An autumn under the threat of violent weather episodes

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Antoine Orsini, hydrobiologist, professor at the University of Corsica does not hide his concern about the appearance of strong storms or even “medicine” in the coming weeks. In question, an overheated Mediterranean Sea. Average water temperature: 24° A high-risk autumn. Such was the meteorological hypothesis put forward, reluctantly, by Antoine Orsini – hydrobiologist, professor-researcher at the University of Corsica, at the Espace Diamant stage in Ajaccio this Monday, during a conference on the subject of climate change*. “I’d like to be wrong,” he insists. But the problem is that the overheating of the Mediterranean during this summer heat wave 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 continent was 24°. It was 27° between Sardinia and Sicily. So we have a very warm sea just as the first cold weather arrives. This type of situation always worries me”, he continues. A mood that seems legitimate given the violent weather that hit Italy’s Ancona province on the night of September 15-16, killing at least 11. Weather phenomena observed during the fall of 2020 and then into 2021 will likely raise questions. At the end of September 2020, bathers continue to enjoy the beach and the water at 22°. On 2 October, a deluge hit the La Vésubie, Roya and Tinée valleys inland Nice, causing deadly flooding. Unusual rainfall is observed. “As an example, in a surface of 25 km2 fell 570 mm or liters of water per m2, which represents a total of 14 million m3, that is to say, half the capacity of the dam of Calacuccia. In another part of this territory with an area of ​​100 km2, 400 mm fall per m2, which corresponds in total to 40 million m3, that is to say, a volume greater than the content of the Tolla dam”, he recalls. Hot air versus cold fall A year earlier, at the end of September 2020, strong winds and heavy rain swept over Greece. On the 28th and 29th of October it was Sicily’s turn to find itself in the eye of the cyclone or rather the “medicine” dubbed “Apollo”, “a Mediterranean hurricane, half tropical cyclone, half storm classic and a sign that the Mediterranean basin”. it is becoming more tropical,” observes Antoine Orsini. In some places 500 mm or liters of rain per m2 are recorded. Between the two episodes, on October 1, it was 26° in the water of Sicily at a depth of 40 meters. Fifteen days later, the thermometer lost barely 2°. The Mediterranean continues to show 24°. In 2022, once again, the threat is inscribed in the order of meteorological realities, inasmuch as a mass of warm air might well come to meet a cold drop. To the contrast of the masses of air is added a strong evaporation. And, the result is well known, “the more evaporation there is, more water vapor is available in the atmosphere and the more intense the rains can potentially be. Thus, the warm waters of the Mediterranean autumn provide fuel for the Mediterranean episodes,” explains Météo France. Under these conditions, only a tenacious anticyclone could prevent the formation of violent storms. But nothing is certain. Météo France analyzed the scenario for the months of September, October and November as part of its seasonal forecasts. Climatic anomalies After a scorching summer, the climate trends described by the national meteorologist show “warmer than normal weather in France, due to a context favorable to a shift to the north of Europe of the ocean flow and the circulation of disturbances”, we observe. . With regard to precipitation, on the other hand, “no scenario is proposed for this quarter in France. Likewise, no trend appears in the frequency of weather conditions favorable to Mediterranean episodes. However, the persistence of particularly high sea surface temperatures could contribute to increasing its intensity. The field of possibilities is open. Especially since at the time of global warming, the weather is not an anomaly. Thus, “in March 2022 when it snowed on the island, it was 30 at the level of the polar circle”, recalls Antoine Orsini. Another remarkable fact refers to May 15, 2019. On this day it snowed heavily on the Vizzavona pass. We are far from the spring atmosphere that accompanies the calendar; the fault of the “jet-stream”, the current of air. “The increase in temperature at the poles generates pressure changes. Therefore, this current will be characterized by large oscillations. To simplify, we’ll have the cold air going down the hollow side and the hot air going up the other side. This is, in general, what happened on the famous May 15, 2019. The cold waves are fully compatible with climate change”, analyzes the academic. According to several meteorologists, a Mediterranean episode could affect the south of France and Corsica during this weekend of September 24 and 25. At the moment, uncertainties remain about the intensity and location of the storms. *The sequence was part of the program of Storia d’Acqua, the event dedicated to water and offered until October 30 by the city of Ajaccio in collaboration with the State, the community of Corsica, Kyrnolia and Paris-Match magazine
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