Some of the biggest cold snaps on record come back to the polar vortex. While the term for this cold dome of low pressure has been around since the late 1940s, it only became popular in the mid-2010s. Meteorologist Joe Martucci explains this phenomeon and what factors cause it to be dislodged from its arctic home.
This animation shows the stratospheric polar vortex in the middle stratosphere as it evolves from 1 Dec 2020 through 7 Feb 2021. The quantity shown is called "Potential Vorticity", which is plotted on the 850 K isentropic surface, corresponding to about 30 km in altitude. In a very simple sense, potential vorticity is a dynamical fluid quantity that ties together conservation of mass and conservation of angular momentum in a fluid.
For fluid flows satisfying some special conditions (which are good approximations for flow on isentropic surfaces in the stratosphere), potential vorticity is conserved following the flow, meaning it behaves a bit like an "ink tracer" in water. The polar vortex in this animation is represented by the brightest (highest potential vorticity) region.
On January 5th, 2021, a sudden stratospheric warming began as the polar vortex was displaced off the pole and stretched. The polar vortex disturbances continued through January, and by the beginning of February, you'll notice that the polar vortex is much smaller and weaker than it was at the start of the video.
The stratospheric polar vortex has vertical structure too! Want to see what it looks like around 20 km instead? See this video: https://youtu.be/YvwIOkuZJVs
Want to see a longer animation showing the early development of the polar vortex beginning in October 2020? See this video: https://youtu.be/VsSVEHVkSPA
