Origin of Mediterranean heatwaves discovered

High pressure areas from Africa , where hot, dry air tends to descend , bringing good weather and atmospheric stability, can remain over the Mediterranean Sea for up to five consecutive days, stopping the winds and heating the surface waters : this is how the heat waves that are increasingly hitting the Mediterranean are created. The mechanism has been reconstructed in research published in the journal Nature Geoscience and conducted by the Euro-Mediterranean Center for Climate Change (CMCC), based in Bologna, under the coordination of Giulia Bonino. The results will now allow the phenomenon to be predicted with greater precision .

Working as a team, oceanographers and meteorologists analyzed data collected over 40 years , from 1982 to 2022, relating to 123 major marine heatwave events covering an area of over 100,000 square kilometers . They found that marine heatwaves in the Mediterranean become four to five times more likely when they coincide with the presence of light winds .

The research focused in particular on the phenomenon of warm air intrusions moving towards Europe from Africa . These are high-pressure areas called ' subtropical ridges ', better known as ' African anticyclones '. These intrusions occur mainly in summer , with a frequency of about two days , but can sometimes become persistent .

When subtropical ridges develop over the Mediterranean basin for five consecutive days or more , they cause the prevailing winds to stop, causing the sea to stop radiating heat and the surface waters to warm rapidly . Statistics from events observed in the western, central, and eastern Mediterranean also indicate that when a subtropical ridge and weak winds occur together , the probability of a heat wave increases four to fivefold .

"Our study identifies favorable conditions that lead to marine heatwaves and reveals that they are triggered by persistent subtropical ridges that weaken the strong winds in the area," says Ronan McAdam, CMCC researcher and co-author of the study. "It's very gratifying to identify the mechanisms behind a phenomenon we've been studying for years," says Bonino. The result now lays the foundation for developing more accurate forecasting systems , which could help protect marine ecosystems from future extreme events . Considering that the Mediterranean is warming faster than the global average, knowing precisely when a heatwave is about to hit becomes essential. "These findings," McAdam observes, "are essential for improving forecasting systems and Earth system models, and represent a fundamental step towards effective early warning and mitigation strategies in the basin."