Between midnight and 6 a.m., Monday, October 15, it fell to 140 millimetres of rain in Carcassonne and 295 mm in the city nearby Trèbes, where six people were killed that night. “From 20 mm of precipitation per hour, it is already a strong accumulation ; there, it is colossal. A record “, commented Emmanuel Demaël, forecaster with Météo France, about the “blade of water” which sowed desolation in the Aude, between Trèbes and Conques-sur-Orbiel.

“thunderstorms” online, violent, difficult to predict and very localized, which dump the equivalent of several months of rainfall in a few hours, causing flash floods on a narrow strip of territory : these are the characteristics of the episodes of the mediterranean that experienced in the fall of Languedoc, Provence and the Ardèche – improperly called “episodes cévenols” when they do not refer to regions where the clouds are arrested by the heights of the Cevennes.

Read also :   Storms and floods : eleven dead in the Aude, placed by vigilance red

Related to rising warm and humid air and unstable from the sea, they mostly occur between September and November, when the temperature of the sea has been heating up all summer is the highest, which promotes high evaporation. These weather-related, in fact, all the mediterranean basin : Spain, Italy, Greece, but also the Maghreb, which is more concerned in December and January.

torrential rains increasingly brutal

The experts leave no doubt : these rains, there will be others. They may occur more often, but most importantly they promise to be more brutal. They are already more intense and should be strengthened by the effects of climate change.

scientists – including Robert Vautard, researcher (CNRS) at the laboratory of climate sciences and the environment – have been rotating their models and gave an account of some of their results in two papers published in the journal Climatic Change in April and June.

They believe that, in the south of France, the intensity of precipitation extremes has already significantly increased ” 22 % over the period 1961-2015 “. In addition, from a set of simulations, their work shows that the probability of exceeding an episode of a precipitation extreme occurring once every hundred years “has more than doubled due to temperature” up. And to conclude : “We argue that it is difficult to explain the trends in diagnosed without resorting to the human influence on climate. “

” the warmer the air is, the more it stores humidity : a degree Celsius in addition to translates to 7% moisture content additional, exposes Yves Tramblay, hydrologist at the laboratory HydroSciences (Montpellier university, CNRS, Institute of research for development). Therefore, we can say with certainty that the episodes in the mediterranean will become more intense. “

Dryness in the mediterranean basin

co-Authored with Samuel Somot (Center national weather research) an article on this subject, also published in Climatic Change, September 25, Yves Tramblay details their research, which take into account two scenarios of the intergovernmental panel on climate change (IPCC), the most optimistic forecasting a warming of over 1 °C by 2050. “In both cases, an intensification of rainfall extremes is emerging, summarized perhaps. It is not uniform : it varies between 5 % and 100 % increase, depending on the basin, but is around 20 % on average by the end of this century and even beyond, in the south of France, northern Italy and the Balkans. “

The fall storms should not forget that the mediterranean basin as a whole suffers now from drought. Everywhere the number of days where it rains decreases, and, everywhere, storms are becoming more violent. The flash floods caused by heavy rains such as those of the October 15, have been killed in recent years. In 1999, thirty-five people died in the south of France, of which twenty-six people in the Aude. In 2001, in Algiers, a thousand people have disappeared after an episode of the mediterranean.

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A répresident shows the level reached by the’water during floods à Villegailhenc, près of Carcassonne, in the south of France. The level reached Monday morning by the flood à Trèbes, in the vallée de l’Aude, is without précédent since 1891, the c’isà-dire « not far from a seven-mètres », a déclaré à l’AFP the service Vigicrues.


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