“Atmospheric river”: why does the rain that falls in mainland France come from the West Indies?

“Atmospheric river”: why does the rain that falls in mainland France come from the West Indies?

Finally the end of the drought? After dozens of days without rain between January and February, precipitation has returned to France since Wednesday. During the next ten days, between 30 and 50 mm of water should fall on the north of the Loire, and nearly 80 mm or even 100 m locally in the South, such as in Savoie or in the Hautes-Alpes, according to Agate Météo . The equivalent of more than a month of precipitation according to seasonal norms.

A rain related to “atmospheric river” phenomenon or “rum express”, a rapid circulation of hot and humid air in the form of a fairly narrow corridor 200 to 400 km wide, from tropical latitudes to temperate regions. “More simply, it is very hot and very humid air that comes to us from the tropics, which crosses the entire Atlantic to cause several episodes of strong disturbances”, deciphers meteorologist Patrick Marlière.

Technically, the phenomenon originated in the West Indies, between two weather systems with opposite rotations that draw hot and humid tropical air eastward in a sort of corridor. Fed by the evaporation of ocean water, this high-altitude river empties in several episodes over Western Europe from southern Ireland to Spain.

Floods but no “useful rains”?

The first depression has already fallen on the metropolitan territory between this Wednesday and Thursday, starting with Brittany. The second is scheduled for Saturday March 11 and could have harmful consequences locally. Météo France already anticipates, that day, potential floods “on the western slopes of the Massif Central, the Northern Alps, the Jura and the Vosges”. It remains to be seen how long this can last? “Other depressions will form for another 4 or 5 days, until the middle of next week or even longer,” estimates Patrick Marlière.

These atmospheric river phenomena are happening more and more often over Europe (and also on North America). The last episode in France dates from December 2022 when 81.6 mm of water fell in Lyon, for example. “It is a term that has been used for a short time because it is linked to climate change. The warmer the air, the more these phenomena can contain water vapours. These extreme phenomena are closer and stronger,” explains the meteorologist.

Enough to solve the problem of lack of water and drought observed this winter? “We are not going to deprive ourselves of this water but it is not necessarily useful rain, retorts Patrick Marlière. If it falls in 3 days the equivalent of a month, it is irrelevant. The water will not infiltrate to the water tables, it will be evacuated. The ideal for the hydrological crisis that we are experiencing is that it rains a little every day for three or four months. So make no mistake, putting out the umbrellas doesn’t mean the drought is behind us.

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