The polar vortex is a persistent cyclonic circulation centred over each pole, present in both the troposphere and stratosphere. The Arctic stratospheric vortex is especially relevant: when it weakens or collapses (sudden stratospheric warming, SSW), frigid air can plunge toward mid-latitudes.
Impact on Europe
Polar vortex disruptions are linked to Europe's most severe cold waves. Filomena (January 2021) was partially facilitated by vortex weakening. The jet stream becomes wavier when the vortex is weak, creating blocking patterns that trap cold air over regions for weeks.