An autumn under the threat of violent weather episodes

Antoine Orsini, hydrobiologist, lecturer at the University of Corsica does not hide his concern about the occurrence of strong storms or even “medicane” during the weeks to come. In question, an overheated Mediterranean Sea. Average water temperature: 24°

A high-risk autumn. Such was the meteorological hypothesis posed, reluctantly, by Antoine Orsini – hydrobiologist, teacher-researcher at the University of Corsica, on the Espace Diamant stage in Ajaccio this Monday, during a conference on the theme of climate change*. ” I wish I was wrong “, he insists.

But, the problem is that the overheating of the Mediterranean during this scorching summer and beyond, now creates the possibility of devastating episodes. ” A few days ago, on September 17, the temperature of the sea water between Corsica and the mainland was 24°. It was 27° between Sardinia and Sicily. We therefore have a very warm sea just when the first cold weather arrives. This kind of situation always worries me.he continues.

A state of mind that seems legitimate given the violent weather that hit the province of Ancona in Italy on the night of September 15 to 16, causing at least 11 victims. The meteorological phenomena observed during the autumn of 2020 and then 2021 are likely to raise questions. At the end of September 2020, swimmers continue to enjoy the beach and water at 22°. On October 2, a deluge hit the valleys of La Vésubie, Roya and Tinée in the hinterland of Nice, causing deadly floods. Unusual amounts of rain are observed.

For example, over an area of ​​25 km2it fell 570 mm or liters of water per m2which represents a total of 14 million m3, that is to say half the capacity of the Calacuccia dam. In another portion of this territory with an area of ​​100 km2it falls 400 mm per m2which altogether corresponds to 40 million m3i.e. a volume greater than that contained in the Tolla dam”he recalls.

Hot air versus cold drop

A year earlier, at the end of September 2020, strong winds and heavy rains swept over Greece. On October 28 and 29, it was Sicily’s turn to find itself in the eye of the storm or rather of the “medicine” baptized “Apollo”, “a Mediterranean hurricane, half tropical cyclone, half classic storm and a sign that the Mediterranean basin is becoming more tropical observes Antoine Orsini. In places, 500 mm or liters of rain per m2 are recorded. Between the two episodes, the 1er October, it is 26° in the water off Sicily at a depth of 40 meters.

Fifteen days later, the thermometer lost barely 2°. The Mediterranean is still showing 24°.

In 2022, once again, the threat is inscribed in the order of meteorological realities, insofar as a mass of warm air could very well come to meet a cold drop. To the contrast of the air masses is added a strong evaporation. And, the sequel is well known, “ the more evaporation there is, the more water vapor there is available in the atmosphere and the more intense the rains can potentially be. Warm waters in the fall in the Mediterranean thus provide fuel for Mediterranean episodes “, explains Meteo France. Under these conditions, only a tenacious anticyclone could prevent the formation of violent thunderstorms. But nothing is certain.

Météo France looked at the scenario for the months of September, October and November as part of its seasonal forecasts.

Climate anomalies

After a scorching summer, the climatic trends outlined by the national meteorologist show ” warmer than normal weather over France, due to a favorable context for a shift towards northern Europe of the ocean flow and the circulation of disturbances”, we note. With regard to precipitation, on the other hand, no scenario emerges for this quarter for France. Similarly, no trend appears in the frequency of meteorological situations favorable to Mediterranean episodes. However, the persistence of particularly high sea surface temperatures could contribute to increasing their intensity. “, we analyze. The field of possibilities is open.

Especially since at the time of global warming, the weather is not an anomaly. Thus, “en March 2022 while it was snowing on the island, it was 30 at the level of the polar circle”, recalls Antoine Orsini.

Another remarkable event refers to May 15, 2019. On this day, it snowed heavily at the Vizzavona pass. We are far from the spring atmosphere that goes with the calendar; the fault of the “jet-stream”, the air current. ” The increase in temperature at the poles generates changes in pressure. Therefore, this current will be characterized by large oscillations. To simplify, we will have the cold air which descends in the hollow part and the hot air which rises on the other side. This is generally what happened on the famous May 15, 2019. Cold waves are completely compatible with climate change », Analyzes the university.

According to various forecasters, a Mediterranean episode could affect the south of France and Corsica during this weekend of September 24 and 25. For the time being, uncertainties remain as to the intensity and location of the storms.

*The sequence was on the program of Storia d’Acqua, the event dedicated to water and offered until October 30 by the city of Ajaccio in partnership with the State, the Corsica community, Kyrnolia and the magazine Paris- Match

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