Strong heat for several weeks in a row, then devastating thunderstorms that burst. In Corsica, violent gusts of wind and torrential rains caused damage on Thursday August 18, and above all caused the death of at least five people, killed by falling trees or at sea. Franceinfo explains how the abnormally high temperatures of the Mediterranean Sea, which is inexorably warming, favor these extreme weather phenomena. Indeed, this symptom of the climate crisis, in addition to causing significant damage to biodiversity, influences weather conditions.
The Mediterranean is warming strongly
The Mediterranean basin is today one of the regions of the world most affected by global warming. At the end of July, experts in climate change alerted AFP to the “great marine heat wave” underway since the end of May, with temperatures “exceptional” higher than “4 to 5 degrees” to normals. The warming of the Mediterranean occurs on the surface, but also in the deeper layers.
In July, “from the Balearic Sea (Spain) to Sardinia (Italy), as well as to the east of Corsica and throughout the Tyrrhenian Sea (between Sicily and Corsica), we observe on the surface ( ..) exceptional temperature values between 28 and 30 degrees” which are “higher than normal, around +4 to +5 degrees”, NGO Mercator Ocean International told AFP. This non-profit organization based in Toulouse brings together the main institutes specializing in oceanography from France, Italy, Spain, Great Britain and Norway and pilots the European ocean monitoring service, the Copernicus Marine Service (CMEMS). ).
The heat of the sea fuels violent weather phenomena
“HASToday, we know that the increase in water temperature causes extreme phenomena“highlighted Christian Buchet, director of the Center for Sea Studies at the Catholic Institute of Paris, interviewed for franceinfo. “The warm water of the Mediterranean favors the ascent of the air”explains to franceinfo the meteorologist Nicolas Le Friant. Lot and humid air rises in the atmosphere as soon as the temperatures in the air cool down.
The Mediterranean therefore becomes a reserve of warm water particularly conducive to the formation of storms. “The warmer the Mediterranean, the more we will have an additional reserve of energy, which will help to turn into torrential rainfall”, analysis for franceinfo hydrologist Emma Haziza, specialist in the adaptation of our societies to climate change.
This configuration is conducive to Mediterranean episodes. These storms, accompanied by heavy accumulations of rain which generally occur in autumn, are the result of a confrontation between the upwellings of hot, humid and unstable air from the Mediterranean, and the arrival of cold air from high altitude. This thermal contrast can also give rise to an arcus, a cloud in the shape of an arched roll, a harbinger of violent storms. A cloud of this type was precisely observed in the sky of Corsica, before the devastating bad weather on Thursday.
Heat is stored at sea and diffused for a long time in the atmosphere
The heat of the sea currently observed will have an impact on the coming months. “All the heat stored by the sea during the summer will stay for a very long time, and this inertia can influence the atmosphere for a longer time”warns on TF1 Florian Pantillon, researcher at the CNRS aerology laboratory in Toulouse. “This combination of a sea that remains very warm with weather that is starting to deteriorate will provide a lot of humidity to the atmosphere and therefore a lot of potential for heavy rains and intense episodes in the fall”adds the scientist.
After a particularly hot summer, the cooling of temperatures in the fall could generate “medicanes”. This contraction of the word “hurricane” (hurricane in French) and “Mediterranean”, designates an extreme storm of short duration which is, as its name suggests, in the Mediterranean. Storms, Mediterranean episodes, storms, “medicanes”… “The warmer the water, the more violent this type of episode is likely to be”warns for Le Figaro carolina jane Muller, CNRS researcher on secondment to the Austrian Institute of Science and Technology.