. This study introduces a new approach, termed hybrid GEMs, to improve IES data analysis by reflecting upwelling strength based on the depth of the 20°C isotherm (D20). The hybrid GEMs consist of one moderate GEM and two combined GEMs derived from three groups of historical hydrographic profiles in the SCTR, categorized by D20 ranges. When applied to the in situ τ measured by a pressure-recording IES at Station K (61°E, 8°S) in the SCTR from May 2019 to December 2021, the absolute dynamic topography from satellite altimetry is used as an index to select the appropriate hybrid GEM based on the consistency between the absolute dynamic topography and D20 variability. The vertical temperature profiles based on hybrid GEMs show significant improvements in both the mean and maximum root mean square errors of the upper 300 m temperature, which are reduced by approximately 29% and 20%, respectively. The hybrid GEM–derived temperature profiles reliably capture temperature variability in the upper 300 m, demonstrating the strong potential of acoustic travel time as an essential observational variable in data-sparse tropical upwelling regions of the Indian Ocean.

How to cite: Lee, E., Na, H., and Nam, S.: A Hybrid Gravest Empirical Mode Method for Reconstructing Temperature Profiles in the Seychelles–Chagos Thermocline Ridge , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6757, https://doi.org/10.5194/egusphere-egu26-6757, 2026.

South Asian summer monsoon (SASM) delivers substantial rains to Indian subcontinent and drives upwelling in the Arabian Sea that sustains one of the world's most productive fisheries there. Both marine upwelling records and terrestrial rainfall records have been established as fundamental archives for reconstructing past SASM variability. However, the upwelling records vary in opposite direction to the terrestrial rainfall records on orbital timescale, leading to a long-standing paradox in the past monsoon variability. To understand this paradox, here we combine paleoclimate records with novel transient climate simulations that explicitly separate the effects of the Northern and Southern Hemisphere insolation forcing. Our results show that the SASM rainfall is governed by Northern Hemisphere (NH) insolation, whereas the Arabian Sea upwelling is dominated by Southern Hemisphere (SH) insolation. When boreal summer occurs at perihelion, insolation is strongly enhanced not only in the NH but also in the tropical-subtropical SH. The former enhances the SASM rainfall through Eurasian warming, while the latter weakens the Arabian Sea upwelling by inducing South African warming and subsequent atmospheric teleconnections. We reconcile the long-standing paradox, and more broadly, reveal that warming in South Africa could exert a significant and previously overlooked remote forcing on the SASM system.

How to cite: Wen, Q., Liu, Z., Wang, T., Ji, C., Liu, J., Cheng, H., Yan, M., Ning, L., Jing, Z., Liu, H., Lei, J., Lu, J., Creutzig, F., and Yin, Q.: Reconciling Contrasting Marine and Terrestrial Responses of South Asian Summer Monsoon Reveal a Remote Control from South Africa, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7310, https://doi.org/10.5194/egusphere-egu26-7310, 2026.

The Arabian Sea is among the most productive ocean basins globally, driven by summer coastal upwelling, winter convective mixing, and aeolian dust inputs that supply nutrients to the euphotic zone. In several aspects, this region shares characteristics with High Nutrient–Low Chlorophyll (HNLC) systems, where mineral dust deposition partially alleviates iron limitation of surface waters, that are ventilated by iron-depleted waters (e.g., the Antarctic Intermediate Waters). Paleorecords indicate that enhanced dust fluxes during the Last Glacial Maximum (LGM) coincided with increased primary productivity in the northwestern Arabian Sea, suggesting a potential role for iron fertilization, although the underlying mechanisms remain poorly constrained.

Here, we reconstruct millennial-scale variations in coccolithophore growth rates in the northwestern Arabian Sea since the LGM, based on the coccolith carbon isotope vital effect (δ¹³C_VE) recorded in sediment core MD00-2354 (61.48°E, 21.04°N). Combined with coccolithophore cell-size estimates at the studied site, and reconstructed iron fluxes in the area, these data allow us to investigate the links between iron availability and phytoplankton growth from 22 to 4 ka.

Our results show that coccolithophore growth rates and cell sizes were significantly increased during the LGM, coincident with maxima in mineral dust and iron fluxes. This pattern suggests that nutrient availability was the primary control on coccolithophore growth at that time. This interpretation is supported by a positive correlation between coccolithophore growth rates and independently reconstructed net primary productivity at the site. A likely mechanism is that increased iron supply during the LGM enhanced phytoplankton nitrogen assimilation, as further supported by ROMS–PISCES model simulations. Comparisons between simulations with and without atmospheric iron deposition indicate that, under increased iron input, the enhancement in nitrogen utilization exceeds that of phosphorus utilization, and is concomitant to elevated primary productivity.

How to cite: Zhou, X., Duchamp-Alphonse, S., Jin, X., Liu, C., Jiang, X., Bassinot, F., and Kissel, C.: Iron fertilization enhanced coccolithophore growth rate in the northwestern Arabian Sea during the Last Glacial Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7753, https://doi.org/10.5194/egusphere-egu26-7753, 2026.

It has been recognized that the low-tropospheric circulations associated with Northeast Asian and the western North Pacific monsoons are closely related to each other via atmospheric teleconnection in boreal summer. This teleconnection can be understood by the responses to the stationary Rossby wave. In addition, the climate mode including the atmospheric teleconnection has a large variability on inter-decadal as well as interannual time scales associated with adjacent climate variability. This study focuses subseasonal climate modes, which were decomposed by the self-organizing map (SOM) analysis. This study suggests that those modes are significantly increased in the last few decades, and the changes in the climate mode are related to upper ocean warming of Northern Indian Ocean due to global warming and changes in the climate variability in the Indian Ocean. This study also shows the possible reasons for the analysis results.

How to cite: Lee, H. and Kwon, M.: Inter-decadal changes in a subseasonal climate variability in the western North Pacific region with Northern Indian Ocean in boreal summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8740, https://doi.org/10.5194/egusphere-egu26-8740, 2026.

As a link between the surface waters and the deep ocean, intermediate water masses play a key role in transmitting atmospheric variations into the deep sea. The paleo-oceanographic reconstructions of intermediate water mass variability are therefore essential to comprehend the pace and extent with which shallow marine changes are transferred into ocean basins. Cold water corals (CWC) thriving in intermediate water depths have been identified as an adequate geochemical proxy archive to reconstructed temporal changes in water mass properties, due to the sustained growth of their carbonate skeleton and their long lifespan (several hundred years).

Two living CWC colonies of Enallopsammia rostrata (Pourtalès, 1878) have been collected in the northern part of the Mozambique Channel around the island of Mayotte, during the research cruise SO306 with the RV Sonne in August 2024. The corals were collected with a ROV from water depths between 600 to 900 meters within the transition zone of South Indian Central Water (SICW) and the underlying Red Sea Water (RSW). The chemical composition (Ca, Li, Mg) of different branches from each colony was analysed using line scan laser ablation inductively coupled mass spectrometry (LA-ICP-MS). U/Th dating enables the determination of ages and calculation of growth rates for the individual colonial parts.

The sclerochronological aligning of the geochemical data along the U/Th-based growth rates enabled a reconstruction of Li/Mg_coral variations until the end of the penultimate century. Pronounced cyclic variabilities in ranges of duration from years to decades could be identified within the Li/Mg-records, due to the spatially high-resolution LA-ICP-MS measurements. However, a significant trend in Li/Mg_coral, that would indicate a continuous change of water temperatures, could not be identified within the two colony records over the reconstructed time period. Water temperatures derived from mean Li/Mg_coral by employing Li/Mg-temperature calibration (Montagna et al. 2014) match well with observed water temperature values between of 6.6°C and 9.4°C, respectively. The reconstructed temperature variability for both colonies show variations on an average range of 3°C (2SD) over multi-year intervals, which can be attribute to a changing extent of influence of the differently temperate water masses around Mayotte.

Our reconstruction shows no long-term temperature increase in the intermediate water masses of the West Indian Ocean during the last century contrary to the anthropogenic warming of the atmosphere and surface ocean. The found temperature variability, however, points to a dynamic and periodic shifting of the different water masses, which suggests a more lateral exchange within intermediate water depths in the northern entry area of the Mozambique Channel.

• Montagna, M. McCulloch, E. Douville, M. L. Correa, J. Trotter, R. Rodolfo-Metalpa, D. Dissard, C. Ferrier-Pagès, N. Frank, A. Freiwald, S. Goldstein, C. Mazzoli, S. Reynaud, A. Rüggeberg, S. Russo, M. Taviani (2014): Li/Mg systematics in scleractinian corals: Calibration of the thermometer. Geochim. Cosmochim. Acta 132, 288–310

How to cite: Kniest, J. F., Raddatz, J., Fietzke, J., Frank, N., Kaiser, T., Freiwald, A., and Flögel, S.: One century of intermediate water masses temperature variability of the West Indian Ocean reconstructed by Li/Mg-thermometer in scleractinian cold-water corals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9018, https://doi.org/10.5194/egusphere-egu26-9018, 2026.

The Last Glacial Maximum (LGM) and the subsequent deglacial period likely featured climate-ocean dynamics and deep-ocean carbon storage states that contrasted with those of today. In this study, we focus on the Indian Ocean and place regional reconstructions in a global context by integrating published results from other ocean basins. Differences in deep-ocean carbon storage between basins across the LGM and deglaciation reflect changes in (1) deep-water mass sources and circulation structure and (2) regional regulation processes within the ocean basin. We reconstruct deep-water oxygen concentrations ([O₂]) between 26 and 10 ka BP using sediments from IODP Site 353-U1445 (Bay of Bengal) and IODP Site 361-U1479 (Cape Basin). Deep-water [O₂] is inferred from the carbon isotope gradient between epifaunal and infaunal benthic foraminifera (Δδ¹³C_epi-in). Changes in biological pump efficiency are assessed from the carbon isotope gradient between planktonic and benthic foraminifera (Δδ¹³C_p-b). Reconstructed [O₂] records are combined with outputs from the TraCE-21ka simulations and CMIP6 EC-Earth3-CC model to estimate respired carbon storage (Pg C).

During the LGM, deep-water [O₂] variations in the Cape Basin and the Bay of Bengal showed broadly synchronous trends, with major inflection points occurring at similar times. However, changes in the Cape Basin systematically preceded those in the Bay of Bengal. This temporal offset indicates a more rapid response in the Cape Basin relative to the Bay of Bengal. From the LGM to the deglaciation, increasing deep-water [O₂] and declining carbon storage in the Cape Basin are closely associated with reduced biological pump efficiency. In contrast, the Bay of Bengal exhibited stronger variability during the deglaciation, with a pronounced response during the Bølling–Allerød (B/A) interval, when deep-water [O₂] sharply decreased. During the B/A stage, the Antarctic Cold Reversal in the Southern Ocean was characterized by weakened AABW formation and reduced deep-water [O₂]. These changes slowed deep-water renewal and enhanced deep-water organic carbon remineralization, which probably resulted in increased deep-water respired carbon storage in the Indian Ocean. The larger LGM–deglacial amplitude in the Cape Basin reflects its location at the confluence of Atlantic, Southern Ocean, and Indian Ocean water masses, resulting in a more rapid and pronounced response to circulation reorganization, whereas the Bay of Bengal exhibits weaker and delayed responses as a distal deep-water reservoir. Estimated respired carbon storage efficiency in the Cape Basin is higher during the LGM by~0.03 mol m⁻³ and ~0.05 mol m⁻³ relative to Heinrich Stadial 1 (H1) and the B/A, respectively. Consistent with this difference, mean respired carbon storage decreased from ~5.51 Pg C (~2.58 ppm CO₂ equivalent) during the LGM to ~2.84 Pg C (~1.33 ppm) and 1.10 Pg C (~0.52 ppm CO₂) during H1 and the B/A, respectively. In contrast, the Bay of Bengal exhibits higher respired carbon storage during the B/A (1.24 Pg C; ~0.58 ppm CO₂ equivalent) than during the LGM (0.51 Pg C; ~0.24 ppm CO₂ equivalent). This study highlights the heterogeneous response of the Indian Ocean deep carbon reservoir during glacial-interglacial transitions.

How to cite: Shen, W. and Zhao, N.: Evolution of deep-ocean carbon storage in the Indian Ocean since the Last Glacial Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15512, https://doi.org/10.5194/egusphere-egu26-15512, 2026.

Six major summer monsoon floods occurred in the Yangtze Basin between 1992–2024 affecting millions of people, compared to only one during 1960–1991. This significant rise in hydroclimatic extremes is closely associated with an approximately 50% increase in variability at the quasi-biennial timescale. In this study, using sea surface height and thermocline depth from the ORAS5 reanalysis and EN4 observational analysis, we demonstrate that the increased quasi-biennial variability in East Asian summer monsoon rainfall over the Yangtze River Basin is strongly coupled with intensified quasi-biennial scale wave dynamics in the Indian Ocean. We provide evidence of fundamental changes in the characteristics of baroclinic waves in the tropical Indian Ocean over recent decades. We find that the mean phase speed of westward-propagating tropical Rossby waves has increased by 70%, along with their overall variance. These shifts are likely associated with changes in large-scale atmospheric forcing. Our findings highlight that evolving Indian Ocean wave characteristics are a key driver of changes in East Asian summer monsoon variability at quasi-biennial timescales and the associated hydrological extremes over East Asia, with important implications for the predictability of East Asian summer monsoon rainfall at these timescales.

How to cite: Dasgupta, P., Nam, S., McPhaden, M. J., Kang, D., Mathew Koll, R., and Jayanthi Sasikumar, S.: Intensified Indian Ocean Rossby Wave Dynamics as a Driver of Increased Quasi-Biennial Summer Monsoon Floods in the Yangtze River Basin , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15991, https://doi.org/10.5194/egusphere-egu26-15991, 2026.

In this presentation, we review the recent research progress based on moored observations of the Leeuwin Current, an eastern boundary current of the south Indian Ocean, off the west coast of Australia, over the past 15 years. The focus will be on the seasonal cycle of the Leeuwin Current, as well as the interannual temperature variability and its drivers, with a focus on the Ningaloo Niño – an austral summer marine heatwave event. In the future climate projections, the Leeuwin Current (along with the Indonesian Throughflow) will become weaker, as shown in both climate model projections and downscaling models. However, the Ningaloo Niño is expected to strengthen in the future climate, with its peak month shifting from February to March in the austral summer. Climate model projections suggest that both enhanced local air-sea coupling and remote forcing from the Pacific may induce such a strengthening of the warming events.

How to cite: Feng, M.: Observations of the Leeuwin Current and variability, and future projections of Ningaloo Niño marine heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16944, https://doi.org/10.5194/egusphere-egu26-16944, 2026.

The Bay of Bengal (BoB) is characterized by intense stratification driven by monsoonal freshwater flux coupled with high irradiance levels, which limits nutrient supply to the euphotic zone and restricts oxygen ventilation. While the northern part of the Bay remains hypoxic, ubiquitous mesoscale eddies provide a mechanism to break this stratification, pumping nutrients into surface layers and transporting shelf-associated microbial communities to the central bay. The response of these communities to the dynamics of this region remains however poorly understood.

The objective of the present study was to analyse the response of two distinct microbial communities to the environmental dynamics of this region. We conducted a series of onboard incubations using natural microbial communities collected from surface and deep chlorophyll maximum (DCM) waters during a cruise aboard the R/V Thomas G. Thompson in the summer months of 2025. Two distinct treatments were established: a control representing standard stratified conditions (characterized by an abrupt oxygen gradient and low irradiance at depth), and an experimental treatment designed to simulate the nutrient injection and mixing typically induced by cyclonic eddies, under well-oxygenated conditions and the local photoperiod. We monitored physiological parameters as chlorophyll-a concentration, the maximum photosynthetical quantum yield (F_v/F_m) and intracellular nitrate pools.

Our findings indicate that both communities (surface and DCM) exhibited similar response patterns under stratified conditions, with no significant growth and intracellular nitrate levels remaining lower (≈ 0.1 µM) than the freely dissolved pool (0.9-1.2 µM). In contrast, nutrient enrichment from bottom waters resulted in a rapid community response. The surface community exhibited a rapid uptake of nitrate within the first hours of incubation, resulting in an increase in the intracellular pool, which was followed by a gradual consumption over the following days. These results demonstrate the physiological plasticity of the community in response to a highly dynamic environment, with the capacity to utilize episodic nutrient enrichment within this highly variable system. Such plasticity may have significant implications for the nitrogen biogeochemical cycle as well as for the overall microbial community composition in the highly dynamic and increasingly deoxygenated North Indian Ocean.

How to cite: Rodríguez-Márquez, J., Bartual, A., Bourbonnais, A., Pachiadaki, M., and Garcia-Robledo, E.: Surface and deep chlorophyll microbial community response to mixing events in the stratified Bay of Bengal (NE Indian Ocean), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17507, https://doi.org/10.5194/egusphere-egu26-17507, 2026.

The Bay of Bengal is considered one of the largest oceanic Oxygen Minimum Zone (OMZ), characterized by oxygen levels that remains persistently near the threshold of anoxia, possibly limiting the widespread nitrogen loss observed in other major OMZs. Planktonic microbial respiration is largely responsible for the formation and maintenance of the hypoxic and anoxic conditions found in the OMZs. Understanding the respiratory kinetics of the planktonic microbial community is therefore essential to predicting the sensitivity of this area to further deoxygenation. During a cruise aboard the R/V Thomson in the summer months of 2025, we investigated the regulation and control of microbial oxygen consumption within the upper 300 m of the water column. We combined high-resolution vertical profiling with experimental rate measurements using high-sensitivity oxygen sensors to characterize the metabolic transition from the upper oxic layer through the oxycline into the nearly anoxic core. Microbial community abundance was quantified via flow cytometry to link biomass density with metabolic activity. Respiratory kinetics were characterized by onboard water incubations with samples subjected to a wide range of oxygen levels. Our results demonstrate a clear vertical stratification in respiratory potential, with the highest rates associated with the upper oxic layer and a progressive decrease as oxygen and chlorophyll levels decreased. However, higher values were also found at intermediate depths within the hypoxic water layers. By fitting oxygen consumption rates to kinetic models, we calculated the apparent half-saturation constant (Km) for the microbial community throughout the water column. These Km values showed a complex distribution, generally reaching their minimum in the oxycline and increasing within the hypoxic zones. This suggests a counterintuitive decrease in oxygen affinity at low oxygen levels, although significant consumption rates were observed even at trace levels of oxygen. This trend may indicate a taxonomic shift in the microbial community or a change in the expression of different types of terminal oxidases, thereby demonstrating adaptation of the microbial community to the episodic oxygen supply characteristic of the interior of the Bay of Bengal.

How to cite: Garcia-Robledo, E., Rodriguez-Marquez, J., Pachiadaki, M., Bourbonnais, A., and Calderon-Caro, J.: Regulation and control of the planktonic microbial respiration in the hypoxic waters of the Bay of Bengal, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17545, https://doi.org/10.5194/egusphere-egu26-17545, 2026.

The Indian Ocean climate response to natural external forcing, such as volcanic eruptions, is still uncertain due to potential model biases and lack of validation from observations and subannual-resolution marine palaeorecords. Here we present monthly temperature and hydrology reconstructions derived from coral Sr/Ca and oxygen isotopes in the northeastern Indian Ocean back to 1774. Our reconstructions reveal anomalous and prolonged cooling and freshening during the early 19^th century (~1809-1824), which we attribute to a cluster of tropical volcanic eruptions that includes the unidentified 1809 and Tambora 1815 eruptions. The regional cooling and freshening were unusually strong compared to the wider Indian Ocean. The eruptions forced negative Indian Ocean Dipole (IOD)-like conditions in our reconstructions, followed by positive IOD-like conditions in subsequent years, regardless of eruption magnitude. Our results and other palaeorecords suggest positive IOD-like mean conditions during the early 19^th century, accompanied by stronger summer rainfall over areas of India and a negative Interdecadal Pacific Oscillation state, were associated with the regional cooling and freshening. Our findings highlight the sensitivity of the northeastern Indian Ocean to external forcing and that available observations, proxy records, and climate model simulations do not capture the full range of regional climate variability, complicating climate change projections for this highly populated region vulnerable to future climate extremes.

How to cite: Camelia, H., Felis, T., Kölling, M., Scheffers, S., and Chavanich, S.: Pronounced volcanic cooling and freshening in the northeastern Indian Ocean during the early 19th century, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20293, https://doi.org/10.5194/egusphere-egu26-20293, 2026.

Uncertainty in Earth System Model (ESM) projections of Indian Ocean biogeochemistry is often attributed primarily to differences in biogeochemical process representations. Here, we demonstrate that large present-day physical biases and divergent future physical climates also play a substantial role. Using a bias-corrected ocean-only model forced by air–sea flux anomalies from multiple CMIP6 models, we show that correcting present-day physical biases strongly amplifies projected summer surface chlorophyll (SChl) changes and substantially improves inter-model consistency.

Across key Indian Ocean upwelling regions, increased upper-ocean stratification driven by heat-flux anomalies consistently reduces SChl, highlighting the role of ocean warming in shaping future biogeochemical change. In contrast, wind-driven changes dominate the SChl response in several regions, particularly off southern India and off Sumatra, emphasizing strong regional differences in physical controls. These results underscore the central importance of monsoonal wind variability and its future evolution for Indian Ocean biogeochemistry, with implications for ecosystem functioning and the predictability of regional climate impacts. 

How to cite: Kwatra, S., Lengaigne, M., Iyyappan, S., Dutheil, C., and Vialard, J.: Physically Driven Uncertainty in Future Indian Ocean Chlorophyll: Roles of Stratification, Winds, and Bias Correction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20467, https://doi.org/10.5194/egusphere-egu26-20467, 2026.

Intermediate water masses in the tropical western Indian Ocean play a key role in subsurface thermohaline circulation by contributing to the redistribution of heat and salt, yet their variability on seasonal to interannual timescales remains poorly understood due to the historical scarcity of sustained in situ observations. We present an observational analysis of intermediate water mass variability based on a continuous subsurface mooring time series collected from 2019 to 2025 at the Seychelles-Chagos Thermocline Ridge (SCTR; 8°S, with the mooring located at 61°E during May 2019-June 2024 and relocated to 65°E thereafter). We focus on the intermediate layer spanning approximately 440-1190 m depth (corresponding to ~27.0-27.4/27.5 sigma-theta), using temperature, salinity, potential density, and spiciness. Pronounced changes in physical properties are observed between the earlier (2019-2021) and later (2022-2025) periods. Relative to the earlier years, the intermediate layer during later period exhibits freshening (34.82 to 34.79 PSU, -0.1%) and warming (7.06 to 7.19 °C, +1.8%), accompanied by a decrease in potential density (27.28 to 27.23 kgm^-3, -0.2%) and a concurrent increase in spiciness (0.64 to 0.66, +1.8%), suggesting potential changes in the relative contributions of intermediate water masses. To examine this possibility, we apply an optimal multiparameter (OMP) analysis to quantify the fractional contributions of Red Sea Overflow Water (RSOW), Indonesian Intermediate Water (IIW), and Antarctic Intermediate Water (AAIW). The OMP results show that RSOW has both the largest fractional contribution and the strongest interannual-scale variability among the three intermediate water masses at the SCTR accounting on average for ~0.59±0.05 of the intermediate layer indicating its dominant role in modulating intermediate-layer variability in the region. In comparison, IIW and AAIW contribute smaller mean fractions (~0.25±0.01 and ~0.13±0.03, respectively) and display comparatively weaker temporal variability. Notably, the mean RSOW fraction decreases during 2022-2025 from 0.62±0.04 to 0.57± 0.03 (-7.3%), whereas the contributions of IIW and AAIW increase from 0.24± 0.01 to 0.25± 0.01 (+7.2%) and from 0.11±0.03 to 0.15±0.02 (+34.6%), respectively. While RSOW remains the dominant intermediate water mass at the SCTR, the increased fractions of IIW and AAIW during the later years indicate an enhanced relative contributions of these water masses during the later period, consistent with the observed freshening and increase in spiciness in intermediate layer. By leveraging a rare continuous mooring time series, this study demonstrates the value of sustained in situ observations for resolving multi-year variability in intermediate water mass composition and properties at the SCTR region.

How to cite: Song, S., Nam, S., and Menezes, V. V.: Observed variability of intermediate water masses in tropical western Indian Ocean from a 2019-2025 subsurface mooring time series, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20563, https://doi.org/10.5194/egusphere-egu26-20563, 2026.

The tropical southeastern Indian Ocean regarded as a pivotal region for Indo-Pacific climate teleconnections, including phenomena such as the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Interdecadal Pacific Oscillation (IPO). However, long-term instrumental climate data are often lacking for tropical oceans. The geochemistry of massive stony corals provides a valuable record of past hydroclimatic conditions that compensates for this lack and surpasses existing data.

Using sub-seasonally resolved coral Sr/Ca and δ¹⁸O records from Browse Island, Australia spanning 1753–2011, this work provides new insights into sea surface temperature (SST) and salinity variability over interannual to multidecadal timescales. The Sr/Ca record reveals robust correlations with instrumental SST, capturing the long-term industrial era warming starting at the end of the Little Ice Age (LIA) and accelerating trends since the early 20^th century, indicative of anthropogenic forcing. The δ¹⁸O_seawater record, reconstructed from paired Sr/Ca and δ¹⁸O data, highlights hydrological variability driven by precipitation-evaporation dynamics, closely tied to the Australian monsoon, and ITF transport. While the imprint of the IOD seems to be reflected more in SST anomalies in the region, the influence of ENSO is recorded in hydrological anomalies due to changes in ocean advection. Long-term trends in δ¹⁸O_sw indicate centennial variability, reflecting complex interactions between monsoon-driven freshwater fluxes and ITF circulation. Freshening since the 1950s is likely caused by the intensified hydrological cycle due to anthropogenic warming. The SST reconstruction tracks the cooling and warming periods indicated by the IPO.  

The findings underscore the influence of interannual and decadal variability, particularly the IOD, ENSO, and the Interdecadal Pacific Oscillation (IPO), on SST and salinity, mediated by the combined effects of monsoon dynamics and ITF transport. Discrepancies between δ¹⁸O_sw and Sr/Ca-SST trends emphasize the need for further investigation into the driving mechanisms of long-term climate variability by pantropical teleconnections.

How to cite: Zinke, J., Krawczyk, H. A., Behera, P., Boom, A., Hambach, B., Pfeiffer, M., Cantin, N., Lough, J. M., and Wilson, P.: A 250-year SST and salinity coral record reflecting Indo-Pacific teleconnections by the Indonesian Throughflow , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21158, https://doi.org/10.5194/egusphere-egu26-21158, 2026.

Ocean submesoscale eddies, characterized by horizontal scales less than the first baroclinic Rossby radius of deformation, are increasingly recognized for their critical roles in marine ecosystems, ocean energy balance, and the Earth’s climate system. Despite extensive research on these submesoscale features through high-resolution simulations and regional observations, our knowledge of their dominant driving mechanism from a global perspective is very limited. Here, we present the global observational evidence for the primary role of mesoscale Lagrangian coherent structures (LCSs) in driving submesoscale eddy generation by synergic application of high-resolution spaceborne synthetic aperture radar data and radar altimeter data. Applying a deep-learning detection system to more than three million global Sentinel-1 and Envisat SAR images, we found that more than 80% of detected submesoscale eddies are clustered within a 10-km range around mesoscale LCSs characterized by high kinetic energy and persistent straining. Further composition analysis quantifies that more than half of submesoscale eddies occur within the ring-shaped areas of coherent mesoscale eddies, where the strong strain is conducive to frontogenesis. Our findings highlight that the generation of submesoscale eddies is attributed to instabilities initiated by strain-induced frontogenesis. This study establishes a new paradigm for locating submesoscales by targeting LCSs, thereby supporting a global evaluation of their contributions to energy balance and material transport.

How to cite: Zi, N., Li, X.-M., and Gade, M.: Ocean mesoscale coherent structures dominate the generation of global submesoscale eddies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-516, https://doi.org/10.5194/egusphere-egu26-516, 2026.

The Arabian Gulf (also referred to as the Persian Gulf) can exhibit energetic mesoscale dynamics despite its semi enclosed geometry. Using the GLORYS12V1 ocean reanalysis (1993–2020), this study assesses longterm patterns in sea surface height (SSH) variability and the dominant forces shaping them. The results reveal two persistent low SSH systems: one located in the northwestern part of the basin and the other in the southern Gulf. These features act as focal points of mesoscale circulation. Both systems intensify during the summer season, when atmospheric forcing and surface buoyancy losses are strongest. Their locations coincide with regions of enhanced mixing and surface cooling, suggesting a possible role in the Gulf deep water formation.

Seasonal circulation analysis indicates a summer cyclonic regime influenced by the Iranian coastal current and the Gulf coastal current, which interact with and modulate the two eddies. SSH and Eddy Kinetic Energy (EKE) fields display interannual alternation in the dominance of each eddy, highlighting their unequal sensitivity to regional dynamical conditions and basin-scale climate variability. Three dominant modes were identified using Empirical Orthogonal Function (EOF) analysis: a basin wide meridional gradient (EOF1), a two eddy dipolar mode linked to wind variability (EOF2), and southern eddy mode (EOF3) associated with the Indian Ocean Dipole Mode Index (DMI). Together, these modes describe a recurring low-frequency exchange of energy between the northern and southern eddies.

How to cite: Alrushaid, T. and Al Senafi, F.: A Dynamic Dipole: Longterm Mesoscale Oscillation in the Arabian Gulf, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-766, https://doi.org/10.5194/egusphere-egu26-766, 2026.

In global ocean circulation and climate models, the bottom-enhanced turbulent mixing is often parameterized by assuming that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a non-hydrostatic two-dimensional numerical model in the horizontal-vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett-Munk (GM) background internal wave field, we find that ζ of the internal lee wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U₀, the horizontal wavenumber k_H, and the height h_T of the seafloor topography. When the steepness parameter (Sp=Nh_T/U₀: N is the background buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM background internal wave field to create a turbulent mixing region with ζ that extends further upward from the seafloor as U₀ increases, but is nearly independent of k_H. In contrast, when Sp exceeds 0.3, the inertial oscillations (IOs) gradually develop at heights not far above the seafloor topography, inhibiting the upward propagation of the bottom-generated internal lee waves. By interacting with the background IOs, the upward propagating internal lee waves dissipate some of their energy, but simultaneously contribute the rest of their energy to amplify the IOs. The oscillatory flow, consisting of the superposition of the steady flow and the IOs, efficiently generates upward propagating internal lee waves during the period centered on the time of its maximum, when it becomes transiently stationary.

How to cite: He, Y. and Hibiya, T.: The vertical structure of internal lee wave-driven benthic mixing hotspots, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2202, https://doi.org/10.5194/egusphere-egu26-2202, 2026.

Two-dimensional (2D) estimates of the upper-ocean vertical velocity w have been commonly performed based on single hydrographic distance–depth sections. However, biases of these estimates have seldom been investigated. We conduct such an investigation employing a 2-month dataset (including temperature, salinity, and horizontal velocity) at a typical front, the Almeria–Oran Front in the Mediterranean Sea, which was collected by a glider fleet piloted in parallel across-front sections. Specifically, using daily objective maps constructed from the dataset, we perform three-dimensional (3D) and 2D estimates of the balanced w (w_3D and w_2D) through the quasigeostrophic omega equation and evaluate w_2D against w_3D justified previously. Results show a significantly biased w_2D that is estimated assuming a straight front without curvature. Generally, in the 400-m upper ocean, w_2D and w_3D have a weak spatial correlation of 0.4–0.6; w_2D also presents a notably different magnitude, less than 50% of w_3D (even less than 20% in many cases). We find a pronounced curvature-induced shearing deformation (of horizontal density gradients by geostrophic flows) effect destroying the geostrophic balance and so is the associated w to restore the balance; precluding this effect in w_2D leads to the biases. These biases are also analyzed using the potential vorticity conservation principle: As the curvature causes the across-section vorticity advection, water parcels advected by the across-section flow change their vorticity; they have to be vertically compressed/stretched, requiring w that is neglected in w_2D. Therefore, the biased w_2D may be insufficient for understanding the vertical heat transport and its impact on the climate system.

How to cite: Liu, L., Jing, Z., and Xue, H.: Revisiting the Two-Dimensional Estimate of Ocean Vertical Velocity Using Underwater Glider Fleet Observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2681, https://doi.org/10.5194/egusphere-egu26-2681, 2026.

Internal tides (ITs) play a fundamental role in ocean dynamics by transferring tidal energy through cascades to small-scale turbulence, ultimately driving diapycnal mixing that sustains the deep-ocean circulation and regulates biogeochemical cycles. While ITs energy sinks are traditionally attributed to topographic interactions, the impact of surface wind forcing on this energy pathway remains a significant area of uncertainty. To address this knowledge gap, this study employs a dynamical decomposition approach applied to realistic ITs-resolving numerical simulations to quantify the wind impact on ITs in the global ocean. Our analysis reveals that wind forcing globally imposes a strong damping effect on ITs, with a median magnitude of the wind work on ITs of O (10^-4)  W/m². Globally, this wind damping accounts for a non-negligible fraction of the total ITs energy sink, substantially influencing the distributions of ITs and the diapycnal mixing they induce. To provide observational constraints beyond numerical simulations, we develop a scaling approach to estimate wind damping of ITs by projecting the ITs velocity onto the wind direction and evaluating the net wind work over a tidal cycle. Our findings collectively suggest that wind damping constitutes a critical component of the ITs energy budget. This challenges the conventional paradigm of predominantly topography-driven energy sinks and underscores the necessity of integrating atmospheric forcing into a holistic understanding of ITs energy budget.

How to cite: Liu, Y., Liu, Z., Wang, D., and Wang, C.: Wind Damping of Internal Tides in the Global Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3315, https://doi.org/10.5194/egusphere-egu26-3315, 2026.

Observational data are fundamental for understanding geophysical dynamics, yet constraints such as cost or environmental conditions often result in sparse and noisy data. Recovering physical quantities such as energy spectra from such data constitutes a classic ill-posed inverse problem. Traditional approaches typically rely on interpolation to regular grids, which can introduce errors, especially for shallow spectra.

This study proposes a random recovery framework that infers spectra from the second-order statistics of observations under suitable assumptions. Observation noise is reduced using the Best Linear Unbiased Estimator (BLUE), while shrinkage techniques are employed to obtain stable and invertible covariance estimates under limited sampling. To achieve robust solutions without interpolation, we introduce a high-order L² regularization, using the discrepancy principle to determine the optimal regularization parameter. For high-dimensional settings, we apply hard clustering to group similar spectra, thereby reducing the number of unknowns and enhancing recovery efficiency.

Numerical experiments demonstrate that this method offers a robust and practical approach for spectral recovery without interpolation, making it particularly suitable for sparse and noisy observations.

How to cite: Liang, H., Jäger, J., Kutsenko, A., and Oliver, M.: Interpolation-free method to recover spectra from sparse observations of random fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3458, https://doi.org/10.5194/egusphere-egu26-3458, 2026.

Within the scope of the international project NA-VICE, an oceanographic campaign was conducted from the Azores to Iceland in the summer of 2012. During this campaign, a Mediterranean Water Eddy (meddy) was identified at 41.3ºN, 27.1ºW. Meddies are deep subsurface anticyclones containing anomalously warm, saline modified Mediterranean Water. Generated at the topographically trapped Mediterranean Undercurrent, they can transport salt and heat great distances from the eastern boundary, accounting for at least half of the Mediterranean Water salt flux into the North Atlantic. Despite their significant role in forming the Atlantic intermediate water masses, their life cycle remains poorly understood. In this study, a meddy was identified using vertical CTD casts and ADCP data. Its core was located between 900 and 1100 m and was characterized by a salinity anomaly of 0.26, a temperature anomaly of 2.4 °C, and a negative anomaly in the buoyancy frequency. Tracing the meddy via its sea-surface manifestation and an Argo float suggests that it originated on the Irish continental slope and moved southwest toward the Mid-Atlantic Ridge. The existence of meddy generation sites well north of the Iberian Peninsula, not listed in recent studies, implies that meddies’ contribution to the heat and salt budget of the North Atlantic mid-depths might be underestimated. The detailed ADCP observations during the cruise suggest that, at that time, the meddy was interacting with a strong surface cyclone, which trapped a portion of the Mediterranean Water from the meddy, thus contributing to its decay. Future studies should investigate the additional contribution of meddies generated north of the Iberian Peninsula to the thermohaline balance of the interior North Atlantic.

How to cite: Serpa, T., Bashmachnikov, I., Relvas, P., and Martins, A.: A Northern Meddy at the Mid-Atlantic Ridge, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7682, https://doi.org/10.5194/egusphere-egu26-7682, 2026.

The southern coast of Korea is characterized by a complex Rias coast and a barotropic flow regime dominated by strong tides. Under the influence of tidal forcing, complex current patterns develop regularly, leading to the generation of coastal eddies with spatial scales ranging from 0.1 to 10 km. These submesoscale eddies serve as an intermediary between mesoscale dynamics and small-scale turbulence, playing an important role in energy transfer. The goal of this study is to understand the dynamics of eddies observed during a field campaign near the Yokji region and Nodae Island using an eddy-resolving numerical model (with a grid resolution of 30 m). A numerical hydrodynamic model for the region was developed by the Deflt3D model and validated using Acoustic Wave and Current profiler data, velocity fields estimated from unmanned aerial vehicle imagery, and Sentinel-2 true-color imagery. To characterize eddy generation and interaction processes, the barotropic vorticity diagnostics for the depth-integrated flow are used. The local dynamics of submesoscale eddies highlight that the nonlinear advection is the leading local source of vorticity over the study area, followed by secondary contributions from bottom pressure torque and bottom drag curls interacting with topography. We employed a coarse-graining approach to estimate multiscale energy fluxes and kinetic energy transfer. The analysis suggests a tendency for localized upscale energy transfer into adjacent larger-scale background current during the eddy dissipation phase, and that barotropic instability near the cape is a potential contributor to the observed eddy generation. This framework will offer broader applicability for understanding submesoscale energetics and instability processes in tidally dominated shallow coastal systems.

How to cite: Kim, S.-Y., Choi, J.-G., and Jo, Y.-H.: Submesoscale Eddy Dynamics and Energy Transfer in a Tidally Dominated Coastal System, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9204, https://doi.org/10.5194/egusphere-egu26-9204, 2026.

The multifaceted role of oceanic mesoscale eddies in the coupled climate system remains a focus of scientific discussion and research. In this study, we present a set of simulations utilizing the same model and resolution to disentangle the role of mesoscale eddies for global and local climate by applying an eddy backscatter parameterization to enhance or suppress eddy variability.

 Mesoscale eddies can be seen as the oceanic high- and low-pressure systems, acting as drivers of ocean weather by introducing chaotic small-scale features into the large-scale flow. By redistributing heat, momentum, and tracers such as nutrients or dissolved trace gases, they influence and shape ocean circulation patterns and affect marine productivity through vertical mixing. Eddies also interact with the atmosphere, modulating heat and moisture exchange, which contributes to variations in wind patterns, storm tracks, and ultimately influences regional and global climate dynamics.

 In this study, we conduct three distinct sets of ensemble simulations using the AWI-CM3 climate model to investigate the impact of ocean weather on climate variability. All configurations use the same spatial resolution but different levels of eddy activity due to different parameterization calibrations. One configuration is largely resolving the eddy variability, one simulation is substantially suppressing it and a reference configuration is somewhere in between using standard model parameters for AWI-CM3. With a resolution of 30 km in the atmosphere and 10–60 km in the ocean, these simulations are sufficiently detailed at the coupling interface to directly resolve air-sea interactions at the feature level. The ensemble simulations span the period from 1950 to 2015. They are then used to study the effect of eddy activity on long-term variability in large-scale ocean and atmospheric dynamics. A special focus is on mesoscale atmosphere-ocean interactions along eddy active regions such as the western boundary currents, highlighting substantial changes in local heat fluxes as well as large-scale dynamics, both in terms of climatic means and temporal variability and including changes in, e.g., Gulf Stream position and strength and its consequences for the atmospheric circulation.

How to cite: Juricke, S. and Hutter, N.:  Ocean eddies in the climate system: Disentangling the role of mesoscale eddies in atmosphere-ocean interactions and global climate variability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10743, https://doi.org/10.5194/egusphere-egu26-10743, 2026.

Internal solitary waves (ISWs) are widely distributed in marginal sea areas. When propagating over steep and complex continental slopes and shelves, ISWs will undergo deformation and breaking, thereby enhancing vertical mixing and energy transfer in the inner ocean and playing an important role in Earth’s biogeochemical cycles. In marginal seas, basin-scale topography not only influences the along-propagation direction evolution of ISWs, but also affects their transverse diffraction, leading to asymmetric structures along the basin axis observed in satellite imagery.

In this work, a theoretical model based on the variable-coefficient Kadomtsev-Petviashvili (vKP) equation is developed to reveal the influence of continental slopes within basin topography on the transverse diffraction of ISWs, and to explain the mechanisms governing the 2+1-dimensional propagation and evolution of ISWs over the slopes of the Sulu Sea. Using the climatological annual-mean density and bathymetry of the Sulu Sea, two idealized configurations are constructed in which topographic variations are isolated to either the along-propagation direction or the cross-propagation direction. The results show that all the dynamical coefficients vary most significantly over the slope region with water depths between 300-3000 m.

Numerical simulations of the vKP model indicate that cross-propagation direction slope variations play a dominant role in shaping the two-dimensional spatial distribution of ISWs. The asymmetric distribution of ISWs along the basin axis is primarily caused by phase speed differences induced by depth variations. In contrast, both the along- and cross-propagation directions slope variations influence the waveform evolution of ISWs. Enhanced nonlinearity in shallower regions leads to waveform steepening and larger amplitudes, whereas weaker nonlinearity and reduced amplitudes are found on the deeper side.

Furthermore, based on the simulation results, the spatial distributions of ISW energy and energy flux in the Sulu Sea are estimated. Although ISWs on the deeper side exhibit smaller amplitudes, their energy flux is significantly stronger, reaching approximately 10 kW/m, which is twice the flux on the shallow side.

How to cite: Ruan, X.: Influence Mechanism of Continental Slope on the 2D Propagation and Evolution of Internal Solitary Waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11738, https://doi.org/10.5194/egusphere-egu26-11738, 2026.

The representation of the Meridional Overturning Circulation (MOC) remains a major source of uncertainty in climate models, since different models show largely different upwelling pathways and magnitudes. The present paradigm is that the MOC consists of a quasi-adiabatic middepth overturning that is mainly Southern Ocean wind-driven with little interior diabatic transformation,  and a deep cell that is mixing driven. However, often the mixing and associated diapycnal transports are diagnosed without explicitly accounting for spurious numerical mixing, which may affect water mass transformation in models.

Here, we assess the role of such numerical mixing in shaping diapycnal transport and overturning circulation in models by using the unstructured-mesh ocean model FESOM2 in an idealized Neverworld2 configuration. We do this in two idealized configurations: one with parameterized eddies and one in which eddies are resolved (the latter being finer). By comparing vertical mixing profiles and their horizontal distributions, and by exploiting FESOM2's discrete variance decay and water-mass transformation diagnostics, we identify potential sources of spurious mixing and quantify its contribution to the diapycnal upwelling in the model. 

How to cite: Beauregard, G., Griesel, A., Pollmann, F., Eden, C., Scholz, P., and Danilov, S.: How much Overturning is Numerical? Identifying Numerical Mixing in Idealized Ocean Model Experiments. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12320, https://doi.org/10.5194/egusphere-egu26-12320, 2026.

   Interferometric missions such as SWOT, together with conventional nadir altimetry missions, provide unprecedented observations of sea surface height variability relevant to climate studies. However, the accurate exploitation of these measurements requires the correction of high-frequency signals associated with wind- and gravity-driven processes, among which internal tides and internal solitary waves represent a significant source of variability.  Internal tides, also referred to as baroclinic tides, are internal gravity waves that oscillate at tidal frequencies in the ocean interior and produce sea surface height signatures of a few centimeters. For the first vertical mode, internal tides typically exhibit horizontal wavelengths ranging from about 50 to 250 km, while higher modes are characterized by smaller spatial scales. As internal tides propagate, they may lose phase coherence and undergo nonlinear steepening, resulting in organized packets of internal solitary waves with typical horizontal scales of 1–15 km, which are clearly resolved and observed by the high spatial resolution of SWOT.

   Previous studies have demonstrated that global internal tide (IT) atlases are effective at correcting the stationary component of internal tide signals in altimetric observations (Carrère et al., 2021). In this study, we evaluate the performance of three recent global IT atlases—HRET22 (Zaron, 2024), ZHAO30yr (Zhao, 2025), and MIOST-IT24 (Tchilibou et al., 2025)—for the removal of stationary internal tide variability in SWOT and conventional nadir altimetry measurements.

   Our results show that these atlases reduce up to approximately 20% of sea level anomaly (SLA) variance at horizontal scales between 50 and 200 km. At the global scale, MIOST-IT24 generally outperforms HRET22, while ZHAO30yr exhibits the best performance in specific regions, notably the eastern Pacific, the Atlantic Ocean, and the northern Madagascar region.

    However, these global internal tide models have little to no impact at spatial scales below 50 km, which are primarily associated with higher vertical modes and internal solitary waves. This limitation highlights the need for the oceanographic community to develop new correction strategies and methodologies capable of addressing small-scale and nonlinear internal wave signals, including solitons, in high-resolution altimetric observations.

How to cite: Carrere, L., Tchilibou, M., Dabat, M.-L., Lyard, F., Ubelmann, C., and Dibarboure, G.: Use of new baroclinic tide models to improve the correction of internal tides in SWOT and nadir altimeter data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12490, https://doi.org/10.5194/egusphere-egu26-12490, 2026.

Ocean mesoscale eddies are ubiquitous features of the open ocean and strongly influence the ocean’s physics, chemistry, and biology. Mesoscale eddies play a critical role in the climate system and regulating the exchange of heat and carbon with the atmosphere. They also play a significant role in the redistribution of heat, salt, carbon, and nutrients around the ocean. Thus, proper modeling of eddies in both historical and future climates is crucial to accurately capturing the Earth system. Climate projections using global coupled models with eddy ocean components have recently started to become more widely used. Despite their critical role in understanding and forecasting climate characteristics, these so-called eddy-permitting models have not been rigorously explored to verify that resolved eddies are realistic, and thus any downstream scientific testing of hypotheses in biogeochemistry, ocean physics or other associated Earth systems impacted by eddies hinge on this critical assumption. This presentation compares the characteristics and behavior of observed eddies in ¼ degree satellite altimetry data with eddies detected in ¼ degree reanalysis data and ocean model output. 

When compared to eddies observed in satellite altimetry data, eddies in reanalysis data and ocean model output are missing almost 30% of the number of eddy trajectories. Further, many characteristics of eddies in reanalysis data and ocean model output differ from eddies observed in altimetry data. At a high level, eddies in reanalysis data and ocean model output generally live longer, are larger, and are weaker than observed eddies in satellite altimetry data. These comparisons are made both locally and in the global aggregate to assess the differences in both the global distribution of eddy characteristics as well as differences in the regional eddy behavior. 

How to cite: Lombardi, B., Grooms, I., and Kleiber, W.: Mesoscale Eddy Verification in an Eddy-Permitting Ocean Models and Reanalysis Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13777, https://doi.org/10.5194/egusphere-egu26-13777, 2026.

Global ocean models at resolutions that do not resolve mesoscale eddies lack variability, not just on scales that they cannot represent because they are below the grid scale, but also on resolvable scales. This research develops a backscatter parameterizations that increases variability on the resolved scales of a non-eddying model. The parameterization acts on the model's momentum equations, and sets the rate of backscatter, viz. the rate at which energy is injected to the resolved scales, proportional to the rate at which the Gent-McWilliams (GM) parameterization removes energy from the resolved scales. This models the physical process whereby mesoscales convert large-scale potential energy to kinetic energy, and then transfer that kinetic energy towards larger scales. These parameterization is implemented in the MOM6 ocean model, and results are presented on its impact in simulations at nominal 2/3-degree resolution. Stochastic GM+E acts primarily in the Southern Ocean, the North Atlantic Current, and the Kuroshio Extension, where it impacts SST variability and southern-hemisphere sea ice extent.

How to cite: Grooms, I., Agarwal, N., Marques, G., Pegion, P., and Yassin, H.: Stochastic GM+E: An energetically-informed stochastic backscatter scheme for ocean models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14014, https://doi.org/10.5194/egusphere-egu26-14014, 2026.

Oceanic energy budgets and mixing parameterizations are largely framed around tidal forcing, reflecting the availability of long-term, global datasets for barotropic and baroclinic tides. In contrast, internal waves, particularly Internal Solitary Waves (ISWs), remain poorly represented in global energy frameworks, despite their recognized role in transferring energy across scales, driving localized mixing, and modulating stratification. This imbalance is not only conceptual but observational: the lack of consistent, global datasets has limited the integration of internal wave processes into large-scale circulation and climate-relevant ocean models.

ISWs are nonlinear internal waves that propagate over long distances in stratified oceans, linking mesoscale and large-scale forcing to small-scale turbulence. Beyond their surface expressions observable from space, ISWs involve strong internal currents and large vertical displacements of isopycnals, with implications for offshore operations, marine structures, navigation, and ocean energetics. However, their transient nature and wide spatial extent make them particularly challenging to observe systematically, resulting in fragmented and geographically biased observational records.

We present the Internal Waves Service (IWS), which provides a first step towards addressing this gap, as a global, open, service-oriented framework for the systematic detection, mapping, and archiving of ISWs from satellite Earth Observation data. The service currently exploits synthetic aperture radar (SAR) imagery acquired by Sentinel-1 in Wave Mode, which provides unique, globally distributed observations of ISW surface signatures. Unlike traditional studies focused on specific regions or short time periods, the IWS processes all Sentinel-1 Wave Mode acquisitions on a continuous basis, enabling consistent global mapping of ISW presence and absence.

ISW detection is performed using an AI-assisted classification framework applied to SAR vignettes, supported by expert validation and iterative model refinement. The resulting products form a persistent, standardized dataset documenting spatial patterns and temporal variability of ISW activity across ocean basins.

By consolidating previously fragmented observations into a coordinated global dataset, the IWS provides a new observational basis for assessing the role of internal waves within ocean energy pathways. This systematic mapping supports comparative analyses, facilitates model evaluation, and opens the door to more consistent integration of internal wave processes alongside tides in multiscale ocean dynamics and energy budgets. Developed as a community-driven initiative involving 24 research institutions across 12 countries, the IWS is designed to evolve towards broader sensor integration and enhanced spatial coverage, strengthening its relevance for ocean modelling and climate studies.

How to cite: Santos-Ferreira, A., Pinelo, J., da Silva, J. C. B., Johannessen, J. A., Magalhaes, J. M., Forget, G., Gonçalves, J., Chapron, B., Gommenginger, C., Pinheiro, M., Carr, M., Buijsman, M., Oikonomoy, E., Stastna, M., Shutler, J., Pineda, J., Terletska, K., Hartharn-Evans, S., Holt, B., and Collard, F. and the Internal Waves Service (IWS) Consortium: Internal Waves Service: Towards Systematic Global Observation of Internal Solitary Waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14433, https://doi.org/10.5194/egusphere-egu26-14433, 2026.

Western boundary undercurrents (WBUCs), present beneath almost all western boundary currents, are significant for transporting subsurface mass and energy and connecting regional circulations. Observations suggest that WBUCs can have strong ageostrophy and intra-seasonal variability, but many dynamic details remain unclear. Here, we analyze a representative model idealized from the Kuroshio Current and Luzon Undercurrent (KC and LUC); other instances like the Gulf Stream and the Deep Western Boundary Current are also feasible. A cross-shore mean flow section, extracted from realistic simulations, serves as the only input. We employ biglobal instability analysis (BIA) and high-resolution (up to 500m) regional simulations to reveal the nonlinear instability of the WBUC, its interaction with the upper-layer, and the induced mesoscale and submesoscale turbulence. First, we solidly verify BIA by showing that the predicted evolution of dominant eigenmodes closely agrees with the model results. Second, an upper-layer (depth < 500m) KC mode and a middle-layer (500 to 1500m) LUC mode are identified. The nonlinear instability of the KC mode leads to strong variability and periodic reversal of the LUC. The subthermocline-eddy-like LUC mode has stronger nonlinearity, but negligibly affects the upper layer. Qualitative and, in some cases, quantitative agreement with observations is obtained. The kinetic energy spectra for the subthermocline can exhibit the k scaling as the upper layer, jointly driven by the KC and LUC instability. Moderate centrifugal instability is identified for the LUC near the topography, leading to locally enhanced submesoscales and eddy fluxes. The present model has the potential to serve as a benchmark for global WBUCs, providing theoretical explanations to observational trends and helping improve the modelling for multi-layer circulations. 

How to cite: Chen, X. and Gan, J.: On the nonlinear instability and submesoscale turbulence for western boundary currents and undercurrents , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15575, https://doi.org/10.5194/egusphere-egu26-15575, 2026.

We analyze barotropic and baroclinic instabilities of axisymmetric vortices in a hierarchy of quasigeostrophic models. Revisiting the classical vortex profiles of Carton and McWilliams (1989), we show that these simple vortex profiles possess very low-dimensional unstable manifolds in the nondissipative limit.  Using numerical simulations of the potential vorticity together with analytical calculations, we construct systematic approximations of these unstable manifolds. We then derive reduced-order models using the theory of extended normal forms on the low-dimensional reduced dynamics on these manifolds. The resulting Stuart–Landau–type amplitude equations, obtained in both data-driven and equation-driven settings, capture growth rates, frequencies, and the early nonlinear evolution leading to vortex deformation and breakup. This yields an interpretable and low-dimensional predictive description of the dynamics of vortex instabilities.

How to cite: Kaszas, B. and Thomas, L. N.: Low-Dimensional Invariant-Manifold Models of Vortex Instability , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15715, https://doi.org/10.5194/egusphere-egu26-15715, 2026.

Turbulent mixing plays a key role in regulating heat and salt distribution in polar oceans, influencing water-mass transformation and ice-ocean interactions, yet direct observations of mixing south of 60°S remain scarce. As a result, the magnitude and drivers of vertical mixing in the Southern Ocean, particularly under and near sea ice, remain poorly constrained. Here, we present microstructure turbulence observations collected over three consecutive austral summers (2023-2025) in the King Haakon VII Sea (the eastern part of the Weddell Gyre, Southern Ocean). We observe enhanced sub-surface mixing over the continental slope, with mean dissipation rates an order of magnitude higher than typical values in the open ocean below the mixed layer.

This elevated mean dissipation on the continental slope is strongly influenced by a single episodic extreme event, reaching dissipation rates of up to 3 × 10⁻⁶ W kg⁻¹ mid-depth. Excluding the extreme event, we still observe enhanced mixing on the slope, with mean dissipation about three times higher than the open ocean values below the mixed layer. The observed elevated dissipation is associated with peaks in velocity shear and occurs when tidal inversion models predict periods of large tidal acceleration during the spring-tide cycle. Comparisons with internal-tide mixing model outputs further suggest that the enhanced slope mixing is driven by tides interacting with the steep continental slope.

Based on our observations, the continental slope mixing acting on the local temperature gradients produces a mean upward heat flux of approximately 3 W m⁻² into the base of the Winter Water layer, with peak values of 10 W m⁻². The extreme mixing event occurred within the Winter Water itself, where temperature gradients are weak. However, if the same extreme turbulence were to occur on the stronger thermal gradients at the base of the Winter Water layer, it could generate vertical heat fluxes of up to 124 W m⁻² from the underlying warm waters into the Winter Water layer.

Model-based estimates further suggest that tidal mixing along the Antarctic continental slope could drive a circumpolar mean heat flux of approximately 9 W m⁻² into the base of the Winter Water. These results highlight continental slope mixing as a mechanism for upper-ocean heat redistribution, with implications for Antarctic sea-ice formation, melt processes, and polar ocean heat budgets more broadly.

How to cite: Hus, J. J., Meyer, A., Hattermann, T., Peña-Molino, B., and de Lavergne, C.: Observed Enhanced Mid-Depth Mixing on the Antarctic Continental Slope Drives Heat Flux Into the Winter Water Layer., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16047, https://doi.org/10.5194/egusphere-egu26-16047, 2026.

The ocean is forced at large scales by fluxes of momentum and buoyancy. Yet, climate equilibrium can only be reached through the dissipation of these energy sources, which occur at much smaller scales. Understanding how energy is transferred across this wide range of scales and the routes to dissipation are therefore key to understand the ocean response to future climate scenarios and remains a central challenge in physical oceanography. While the inverse kinetic energy cascade associated with geostrophic turbulence has been extensively studied, the direct cascade of kinetic energy and the processes that enable energy transfer towards dissipative scales remain incompletely understood and poorly constrained in global ocean models.
In this talk, we review some of the recent work achieved in identifying and quantifying the processes leading to cross-scale energy fluxes using flow decomposition methods and spectral fluxes analyses, applied to realistic high-resolution simulations forced with eddies and internal waves. We show how the interaction between eddies and internal waves are central in enhancing the direct energy cascade –as opposed to the common paradigm relying on interactions among internal waves–, and in reducing the inverse energy cascade. We describe as well the physical processes underlying these interactions.
These studies establish eddy–internal wave interactions as a fundamental component of the ocean energy budget, with implications for mixing, dissipation, and the parameterization of sub grid scale processes in ocean models.

How to cite: Delpech, A.: Eddy-internal waves interactions and their contribution to cross-scale energy transfers in the ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17830, https://doi.org/10.5194/egusphere-egu26-17830, 2026.

Internal Solitary Waves (ISW) are fundamental components of coastal ocean dynamics, playing a pivotal role in sediment transport, nutrient mixing, and the dissipation of tidal energy. These non-linear oscillations, driven by gravitational forces within stratified water columns, manifest on the ocean surface as curvilinear bands of varying roughness. This modulation of surface roughness makes them detectable by Synthetic Aperture Radar (SAR) sensors despite their negligible slope on the surface. However, while SAR offers high-resolution, all-weather monitoring capabilities, the automated quantification of ISW characteristics remains a complex challenge. Difficulties arise not only from signal contamination by atmospheric and oceanic features such as wind and rainfall, but also from the high geometric variability of the wavefronts themselves.

We propose a comprehensive analytical framework for detecting, segmenting, and characterizing internal waves using Sentinel-1 SAR observations. The methodology is developed and tested on a dataset of Level-1 Ground Range Detected Interferometric Wide Swath (GRD IW) images acquired along the northeast coast of South America. This region is oceanographically unique due to the intense stratification induced by massive coastal river discharge, specifically from the Orinoco and the Amazon rivers.

Our approach employs a two-stage process that synergizes deep learning with geometric modeling. In the first stage, a U-Net architecture segment the observation into two classes: the wave packets and the specific leading waves. The model is trained to predict a distance map relative to the feature boundaries rather than a simple binary mask. This pixel-wise regression, performed at a resolution of 50 m/px, is validated against manual annotations, providing a robust identification capability where no comparable high-resolution groundtruth exists.

Following segmentation, the second stage focuses on physical characterization. The leading wave of each detected packet is modeled using an adaptive polynomial function. Optimized via gradient descent, this function fits the curvilinear shape of the wavefront. This mathematical representation allows for the precise computation of wave orientation and propagation direction. Subsequently, the wave packet is projected onto a geometry orthogonal to the leading wave to obtain a curvature-independant representation of the wave packet. A Fourier Transform is applied to this projection to calculate the dominant wavelength. Furthermore, by analyzing the spacing and propagation direction, deducing the generation chronology of successive packets produced by tidal cycles.

Results demonstrate strong agreement between automated detections and regional dynamics. The spatial distribution reveal detection in the vicinity of . In the vicinity of Trinidad and Tobago, the spatial distribution highlights generation hotspots northwest of straits and continental shelf. Activity peaks during the autumn months, coinciding with the maximum discharge of the Orinoco River, inline with a strong modulation by stratification stability. Statistical analysis reveals a mode wavelength of approximately 350 meters, but is biased by the manual segmentation dataset. While challenges remain regarding overlapping wave fields, this tool provides a robust pathway for monitoring internal wave energetics, offering significant potential for synergy with altimetry missions, such as SWOT, and in-situ coastal management.

How to cite: Colin, A. and Husson, R.: Internal Solitary Waves characterization on Sentinel-1 observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19271, https://doi.org/10.5194/egusphere-egu26-19271, 2026.

The dynamic processes that generate a continuous oceanic energy spectrum remain poorly understood. We investigate the relative roles of tides, mesoscale eddies, high-frequency winds, and nonlinear interactions using a submesoscale-resolving ICON simulation of the South Atlantic, which captures substantially more ocean variability than conventional eddy-resolving models. Model realism is demonstrated by comparing simulated energy spectra with observations from a dedicated field campaign and satellite data. Sensitivity experiments isolate individual processes, including simulations without tidal forcing to suppress tidal waves, without high-frequency winds to suppress near-inertial waves and without mesoscale eddies to reduce wave–mean-flow interactions. Frequency–wavenumber spectra are used to distinguish random variability from wave-driven variability by identifying elevated energy along the first modes of the dispersion relation. We find that tides and high resolution are essential to reproduce realistic energy levels in the internal wave band. Suppressing near-inertial waves reduces energy between tidal peaks while enhancing energy at the peaks, highlighting the importance of wave–wave interactions in sustaining a continuous spectrum. In contrast, suppressing mesoscale eddies has a weaker effect, suggesting that wave–mean-flow interactions play a less significant role.

How to cite: Epke, M. and Brüggemann, N.: Drivers of the Continuous Oceanic Energy Spectrum at High Frequencies: A realistic modeling approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21105, https://doi.org/10.5194/egusphere-egu26-21105, 2026.

Mixing and Water Mass Transformations (WMT) in the ocean are key to the dynamics, ranging from small-scale dissipation to large-scale overturning circulations. Therefore, the quantification of mixing and WMT in ocean models is of fundamental importance and enables a more detailed analysis of oceanic processes. In this presentation different diagnostic approaches developed during the last years are reviewed and compared. These diagnostic methods also offer to investigate the contributions originating from discretization errors in the numerical transport schemes, causing spurious numerical mixing and spurious overturning circulations. Subtleties of the methods and recent refinements will be presented to provide an accurate analysis framework that is consistent with analytical theories and the discrete model equations.

How to cite: Klingbeil, K.: Analyzing Mixing and Water Mass Transformations in Ocean Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21305, https://doi.org/10.5194/egusphere-egu26-21305, 2026.

Global mean surface temperature (GMST) reached a new record in 2024, exceeding pre-industrial levels by approximately 1.5 °C for the first time. The long-term rise in GMST is driven by Earth’s radiative imbalance at the top of the atmosphere, caused by human-induced increases in heat-trapping greenhouse gases. Superimposed on this long-term warming trend are natural variations like those associated with El Niño and La Niña. Following an unusual triple-dip La Niña from 2020 to 2023, a strong El Niño developed in boreal spring 2023 and persisted through mid-2024. From the final year of the La Niña (July 2022–June 2023) to the subsequent year encompassing the 2023–24 El Niño (July 2023–June 2024), GMST rose by an unprecedented 0.36 °C to 1.5 °C above the 1850-1900 average. This presentation demonstrates that the primary driver of this abrupt increase in GMST was the release of heat previously stored in the ocean during the prolonged La Niña, which was rapidly transferred to the atmosphere during the 2023–24 El Niño event.

How to cite: McPhaden, M., Jiang, N., Zhu, C., Lian, T., Hu, Z.-Z., Zhou, C., and Chen, D.: The 2023-24 El Niño Boosts Global Mean Surface Temperatures to a New Record High 1.5°C Above Preindustrial Levels, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2013, https://doi.org/10.5194/egusphere-egu26-2013, 2026.

The springtime North Tropical Atlantic (NTA) sea surface temperature (SST) anomaly serves as a crucial extratropical precursor to the El Niño–Southern Oscillation (ENSO), helping to alleviate the spring predictability barrier of ENSO. While the influence of the NTA on ENSO has been gradually strengthening since the mid-20th century, this trend cannot be explained by global warming. This study reveals that the Victoria Mode (VM) in the North Pacific is the key driver of this intensification. Since the mid-20th century, the negative phase of the VM has progressively strengthened, which in turn has enhanced the coupled subtropical northeasterly trade winds. This enhancement has intensified air-sea coupling over the subtropical northeastern Pacific. Consequently, atmospheric anomalies excited by the spring NTA are now more likely to imprint significant SST anomalies onto this critical hub region in the subtropical northeastern Pacific. Through intensified local air-sea interactions, these anomalies are then further transmitted into the tropical Pacific, ultimately triggering ENSO events. Our findings demonstrate that the influence of the North Tropical Atlantic on the tropical Pacific is largely modulated by the background climatic state of the North Pacific.

How to cite: Zheng, Y.: Amplification of the NTA's Impact on ENSO: The Important Modulation by VM, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2033, https://doi.org/10.5194/egusphere-egu26-2033, 2026.

The El Niño–Southern Oscillation (ENSO) arises from ocean–atmosphere interactions in the tropical Pacific and is a major source of global seasonal climate predictability. Canonical theories describe ENSO as a cyclic phenomenon, with ocean dynamics favouring transitions between warm (El Niño) and cold (La Niña) phases, and atmospheric noise introducing irregularities. Here, we show that ocean dynamics rarely favour such transitions. Following La Niña and moderate El Niño events, opposing wave signals from the central and western Pacific weaken the ocean’s memory, inhibiting consistent phase reversals. In contrast, extreme El Niño events—such as those in 1982, 1997, and 2015—trigger strong, nonlinear atmospheric responses that generate distinctive ocean heat content anomalies and set up a robust transition to a two-year La Niña. We propose revising the canonical recharge oscillator framework to account for this behaviour and explain ENSO’s dominant 3–7 year timescale as emerging from transitions between extreme El Niño and multi-year La Niña events. Overall, these results indicate that extreme El Niño events uniquely provide two-year ENSO predictability, while in other cases, predictability stems from external forcing that generate imbalances between heat content anomalies in the central and western Pacific.

How to cite: Liu, F., Vialard, J., Han, S., Planton, Y., Lengaigne, M., Gangiredla, S., Zhao, S., Guilyardi, E., Ethé, C., Person, R., Voldoire, A., Jin, F.-F., V Fedorov, A., and J. McPhaden, M.: ENSO cycles mostly after extreme El Niño events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2093, https://doi.org/10.5194/egusphere-egu26-2093, 2026.

Linear inverse models (LIMs) are widely used in climate science to diagnose dynamical relationships among climate variables and to predict large-scale variability such as El Niño-Southern Oscillation (ENSO). Recent extensions from stationary to cyclostationary LIMs (CS-LIMs) incorporate seasonally varying dynamics, but most formulations still assume white-noise forcing, which implicitly requires that the system state and the random forcing are well separated in frequency. This assumption limits their ability to represent realistic atmosphere-ocean interactions.

In this study, we further advance the cyclostationary LIM framework by introducing Ornstein-Uhlenbeck colored noise to represent persistent atmospheric stochastic forcing. We refer to this extension as CS-Colored-LIM. Incorporating persistent noise enables a more physically consistent representation of unresolved atmospheric variability and its cumulative influence on the coupled system.

We compare the newly developed CS-Colored-LIM and conventional LIMs in terms of their Niño 3.4 forecast skill and their ability to capture essential ENSO features and the influence of stochastic forcing. Our analysis demonstrates that CS-Colored-LIM accurately reproduces the seasonal cycle of ENSO variability, providing a framework for studying ENSO phase locking and the spring prediction barrier. Moreover, despite the submonthly characteristic timescale of persistent noise, its cumulative contribution to Niño 3.4 evolution exceeds 10%, revealing the non-trivial role of persistent stochastic forcing.

Forecast experiments show that cyclostationary formulation improves short-range prediction skill (≤ 12 months) through better representation of month-to-month variability, while colored noise enhances longer-lead performance (>12 months) by accounting for persistent atmospheric forcing. CS-Colored-LIM benefits from both effects, yielding statistically significant improvements in correlation skill, more reliable ensemble forecasts, and enhanced prediction of major ENSO events, compared to conventional LIMs. Consequently, CS-Colored-LIM provides a simple yet powerful framework for long-range ENSO diagnosis and prediction, offering new insights into the interaction between seasonally varying dynamics and persistent stochastic forcing.

How to cite: Lien, J., Ando, H., Richter, I., and Kido, S.: Diagnosing and predicting ENSO using cyclostationary linear inverse models with persistent stochastic forcing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2201, https://doi.org/10.5194/egusphere-egu26-2201, 2026.

The Warm Pool Dipole (WPD), a seesaw pattern of sea surface temperature anomalies (SSTAs) between the southeast Indian Ocean and the central-western Pacific, can strongly influence regional precipitation. This study explores its dynamical impact on South China spring rainfall and the decadal variability of this relationship. Observations and model simulations show that cold SSTAs in the central-western Pacific may excite a westward-propagating Rossby wave, while warm SSTAs in the southeast Indian Ocean induce low-pressure anomalies over the eastern Indian Ocean. These may both strengthen easterly anomalies over the western Pacific. The associated anticyclone further intensifies the western North Pacific subtropical high, increasing moisture transport to South China. Notably, the WPD’s influence weakened significantly after around 2000, becoming negligible compared to the period before. This shift is attributed to a diminished atmospheric response linked to the reduced intensity of the WPD itself in recent decades. This work identifies a new potential predictability source for seasonal forecasting of spring rainfall in South China.

How to cite: Xing, W.: Mechanisms and Decadal Variability of the Warm-Pool Dipole Mode’s Influence on South China Spring Rainfall, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3267, https://doi.org/10.5194/egusphere-egu26-3267, 2026.

El Niño prediction uncertainty is highly sensitive to initial-condition uncertainties. The present study explores the sources and dynamics of initial uncertainty using the Coupled Conditional Nonlinear Optimal Perturbation (C‑CNOP) method. By imposing physically consistent and rapidly growing coupled initial perturbations, a series of ensemble forecast experiments were conducted for El Niño events from 1982 to 2023, with initializations in different seasons. The resulting ensembles demonstrate high reliability for predictions initialized in January, April, and July, effectively characterizing prediction uncertainty. Conversely, October-initialized predictions show persistent under-dispersion, as perturbation growth is suppressed by an overly stable model background state, indicated by a low Bjerknes stability index. Building on the reliable framework, key sensitive regions were identified across the Pacific, Indian, and Atlantic Oceans, where initial uncertainties significantly contribute to prediction uncertainties in the tropical central‑eastern Pacific, with patterns that vary seasonally. Beyond reaffirming the significant impact of extratropical North Pacific initial uncertainties, the results also highlight the role of mid-latitude South Pacific regions. Cross‑basin remote effects are also identified. Specifically, interactions between the tropical Atlantic and Pacific are confirmed for January and July initializations, alongside a reaffirmed influence from the tropical Indian Ocean. Statistical evidence also suggests potential pathways originating from the mid-latitude South Atlantic in January-initialized predictions and the subtropical South Indian Ocean in April-initialized predictions. Validation experiments demonstrate that reducing initial errors in these identified regions enhances prediction performance and reduces overall prediction uncertainty. Moreover, utilizing perturbation information from these regions to select ensemble members improves both deterministic and probabilistic prediction performance. These findings clarify the initial sources of El Niño prediction uncertainty and provide a practical foundation for optimizing targeted observation strategies.

How to cite: Hou, G. and Duan, W.: Dominant Initial Uncertainty Sources for El Niño Forecasting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3348, https://doi.org/10.5194/egusphere-egu26-3348, 2026.

Despite substantial progress over the past four decades, accurately predicting the spatiotemporal structure of the El Niño–Southern Oscillation (ENSO) remains a persistent challenge for dynamical models. While deep learning models have demonstrated improved prediction skills, their performances are constrained by biases in climate models used for training and lack dynamic interpretability. Here we construct a novel hybrid model that integrates deep learning techniques into a dynamical model, enabling information exchanging during integration. Training on physical-informed data, the model continuously adapts and improves forecasts, achieves unprecedented ENSO prediction skills, particularly in El Niño diversity and the spring predictability barrier. Moreover, as the hybrid model requires only a small volume of data by training on observations, it circumvents biases in climate models. Enhanced prediction skills arise primarily from improved representation of the leading feedbacks associated with ENSO. Our results suggest that training models with physical-informed data is an effective approach for ENSO prediction.

How to cite: Feng, J., Lian, T., Liu, T., and Chen, D.: Achieving explainable ENSO prediction using small data training, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3457, https://doi.org/10.5194/egusphere-egu26-3457, 2026.

There is a long history of studies of potential interactions between the El Niño–Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO). Some suggested that ENSO may modulate QBO phase transitions or amplitude, although identifying a straightforward correlation of the two variability modes has been elusive. Recent studies showed some interesting connections between the surface composites of the two modes, sea surface temperature in particular. However, the observed record is short and noisy, raising the question whether such patterns reflect a robust dynamical coupling or a statistical artifact. In this talk, I will show the observed patterns from ERA5 show and therefore imply. Additionally, I will then discuss how the various high-top CMIP6 models produce (or do not produce) the observed signal. By comparing model output with observations, we assess the extent to which apparent correlations are reproducible by this physical mechanism or can be identified as an artifact.

How to cite: Koepnick, K., Harnik, N., Randall, D., and Tziperman, E.: ENSO-QBO correlations: a robust dynamical coupling or a coincidence due to the short record? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3547, https://doi.org/10.5194/egusphere-egu26-3547, 2026.

The representation of El Niño-Southern Oscillation (ENSO) atmospheric teleconnections is investigated in a new set of multi-decadal simulations that combine an eddy-resolving ocean with a high-resolution atmosphere, both at an unprecedented horizontal resolution of ~10 km. The experiments were produced using three different models run under a coordinated protocol within the European Eddy-RIch Earth System Models (EERIE) project. The impact of high resolution on ENSO teleconnections is unsettled, and no assessment has been carried out so far using climate simulations at the 10-km scale employed here.

Model fidelity is evaluated using a set of diagnostics designed to capture key components of ENSO teleconnections, including the tropical atmospheric response, Rossby wave generation, extratropical tropospheric and stratospheric circulation anomalies, and associated surface signals. These diagnostics are further applied to atmosphere-only experiments at low (~30 km) and high (~10 km) resolution, which are used to assess the impact of atmospheric resolution alone.

The coupled EERIE simulations show heterogeneous results relative to previous models with coarser grids (maximum 25 km). While the overall performance is positive, it depends on the season, region, and model configuration. Consistent with this, the atmosphere-only experiments suggest only modest gains from enhanced atmospheric resolution. The results are placed in the context of uncertainty in the ENSO response associated with internal variability and sampling, which may hinder potential benefits.

How to cite: Mezzina, B., Roberts, C. D., Aengenheyster, M., Ghosh, R., Roberts, M. J., and Batlle Martin, M.: ENSO teleconnections in eddy-rich climate models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5711, https://doi.org/10.5194/egusphere-egu26-5711, 2026.

El Niño-Southern Oscillation (ENSO) is the dominant atmosphere-ocean coupled mode of year-to-year variations in the tropical Pacific. It shows diverse spatiotemporal characteristics and casts major influences on seasonal predictions of global weather-climate extrema. Despite numerous dynamical and statistical models for ENSO prediction and predictability studies, they are commonly subjected to one-to-three issues among less skillful simulation of El Niño diversity, huge requirements of computational resources and a low robustness in statistics. Here, an efficient deep-learning model involving nonlinear coupling of multiple variables is independently developed to study the predictability of two types of El Niño events related to initial uncertainty, which is the first kind of predictability problem. The model can skillfully simulate statistically robust features of observed El Niño diversity in terms of periodicity, amplitude, and seasonal phase-locking. Using this model, we have revealed mathematically several new types of fastest-growing initial errors in two types of El Niño predictions based on a novel concept of conditional nonlinear optimal perturbation (CNOP), especially including one that can strengthen central Pacific types of events, which is rarely investigated before. Moreover, CNOPs are superimposed into a numerical model, GFDL CM2p1, for comprehensive validation and growth mechanism mining, which demonstrates the consistent dynamical evolution of initial errors in both numerical and AI models. Our study represents the first attempt to explore the first kind of ENSO predictability problem from perspectives of nonlinear error-evolving dynamics using a data-driven model. This is of great importance as it offers us sufficient confidence to perform ENSO-related (such as the Madden-Julian Oscillation, etc) mechanisms and predictability studies in the future without strongly relying on dynamical numerical models.

How to cite: Yang, Z., Ren, X., Wu, X., and Wang, G.: The First Kind of Predictability Study for El Niño Prediction in a Multivariate Coupled Data-Driven Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6072, https://doi.org/10.5194/egusphere-egu26-6072, 2026.

The El Niño-Southern Oscillation (ENSO) is a dominant source of global climate variability. However, climate models exhibit persistent and substantial spread in simulating the ENSO-related cloud radiative feedback (CRFB), which directly limits the fidelity of ENSO amplitude and period projections. This study re-evaluates the representation of ENSO-related CRFB in CMIP6 models, which generally exhibit a weak amplitude and a westward shift of the negative feedback center in the tropical Pacific. To identify the sources of model uncertainty in ENSO CRFB, we analyzed experiments conducted with the atmospheric models Community Earth System Model (CESM) and a modified version of the Max Planck Institute for the Meteorology Earth System Model at low resolution (MPI-ESM-LR). Results show that compared to CESM, MPI-ESM-LR fails to accurately simulate mid-level cloud properties, which largely govern the cloud radiative effect. In contrast, biases in mean sea surface temperature (SST) and ENSO amplitude also considerably influence the simulation of CRFB. The CRFB bias in CMIP6 is strongly linked with that in the corresponding models from the Atmospheric Model Intercomparison Project (AMIP), further indicating the important role of atmosphere model (especially the cloud and convective parameterization) in simulating the CRFB on ENSO.

How to cite: Xu, W., Zheng, X.-T., and Hu, K.: Revisiting the modelled cloud radiative feedback on ENSO — the source of model uncertainty, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6165, https://doi.org/10.5194/egusphere-egu26-6165, 2026.

Andam Sea to South China Sea (SCS) transient mesoscale water flux through the Singapore Strait, defined as reflux, reverses the annual SCS-Malacca Strait throughflow. Despite its dynamical significance, this process has received little attention. This study comprehensively examines its climatic features, driving factors and underlying mechanisms. Results indicate that reflux mainly occurs in summer and is rare in winter. Three types - WCE-, CE-, and E-type - are classified based on the extent of eastward intrusion across the Strait. The proposed physical mechanism is as follows: strong westerly winds drive surface water eastward, causing water accumulation along the western coasts of the Malay Peninsula and Sumatra and thereby elevating sea surface height (SSH) in the Malacca Strait. As SSH increases, the SSH gradient across the Strait reverses, initiating eastward flux. Meanwhile, strong westerly winds blocked by Sumatra deflect the southeastward flow northwestward around the Sunda Strait, intensifying the northward current at the eastern exit of the Singapore Strait, which enhances local Ekman transport and facilitates reflux. Although the same physical process operates in both seasons, the causes of strong westerly winds over the tropical eastern Indian Ocean differ. Summer reflux is favoured by the intensified southwest monsoon, whereas wintertime events are modulated by La Niña conditions, when warm waters and atmospheric heating near Sumatra induce a Gill-type low-level response to the equatorially symmetric heat source. Furthermore, while the considered three reflux types share the same fundamental mechanism, stronger atmospheric and oceanic forcing generates more intense and spatially extensive reflux events.

How to cite: Wang, Z., Ma, P., Xu, X., and Tkalich, P.: Water flux from the Andaman Sea to the South China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6312, https://doi.org/10.5194/egusphere-egu26-6312, 2026.

Understanding changes in the pattern of tropical Pacific sea surface temperature (SST), especially its zonal contrast, in response to greenhouse gas forcing is essential for predicting future climate change. Among various mechanisms proposed, the ocean dynamical thermostat (ODT) associated with eastern Pacific upwelling may act to intensify the zonal SST contrast during the early stage of warming. However, its physical processes and efficiency in transient response remain controversial. Here we revisit the ODT mechanism by diagnosing the mixed-layer heat budget in both a simple coupled model (Zebiak–Cane model) and a complex GCM (MIROC6 abrupt 4xCO2 experiment). Following Clement et al. (1996), we decompose the ODT into two processes: ocean dynamical adjustment (ODA) due to mean upwelling and thermocline feedback (THF) due to anomalous upwelling, to investigate their roles in the SST pattern response to imposed surface heating. The SST pattern evolution is very different between the two models: initial eastern Pacific warming in the Zebiak–Cane model is quickly offset by ODA, enhancing the zonal SST contrast and triggering THF and horizontal advection that further cool the east, but MIROC6 exhibits a weakening of the zonal SST contrast from the beginning because the ODA cooling is overwhelmed by other processes. The contrast in the initial response is critical in their long-term response and it is explained mainly by differences in mean ocean currents and the spatial homogeneity of the radiative forcing.

How to cite: Li, W. and Watanabe, M.: Revisiting Ocean Dynamical Thermostat Mechanism for the Tropical Pacific SST response to Global Warming, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8356, https://doi.org/10.5194/egusphere-egu26-8356, 2026.

The dynamics of the El Niño Southern Oscillation (ENSO) vary from decade to decade in both observations and CMIP models. This complicates both ENSO prediction and the interpretation of any climate change signal. We aim to identify what processes are necessary to produce this decadal modulation, as quantified by ENSO growth rate and phase speed, using a set of recharge oscillator models with varying levels of nonlinearity.

We find that a linear recharge oscillator model with Gaussian white noise is sufficient to produce decadal modulation consistent with both observations and CMIP models. Although ENSO exhibits nonlinearity in other metrics, adding nonlinear terms to the recharge oscillator model does little to change the magnitude of decadal modulation. 

We show that the white noise forcing in recharge oscillator models impacts both ENSO emergent characteristics (e.g. amplitude) and representations of ENSO dynamical behaviour (e.g. growth rate, phase speed). Furthermore, we demonstrate that CMIP-class models do not produce ENSO predictability beyond the timescale expected from a linear recharge oscillator model. Together, these findings suggest future research should focus on the shorter-timescale processes influencing ENSO, including the sub-monthly processes typically modelled as noise, and highlight that short-timescale processes play an underappreciated role in influencing ENSO on much longer decadal timescales.

How to cite: Jeffree, J., Maher, N., Quinn, C., and Dommenget, D.: Decadal modulation of ENSO dynamics emerges primarily from white-noise forcing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8466, https://doi.org/10.5194/egusphere-egu26-8466, 2026.

Tropical land precipitation(TLP) plays a crucial role in tropical ecosystems, human activities and regional energy and hydrological cycles. To improve the predictability of TLP, it is essential to understand not only its variability but also its temporal persistence, which is a fundamental aspect of climate predictability. However, the persistence characteristics of TLP remain understudied. Using multiple precipitation datasets, this study reveals a rapidly declines in TLP persistence in June, regardless of the initial month. This phenomenon, termed the June persistence barrier (PB), is robust across datasets. Further analysis shows that sea surface temperature (SST) anomalies in the central and eastern tropical Pacific, associated with the El Niño–Southern Oscillation (ENSO), are the primary driver of the June TLP PB. ENSO-related SST exhibits a PB in May–June. When the linear influence of ENSO is removed, the TLP PB disappears, and persistence weakens significantly. Mechanistically, SST primarily affects the large-scale Walker circulation, which in turn causes quasi-consistent changes in vertical motion over tropical land. These changes directly influence local moisture transport, ultimately leading to TLP anomalies. The seasonal persistence of SST enables a sustained remote influence through the atmospheric bridge, linking oceanic variability to land precipitation. These findings not only deepen our understanding of the intrinsic variability of TLP but also provide a potential theoretical basis for the future seasonal prediction of TLP using its persistence.

How to cite: An, R., Li, J., and Feng, J.: June persistence barrier of tropical land precipitation and its relationship with ENSO, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8517, https://doi.org/10.5194/egusphere-egu26-8517, 2026.

The Malacca and Singapore Straits (MSS) is one of the world’s most critical maritime corridors, supporting intensive navigation, port operations, and coastal activities. Mean ocean flow and volume transport in the MSS play a key role in environmental risk assessment, particularly for predicting the transport pathways of accidentally spilled oil or hazardous substances. The volume transport in the MSS is influenced by both local atmospheric forcing and remote drivers, including the South China Sea Throughflow (SCSTF), and exhibits pronounced variability across multiple time scales. In this study, a high-resolution dataset of ocean flow simulated by the NEMO ocean model over the Maritime Continent, forced by ORAS5 ocean reanalysis and ERA5 atmospheric data, is used to analyze volume transport variability in the MSS. The simulated volume transport in the MSS is investigated following comprehensive validation against observations in key passages, including the Luzon Strait, Taiwan Strait, and Karimata Strait. At interannual time scales, MSS volume transport shows moderate correlations with both the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Strong seasonal variability is evident, driven by monsoon winds, with monthly climatology showing predominantly westward transport throughout the year, stronger in boreal winter and weaker in summer. Despite this climatological pattern, analysis of daily mean data reveals frequent eastward transport events during summer at sub-mesoscale time scales. These eastward transport events exhibit strong seasonality and show significant correlations with ENSO, with enhanced eastward transport occurring in summers following strong El Niño events. During these events, daily mean surface currents can reach magnitudes comparable to tidal currents in many parts of the strait. These results underscore the importance of accounting for short-term current variability when assessing pollutant transport and associated environmental impacts in the MSS.

How to cite: Ma, P., Wang, Z., Chow, J. H., and Tkalich, P.: Variability of Volume Transport in the Malacca and Singapore Straits and its Implications for Environmental Transport, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8939, https://doi.org/10.5194/egusphere-egu26-8939, 2026.

The dynamics of the El Niño–Southern Oscillation (ENSO) can be succinctly represented by the Recharge Oscillator (RO) framework. In this presentation, we systematically explore how ENSO properties depend on RO parameters and assess the framework’s ability to diagnose both present-day variability and externally forced changes.

We first show that parameter regimes producing self-sustained oscillations yield unrealistically low Niño3.4 kurtosis compared to observations, indicating that ENSO is more consistently represented as a stochastically forced, linearly stable system. Rather than relying on standard fitting approaches, we conduct a broad exploration of RO parameter space to identify configurations that simultaneously reproduce observed ENSO amplitude, seasonality, skewness, and lagged autocorrelation. The simplest and most realistic configuration is a strongly damped oscillator, with a decay timescale shorter than the dominant ENSO period, forced by multiplicative white noise and modulated by weak deterministic nonlinearities.

These simulations generate interdecadal ENSO fluctuations comparable in magnitude to those observed, raising questions about the interpretability of slowly evolving RO parameters inferred from single realizations. Using idealized twin experiments, we show that fitting the RO to individual time series produces spurious interdecadal parameter shifts that appear to “explain” ENSO variability through changes in the Bjerknes–Wyrtki–Jin index, but do not reflect a forced response.

We then impose 20–40% linear trends in selected RO parameters over 200 years and test their recoverability using ensemble fits. About 50 ensemble members are sufficient to robustly detect linear parameter changes and most ENSO property trends over 40-year windows, while detecting changes in skewness and nonlinear parameters associated with extreme ENSO events requires 100 members or more. These idealized experiments demonstrate that ensemble simulations are essential for diagnosing externally forced changes in ENSO dynamics and provide a proof of concept for applying the RO framework to large-ensemble climate model experiments.

How to cite: Vialard, J., Han, S., and Fedorov, A.: Exploring present and future ENSO dynamics within the recharge oscillator framework, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9539, https://doi.org/10.5194/egusphere-egu26-9539, 2026.

Most state-of-the-art climate models (CMIP6) struggle to simulate the non-linear asymmetry of the El Niño-Southern Oscillation (ENSO), specifically the positive skewness where El Niño events are stronger than La Niña. While previous studies have attributed this deficiency to atmospheric parameterizations or equatorial cold tongue biases, we identify a distinct oceanic driver originating in the Eastern Pacific. We show that a systematic coastal warm bias in the eastern tropical Pacific is actively transported westward by the South Equatorial Current, creating an "advective bridge" that warms the central Pacific cold tongue. This advected heat elevates the background state, saturating the deep convection threshold and effectively removing the thermodynamic "floor" required for non-linear atmospheric feedbacks. Critically, we demonstrate a physical dissociation: while central Pacific trade wind biases primarily control ENSO amplitude, it is the upstream advective warming that determines ENSO skewness. These results suggest that improving the representation of eastern boundary current dynamics is a prerequisite for capturing the non-linear character of future ENSO variability.

How to cite: Yu, D., Dommenget, D., and Müller, W.: The role of Eastern Pacific Coastal Warm Bias on the ENSO Nonlinearity Bias in CMIP6 Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9672, https://doi.org/10.5194/egusphere-egu26-9672, 2026.

The seasonal persistence barrier (PB) is a critical factor limiting the prediction skill of the El Niño-Southern Oscillation (ENSO). While previous studies have predominantly focused on the Niño3.4 region (Eastern Pacific El Niño), the increasing frequency of Central Pacific (CP) El Niño events since the 1990s necessitates a distinct examination of the Niño4 region, which exhibits unique predictability characteristics. Using long-term reanalysis datasets, this study investigates the PB of sea surface temperature (SST) in the Niño4 region. Climatologically, the PB for the Niño 4 index occurs during June–July, but it exhibits significant decadal variability. We identify that the Interdecadal Pacific Oscillation (IPO) plays a crucial role in modulating the timing of the PB (PB month). Our analysis reveals a robust relationship where the IPO phase regulates the seasonal locking of the prediction barrier. Specifically, during positive IPO phases, the PB tends to occur earlier, whereas distinct timing characteristics are observed during negative phases. Mechanistically, a diagnostic analysis based on the Bjerknes Stability Index (BJI) demonstrates that the IPO background state alters the seasonal cycle of the zonal advective feedback (ZA) in the equatorial central Pacific. This modulation shifts the seasonal peak of the total ENSO growth rate, thereby determining the timing of the steepest decline in prediction skill. These findings offer new insights into the decadal variability of CP-ENSO predictability and highlight the importance of background state modulation in ENSO forecasting. 

How to cite: Hou, M.: Decadal Modulation of the Seasonal Persistence Barrier for Central Pacific El Niño by the Interdecadal Pacific Oscillation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10095, https://doi.org/10.5194/egusphere-egu26-10095, 2026.

El Niño and the Southern Oscillation (ENSO) have a worldwide impact on seasonal to yearly climate. However, there are decadal variations in the seasonal prediction skill of ENSO in dynamical and statistical models; in particular, ENSO prediction skill has declined since 2000. The shortcomings of models mean that it is very important to study ENSO seasonal predictability and its decadal variation using observational/reanalysis data. Here we quantitatively estimate the seasonal predictability limit (PL) of ENSO from 1900 to 2015 using Nonlinear local Lyapunov exponent (NLLE) theory with an observational/reanalysis dataset and explore its decadal variations. The mean PL of sea surface temperature (SST) is high in the central/eastern tropical Pacific and low in the western tropical Pacific, reaching 12–15 and 7–8 months, respectively. The PL in the tropical Pacific varies on a decadal timescale, with an interdecadal standard deviation of up to 2 months in the central tropical Pacific that has similar spatial structure to the mean PL. Taking the PL of SST in the Niño 3.4 region as representative of the PL in the central/eastern tropical Pacific, there are clearly higher values in the 1900s, mid-1930s, mid-1960s, and mid-1990s, and lower values in the 1920s, mid-1940s, and mid-2010s. Meanwhile, the PL of SST in the Niño 6 region—whose average value is 7 months—is in good agreement with the PL of most regions in the western tropical Pacific, with higher values in the 1910s, 1940s, and 1980s and lower values in the 1930s, 1950s, and mid-1990s.In the framework of NLLE theory, the PL is determined by the error growth rate (representing the dissipation rate of the predictable signal) and the saturation value of relative error (representing predictable signal intensity). We reveal that the spatial structure of the mean PL in the tropical Pacific is determined mainly by the error growth rate. The decadal variability of PL is affected more by the variation of the saturation value of relative error in the equatorial Pacific, whereas the error growth rate cannot be ignored in the PL of some regions. As an important source of predictability in ENSO dynamics, the relationship between warm water volume and SST in the Niño 3.4 region has a critical role in the decadal variability of PL in the tropical Pacific through the error growth rate and saturation value of relative error. This strong relationship reduces the error growth rate in the initial period and increases the saturated relative error, contributing to the high PL.

How to cite: Hou, Z. and Li, J.: Investigating decadal variations of the seasonal predictability limit of sea surface temperature in the tropical Pacific, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12134, https://doi.org/10.5194/egusphere-egu26-12134, 2026.

The El Niño-Southern Oscillation (ENSO) is the main source of climate variability in the tropical Pacific, affecting global patterns and extreme events like heatwaves, with significant consequences. ENSO is marked by warmer (El Niño) and cooler (La Niña) sea surface temperature (SST) anomalies over the eastern Pacific, with a typical seasonal cycle: development in fall, a winter peak, and rapid decay in spring. However, some ENSO events last beyond this canonical cycle, extending into northern summer. A notable recent example occurred in 2018-2019, when El Niño conditions persisted until July 2019 and were implicated in driving a record-breaking marine heatwave in the North Pacific. Although prolonged El Niño events have occurred in the recent past and are projected to become more frequent under future climate conditions, their summer impacts, particularly their role in modulating heatwave characteristics, remain poorly understood.

This study compares the frequency, duration, and intensity of summer heatwaves in the Americas following summer-persistent El Niño events with those following normal El Niño events. We define a summer-persistent El Niño as an event in which SST anomalies in the Niño3.4 region remain above 0.5 °C through June of the decaying year. Heatwaves are defined as periods of at least three consecutive days with daily maximum temperatures above the 90^th percentile of the 1961-1990 reference period. To assess these relationships, we analyse reanalysis datasets (ERSSTv5, NCEP20CR, ERA5) and perform AMIP-type simulations using the atmospheric component of the Earth System Model EC-Earth3. For ENSO-neutral, conventional El Niño, and summer-persistent El Niño conditions, monthly SST composites are generated that capture the annual cycle by averaging all historical events. Based on these composites, we construct a 10-member ensemble, each spanning a six-year simulation. To further isolate ENSO-related forcing, we perform sensitivity experiments by uniformly increasing SSTs across the ENSO-active region by 1 °C.

We identify six summer-persistent El Niño events since 1895. These events are associated with reduced heatwave activity over the western United States, characterised by less frequent, shorter, and cooler events, linked to a sustained but distorted Pacific-North American (PNA)-like pattern. In contrast, northeastern South America experiences pronounced positive heatwave anomalies, with more frequent, longer-lasting, and more intense heatwaves than those observed following conventional El Niño events. These regional differences are driven by a prolonged weakening and eastward shift of the Walker Circulation, accompanied by intensified descending motion over South America. Collectively, these findings underscore the extended influence of ENSO beyond its typical spring termination and highlight the importance of accounting for ENSO persistence in seasonal heatwave forecasts and climate adaptation strategies in ENSO-sensitive regions.

How to cite: Schultze, A., Lu, Z., Wang, Z., Karami, M. P., Zhang, Q., Zheng, M., and Pugh, T. A. M.: Prolonged El Niño conditions modulate heatwaves across the Americas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12299, https://doi.org/10.5194/egusphere-egu26-12299, 2026.

The tropical Pacific Ocean plays an outsized role in global climate, affecting, for instance, temperature and precipitation globally, as well as cyclone genesis and location. The equatorial zonal gradient in tropical Pacific sea surface temperatures (SSTs) has strengthened toward a more La Niña-like state in observations over the 20^th and 21^st centuries. Confirming whether climate models are capable of matching the observed gradient strengthening through some combination of a forced response and internal variability is therefore an active topic of research. While some studies have demonstrated that models can skillfully reproduce observed trends in the tropical Pacific SST gradient, others have argued that models fail to simulate these trends. However, these prior papers have focused on different and specific periods in the observational record to perform their assessments, with implications for the nature of the trends identified and the characterization of correspondence between the models and observations. Moving beyond assessments over a single time interval, we perform a comprehensive analysis over all trends in intervals of twenty years or longer from 1870 to 2024. We compare the observed trends from 5 observational datasets with simulated trends from 14 CMIP6 large ensembles (337 ensemble members in total). We demonstrate that models are not able to match many long-term trends in the observed gradient, especially those that end more recently. Models that are able to match these trends do so through excessive internal variability that compensates for their gradient-weakening forced responses. We additionally find that trends in the observed gradient strengthen at an increasing rate with time, a forced response that is in contrast to the behavior of most models.

How to cite: Byrne, H., Seager, R., and Smerdon, J.: CMIP6 models cannot capture long-term forced changes in the tropical Pacific sea surface temperature gradient, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12841, https://doi.org/10.5194/egusphere-egu26-12841, 2026.

The zonal gradient in tropical Pacific sea surface temperatures (ΔSST_west-east) plays a major role in global climate, from modulating the rate of global warming and ocean-atmosphere CO₂ fluxes, to influencing tropical cyclone genesis and regional precipitation patterns. While observations estimate that this gradient has strengthened over the historical record, most coupled climate models, from all generations up to the most recent (CMIP6), simulate a forced weakening of the zonal SST gradient over this period, a discrepancy that has been variously attributed to representations of internal variability, model mean-state biases and incorrect forced responses. In a previous study, we demonstrated that CMIP6 models fail to capture most recently-ending observed trends in ΔSST_west-east and identified signs that the observed strengthening is consistent with a forced response to rising atmospheric greenhouse gases. We additionally classified the 14 analyzed CMIP6 models into two groups according to whether they simulate a forced weakening of the gradient or a more ambiguous response over the historical period. In this study, we characterize how the forced trends in these model groups evolve under different 21st-century Shared Socioeconomic Pathways and find that even models that simulate slight gradient strengthening over the historical period ultimately simulate weakening gradients under most projections, a transition that occurs roughly in the contemporary period. To better understand why the two model groups show contrasting gradient behavior over the historical period, and more consistent behavior under projected scenarios, we conduct an empirical orthogonal function analysis to investigate the contributions of greenhouse gas (GHG) and aerosol forcings to ΔSST_west-east changes in the historical period. We further validate these analyses through use of single forcing large ensembles. We find that both greenhouse gas and aerosol modes are near ubiquitous within the model group, with these modes contributing in opposite senses to gradient changes between the model groups. A similar analysis on 4 observational products provides evidence for the influence of both GHG and aerosol modes on changes in observed ΔSST_west-east. Taken together, these findings quantify contributions from both GHG and aerosol forcing to changes in observed and modeled ΔSST_west-east, providing increased understanding of real-world ΔSST_west-east changes and the origin of model responses that yield unrealistic gradient changes over the historical period.

How to cite: Byrne, H., Seager, R., and Smerdon, J.: Greenhouse gas and aerosol forcings contribute differently to changes in the tropical Pacific sea surface temperature gradient in models and observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12965, https://doi.org/10.5194/egusphere-egu26-12965, 2026.

The Peruvian upwelling system is one of the most productive coastal marine ecosystems, powered by persistent winds that bring cold, nutrient-rich water to the surface. However, it can be severely disrupted by rapid coastal warming episodes known as Coastal El Niño events, which alter marine ecosystems and regional climate. Although often linked to ENSO, evidence from events like 1925 and 2017 shows that some warmings arise without basin-wide equatorial warming or strong tropical Pacific coupling, prompting the need to assess how common they are and whether they represent a recurring climate mode.

Using an objective, pattern-based method rather than traditional Niño indices, this study identifies coastal warming events defined by an eastern-Pacific warming and central-Pacific cooling dipole while the canonical ENSO mode remains weak. This approach reveals that ENSO-independent coastal warmings are more frequent and diverse than previously thought, typically driven by subtropical atmospheric variability that weakens both far-eastern equatorial trade winds and alongshore coastal winds, reducing upwelling, deepening the nearshore thermocline, and amplifying surface warming; some events may later transition into full El Niño as equatorial feedbacks develop. Overall, coastal warming emerges as a distinct mode of tropical Pacific variability triggered by remote atmospheric forcing and strengthened by local ocean–atmosphere processes.

How to cite: Soto, J., Khodri, M., and Chamorro, A.: Coastal El Niño events off Peru associated with cold ENSO background conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14196, https://doi.org/10.5194/egusphere-egu26-14196, 2026.

Decadal variability in the Pacific plays an important role in shaping global climate and can modulate the expression of anthropogenic warming, as illustrated by its contribution to the early 21st-century global warming hiatus. The Interdecadal Pacific Oscillation (IPO) describes multi-decadal sea surface temperature variability across the Pacific, yet the processes driving its variability remain poorly understood. Although the Pacific Decadal Oscillation (PDO) and South Pacific Decadal Oscillation (SPDO) are often synchronized by tropical forcing, they are not always equal. Examining their covariance may provide critical insight into the mechanisms underlying IPO variability. In this study, we analyze a new paleo-based multivariate reanalysis of the Norwegian Climate Prediction Model (NorCPM) that assimilates annually resolved paleoclimate records from the past centuries while including transient external forcing. The dataset spans the 1500–2010 period, allowing us to overcome limitations of previous IPO studies associated with short observational records, especially in the Southern Hemisphere. The temporal evolution of PDO–SPDO covariance over the last five centuries is assessed using low-frequency component analysis to isolate the internal variability and examine how this relationship varies under differing backgrounds states. The results indicate that while the PDO and SPDO are predominantly in phase, their covariance is highly non-stationary, with periods of weakened or reversed coupling. Examining this covariance provides additional context for understanding how Pacific basin–scale interactions contribute to IPO variability.

How to cite: Richer, F., Svendsen, L., and Dalaiden, Q.: North–South Pacific Decadal Co-Variability over the last 500 years informed by a new paleo-reanalysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14504, https://doi.org/10.5194/egusphere-egu26-14504, 2026.

Recently, we have developed two approaches (a climate network [1] and a complexity-based approach [2]) that allow forecasting the onset of El Niño events about 1 year in advance. The complexity-based approach additionally enables forecasting the magnitude of an upcoming El Niño event. These methods successfully forecasted the onset of an Eastern Pacific El Niño for 2023/24 and the subsequent record-breaking warming of 2024 [3]. Here, we propose the interannual relationship of the Oceanic Niño Index as an additional predictor for forecasting La Niña and neutral events. Combining the three approaches therefore enables probabilistic forecasting of all three phases of ENSO dynamics about 1 year in advance. Based on these approaches, in December 2024 we correctly forecasted with 91.4% probability the absence of an El Niño in 2025 [4]. With 69.6% probability, we predicted a neutral event as the most likely outcome for boreal winter 2025/26.

[1] Ludescher, J., Gozolchiani, A., Bogachev, M. I., Bunde, A., Havlin, S., Schellnhuber, H. J., (2013). Improved El Niño forecasting by cooperativity detection. Proc. Natl. Acad. Sci. U.S.A. 110(29), 11742.

[2] Meng, J., et al. (2020). Complexity-based approach for El Niño magnitude forecasting before the spring predictability barrier. Proc. Natl. Acad. Sci. U.S.A. 117(1), 177.

[3] Ludescher, J., Meng, J., Fan, J., Bunde, A., Schellnhuber, H. J., Very early warning of a moderate-to-strong El Niño in 2023, https://doi.org/10.48550/arXiv.2301.10763

[4] Ludescher, J., Meng, J., Fan, J., Bunde, A., Schellnhuber, H. J., Climate network and complexity approach predict neutral ENSO event for 2025, https://doi.org/10.48550/arXiv.2502.00643

How to cite: Ludescher, J., Meng, J., Fan, J., Bunde, A., and Schellnhuber, H. J.: Predicting ENSO dynamics with network & complexity analyses, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14829, https://doi.org/10.5194/egusphere-egu26-14829, 2026.

The El Niño-Southern Oscillation (ENSO) is the leading source of predictable, internally generated, large-scale climate variability. Centered in the tropical Pacific, ENSO influences global precipitation, atmospheric circulation, and temperature patterns via atmospheric teleconnections. ENSO events can trigger climate extremes, in turn devastating communities and costing billions of dollars. Over recent decades, substantial progress has been made in early and accurate seasonal forecasting of ENSO events. However, relatively less is known about how ENSO teleconnections vary in space and time, so called nonstationarity, which limits our ability to confidently relate these forecasts to expected impacts.

Assessing ENSO teleconnection nonstationarity is challenging because the instrumental record is relatively short. Comprehensive physical climate models help to address these limitations, but intrinsic biases undermine their utility for this purpose. By contrast, statistical climate models are trained on observations and can therefore provide a valuable complementary perspective.

Linear Inverse Models (LIMs) are efficient, linear statistical climate models that are computationally inexpensive and straightforward to modify. Here we develop state-of-the-art LIMs that simulate ENSO asymmetry and diversity (Martinez-Villalobos et al., 2025), the seasonal cycle (Shin et al., 2021), and ENSO’s teleconnected impacts in the extratropics (Ault et al. 2018). Utilizing the LIMs we provide the most confident estimates to-date of intrinsic nonstationarity in ENSO teleconnections. Furthermore, by altering the LIM we assess the role for various processes in driving nonstationarity, with a particular focus on the influence of ENSO asymmetry, the seasonal cycle and phase locking, and inter-basin interactions. Our results have implications for seasonal forecasting, characterizing ENSO impacts in a changing climate, and validating the comprehensive physical climate models that are the basis of future projections.

How to cite: Coats, S., Heller, S., and Atwood, A.: Exploring nonstationarity of ENSO teleconnections using state-of-the-art Linear Inverse Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15270, https://doi.org/10.5194/egusphere-egu26-15270, 2026.

The El Niño-Southern Oscillation (ENSO) is among the most well understood climate phenomena. The Recharge Oscillator (RO) theory is widely used to conceptualize the key ENSO physics in observations and state-of-the-art models. It is well known that the ENSO-associated equatorial zonal wind anomalies shift southward in boreal winter, contributing to ENSO termination. Thus far, this effect has not been explicitly incorporated into the RO framework. Here we derive a new form of the RO, which incorporates the seasonal meridional migration of the zonal wind anomalies under the low-frequency limit. In our theory, wind stress centered off the equator forces equatorial waves and acts with a delayed effect on SST. Meanwhile, ENSO is stabilized as the central latitude of the zonal wind anomalies shift southward, owing to exponentially weakened thermocline feedback. A stochastic RO simulation with a prescribed observed southward wind shift reproduces ENSO seasonal synchronization and combination tones.

How to cite: Iwakiri, T., Stuecker, M., Jin, F.-F., and Zhao, S.: ENSO Recharge Oscillator Theory Integrating the Southward Wind Shift, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15517, https://doi.org/10.5194/egusphere-egu26-15517, 2026.

There is an emerging consensus that ENSO amplitude may change non-monotonically in response to external forcing, increasing over the course of the 20th century and decreasing in the 22nd and 23rd centuries. While the physical explanations for these asymptotic short-term and long-term cases are well-supported by climate model simulations, projections of ENSO’s 21st century changes vary widely between models. In this work, we investigate why ENSO amplitude begins to decline in the mid-21st century in the CESM2 model. We find that almost all of the amplitude decrease results from a weakening of the strongest El Niño events. While strong El Niños' intensity begins decreasing in ~2010, La Niñas’ intensity continues increasing for several decades afterwards. The net result of these opposing changes is a ~2030 maximum in overall ENSO amplitude and a reverse in ENSO asymmetry by the 21st century (La Niñas become stronger than El Niños, opposite to the 20th century). We show that this asymmetric change in ENSO intensity is consistent with a weakening of the zonal temperature gradient, which increases the zonal variability of the Walker circulation and limits the potential intensity of El Niños but not La Niñas. Overall, our analysis suggests that changes in asymmetry may have a leading-order effect on overall ENSO amplitude in the 21st century, and that El Niño intensity may not always be a useful upper bound on La Niña intensity.

How to cite: Carr, T., Gebbie, G., Gonzalez, A., Ummenhofer, C., and Vialard, J.: Towards understanding ENSO's non-monotonic projections in the CESM2 climate model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15911, https://doi.org/10.5194/egusphere-egu26-15911, 2026.

 Accurately predicting the El Niño-Southern Oscillation (ENSO) remains a central challenge in seasonal climate forecasting. Statistical approaches, such as the Linear Inverse Model (LIM) and the Model Analog (MA), have been widely applied to predict sea surface temperature and sea surface height anomalies in the tropical Indo-Pacific, but each approach has intrinsic limitations. Although their weighted combination, MA-LIM, improves forecast skill over LIM and MA individually, it is still affected by residual biases from both methods. To address this limitation, this study introduces a new statistical prediction framework, NEW MA-LIM, which more optimally unifies MA and LIM by explicitly modeling the temporal evolution of MA forecast errors within the LIM operator and applying dynamic corrections at each forecast lead. Hindcast experiments for the period 1961–2023, using observational datasets and 15 CMIP6 preindustrial control simulations, show that NEW MA-LIM consistently outperforms LIM, MA, and MA-LIM across 1–12 month leads. In particular, it substantially alleviates the spring predictability barrier. A key finding is that MA forecast errors exhibit significant linear predictability, and their spatiotemporal patterns can be effectively reproduced by LIM. This enables more reliable ENSO prediction within a low-dimensional dynamical framework.

How to cite: Kim, S. and Shin, J.: Error correction of model analog forecasts using a linear inverse model for improving statistical ENSO prediction skill, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16264, https://doi.org/10.5194/egusphere-egu26-16264, 2026.

In this study we review the current knowledge of the impacts associated with the El Niño Southern Oscillation (ENSO) in Australia. Among the major large-scale modes of variability, ENSO is the dominant phenomenon influencing seasonal mean rainfall and temperature, producing a spatially coherent pattern across nearly two-thirds of Australia. Its influence typically amplifies during multi-year events and varies on multidecadal cycles. When co-occurring with other climate modes of variability, such as Indian Ocean Dipole, Southern Annular Mode and Madden–Julian Oscillation, ENSO’s combined influence can explain about 50% of seasonal rainfall variability in parts of eastern and northern Australia during spring.

The main large-scale mechanisms that explain ENSO’s influence in Australia are (1) via changes in tropical atmospheric circulation associated with the Southern Oscillation and its related changes in sea level pressure, (2) through the modulation of the Pacific South American (PSA) pattern, and (3) indirectly via changes in the Indian Ocean sea surface temperatures (SST), which trigger Rossby wave trains to the Australian extra-tropics. These mechanisms affect the intensity and persistence of weather systems that control rainfall, particularly in eastern Australia. Their impacts are further modulated by local SST and land-atmosphere processes that can alter evaporation, humidity and moisture advection inland, thereby modulating rainfall response during ENSO events.

Although most studies published in the literature have focused on addressing the El Niño impacts, it is the La Niña phase of ENSO that produces a more consistent and arguably more societally impactful change across Australia. The ENSO-Australian rainfall relationship is asymmetric and stronger for La Niña. It is also the Central Pacific-type of ENSO event that typically produces stronger impacts on Australia. We will discuss the links between ENSO diversity, weather patterns, and associated extreme events, such as droughts and floods.

In a warmer climate, the ENSO-Australian rainfall relationship is projected to intensify by about 10-20%, consistent with many other regions across the globe. ENSO-driven precipitation and surface temperature variability is projected to strengthen in September to November over southeastern and southern Australia, while the largest changes are projected to occur during the warm season from December to February over most of western and northern Australia.

Despite considerable improvements in ENSO predictability and seasonal outlooks over the past four decades, predicting its impacts remains challenging because of large internal atmospheric variability. In addition, the observed cooling trend in the Pacific Ocean directly challenges the accuracy of El Niño-like warming projections in a future warming climate. These evolving ENSO features highlight the need for strategic research, sustained in situ monitoring, reduced model biases, and improved understanding of the anthropogenically induced changes in Pacific temperatures to support adaptation strategies.

How to cite: Taschetto, A. S. and McGregor, S. and the great team: A review of the El Niño Southern Oscillation impacts on Australian climate, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19177, https://doi.org/10.5194/egusphere-egu26-19177, 2026.

Canonical El Niño (EN) events typically peak in boreal winter and then rapidly decay in the ensuing months, transitioning to a La Niña (LN) event or a neutral condition by the following winter. Strikingly, an EN event that peaked in 1986/87 boreal winter, unexpectedly persisted into the following year, generating a rare two-year consecutive EN event. However, the 1986/87~1987/88 two-year EN event receives few attention. This study reveals that this two-year EN event encompasses some critical yet commonly overlooked processes for the formation and development of EN event. Specifically, the high-frequency (HF) westerly wind anomalies, induced by the tropical cyclones (TCs) and Madden-Julian Oscillation (MJO) events, were the pivotal drivers of the unexpected re-ignition in the second year. During the 1986/87 winter, the unexpected emergence of four TCs induced vigorous westerly wind anomalies over the western equatorial Pacific (WEP), disrupting the anomalous anticyclone circulation over the western North Pacific (WNPAC) and the associated easterly wind anomalies over WEP that were anticipated during the 1986/87 winter. Such unexpected westerly wind anomalies helped maintain the EN warming through December 1986 to February 1987. Subsequently, a series of HF westerly wind anomalies, induced by TCs and MJO events in April, May and July 1987, reinvigorated the waning warming, pulling it back into a fledged EN event by the end of 1987. Gaining insights into the formation mechanism behind this unique two-year EN event can deepen our understanding of ENSO dynamics and provide implications for enhancing the accuracy of EN prediction.

How to cite: Chen, L., Song, M., Zhao, J., Lin, P., Jiang, L., Wang, L., and Zhi, H.: Impacts of Tropical Cyclones and MJO on El Niño Evolution: Revisiting the Formation Mechanism of the 1986/87~1987/88 Two-year El Niño, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19881, https://doi.org/10.5194/egusphere-egu26-19881, 2026.

The temperature contrast between the western and eastern equatorial Pacific Ocean (hereafter zonal temperature gradient) plays a central role in driving the Walker Circulation, an atmospheric circulation pattern that affects climate across the globe. Over the past forty years, observations show that this zonal temperature gradient has strengthened. However, fewer than 1% of simulations from the latest generation of climate models reproduce this observed trend.

Two possible explanations have been proposed for this discrepancy. First, the strengthening could be a response to human-driven climate change that models fail to represent accurately. Second, it could reflect natural, long-term fluctuations of the climate system that models underestimate. Here we examine the second possibility.

We show that climate models underestimate the magnitude of low-frequency natural variability in the tropical Pacific, partly because they rarely simulate extreme El Niño events. El Niño is a recurring climate phenomenon in which the zonal temperature gradient weakens. During extreme events, this gradient can nearly vanish, which substantially increases temperature variability on decadal timescales. When we statistically account for the rarity of such extreme events in models, agreement with observations improves only modestly: even after correction, only about 5% of simulations reproduce the observed strengthening.

These results indicate that it is very unlikely that the recent strengthening of the Pacific temperature contrast arises from natural variability alone. This finding instead points to potential deficiencies in how climate models represent the tropical Pacific’s response to global warming.

How to cite: Planton, Y., Vialard, J., Fedorov, A., Lengaigne, M., McGregor, S., and Stuecker, M.: Internal Variability Insufficient to Explain Recent Equatorial Pacific Trends, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20872, https://doi.org/10.5194/egusphere-egu26-20872, 2026.

Climate change, in particular the rise in tropical sea surface temperature, is the greatest threat to coral reef ecosystems today with associated climatic extremes affecting the livelihood of tropical societies. The interaction between the tropical ocean basins plays a key role in modulating climate variability on interannual to decadal timescales. These timescales are of strong relevance to societies and ecosystems, because they control the time interval for recovery between extreme events. Throughout the tropical oceans, a key archive for reconstructions of temperature and hydrology are massive shallow-water corals. Annually to monthly resolved coral proxy records are critical for our understanding of tropical ocean-atmosphere interactions. The DFG Priority Programme “Tropical Climate Variability and Coral Reefs” (SPP 2299) aims to enhance our understanding of tropical marine climate variability and its impact on coral reef ecosystems in a warming world, by quantifying climatic and environmental changes during both the ongoing warming and past warm periods on timescales relevant for society. Ultra-high resolution (monthly to weekly) geochemistry of the coral skeleton is a valuable tool to understand the temporal response of corals to ongoing climate change. Developing reconstructions of past tropical climate and environmental variability, in conjunction with advanced statistical methods, earth system modelling and observed ecosystem responses allows improved projections of future changes in tropical climate and coral reef ecosystems. We present examples (1) for modes of tropical climate variability affecting coral reef ecosystems, such as interactions of the IOD and ENSO, (2) for thermal stress signatures in coral geochemical and isotopic records, and (3) highlight knowledge gaps and future directions in this emerging field, contributing to a better understanding of the response of coral reef ecosystems and tropical climate variability to ongoing and future climate change.

How to cite: Felis, T., Hargreaves, J. A., and Pfeiffer, M. and the SPP 2299 Team: Tropical Climate Variability and Coral Reefs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20994, https://doi.org/10.5194/egusphere-egu26-20994, 2026.

How to cite: Landi, N. and Liguori, G.: Revisiting null hypotheses for Indian Ocean interannual variability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21356, https://doi.org/10.5194/egusphere-egu26-21356, 2026.

Marine heatwaves (MHWs) are intensifying globally, and have become more frequent in the Arctic under ongoing climate warming. Yet, they remain little studied in the high Arctic, despite rapid environmental changes and distinctive regional features such as extensive sea ice and strong salinity stratification. These conditions likely produce polar-specific driving mechanisms and impacts, which are especially unclear for MHWs occurring below the Arctic surface.

Here, we review and synthesise scattered yet valuable insights from across disciplines to address two key questions: (i) What are the drivers of Arctic MHWs, and (ii) what are their ecological and biogeochemical impacts? We extend this review beyond the surface to the largely overlooked subsurface dimension. We clarify where knowledge is well-established, and where knowledge remains speculative but supported by indirect evidence. In particular, we highlight Arctic-specific processes associated with MHWs, and outline plausible yet undocumented feedback mechanisms. We conclude by offering methodological and scientific recommendations to guide future research.

By integrating cross-disciplinary information, we aim to advance a more comprehensive understanding of Arctic MHWs and their potential consequences for this rapidly changing ocean.

How to cite: Athanase, M., Gou, R., Köhn, E., Richaud, B., and Simon, A.: Towards Uncovering Arctic Marine Heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-796, https://doi.org/10.5194/egusphere-egu26-796, 2026.

The tropical cyclone (TC)–heatwave compound hazards are two seemingly contrasting climate hazards that pose unprecedented recovery challenges to coastal communities when flood-induced power outages result in prolonged exposure of vulnerable populations to extreme humid heat. Landfalling tropical cyclones at the coast can cause catastrophic damage due to strong winds and flooding from storm surges and extreme precipitation-induced inundations. Further, storms often precede or follow extreme humid heat stress. However, a detailed investigation of the causal drivers of landfalling TCs and their subsequent impact on humid heatwave development in coastal cities remained unexplored. The eastern coastal regions in peninsular India, bordering the Bay of Bengal, frequently experience humid heat stress compared to other parts of the country due to large-scale subsidence, causing persistently high temperature and humidity levels. The densely populated metropolitan cities, Kolkata and Chennai, both with populations exceeding 10 million, experience risks of extreme heat stress, e.g., dangerous heat stroke events and higher likelihood of discomforts. Considering a 43-year (1982−2024) analysis period in a probabilistic framework, we analyze the spatiotemporal compounding patterns of marine heatwave (MHW)–TC–heatwave coupling for 259 landfalling TCs, including 37 rapidly intensified (intensity changes of 30 kts/24 hr) landfalling storms that move across the coast, within t ∈ [–15, +15] days of the occurrence of peak heatwave intensity over land and ocean (i.e., marine heatwave) when the storm passes within a 500 km radius distance. The MHWs favors the development of a warm thermal environment and increases the likelihood of rapid storm intensification. Over the Bay of Bengal, MHWs are spatially extensive, showing significant upper tail correlation between MHW and TC peaks over 86% of the ocean. Approximately 39% of the ocean shows a significant preconditioning of strong to severe MHWs on landfalling TCs. For selected urban sites across the coast with major economic activities (trade, finance, and maritime transportation), our results showed that the trend in TC-compounded daily maxima wet-bulb temperature (T_wmax) shows a statistically significant (at a 5% level) upward trend at the rate of 0.18/decade. The T_wmax anomalies even exceeded +3-standard deviations (s.d.) from the normal in ~12% (5/43) of years, and these events are compounded by tropical storms (maximum sustained windspeed < 63 kts). Moreover, the magnitude of temperature anomalies is dominant during the post-monsoon season. The 95^th quantile of T_wmax anomalies following a week of storms’ passage is ~24% greater than that of the corresponding T_wmax anomalies ahead of the storms, suggesting a significant heavy tail behaviour of temperature extremes after TC landfall. An event coincidence analysis of TC-heatwave coupling in heavily urbanized areas shows up to 50% coincidence of TC being followed by humid heatwaves, considering a few days of time lag after TC landfall, while during TC-heatwave coincidence (at a lag of zero days), such probability varies only from 5−20%. Recognizing the causal interaction between MHW−landfalling TC−and coastal urban heatwave event chain adds value to heatwave adaptation planning during post-TC landfall events, which is often overlooked in practice.     

Reference: Ganguli, P., Lin, N. npj Natural Hazards. 2(1), 1-15 (2025).

How to cite: Ganguli, P. and Lin, N.: Escalating Coastal-Urban Heatwaves Preconditioned by Marine Heatwave–Tropical Cyclone Hazard Cascade , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1358, https://doi.org/10.5194/egusphere-egu26-1358, 2026.

One of the devastating effects of global warming is potentially more frequent and stronger extreme events. In particular, extreme warm events in the ocean, known as marine heatwaves (MHWs), can have severe and even irreversible impacts on marine ecosystems, underscoring the imperative need to quantify their anthropogenic changes based on observations. In situ temperature profiles over the past three decades reveal an outsized global increase (over 10% per decade) in intensity of MHWs, whereas both ocean reanalysis and climate simulations during the same period suggest that the changes should be an order of magnitude smaller. Here we show that the paradox arises primarily from an artificial trend of MHWs caused by increasing amount of temperature profiles with time. Sparsity of temperature profiles in the early period systematically underestimates the intensity of MHWs, while subsequent densification of temperature profiles alleviates such underestimation, introducing an artificial positive trend of MHW intensity. This artificial trend is more dominant in historically observation-sparse regions like the Southern Ocean. Our findings indicate that the augmented in situ ocean observing capacity with time may severely contaminate the genuine response of extreme events to the global warming so that careful handling of these observations is essential to reach valid conclusions.

How to cite: Wang, Z., Jing, Z., and Sun, H.-X.: Augmented In Situ Ocean Observing Capacity Could Cause an Artificial Intensification of Extreme Warm Water Events Globally , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2144, https://doi.org/10.5194/egusphere-egu26-2144, 2026.

The East/Japan Sea is a semi-enclosed marginal sea that has experienced rapid sea surface temperature (SST) warming exceeding 0.9 °C since the 1980s, which has intensified the occurrence of marine heatwaves (MHWs). Its semi-enclosed nature amplifies the influence of external climate forcing, making reliable projections of future SST and MHW changes essential for assessing ecological and socio-economic impacts. Here, we investigate future SST and MHW changes using high-resolution (1/8°) dynamical downscaling simulations based on the Regional Ocean Modeling System (ROMS), driven by seven Coupled Model Intercomparison Project Phase 6 (CMIP6) models under four Shared Socioeconomic Pathway scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) for 1982–2100. The ROMS ensemble outperforms the CMIP6 ensemble in reproducing observed SST and MHW characteristics for the historical period of 1985–2014, particularly during winter, due to improved simulation of the transport of the Tsushima Warm Current through the Korea Strait and associated regional circulations. Future projections (2071–2100) indicate that SST warming and MHWs in the central East/Japan Sea will become stronger, longer-lasting, and more spatially heterogeneous in ROMS, in contrast to the more uniform patterns projected by CMIP6, especially under high-emission scenarios. This spatial heterogeneity is associated with intensified transport of the Tsushima Warm Current and a strengthened East Korean Warm Current, which enhance heat advection along their pathways into the basin interior. These results highlight the added value of high-resolution dynamical downscaling for understanding and preparing for future SST and MHW changes in the East/Japan Sea, providing insights for regional climate impact assessment and adaptation planning.

How to cite: Oh, S.-G., Lim, K.-G., Kang, Y.-K., Son, S.-W., and Cho, Y.-K.: Revealing Enhanced Signals of Future Marine Heatwave Changes in the East/Japan Sea through a High-Resolution Dynamical Downscaling Ensemble, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2283, https://doi.org/10.5194/egusphere-egu26-2283, 2026.

Increasing frequency, intensity, and duration of marine heatwaves (MHWs) is an essential indicator of regional warming in the Eastern Mediterranean Sea. Moreover, desert dust intrusions are frequently observed over the sea: they are characterized by the arrival of warm air masses containing dust aerosol from the desert. In this study we found the effect of desert dust intrusion on the detection of marine heatwaves by satellite SST retrievals. This effect has not yet been investigated in previous studies. Our approach was based on the separate use of microwave (MW) and infrared (IR) satellite radiometry of nighttime sea surface temperature (SST). Satellite MW radiometry was represented by a product developed by the GHRSST group: it is based on SST retrievals from multiple satellite MW sensors. On the other hand, satellite IR radiometry was represented by MODIS-Aqua nighttime SST retrievals. Our analysis, for the first time, provides observational evidence that there was no effect of dust intrusion on the detection of MHWs by satellite MW radiometry, despite the fact that the aerosol optical depth (AOD) ranged within an extremely wide interval of 0.3 to 5. As for IR radiometry, we found an inverse correspondence between daily variations in both IR-based SST and AOD. The inverse correspondence indicates that IR-based SST was profoundly influenced by desert dust causing negative biases in daily variations in IR-based SST. This dust-induced artificial "cooling effect" in satellite IR data masked actual MHWs. As a result, in the presence of a strong dust intrusion (AOD of up to 5), satellite IR radiometry was incapable of detecting MHWs. This was in contrast to MW radiometry which was capable of detecting MHWs. An essential point of our study is that, even in the presence of weak dust intrusion (AOD ranged from 0.3 to 0.4), IR-based SST was incapable of detecting MHWs due to the occurrence of erroneous short-term sharp drops in IR-based SST. This failure was because of dust appearance at high altitudes. Dust-related IR radiation, emitted by dust particles at high altitudes was interpreted by satellite IR sensors as SST cooler that it actually was. Our findings highlight the importance of analyzing physical factors responsible for interruptions of MHWs - namely, whether these interruptions are actual SST changes or indeed dust-induced artifacts. The failure of satellite IR radiometry to detect MHWs reduces the capability to detect MHWs by SST datasets integrating MW and IR radiometry. It was proved by the MUR Global Foundation SST analysis developed by the GHRSST group. We found an underestimation in the presence of MHWs in the Eastern Mediterranean Sea.

Reference: Kishcha and Starobinets (2026) Effect of desert dust intrusion on the detection of marine heatwaves. Remote Sensing, https://www.mdpi.com/2072-4292/18/1/48 .

How to cite: Kishcha, P. and Starobinets, B.: Effect of desert dust intrusion on the detection of marine heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2455, https://doi.org/10.5194/egusphere-egu26-2455, 2026.

Marine heatwaves (MHWs) disrupt marine ecosystems and drive significant socioeconomic impacts. In biologically productive, human-dominated coastal waters, MHWs can cause disproportionately large effects relative to the open ocean. In such systems, MHW-induced changes in physical and biological processes have the potential to strongly modulate air–sea CO₂ exchange. However, despite growing evidence that MHWs perturb regional CO₂ fluxes, their integrated impact on the global coastal carbon sink has not yet been quantified. Using four observation-based CO₂ flux datasets, prioritizing a global coastal product, we find that MHWs enhance net CO₂ uptake in global shelf seas by 4.3 ± 0.2% during 1985–2020, while uptake declines in the open ocean. The enhancement is driven primarily by polar and subpolar shelf seas, where frequent MHWs coincide with sea-ice loss and reductions in non-thermal dissolved inorganic carbon (DIC), outweighing reduced uptake or enhanced outgassing in lower-latitude, warming-dominated regions. Simulations with a global ocean biogeochemical model indicate that the observed DIC reductions are primarily driven by enhanced biological carbon fixation. Our results reveal region-specific MHW impacts on coastal carbon dynamics and underscore the critical role of high-latitude shelf systems in the global carbon budget under ongoing climate change.

How to cite: Hu, Z., Mathis, M., Li, H., Ilyina, T., Voynova, Y., Macovei, V., Zhou, F., Meng, Q., Schrum, C., and Zhang, W.: The impact of marine heatwaves on global coastal carbon sink, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2603, https://doi.org/10.5194/egusphere-egu26-2603, 2026.