Disclaimer: I am in no way an expert on the polar vortex. This blog post represents what I have discovered as I’ve researched the subject, trying to understand how the extreme cold conditions we are experiencing now occurred.
Actually, there are two polar vortices in the Northern Hemisphere, stacked on top of each other. The lower one is usually and more accurately called the jet stream. It’s a meandering river of strong westerly winds around the Northern Hemisphere, about 7 miles above Earth’s surface, near the height where jets fly.https://www.marketwatch.com/story/how-this-brutal-polar-vortex-fits-with-global-warming-2019-01-29
The jet stream exists all year, and is responsible for creating and steering the high- and low-pressure systems that bring us our day-to-day weather: storms and blue skies, warm and cold spells. Way above the jet stream, around 30 miles above the Earth, is the stratospheric polar vortex. This river of wind also rings the North Pole, but only forms during winter, and is usually fairly circular.
The polar vortex always exists around the North Pole in winter. So what causes the vortex to split, moving frigid arctic air south, away from the pole? It is thought that a Sudden Stratospheric Warming (SSW), i.e. a sudden rise in the arctic temperatures will decrease the difference in air temperatures between the arctic regions and the mid latitudes (Canada, United States). The arctic is warming twice as fast as the rest of the earth, likely because of decreasing areas of sea ice. So although it is counterintuitive, global warming is implicated in the migration of the polar vortex away from the North Pole.
When an SSW occurs, it often influences the Polar Vortex immediately after, and can move the Polar Vortex off the North Pole.https://www.mlive.com/weather/2018/12/sudden-stratospheric-warming-to-likely-cause-polar-vortex-disruption-what-it-means-for-us.html
The stratosphere is the layer of the atmosphere just above our main weather atmosphere, the troposphere. Here’s a look at the atmosphere divided into layers.
A Sudden Stratospheric Warming is when the stratosphere warms significantly in just a few days. Dr. Judah Cohen from Atmospheric and Environmental Research (AER), says the stratosphere above the North Pole is forecast to warm around 140 degrees in the next few days. Yes you read that correctly – a 140 degree warm-up.https://www.mlive.com/weather/2018/12/sudden-stratospheric-warming-to-likely-cause-polar-vortex-disruption-what-it-means-for-us.html
The stratosphere has a circulation generally centered over the North Pole, known as the Polar Vortex. There is also a Polar Vortex in the troposphere, and it usually sits under the stratospheric Polar Vortex. Think of this vortex like the swirly that develops when you drain the bathtub.
Researcher Cohen and others have found that around the time of a SSW event the Polar Vortex is moved off the North Pole, which is called a Polar Vortex displacement. The Polar Vortex may stay in one piece, or split up into two or three pieces.
Just above is the forecast showing the SSW between December 24 and December 28. You can envision how warm air developing over the North Pole would force a cold pocket of air off the polar region and to lower latitudes, like southeastern Canada and the eastern U.S.https://www.mlive.com/weather/2018/12/sudden-stratospheric-warming-to-likely-cause-polar-vortex-disruption-what-it-means-for-us.html