The polar vortex does not come to visit; its weakness lets Arctic air escape
When a brutal cold snap hits North America, headlines often blame the polar vortex. Scientists say that is not quite right. The vortex is a permanent ring of cold, spinning air around each pole. The frigid outbreaks happen when it weakens and wobbles, letting masses of Arctic air spill towards the mid-latitudes.
There are really two polar vortices, and they are easy to confuse. The stratospheric one is a belt of fast, cyclonic winds roughly 15 to 50 kilometres up, poleward of 50 degrees latitude. It forms each autumn as the polar night sets in and the temperature gap between pole and tropics widens, with the Coriolis effect spinning it up, and it collapses in spring. The tropospheric vortex sits lower, from the ground to about 10 to 15 kilometres, bounded by the jet stream, and it lasts all year. Both turn in the same direction as Earth and both peak in winter.
A strong Arctic vortex in the lower atmosphere is compact and nearly circular, holding cold air tightly in place behind the polar front. As it weakens it can split into smaller vortices, the strongest often near Baffin Island and others above northeast Siberia. At its feeblest, the circulation turns disorganised, the jet stream buckles, and cold air plunges south, producing sudden, sharp temperature drops. In the stratosphere, a midwinter breakdown called a sudden stratospheric warming can also shake up weather at the surface.
The phenomenon has a long history. The tropospheric vortex was described as early as 1853, and sudden stratospheric warmings were discovered in 1952 using weather balloons flying above 20 kilometres. The term became a household phrase during the North American winter of 2013 to 2014. In late January 2019 a deep freeze across the United States and Canada, with wind chills near minus 45 degrees Celsius in the Midwest, closed hundreds of schools and was linked to around 21 deaths.
The vortices matter beyond cold snaps. Ozone depletion is heaviest inside them, especially over the Southern Hemisphere, peaking in spring. Similar vortices also spin around other rotating planetary bodies with low axial tilt.
Source: Polar vortex
