The Mediterranean Sea, renowned for its unique biodiversity and vital economic importance, finds itself increasingly vulnerable to marine heatwaves—a phenomenon characterized by prolonged periods of anomalously high sea surface temperatures. Among these extreme events, the 2022 marine heatwave stands out as a record breaker, exemplifying the intensity and rapidity with which such temperature anomalies can manifest. This susceptibility arises from a complex interaction of atmospheric and oceanographic processes, particularly the delicate balance of air-sea heat fluxes and regional ocean dynamics. Recent advances in climate research have begun to unravel the underlying mechanisms driving these marine heatwaves, promising enhanced predictive capabilities that could safeguard fragile marine ecosystems and the coastal communities reliant upon them.
In a groundbreaking study published in Nature Geoscience, researchers from the Euro-Mediterranean Center on Climate Change (CMCC) present compelling evidence linking the onset of Mediterranean summer marine heatwaves to the persistence of subtropical atmospheric ridges. These ridges, colloquially referred to as African anticyclones due to their origination of warm, dry air masses over the African continent, have been identified as critical atmospheric features that extend beyond merely elevating surface air temperatures. By meticulously analyzing hundreds of marine heatwave occurrences via high-resolution satellite data and sophisticated hierarchical clustering techniques, the study elucidates how these atmospheric structures disrupt typical weather patterns to create the perfect conditions for ocean warming.
Subtropical ridges are not rare phenomena; they occur frequently throughout the summer months with a typical frequency of approximately once every two days. However, the key factor that differentiates a routine atmospheric event from one capable of triggering a marine heatwave is the persistence of these ridges. When these high-pressure systems linger uncharacteristically over the Mediterranean basin for durations exceeding five consecutive days, they impose a quasi-stationary state that arrests the regular eastward progression of weather fronts. This stagnation leads to a critical suppression of prevailing wind patterns, particularly the reduction or near-elimination of winds that usually facilitate the ocean’s thermal regulation through heat exchange with the atmosphere.
The physical mechanism at play involves the delicate interplay between wind-driven oceanic heat loss and the thermal inputs from solar radiation. Typically, strong winds enhance latent and sensible heat fluxes, enabling the sea surface to dissipate the absorbed solar energy into the overlying atmosphere, maintaining a relatively stable temperature regime. Under the prolonged influence of persistent subtropical ridges, wind speeds fall dramatically, stifling this heat dissipation process. The resultant inhibition of oceanic heat loss allows surface waters to warm rapidly, fueling the emergence and intensification of marine heatwaves.
Quantitatively, the study reveals startling statistics: in the Western, Central, and Eastern Mediterranean sub-basins, 63.3%, 46.4%, and 41.3% of marine heatwave events respectively coincide with conditions characterized by both the presence of subtropical ridges and reduced wind speeds. These percentages are particularly striking given that such combined atmospheric scenarios occur during a mere 8.6% to 14.6% of all summer days. This disproportionate representation underscores the amplifying effect these meteorological conditions exert on the probability of marine heatwave genesis.
Further examination into the heat budget of affected regions clarifies the dominant role of wind-mediated heat fluxes. The reduction in wind speed during these persistent ridge events correlates with a substantial decrease—exceeding 70%—in the total ocean-to-atmosphere heat flux within the impacted areas. This alteration in the thermal exchange balance not only fosters the initial formation of heatwaves but also sustains them by limiting oceanic cooling, thereby allowing water temperatures to soar beyond climatological norms.
The collaborative effort behind this research, bridging the expertise of atmospheric scientists and oceanographers, epitomizes the multidisciplinary approach necessary to tackle complex climate phenomena. Through the integration of high-resolution ERA5 reanalysis data with the CMCC’s in-house marine heatwave database, the research team was able to capture subtle meteorological and oceanographic signatures preceding marine heatwave events. These insights enable the advancement of early-warning systems that transcend simplistic temperature threshold models, instead focusing on the physical processes—the atmospheric triggers—that truly govern marine heatwave behavior.
Within three distinct Mediterranean clusters analyzed—comprising 26 events in the Western Mediterranean, 18 in the Central Mediterranean, and 14 in the Eastern Mediterranean—the interplay between subtropical ridges and weak wind regimes dramatically escalates the likelihood of marine heatwave development. The team quantifies this increased risk, noting that when both conditions co-occur, the probability of a heatwave forming multiplies by four to five times. This statistical relationship affords meteorologists and oceanographers invaluable predictive power that could be harnessed to mitigate environmental and economic damages.
The urgency of such mitigation is underscored by observations in highly affected locations like the Gulf of Lion, where subsurface water temperatures surged by nearly 7°C within a scant two-day window during the most extreme heatwave episodes. This rapid temperature escalation highlights the ocean’s sensitivity to atmospheric forcings and the critical need for real-time, accurate forecasts to guide response efforts for fisheries, tourism, and biodiversity conservation.
Researchers emphasize that improving forecasting models to incorporate the persistence and dynamics of subtropical ridges represents a pivotal step forward. Current climate models often fall short in resolving the temporal and spatial nuance of these ridges, limiting their efficacy in predicting marine heatwaves. The newly discovered physical link between persistent atmospheric patterns and oceanic heat accumulation presents an opportunity to refine Earth system models, enhancing their skill and reliability.
Given that the Mediterranean Sea is warming faster than the global ocean average, the stakes of these advancements are high. Accurate characterization and prediction of marine heatwaves will become ever more vital as climate change continues to alter atmospheric circulation and oceanic conditions. The CMCC’s innovative approach, leveraging clustering analysis and high-resolution reanalysis products, exemplifies how data-intensive methodologies can unlock new understanding of climate extremes.
This research forms an integral component of the EU-funded ObsSea4Clim project, which aims to develop robust climate indicators and observational tools to support climate assessments across the Mediterranean. Additionally, the findings will directly inform the ongoing development of CMCC’s Mediterranean Forecasting System—a state-of-the-art platform that provides operational forecasts critical to a broad spectrum of stakeholders spanning from policy-makers to local communities.
Ultimately, this study represents a paradigm shift in our comprehension of marine heatwaves. By illuminating the subtle yet profound influence of persistent subtropical ridges on the Mediterranean’s marine thermal environment, it not only deepens scientific understanding but also opens the door toward actionable climate resilience. As lead author Giulia Bonino remarks, identifying the physical mechanics behind these temperature anomalies is a gratifying achievement that lays the foundation for more accurate, physics-based forecasting in a rapidly warming world.
Subject of Research: Mediterranean summer marine heatwaves and their atmospheric drivers.
Article Title: Mediterranean summer marine heatwaves triggered by weaker winds under subtropical ridges
News Publication Date: 14-Aug-2025
Web References:
- https://www.cmcc.it/article/marine-heat-wave-in-the-mediterranean-observations-and-predictions
- https://www.nature.com/articles/s41561-025-01762-9
- https://essd.copernicus.org/articles/15/1269/2023/
- https://www.cmcc.it/projects/obssea4clim-ocean-observations-and-indicators-for-climate-and-assessments
- http://dx.doi.org/10.1038/s41561-025-01762-9
References: Bonino, G., McAdam, R., et al. (2025). Mediterranean summer marine heatwaves triggered by weaker winds under subtropical ridges. Nature Geoscience. DOI: 10.1038/s41561-025-01762-9
Keywords: Ocean surface temperature, Marine heatwaves, Subtropical ridges, African anticyclones, Mediterranean Sea, Air-sea heat flux, Climate modeling, Early warning systems
