What is the polar vortex?

The term of polar vortex is used to describe a depression that forms each winter above the north and south poles at about 30 km altitude. This cold pocket settles from the average troposphere (8 to 12 km altitude) up to the stratosphere (10 to 50 km altitude). It weakens in summer, but intensifies in winter, because sunlight no longer reaches high latitudes during the polar night. In this context, the circulation of strong, swirling winds from west to east, which characterizes the vortex over the Arctic in the Northern Hemisphere, then becomes very rapid. There is then formed what is called the Stratospheric Jet of the polar nightthe equivalent of JetStream located much lower, about 10-12 km above sea level.

The vortex is however not stable and can present important deformations related to strong and sudden modifications of its temperature, causing rapid descents of cold air towards temperate latitudes in North America or Europe. We can therefore speak of two polar vortices: the most common, the stratospheric polar vortex, and the tropospheric polar vortex, which is the one that interests us the most, because it is synonymous with the risk of cold air invasion.

Two vortexes with very different consequences

The polar vortex: operating principle © The Weather Channel

Remember that there is formed on the surface of the Earth, between the cold air near the poles, and the warmer air at the level of the tropics, a zone of conflict from which depressions are born, and where strong winds circulate. characterizing the jet stream. The latter sometimes takes very sinuous forms linked, during the winter in the northern hemisphere, to the expansion of the tropospheric vortex which causes the jet stream to descend towards the south and with it a mass of icy air.

In the case of the stratospheric vortex, higher in altitude than the tropospheric vortex, the jet stream is stronger and takes on a round or oval shape. This configuration prevents the confrontation of air masses and acts as a shield. The coldest air thus remains confined to the Arctic regions, and is therefore less likely to plunge deeply into North America, or even into Eurasia. Moderate shifts can however occur and cause descents of polar air towards the south, but they remain of short duration.

On the other hand, it happens that the polar vortex weakens, that is to say that the speed of circulation of winds at altitude slows down, resulting in a wave phenomenon, like a river that lacks flow. This phenomenon is most often related to the sudden development of a stratospheric warming in the heart of winter. The polar vortex thus heats up suddenly, with temperatures sometimes rising to 60°C in just a few days, from -70°C to -10°C. It occurs when the stratospheric jet begins to blow in the opposite direction to the Jet Stream, located below in the atmosphere. This has the effect of weakening it and slowing the winds aloft. It then begins to oscillate until it descends towards the middle latitudes. This is how very cold air finds itself trapped in the troposphere, this layer of the atmosphere in which we live, and can thus invade regions usually little affected by these extreme temperatures.

Episodes associated with the recent polar vortex

The areas of the globe that may be affected are diverse. North America is therefore frequently subject to these major cold spells.

The latest polar stall for North America is very recent. It dates from end of December 2022. It was around Christmas that millions of Americans suffered the onslaught of a polar air mass (-30°C at 1500 m altitude) which caused a dizzying drop in temperatures, reaching up to – 31.1°C in Denver (Colorado) or even more surprisingly -9.4°C in Houston (Texas), yet located on the shores of the Gulf of Mexico. At the same time, Western Europe was bathed in great mildness, which we experienced in France between Christmas and New Year’s Day with records. These two simultaneous temperature excesses on either side of the Atlantic Ocean were also linked, due to the undulatory movement of the jet stream, the icy air descending in North America, and the mild air rising towards the ‘Western Europe.

Europe is also sometimes affected, like Asia, by these polar stalls. In Western Europe, the last dates back to March 2018. The temperature then dropped to -42°C on February 28 in Folldal-Fredheim in Norway. Many cold records had fallen all over Europe, with up to -10.2°C in Frosinone in Italy. In France, the coldest day of this cold wave was February 27 with a deviation from normal of -3.2°C. On the morning of February 28, severe frosts (<-5°C) were observed over 80% of French territory, including certain coastal areas.

The last major cold spell linked to a polar stall in France, however, dates from February 2012. A powerful anticyclone had positioned itself between Scandinavia and Russia before extending towards Western Europe. This cold spell, between February 4 and 12, had plunged temperatures between -10 and -14°C daily, and locally down to -16 to -18°C in certain plains. It remains the 5th most severe cold snap observed since 1947.

What is the impact of global warming on the evolution of the polar vortex?

The impact of global warming is not yet well defined. If the Arctic zone has warmed up twice as fast as the rest of the planet over the past thirty years, resulting in a more marked summer melting of the ice and an unstable stratospheric polar vortex favoring descents of polar mass in winter in our latitudes, experts are not unanimous on the causal links. Further research will therefore be needed to better understand the future consequences of climate change on the behavior of the polar vortex.

The oscillations of the polar vortex are therefore often responsible for cold spells at mid-latitudes, but not always. Some cold spells occur without stratospheric warming. As for global warming, its most striking effects are progressively less cold winters, even if certain studies warn of a possible resurgence of cold spells due to the weakening of the vortex, resulting in very large temperature variations between blows. of cold and significant thaws.

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