FOCUS – This depression, which forms each winter above the poles, was the cause of the historic cold spell in the United States in December.
The freezing cold of winter has returned after an exceptionally mild wave that deprived ski resorts of snow. The phenomenon of “polar vortex“, at the origin of the historic cold episode of December in the United States, could it lower the thermometer further for the rest of the season?
What is the polar vortex?
The polar vortex is a cold pocket that forms every winter above the poles, at very high altitude, around 30 kilometers, in the stratosphere. A belt of wind and air maintains this cold depression around the poles. But it happens that the vortex deforms and flows of cold air rush beyond the air belt.
«Geography conditions these vortex stalls», explains the forecaster Régis Crépet of The Weather Channel* . «In the United States, for example, the cold can rush along the Rocky Mountains to the Gulf of Mexico which is, by comparison, at the level of Morocco on the other side of the Atlantic.“. The country has experienced a dizzying drop in temperatures, reaching -31.1°C in Denver (Colorado) or even more surprisingly -9.4°C in Houston (Texas).
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Cold snap in Western Europe
In Western Europe, on the other hand, much less prone to shifts in this polar vortex, the seas and oceans that surround the continent soften the climate. And the cold waves (declared as such from defined low temperature thresholds) caused by the deformations of the vortex are therefore much rarer. “The last was in February 2018 when the temperature dropped to -42 in Norway 250 kilometers north of Oslo», raises The Weather Channel in a note.
And the last major cold spell in France dates back to February 2012. A polar stall caused temperatures in France to fall below -10°C for several days.
«But the current cold is not due to this fluctuation of the vortex“, explains Régis Crépet. The classic cold weather of the season, with an installed anticyclone, should also soften in the weeks to come. “There will be a north-westerly wind coming from the ocean then a westerly wind loaded with humidity which will bring us a rather mild and very disturbed month of February», warns the meteorological expert.
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Is global warming responsible?
The current cold is therefore a classic winter weather. But vortex deformations in Western Europe may be more frequent in the future due to global warming. “The Arctic zone has warmed twice as fast as the rest of the planet over the past 30 years», note The Weather Channel, which led to a more marked melting of the ice and a more unstable polar vortex favoring the descents of polar masses in winter at our latitudes. But “the experts are not unanimous on the links of cause and effect».
Research is still needed to analyze these causal links. Especially since the behavior of the vortex is still a source of questioning, especially during “sudden stratospheric warming», a still poorly known phenomenon during which the temperature softens very markedly at high altitude, causing the cold air to fall heavier towards the ground.
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The effects of global warming remain especially visible through progressively less cold winters. But “some studies warn of a possible resurgence of cold waves due to the weakening of the vortex, resulting in very large temperature variations between cold snaps and significant mild spells“, noted The Weather Channel.
*The Weather Channel is a property of the Figaro group.
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