Overseas
The Mediterranean Sea is particularly vulnerable to marine heatwaves – such as the record-breaking 2022 event, marked by exceptionally high sea surface temperatures – due to the interaction between air–sea heat fluxes and local oceanographic processes, with significant impacts on marine ecosystems and coastal communities.
A new study led by CMCC brings the scientific community closer to identifying the triggers of these extreme events. By analyzing hundreds of marine heatwaves detected through advanced satellite data and clustering analysis, the research shows that persistent subtropical ridges – intrusions of hot air from the African continent into Europe, often informally referred to as “African anticyclones” – play a far greater role than rising atmospheric temperatures alone.
Although subtropical ridges occur frequently in summer, forming roughly every two days, it is their persistence that creates the critical conditions for marine heatwaves. At the onset of such events, these ridges become stationary, halting the usual eastward movement of weather systems.
When ridges settle over the Mediterranean basin for five or more consecutive days, prevailing winds weaken, the sea stops dispersing heat, and surface waters warm rapidly.
“Our study identifies the favorable conditions that precede marine heatwaves and shows that they are triggered by persistent subtropical ridges that weaken strong winds in the area,” explains Ronan McAdam, CMCC researcher and co-author of the study.
The findings reveal that 63.3%, 46.4% and 41.3% of marine heatwaves in the western, central, and eastern Mediterranean respectively occur during periods characterized by both subtropical ridges and weak wind conditions – a striking concentration, given that such conditions only occur 8.6% to 14.6% of all summer days.
When subtropical ridges persist for several days, the resulting decline in wind speed reduces the heat loss from ocean to atmosphere. This loss accounts for more than 70% of the total heat flux in affected regions, driving most of the sea temperature rise.
“It is very rewarding to identify the mechanisms behind a phenomenon we have been studying for years,” says Giulia Bonino, lead author of the study.
Furthermore, the probability ratios across the three Mediterranean clusters – 26 events in the western basin, 18 in the central, and 14 in the eastern – show that when a subtropical ridge and weak winds occur simultaneously, the likelihood of a marine heatwave increases by four to five times.
This statistical relationship lays the foundation for more accurate forecasting systems, which could help protect marine ecosystems and the industries that depend on them from future extreme events. For instance, in the Gulf of Lion, subsurface temperatures rose by nearly 7°C in just two days during the most extreme heatwaves, highlighting how rapidly these events can develop and the urgent need for precise forecasts and effective responses.
“It was an excellent collaboration between oceanographers and meteorologists – combining expertise and passion makes the difference,” adds McAdam. By integrating high-resolution ocean data with meteorological insights, the team demonstrated that early-warning systems can go beyond simple temperature thresholds, incorporating the physical dynamics that truly trigger these events.
As the Mediterranean Sea warms faster than the global average, knowing exactly when a marine heatwave is about to strike becomes crucial. “Our work highlights previously unidentified processes that are essential for accurately representing Mediterranean marine heatwaves,” McAdam concludes. “These results are vital to improving forecasting systems and Earth system models, marking a fundamental step towards effective early-warning and mitigation strategies in the region.”
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