Definition and Methods: novel physics and impact-based definitions which challenge the now-traditional statistical framework; observational and modelling requirements for reliable monitoring; AI/ML-based detection and prediction for surface and subsurface MHWs.
Impacts: Socio-economic damage to marine activities and industries including but not limited to tourism, fisheries, aquaculture; discussions with stakeholders.
Mitigation/Adaptation: forecasting efforts on short-term to decadal timescales; projections of future changes; studies of precursors and predictability.
Interactions: compound and concurrent events, ecosystem and biogeochemical implications (e.g. on nutrient/oxygen availability/harmful algal blooms/ocean acidification and trophic web), impacts on atmospheric circulation/extreme weather events (e.g. Tropical Cyclones/Monsoon Circulation).
This session welcomes studies across all spatial scales; studies on coastal scales and processes are particularly welcome.
Orals: Fri, 8 May, 10:45–12:30 | Room 1.34
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.
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.
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) are anomalously warm seawater events that can persist for months, harming marine ecosystems and biodiversity. The western North Pacific (WNP) and its marginal seas host productive coastal ecosystems and densely populated coasts that have already experienced substantial ecological and socio-economic impacts from MHWs. This study examined the interannual variability of MHWs in the WNP and its marginal seas using 41 years of satellite-based sea surface temperature and reanalysis data. Spectral analysis identified interannual variability (< 6 years) distinct from the long-term warming trend. Nearly half of the interannual peaks coincided with El Niño to La Niña transition periods, during which MHWs persisted throughout the year over the WNP. During the mature phase of El Niño (September-January), southerly wind anomalies associated with the Philippine Sea Anticyclone and Kuroshio Anticyclone intensified downward turbulent heat fluxes, triggering MHWs. As El Niño weakened and La Niña developed (June-September), positive temperature anomalies in the WNP thermocline weakened the vertical temperature gradient, resulting in positive anomalies in the entrainment and vertical diffusion terms and sustaining MHWs during summer. These subsurface temperature anomalies were maintained by negative wind stress curl associated with a westward-extended Western North Pacific Subtropical High and by baroclinic eddies propagating westward along the Subtropical Countercurrent. In contrast, during peak years not associated with an El Niño to La Niña transition, MHWs appeared primarily in summer (July-September), the East Asian rainy season, over the WNP and its marginal seas. An anticyclonic circulation embedded within the circum-global teleconnection pattern may suppress convective activity, thereby enhancing downward shortwave radiation and leading to MHW occurrences. This study provides an integrated understanding of how MHWs in the WNP and its marginal seas are influenced by various large-scale climate drivers on the interannual timescale.
How to cite: Park, H.-J. and Na, H.: Large-scale climate drivers of the interannual variability of marine heatwaves in the western North Pacific and its marginal seas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16381, https://doi.org/10.5194/egusphere-egu26-16381, 2026.
In 2023, the North Atlantic experienced a marine heatwave (MHW) of unprecedented intensity and spatial extent, with sea-surface temperatures breaking records across large parts of the basin. This event raises the question of whether it reached the upper bound of MHW severity under current climate conditions. Here, we apply an ensemble boosting framework to MHWs for the first time, generating physically consistent storylines of the 2023 event conditioned on the observed pre-warming of the North Atlantic. This approach explores the upper tail of physically plausible extremes within a modelled climate system, revealing unrealised but credible extreme outcomes.
The boosted ensemble indicates that the 2023 MHW could plausibly have been both more intense and longer-lived than observed. In extreme storylines, events can persist for up to 579 days and reach a peak intensity of 4.5°C above climatology, compared with 240 days and 2.9°C in observations, likely implying substantially greater risks for marine organisms and ecosystems. Importantly, these amplified outcomes arise from the same mechanisms as the observed one, suggesting that no novel drivers are required. The most extreme storylines are predominantly atmospherically driven. They are characterised by persistently weak near-surface winds and shallow mixed-layers that reduce turbulent heat loss, together with sustained high-pressure conditions over the Euro-Atlantic region, particular in late spring, that suppress cloud cover and enhance shortwave radiative heating. Depending on their persistence and strength, these same drivers can give rise to either exceptionally intense or exceptionally long-lived MHWs. While events of this level of extremity are very rare under current climate conditions, we show that they become more apparent under global warming levels of 2.5 °C or more. By explicitly sampling the physically consistent upper tail, ensemble boosting provides a new perspective on extreme MHWs in a warming North Atlantic.
How to cite: Gregory, C., Hofmann Elizondo, U., Guinaldo, T., and Frölicher, T.: Unrealised extreme storylines of the 2023 North Atlantic marine heatwave revealed through ensemble boosting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11207, https://doi.org/10.5194/egusphere-egu26-11207, 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.
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 (Twmax) shows a statistically significant (at a 5% level) upward trend at the rate of 0.18/decade. The Twmax 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 95th quantile of Twmax anomalies following a week of storms’ passage is ~24% greater than that of the corresponding Twmax 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.
Marine heatwaves (MHWs) — transient events of anomalously high sea surface temperatures — pose severe threats to marine ecosystems. This is particularly critical for Large Marine Ecosystems (LMEs), which, despite covering only 22% of the global ocean, account for up to 95% of the world’s fisheries catch. However, the coastal complexity of LMEs and biases in coarse-resolution climate models have hindered a precise understanding of how climate change modulates MHWs in these vital regions.
This study employs high-resolution, eddy-resolving Earth system model projections to systematically investigate the future evolution of MHWs. Our high-resolution framework significantly improves the simulation of key MHW characteristics by better resolving mesoscale oceanic processes. For instance, it reduces the global bias in simulated mean annual surface MHW frequency by approximately 50% compared to conventional low-resolution models (from -0.60 to -0.31 events per year), thereby providing a more reliable basis for future projections.
Under a high-emission scenario (RCP8.5), projections using the conventional "fixed historical threshold" reveal a dramatic increase in the intensity and annual occurrence days of both surface and subsurface MHWs by the end of the century. For example, the global mean surface MHW intensity is projected to increase by about 1.2 °C. However, this conventional method conflates the effects of long-term mean warming and increased temperature variability, obscuring the critical role of the latter.
To isolate the contribution of enhanced temperature variability, we propose and apply a "future threshold" method, which defines MHWs relative to the shifting long-term mean climate. Strikingly, even after removing the background warming signal, surface and subsurface MHWs are projected to intensify globally, with the surface mean annual MHW days increasing by approximately 2.8 days. This underscores the pivotal role of amplified ocean temperature variability in driving future MHW increases.
This effect is especially pronounced in coastal LME regions. Our "future threshold" analysis indicates that in 83% of LME areas, the intensification of subsurface MHWs surpasses that of surface MHWs, primarily due to greater increases in subsurface temperature variability. This finding suggests that global warming is eroding the vertical thermal refuge for marine organisms, as their adaptive capacity to escape surface heat by moving deeper is increasingly constrained.
Furthermore, we document a profound increase in compound MHW events, where extreme heat co-occurs at the surface and subsurface simultaneously. The projected increase in the frequency of such compound events is about tenfold greater than that of single-layer events. This intensified coupling, also driven by enhanced variability, indicates a move towards more pervasive and vertically extensive marine heat stress. Coupled with concurrent stressors like ocean acidification and deoxygenation, these compound events represent a multi-dimensional threat to deep-sea ecosystem stability and biodiversity.
In summary, high-resolution modeling reveals that global warming threatens marine ecosystems via dual drivers: persistent mean warming and, more critically, amplified temperature variability. This drives an escalation of surface, subsurface, and compound marine heatwaves, especially in crucial Large Marine Ecosystems, underscoring an urgent need for mitigation strategies to protect ocean health and resources.
How to cite: Guo, X.: Climate Change Intensifies Surface-Subsurface Compound Marine Heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17648, https://doi.org/10.5194/egusphere-egu26-17648, 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.
Marine heatwaves (MHWs) have become more frequent and intense under ongoing climate warming, posing increasing risks to coastal ecosystems. This study investigated the characteristics of summer MHWs in a semi-enclose sea (i.e., the Bohai Sea) from 2010 to 2019 and evaluated nutrient and phytoplankton responses under varying sea surface temperature (SST) changes through numerical sensitivity experiments. The higher SSTs (25–28 °C) and longer-lasting MHWs were consistently detected in shallow nearshore regions (<10 m) of Laizhou, Bohai, and Liaodong Bays. Maximum MHW intensities reached 6 °C above climatology within the nearshore regions. Nutrients and phytoplankton biomass displayed pronounced spatial heterogeneity, with elevated Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP), and phytoplankton biomass in nearshore zones (< 10 m) and substantially lower concentrations offshore. Sensitivity experiments demonstrate an asymmetric ecological response to SST changes. Warming SST consistently elevated DIN and strongly suppressed phytoplankton biomass across the basin, particularly in shallow coastal regions, reflecting that summer SST already approaches or exceeds the thermal optimum for phytoplankton. In contrast, cooling produced weaker and more heterogeneous effects. Initial low temperatures under SST reduction suppressed phytoplankton growth and nutrient uptake, but biomass gradually recovered as temperatures moved toward optimal levels, leading to moderate DIN declines later in summer. Results suggest that continued warming and intensified MHWs promote nutrient accumulation and suppress phytoplankton biomass in this semi-enclosed shallow sea, potentially disrupting biogeochemical cycles. These findings provide valuable insights for assessing ecosystem vulnerability under future climate change.
How to cite: Kang, X., Sun, R., and Yin, H.: Phytoplankton responses of an semi-enclosed sea to SST perturbation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2647, https://doi.org/10.5194/egusphere-egu26-2647, 2026.
This study investigates rapid Arctic Ocean changes driven by global warming, focusing on the Barents Sea, Greenland Sea, and Norwegian Sea region (68–82° N, 5–60° E). Using the Marine Heatwave (MHW) index and three sea ice thickness (SIT) datasets, we systematically analyze the joint variability of MHWs and SIT over the period 1985–2025. The results show a pronounced stepwise intensification of MHWs over the past four decades, with the decadal mean category increasing from 0.05 during 1985–1994 to 0.38 during 2015–2025, representing an overall sevenfold increase, and reaching a record-high annual mean of 0.68 in 2025. Climatological analysis further indicates that MHW intensity is markedly elevated during the warm season (May to August) and late autumn (October to November), peaking in November at approximately 0.25. During these periods, interannual standard deviations commonly exceed 0.20, highlighting not only strong background heatwave conditions but also enhanced temporal variability. Based on the 1993–2024 monthly climatology derived from multi-source SIT data, sea ice thickness generally ranges from 0.6 to 1.5 m during the cold season (January to May), increases toward late winter and spring, rapidly thins during the melt season to summer minima of approximately 0.4 to 0.5 m, and partially recovers to about 0.4 to 1.1 m during late autumn and early winter. Interannual variability is most pronounced during the melt season, with summer standard deviations reaching 0.24 to 0.37 m, indicating a high sensitivity of marginal ice zones to atmospheric and oceanic forcing. Overall, MHWs exhibit a strong, accelerating long-term intensification across the study region, forming a persistently elevated heatwave background over recent decades. In contrast, SIT shows only modest thinning or episodic fluctuations, with magnitudes substantially smaller than the increase in MHW intensity. Systematic differences among SIT datasets in absolute thickness, seasonal amplitude, and interannual variability reflect contrasting assumptions in data assimilation, satellite retrievals, and ice rheology. These results underscore the importance of multi-dataset intercomparison for quantifying uncertainties and resolving regional differences in ice type when assessing coupled heat and ice variability in Arctic marginal seas.
How to cite: Ho, H.-Y. and Hsu, P.-C.: Spatiotemporal Evolution of Marine Heatwaves and Sea Ice Thickness in the Nordic Seas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3375, https://doi.org/10.5194/egusphere-egu26-3375, 2026.
Marine heatwaves (MHWs) are prolonged extreme ocean warming events with profound socioeconomic and ecological consequences. While accurate sub-seasonal MHW prediction is critical for proactive marine management, its achievable performance, particularly in global biodiversity hotspots vital to ecosystem-dependent economies, remains poorly understood. Here, we evaluate MHW sub-seasonal prediction skill across the global ocean using current state-of-the-art operational prediction models, emphasizing marine biodiversity hotspots. We show that MHWs are predictable up to two weeks in advance on average globally, with a twofold improvement potential under perfect-model assumptions. The prediction skill for MHWs is most notable as it approaches the model's upper limits in regions influenced by climate modes. However, in nearly two-thirds of marine biodiversity hotspots, the skill remains relatively low, lasting less than two weeks. La Niña conditions generally enhance prediction skill across most marine hotspot regions, primarily in the western Pacific Ocean, whereas El Niño conditions, although extending predictability up to six weeks in the tropical central-eastern Pacific and tropical western Indian Ocean, exhibit significant skills along the western coast of the Americas for global biological hotspot seas. These findings highlight the need for model refinements prioritizing biodiverse regions and leveraging specific climate events to develop more accurate MHW forecasts.
How to cite: Liang, K., Liu, F., Holbrook, N., Zhang, L., Hu, S., and Qiu, Y.: Sub-seasonal prediction for global marine heatwaves and their implications for Biodiversity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5655, https://doi.org/10.5194/egusphere-egu26-5655, 2026.
Marine heatwaves (MHWs) and marine cold spells (MCSs) represent critical ocean thermal extremes with profound ecological and socioeconomic consequences, yet their regional characteristics and drivers remain poorly constrained in many tropical, biodiversity-rich regions. The Philippines, located at the center of global marine biodiversity, is particularly vulnerable to such extremes but has received limited attention in high-resolution analyses, especially during recent exceptionally warm years. In this study, we investigate the spatiotemporal characteristics of MHWs and MCSs in Philippine waters using the Global Ocean Physics Reanalysis from the Copernicus Marine Environment Monitoring Service (CMEMS) at 0.083° × 0.083° spatial resolution. Events are detected following percentile-based definitions using a recent climatological baseline period (1993–2022), with particular emphasis on the warmest years on record (2023–2025).
Our analysis reveals pronounced regional contrasts in the frequency, intensity, and duration of MHWs and MCSs across the Philippine seas, reflecting the strong influence of local ocean–atmosphere interactions and basin-scale circulation. MHWs exhibit increasing persistence and intensity during recent years, while MCSs display asymmetric behavior consistent with long-term ocean warming. The enhanced spatial resolution captures fine-scale coastal and shelf processes that are unresolved in coarser products, reducing uncertainties in the detection of thermal extremes and highlighting localized hotspots of extreme warming and cooling.
This work aims to improve understanding of how large-scale ocean–atmosphere variability manifests as regional thermal extremes in a tropical, archipelagic setting. By providing an updated, high-resolution characterization of both warming and cooling extremes during the warmest recorded years, the study contributes to ongoing efforts to improve monitoring, predictability, and risk assessment of ocean thermal extremes in biodiversity-rich and socioeconomically vulnerable regions such as the Philippines.
How to cite: Concolis, B. M. and Zorita, E.: Ocean Thermal Extremes in the Philippines during the Warmest Recorded Years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7695, https://doi.org/10.5194/egusphere-egu26-7695, 2026.
In recent years, intensified global warming has led to increasingly frequent marine heatwaves (MHWs) in the tropical Indian Ocean, exerting severe impacts on marine ecosystems and coastal socio-economic systems. Using the ECMWF Subseasonal to Seasonal (S2S) reforecast data and NOAA OISST observations, this study systematically evaluates the subseasonal forecast performance of MHWs in the tropical Indian Ocean. For deterministic forecasts, the days and cumulative intensity of MHWs are overestimated near the equator (by up to 40% and 25%, respectively, primarily dominated by false positives), whereas they are underestimated in off-equatorial regions (by up to 50% and 45%, respectively, mainly due to false negatives). Both overestimation and underestimation become more pronounced with increasing forecast lead time. The mean intensity of MHWs is underestimated across the entire region, with the underestimation increasing from about 9% to 14% with lead time. Moreover, forecast biases are more pronounced for strong MHW events than for weak ones. For probabilistic forecasts, MHW forecasting exhibit relatively high skill at lead times of 1–7 days, with predominantly positive Brier Skill Scores and AUC values exceeding 0.80. Although Brier Skill Scores are generally lower near the equator than in off-equatorial regions, AUC values are comparable between the two regions. Overall, the ECMWF S2S system shows promise for subseasonal forecast of MHWs in the tropical Indian Ocean, but notable deficiencies remain for intense events and in specific regions.
How to cite: Meng, L., Li, X., Yan, Y., and Song, X.: Evaluation of the Subseasonal Forecast Performance of Marine Heatwaves in the Tropical Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7731, https://doi.org/10.5194/egusphere-egu26-7731, 2026.
Contrasting mechanisms of two rapid intensifications in Typhoon Hinnamnor (2022) under marine heatwave conditions
Kai Liu1, Xiaojiang Song1, Xingrong Chen1
1.National Marine Environmental Forecasting Center, Beijing 100081, China
Previous studies indicate that tropical cyclone (TC) rapid intensification (RI) predominantly occurs between 8°N and 20°N in the Northwest Pacific. However, under ongoing global warming, the increasing frequency of marine heatwaves (MHWs) at mid to high latitudes and the poleward shift of peak TC intensity suggest that MHWs may increasingly facilitate RI beyond this traditional latitude range. Super Typhoon Hinnamnor (2022) provides a unique example, being the only TC during 1982-2023 to maintain strong intensity north of 25°N while undergoing two RI events under pronounced MHW conditions.
This study reveals distinct roles of MHWs modulated the two RI events of Hinnamnor through fundamentally different upper-ocean vertical structures. The first RI, lasting 18 hours and intensifying the storm from a strong tropical storm to a super typhoon, was primarily ocean-driven. The MHW penetrated into the subsurface, deepening the warm layer and sustaining high upper-ocean heat content (UOHC). A thick barrier layer, preserved by relatively fast storm motion, enabled a subsurface “heat pump” effect that supported sustained intensification. In contrast, the second RI was short-lived (6 hours) and involved only a one-category intensification. A shallow mixed layer confined warming to the surface, enhanced vertical mixing and cold-water upwelling, prematurely terminating the second RI.
How to cite: Kai, L., Xiaojiang, S., and Xingrong, C.: Contrasting mechanisms of two rapid intensifications in Typhoon Hinnamnor (2022) under marine heatwave conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9077, https://doi.org/10.5194/egusphere-egu26-9077, 2026.
Marine heatwaves (MHWs) have intensified globally in recent years, driving widespread coral bleaching, ecosystem degradation, and escalating economic losses. In some coastal regions, coral bleaching rates have exceeded 80%, while fisheries-related damages have been estimated at over USD 3.1 billion annually. Despite these impacts, the characteristics and underlying mechanisms of nearshore MHWs remain poorly constrained, largely due to the lack of high-resolution sea surface temperature (SST) observations. Thermal infrared satellite products are subject to persistent data gaps caused by cloud cover, particularly in coastal environments where strong land–sea interactions, multiscale physical processes, and pronounced spatial heterogeneity limit conventional MHW detection.
A physics-informed deep-learning framework is developed to reconstruct all-weather, high-resolution SST fields for nearshore regions. By integrating physical constraints with multi-source geophysical predictors, the approach generates a 2 km-resolution SST dataset with high accuracy, achieving a root-mean-square error of 0.30 °C, a mean bias of 0.01 °C, and a coefficient of determination (R²) of 0.99 against independent reference observations. The reconstructed SST fields enable robust identification of nearshore MHWs and resolve fine-scale thermal structures that are not captured by existing coarse-resolution datasets.
Based on the reconstructed SST product, the spatiotemporal evolution of nearshore MHWs is systematically characterized, and associated physical and ecological implications are examined. Case studies in the South China Sea and the Mediterranean Sea reveal multiple unprecedented extreme events in recent years, including record-breaking MHWs in the Mediterranean during the past three years in terms of both intensity and spatial extent. High-resolution analyses further reveal an enhanced spatial correspondence between MHWs and coral reef distributions, indicating intensified thermal stress in ecologically vulnerable coastal zones. Accurate, all-weather, high-resolution SST reconstruction therefore provides a critical basis for advancing nearshore MHW detection and improving assessments of emerging coastal climate risks.
How to cite: Wang, Y., Zhou, X., Song, X., and Chen, Y.: A hybrid physical–deep learning approach for high-resolution detection of marine heatwaves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9961, https://doi.org/10.5194/egusphere-egu26-9961, 2026.
Marine Heatwaves (MHWs), defined as prolonged periods of anomalously warm ocean temperatures, have gained increasing scientific attention due to their significant ecological and socioeconomic consequences. Despite advances in MHW detection and characterisation, the mechanisms driving their onset and evolution remain an active area of research. As part of the Horizon Europe ObsSea4Clim project, this study investigates the influence of atmospheric circulation and climate modes of variability on MHWs in the North Atlantic basin. Preliminary findings underscore the critical role of large-scale atmospheric circulation, particularly the positioning and intensity of high-pressure systems, in modulating air–sea heat flux anomalies. These circulation patterns suppress wind speeds, enhance oceanic heat absorption, and consequently influence the spatial distribution, intensity, and duration of MHWs. To better capture the spatiotemporal coherence of these events, we advance from a pixel-wise to an event-based detection framework, enabling the labelling and ranking of spatially organised, scale-dependent MHWs. Enhancements to the spatial filtering and labelling algorithm further improve the detection of event structure and propagation pathways. Building on previous analyses, we apply non-linear statistical techniques, including Spearman’s rank correlation and Mutual Information, to more robustly quantify relationships between MHW characteristics and atmospheric drivers. Composite analyses for positive and negative phases of the North Atlantic Oscillation (NAO) are refined using a monthly NAO index and complemented with the Eastern Atlantic (EA) index, allowing for a more detailed representation of temporal variability. Furthermore, the statistical significance of these results is tested. Together, these advances deepen understanding of the atmospheric drivers of North Atlantic MHWs and enhance the potential for improved prediction of extreme ocean temperature events.
How to cite: Lopes, B., Silva, F., Paixão, J., Baeta, R., Oliveira, A., and Salge, P.: Atmospheric Drivers of North Atlantic Marine Heatwaves: Event‑Scale Detection and Links to Climate Modes Variability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13309, https://doi.org/10.5194/egusphere-egu26-13309, 2026.
The western coast of Australia is a global hotspot for intense marine heatwaves (MHWs). During the austral summer of 2024/2025, an extreme and persistent MHW affected the North West Shelf of Western Australia, lasting more than seven months and spanning over a large shelf and slope region, influenced by a weak La Niña. The event was associated with severe ecological impacts, including widespread fish kills and coral reef degradation, and was characterised by pronounced warming throughout the upper ocean, with subsurface temperature anomalies extending to depths of approximately 200 m.
We investigate the physical processes underpinning this event, with a particular focus on the interplay between atmospheric forcing and oceanic preconditioning. We examine the hypothesis that a period of elevated ocean heat content along the continental shelf, in the absence of strong atmospheric forcing, first triggered subsurface MHW that remained weak or undetectable at the surface. We also examine whether the emergence of a surface-intensified MHW depends on the timing and magnitude of atmospheric anomalies acting on this preconditioned ocean state.
Enhanced subsurface heat storage, suppressed vertical mixing in the water column, anomalous air–sea heat fluxes, and variability in boundary current transport may interact to promote the surface expression, vertical extent, and persistence of MHWs. Our results suggest that elevated ocean heat content alone does not consistently lead to surface extremes, while its coincidence with favourable atmospheric conditions may contribute to particularly intense and vertically extensive events.
Finally, we place the event in a broader climate context by considering how projected changes in both background ocean heat content and air–sea heat fluxes in CMIP6 simulations may favour more frequent, persistent, and vertically extensive subsurface and surface MHWs along the North West Shelf of Western Australia. Increasing ocean heat content, together with shifts in the magnitude, timing, and persistence of atmospheric forcing, may enhance the likelihood that subsurface warming is expressed at the surface and sustained over longer periods. These insights aim to advance process-based understanding of extreme MHWs and support the development of early warning and ecosystem risk assessment frameworks for vulnerable shelf regions.
How to cite: Pinter, S., jones, N., Rayson, M., Feng, M., and Cuttler, M.: Oceanic Preconditioning and Wind-Driven Amplification of the 2024/2025 Marine Heatwave off Western Australia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16334, https://doi.org/10.5194/egusphere-egu26-16334, 2026.
Marine heat waves in the Mediterranean Sea have become more frequent in the last decade, affecting the regional and European weather system. While extreme warming of the surface ocean is well-documented by remote sensing, the heat transport and modification of the thermohaline structure of the water columns remain elusive. This study examines historical ARGO data (years 2000-2025) and indicates that heat is no longer confined to the upper mixed layers but has penetrated deeper layers. Preliminary analysis shows that after a series of record-breaking surface temperatures, a distinct warming trend has emerged in the deep-water masses at 800–1200 meters in the western Mediterranean Sea. Historically more stable, these depths have warmed by 0.4 °C since 2019 relative to the climatological mean between 2000 and 2015. Data shows a general increase in salinity by 0.1 g kg-1. The warming and more saline deeper layers are persistent throughout the seasons. In the upper layer (0-200 meters), extreme warming by > 4°C in the summer time led to a decrease in density by 0.5 kg m-3 compared to the climatological mean. The implications of continued warming, including in the deeper layers, are substantial and include shifts in the Mediterranean Overturning Circulation and thermal persistence, with a long-term "thermal memory". It further affects local and regional weather extremes, with implications for ecosystems and the economy.
How to cite: Wurl, O. and Ribas Ribas, M.: Marine Heat Waves in the Mediterranean: Heat spreads in deep waters, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16349, https://doi.org/10.5194/egusphere-egu26-16349, 2026.
Marine Heatwaves Driven by El Niño and their impacts on coral reefs of the Andaman and Nicobar Islands
Subash Kisku a*, Dr Nibedita Behera a,
Department of Marine Sciences
Berhampur University, Bhanja Bihar, 760007
E-mail: subashkisku@gmail.com
Abstract
Marine heatwaves (MHWs)—prolonged periods of unusually high sea surface temperatures are intensifying with global climate change and pose a growing threat to marine ecosystems. Coral reefs in the Indian region of the North Indian Ocean are particularly vulnerable, with major bleaching episodes observed in recent decades. This study investigates the link between extreme MHWs and coral bleaching events in 1998, 2010, 2016, 2020, and 2024, focusing specifically on coral reef regions in the Andaman and Nicobar Islands from 1991 to 2024. Our objective was to identify a combination of thermal stress criteria that would capture the most severe MHWs and those more elevated and associated with El Niño in this region and encompass all known bleaching-associated events. Using satellite-derived sea surface temperature anomalies, we analysed the intensity, correlation with the Multivariate El Niño/Southern Oscillation Index and ecological consequences of these heatwaves. The results reveal a strong relationship between high-intensity MHWs and widespread coral bleaching, influenced by this region during the years of strong El Niño events. During this event, coral species such as Acropora cerealis, A. humilis, Montipora sp., Favia pallida, Diploastrea sp., and Goniopora sp. Fungia concinna, Gardineroseries sp., Porites sp., Favites abdita and Lobophyllia robusta were severely affected. The coral bleaching data also illustrated that the seasonal peaks from April to July correlated with the documented bleaching episodes and that DHW values exceeding 4°C-weeks were the ones that predicted severe bleaching stress the most. This trend was exacerbated during the El Niño years, which occurred in 1998, 2010, 2016, and 2024. This research provides critical insights into the vulnerability of Indian coral reef ecosystems and underscores the urgent need for region-specific conservation strategies and climate adaptation measures to enhance reef resilience and ensure the survival of these ecosystems in a rapidly warming ocean.
How to cite: kisku, S. and behera, D. N.: Marine Heatwaves Driven by El Niño and their impacts on coral reefs of the Andaman and Nicobar Islands, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16756, https://doi.org/10.5194/egusphere-egu26-16756, 2026.
Ocean memory, defined as the persistence of sea surface temperature (SST) anomalies due to the ocean’s high heat capacity, can last from a few days to several years, affecting both atmospheric and oceanic conditions. Previous research suggests a substantial historical increase in ocean memory. Here we reproduce and reassess this ocean memory increase using multiple satellite-based and reanalysis SST datasets. While all datasets agree on an increase in ocean memory, the magnitude of the trend and its spatial pattern vary considerably. To further interpret these differences, ocean memory is decomposed into high (1–15 days), intermediate (15–365 days), and low (longer than one year) frequency components. This decomposition reveals that the trend from the intermediate-timescale component dominates the historical increase in total ocean memory across all datasets.
Cross-dataset discrepancies in the magnitude and spatial structure of trends in total ocean memory most likely reflect uncertainties in SST estimation, including structural and parametric differences in product construction, rather than genuine physical variability, and may include non-physical artifacts such as inhomogeneities or processing biases. Here we further compare all estimates derived from SST products with that from mooring records, which provide an independent observational benchmark. However, the relatively short temporal coverage of the mooring data makes the comparison inconclusive. In addition, we also evaluate ocean memory trends using a free-running ocean model forced with atmospheric reanalysis (ERA5 and JRA55) which shows positive trends, although area-averaged trend magnitude are 4 to 11 times weaker depending on which SST dataset is used for comparison. In contrast, CMIP6 models show no consistent evidence of such a positive trend, even for future periods under high-emission future scenarios (e.g., SSP5-8.5).
Some satellite-based and reanalysis SST products also contain data artifacts that may influence these trends. For example, dramatic jumps are obvious in temporal autocorrelation in several datasets, which are clearly unrelated to any physical process or climate driver. Overall, the results point to an increase in ocean memory trends yet highlight uncertainties arising from dataset artifacts, limited mooring benchmarks, and the absence of robust model support.
How to cite: Dinesh, A. S., Alexander, L., Gupta, A. S., Li, Z., Malan, N., and Schmaltz, T.: Can we trust ocean memory trends? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18274, https://doi.org/10.5194/egusphere-egu26-18274, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot 1a
EGU26-3225 | ECS | Posters virtual | VPS20
Primary Factors Driving Extreme 2024 Early-spring Marine Heatwaves in the Tropical Atlantic: Shortwave Radiation and Mixed Layer DepthTue, 05 May, 14:57–15:00 (CEST) vPoster spot 1a
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.
