Polar Vortex

The polar vortex is a persistent large-scale cyclonic circulation over each pole, strongest in winter, encircled by westerly winds. The stratospheric polar vortex, centered near 50 hPa in the polar night, is the dominant feature; a separate, weaker tropospheric vortex extends down to the surface. The cold core is confined by the circumpolar westerly jet, the polar-night jet. When the stratospheric vortex weakens or splits during a sudden stratospheric warming, the temperature gradient reverses, the westerlies break down, and cold Arctic air can spill into the mid-latitudes weeks later, driving severe winter cold outbreaks across North America, Europe, and Asia.