A man carries a boy on his back, so carries a child on his back as he walks to an evacuation bus before Hurricane Laura arrives in Lake Charles, Louisiana on August 25, 2020 in amid the coronavirus pandemic. - Andrew CABALLERO-REYNOLDS / AFP

  • Hurricane Laura, which hit the United States on Thursday, could be among the 13 strongest storms to ever hit the country. And while it should lose power inland, the affected regions remain under threat of flooding.
  • More worrying, Laura is only the first major hurricane to hit inhabited areas during the 2020 season. And this one is announced particularly active in the North Atlantic, with up to 25 cyclones expected.
  • The reason ? The start of a La Nina phenomenon in the South Pacific, an intense West African monsoon and warm temperatures in the tropical Atlantic. So many factors which, together, promote the formation of cyclones.

Winds blowing up to 240 km / h. As expected, Laura hit the US coast overnight from Wednesday to Thursday, after gaining power during her crossing of the Gulf of Mexico. Still a tropical storm when it hit Haiti, the Dominican Republic and Cuba a few days ago, Laura is now a category 4 hurricane [winds above 211 km / h] on the Saffir-Simpson scale and s' even approaching category 5, the highest.

Enough to make it "very dangerous", hammer home the authorities in Louisiana, the first affected state, and in Texas, which is about to be visited.

Among the 13 strongest storms to hit the United States

Laura should quickly weaken as she enters land, as hurricanes usually do. "It is the winds, by rubbing the surface of the water, that make a cyclone operate," begins Franck Roux, professor emeritus at the Aerology Laboratory (CNRS / University of Toulouse III), atmospheric physicist. This friction generates evaporation and this water vapor then feeds the large storm systems inside the cyclone, giving it energy. When it passes over land, it no longer has this large source of energy and thus naturally attenuates, especially since it encounters more obstacles on land likely to slow it down. "

Clearly, with Laura, the most spectacular - the winds at 240 km / h - is a priori over. However, the affected regions are not out of the woods. "It is not necessarily the strong winds of a hurricane that cause the most damage, but also the intense rains that accompany it", continues Franck Roux. With, behind, the risk of floods, landslides.

The NHC, the American center for hurricane monitoring, points to this risk of "flash floods occurring in parts of Louisiana" with Laura and potentially ranks the hurricane among the thirteen strongest storms to hit the United States. Fifteen years almost to the day after Katrina, who ravaged the city of New Orleans.

A particularly active 2020 season

And we are only at the end of August, remind Franck Roux like Fabrice Chauvin, researcher at the National Meteorological Research Center (CNRS / Météo France), to better remind that Laura is "only" the first major event of intensity, which more is touching down, with a 2020 season far from over and which promises to be exceptional.

"In the North Atlantic, the cyclone season usually begins in June and ends in November, with generally a peak of activity around September 10," says Fabrice Chauvin. So much for the general framework in which the years follow one another but are not alike. An average season generates twelve tropical storms, including no more than six hurricanes [a tropical storm becomes a hurricane when its winds exceed 117 km / h], of which three will become major hurricanes [of category 3 or more].

The year 2020 will be well above. As early as April, several groups of experts had announced an extremely active coming season in the North Atlantic, until counting on 22 tropical storms expected for pessimistic forecasts. In early August, these first forecasts were even revised upwards by the US Oceanic and Atmospheric Observation Agency (NOAA), now forecasting between 19 and 25 tropical depressions, of which 7 to 11 could turn into hurricanes. And between three and six could reach category 3. More than double, therefore, of a normal hurricane season.

Beginning of La Nina, strong monsoon and warm surface waters

This is because many factors contribute to the hurricane season being indeed very active. Fabrice Chauvin and Franck Roux first of all evoke the multi-year natural climatic phenomena El Nino and La Nina, which take place in the middle of the Pacific Ocean. The first results in abnormally high water temperatures in the eastern part of the South Pacific Ocean, which has the effect of creating unfavorable atmospheric conditions for the formation of hurricanes in the North Atlantic. Quite the opposite of La Nina: unusually cold waters in the Pacific and conditions favorable to cyclonic activity in the North Atlantic. "However, we are currently in a phase of transition from an El Nino phenomenon to a La Nina phenomenon, rather even today towards the beginning of La Nina", slips Fabrice Chauvin.

Another factor: the monsoon in West Africa, very active this year. “There is a lot of rain in the Sahel, which is rather good news for the region,” explains Franck Roux. But these stormy systems that develop in the African monsoon carry the seeds of a cyclone within them when they reach the Atlantic. And can become so if the atmospheric conditions are favorable at that time on the ocean. "

Among these favorable conditions, there are in particular unusually warm sea surface temperatures. And this is precisely the case this year in the tropical Atlantic. “Basically, between Senegal and Venezuela, situates Franck Roux. The anomaly is half a degree. You could say it's not a lot, but it still represents a very significant amount of additional energy that has the effect of fuel for cyclones. "" I watched [Tuesday], it is over 30 ° C on the Gulf of Mexico ", abounds Fabrice Chauvin.

Already at the letter M

To date, the forecasts have proven to be correct: this 2020 hurricane season is indeed very active. The proof, to navigate the succession of cyclones, the tradition is to start the first name of each one with a different letter of the alphabet. Starting with "A" and so on.

On August 4, we were already at “I” with Isaïas, the ninth storm of the season, therefore in the North Atlantic, a stage usually reached at the end of the season. After several very calm weeks - "due to a very strong emission of dusty air coming from the Sahara, which does not create favorable conditions for the formation of cyclones in the North Atlantic", explains Franck Roux, the cyclonic activity has taken back. And here we are at Laura's "L" and even at Marco's "M", another cyclone to ply the Gulf of Mexico, but ultimately not very threatening.

By way of comparison, Fabrice Chauvin refers to the 2005 hurricane season, which had marked the spirits. "There were then 28 baptized cyclones, to the point of having to use the Greek alphabet", says the meteorologist. This 2005 season was also marked by the intensity of its hurricanes, with four category 5, including the infamous Katrina. Again a record.

Active, that's for sure… More intense? Not automatically

"We may not be there in 2020", then continues Fabrice Chauvin. Difficult to predict anyway. Projections focus on expected cyclonic activity. Determining the intensity of cyclones is much more dangerous. “It depends on many other meteorological parameters which change from day to day,” explains Fabrice Chauvin.

When NOAA says it expects seven to eleven hurricanes including three to six major ones, "it's more likely," says Franck Roux. The risk of a tropical storm passing hurricane is on average one in two, the same for that of a hurricane becoming force majeure. "

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