The Mediterranean Sea is particularly susceptible to marine heatwaves – such as the record breaking 2022 heatwave which was characterized by anomalously high sea surface temperatures – due to the interplay of air-sea heat fluxes and local oceanographic processes, leading to significant impacts on marine ecosystems and coastal communities.
A new study , led by CMCC, brings the scientific community one step closer to identifying the driving forces behind these events. Analyzing over hundreds of marine heatwave events identified through advanced satellite data and clustering analysis, the study shows that persistent subtropical ridges – intrusions of warm air from over the African continent into Europe, often informally referred to as African anticyclones – do far more than simply raise air temperatures.
While subtropical ridges occur frequently in summer, roughly every 2 days, their persistence is what creates the critical conditions for marine heatwave formation. During marine heatwave onset, ridge occurrence becomes persistent – the high-pressure system associated with the ridge becomes stationary, disrupting the normal eastward movement of weather systems.
When these ridges settle over the Mediterranean basin for five consecutive days or more, they cause the prevailing winds to still, which then leads the sea to stop shedding heat and surface waters warm rapidly.
"Our study identifies the favourable conditions leading up to marine heatwaves and reveals that they are triggered by persistent subtropical ridges which weaken strong winds in the area," says CMCC researcher and co-author of the study Ronan McAdam .
The findings demonstrate that 63.3%, 46.4%, and 41.3% of marine heatwaves in the Western, Central, and Eastern Mediterranean respectively occur during periods with both subtropical ridges and reduced wind conditions – a remarkable concentration considering these combined conditions only occur during 8.6% to 14.6% of all summer days.
When subtropical ridges persist for several days, the resulting decrease in wind speeds causes a substantial reduction in heat loss from the ocean to the atmosphere. This heat loss accounts for over 70% of the total heat flux in affected regions, and drives the majority of the ocean temperature change.
"It is very satisfying to identify the mechanics behind a phenomena we have been studying for years," says lead author Giulia Bonino .
Furthermore, likelihood ratios across three Mediterranean clusters – 26 events in the Western Mediterranean, 18 in the Central Mediterranean, and 14 in the Eastern Mediterranean – reveal that when a subtropical ridge and weak winds strike together, a heatwave is four to five times more likely to form.
The discovery of this statistical relationship provides the foundation for more accurate prediction systems that could help protect vulnerable marine ecosystems and dependent industries from future extreme events. For example, in the Gulf of Lion subsurface temperatures climbed by almost 7°C over just two days during the most extreme events, illustrating the dramatic speed at which marine heatwaves can develop and the need for accurate predictions and effective responses.
"This was a great collaboration between oceanographers and atmospheric scientists – joining expertise and passion counts," remarks co-author Ronan McAdam . By combining the subtleties of meteorology with high-resolution ocean data, the team shows that early-warning systems can move beyond temperature thresholds to embrace the physics that actually triggers an event.
With Mediterranean seas warming faster than the global average, knowing with accuracy when a marine heatwave is about to strike is essential. "Our work highlights previously unidentified processes that are essential for accurately representing Mediterranean MHWs," says McAdam. "These results are critical to improving forecast systems and Earth system models, representing a key step toward effective early-warning and mitigation strategies in the basin."
