how can we explain the arrival in France of what was a hurricane formed in the Atlantic?

Be careful when approaching depression Kirk. Wednesday October 9, the episode will be associated “to a very strong gust of wind”with gusts of 100 to 130 km/h, and a “intense rainy episode”reaching in one day “quantities [de précipitations] which usually fall within a month”warns Météo-France. Orange vigilance is announced in many departments, in particular in a corridor going from Vendée to the Ardennes. But how can we explain the arrival in France of what was still, a few days earlier, a hurricane in the middle of the Atlantic?

Tropical hurricanes cross the Atlantic Ocean from east to west, from Africa to the Gulf of Mexico. In the United States, all eyes are on Milton, which has grown significantly in a short time towards Florida. Leslie, another hurricane, is currently in the central ocean. “Either these hurricanes exhaust themselves upon encountering the continent or cold waters, or they transform into mid-latitude storms”explains to franceinfo Fabio d’Andrea, researcher at the dynamic meteorology laboratory at the Ecole Normale Supérieure.

In the second, rarer case, hurricanes go in the other direction, from west to east, and can travel further north, outside tropical zones, to the European coasts. “This is the case of Hurricane Kirk, which made what is called an extratropical transition.underlines Fabio d’Andrea.

Hurricanes (also called tropical cyclones) are identifiable by a vortex, a type of whirlpool, with a strong depression of warm air at their heart. This is no longer the case for Kirk, which has transformed into a storm, with a depression of cold air at the heart of the vortex. “The physics of these two phenomena is very differentspecifies Fabio d’Andrea. Wind speeds of 100 to 130 km/h are forecast, which is very strong, and heavy rain. But the Kirk episode will not be extraordinary. For comparison, tropical hurricanes, at their most intense, can have winds reaching 300 km/h.

With global warming, hurricanes are expected to become even more intense. “We have difficulty reproducing these phenomena in our numerical models, but we can predict a change in their ingredients, in particular the sea surface temperature, which is the thermodynamic driver of hurricanes”explains Fabio d’Andrea. “It will very likely increase and hurricanes will draw more energy from the sea.” This increase in temperatures, on the other hand, has no impact on the frequency of these episodes.

“The proportion of intense tropical cyclones (category 4–5) and the maximum wind speeds of the most intense tropical cyclones are projected to increase with increasing global warming.”also highlighted the Intergovernmental Panel on Climate (IPCC) in its latest report. “In the North Atlantic, total precipitation from Hurricanes Katrina, Irma, Maria, Harvey, Dorian and Florence were all made more intense (by 4%, 6%, 9%, 15%, 7.5% and 5%, respectively) by climate change.for its part, reports World Weather Attribution, a network of international scientists.

But will global warming lead to more of these extratropical transitions towards our latitudes? A study published in July in the journal Nature noted an increase in the frequency of these storms resulting from tropical phenomena near Western Europe between 1966 and 2020, while adding that their number could on the contrary decrease by the end of the 21st century. “The question is interesting, but it is still the subject of work”responds Fabio d’Andrea. The consequences of global warming on the intensity of these specific storms also remain uncertain. “One can imagine that they will be a little stronger due to the increased humidity and that the precipitation that follows these storms will be a little stronger, but at this point there is no certainty. “