MADRID, 14 (EUROPA PRESS)
Warm air intrusions from the African continent into Europe have a much greater impact than simply increasing air temperatures on marine heatwaves in the Mediterranean.
This is the conclusion of a new study, led by the CMCC Foundation – Euro-Mediterranean Center on Climate Change, after analyzing hundreds of marine heatwaves identified in the region using advanced satellite data and clustering analysis. The findings are published in Nature Geoscience.
The Mediterranean Sea is particularly susceptible to marine heatwaves, such as the record-breaking 2022 heatwave, characterized by abnormally high sea surface temperatures due to the interaction between air-sea heat fluxes and local oceanographic processes, resulting in significant impacts on marine ecosystems and coastal communities.
PERSISTENCE
Although subtropical ridges—which bring in warm African air—occur frequently in summer, approximately every two days, their persistence creates critical conditions for the formation of marine heat waves. During the onset of a marine heat wave, the formation of ridges becomes persistent: the high-pressure system associated with them becomes stationary, disrupting the normal eastward movement of weather systems.
When these ridges sit over the Mediterranean basin for five consecutive days or more, they calm the prevailing winds, causing the sea to stop releasing heat and the surface waters to warm rapidly.
"Our study identifies the favorable conditions that lead to marine heatwaves and reveals that they are driven by the persistence of subtropical ridges, which weaken the strong winds in the area," says Ronan McAdam, a researcher at the CMCC and co-author of the study.
The findings show that 63.3%, 46.4%, and 41.3% of marine heatwaves in the western, central, and eastern Mediterranean, respectively, occur during periods with subtropical ridges and reduced winds; a remarkable concentration considering that these combined conditions only occur between 8.6% and 14.6% of all summer days.
When subtropical ridges persist for several days, the resulting decrease in wind speed causes a substantial reduction in heat loss from the ocean to the atmosphere. This heat loss accounts for more than 70% of the total heat flux in the affected regions and drives most of the ocean temperature change.
"It's very satisfying to identify the mechanics of a phenomenon we've been studying for years," says lead author Giulia Bonino.
Furthermore, the likelihood ratios in 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 combine, the probability of a heat wave forming is four to five times higher.
The discovery of this statistical relationship lays the groundwork 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 Lions, subsurface temperatures rose by nearly 7°C in 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 an excellent collaboration between oceanographers and atmospheric scientists; the combination of experience and passion is key," says co-author Ronan McAdam. By combining the nuances of meteorology with high-resolution ocean data, the team demonstrates that early warning systems can go beyond temperature thresholds to understand the physics that actually triggers an event.
Since the Mediterranean seas are warming at a faster rate than the global average, it is essential to know precisely when a marine heatwave is about to hit. “Our work highlights previously unidentified processes that are essential for accurately representing Mediterranean marine heatwaves,” says McAdam. “These results are crucial for improving forecasting systems and Earth system models, and represent a key step toward effective early warning and mitigation strategies in the basin.”
