The heat waves in the Mediterranean, long remained mysterious, finally find an explanation thanks to a recent study. The researchers identified the atmospheric phenomena responsible for these marine thermal anomalies.
Every summer, the Mediterranean is adorned with a quiet blue that masks a much more formidable dynamic. Below the surface, an invisible heat rise is slowly transforming the marine ecosystem. Far from spectacular thunderstorms or sudden storms, it is another, more sneaky phenomenon that worries scientists. As the seasons warm up, marine heat waves set in more often, revealing a profound imbalance between the air and the ocean.
When the air freezes, the sea warms up
Episodes of extreme warming of the sea, called marine heat waves, do not occur by chance. According to a recent study published in Nature Geoscience, their triggering is linked to a phenomenon well known to meteorologists, but whose impact on the oceans had so far been underestimated: persistent subtropical ridges. These intrusions of hot air from the Sahara sometimes extend sustainably into Southern Europe, forming high-pressure systems that literally immobilize the atmosphere over the Mediterranean basin.
These subtropical ridges appear frequently in summer, but it is only when they become stationary – for at least five days – that they trigger conditions conducive to a surge in sea temperature. This atmospheric blockage cuts short the renewal of air masses, preventing any natural ventilation of the basin. Far from being an isolated case, this phenomenon has preceded the majority of the most violent marine heat waves recorded in the last forty years.
Marine heat waves, an alchemy between sky and ocean
Warm and still air alone does not explain the speed at which surface waters can gain up to 7 °C in just two days, as shown by surveys in the Gulf of Lion during the summer of 2022. According to the Euro-Mediterranean Center on Climate Change (CMCC), which coordinated the analysis of hundreds of such events, the key lies in the interruption of the natural mechanism of ocean cooling. Evaporation by the wind.
The researchers highlighted a direct relationship between the decrease in wind speed and the loss of heat by the ocean. When the wind falls, the evaporation stops, the heat remains trapped in the surface layers, and the temperature rises. This so-called latent heat flow represents more than 70% of the heat transfer between the ocean and the atmosphere. Without it, the sea becomes an open-air boiler.
This mechanism has been precisely quantified. In most cases, marine heat waves form when subtropical ridges and weak winds coincide. And yet this duo is only present during 8 to 15% of summer days. This rarity reinforces the predictive scope of the phenomenon, because when it occurs, the risk of extreme episodes is multiplied by five.
The forecast challenge in the face of an increasingly unstable Mediterranean Sea
With global warming, marine heat waves are becoming more frequent in the Mediterranean. This region is also recognized as a « hotspot » by scientists. These underwater heat waves cause massive mortality in many species. They also disrupt food chains and weaken fishing like aquaculture. Therefore, anticipating their appearance becomes essential to limit the impacts.
In this context, the work of the CMCC marks an important step forward. Indeed, a new generation of forecasting models is emerging. These models take into account not only the surface temperature, but also the overall atmospheric dynamics. Thanks to this, it becomes possible to spot weak signals announcing a marine heat wave. Among them are the subtropical ridges or the sudden drop in winds. These indicators can thus be used in short-term warning systems.
This step forward in understanding air-sea interactions marks a turning point. It shows that the Mediterranean sky holds an essential part of the thermal memory of the sea. A memory that, if well interpreted, can become a tool to anticipate the most silent… and most destructive disorders.
