"Dangerous, Record-breaking Cold Set for U.S..."
"Bitter Cold, Dangerous Wind Chills, and Heavy Snow..."
"Coldest Temperatures in Decades..."
Anytime you see news headlines like the ones above, you can bet the polar vortex is to blame.
While it sounds like something out of a science fiction disaster film, the term "polar vortex" is actually a technical one. It's the name given to the area of low pressure in the upper atmosphere located near Earth's poles and the strong winds that flow around it. The polar vortex exists year-round, but is strongest during winter when the temperature contrast is greatest between the poles and the mid-latitudes. (It weakens in summer.)
The Arctic and Antarctic Polar Vortices
Earth has two polar vortices--the Arctic polar vortex, which hovers near the North Pole, and the Antarctic polar vortex, which is near the South Pole.
For our interests, we'll be discussing the Arctic polar vortex because it's the one that affects our weather patterns here in the Northern Hemisphere.
Disruption of the Polar Vortex Leads to Arctic Outbreaks
Polar Vortex winds normally blow so fast (100 mph or more) around the low pressure field that they encircle the pole, trapping cold air at the Arctic regions. But every so often, these winds break down, and when this happens, the once-contained cold arctic air becomes free to spill southward into the U.S. (Sometimes it can travel as far south as the state of Florida!)
When a breakdown occurs, it's usually due to one of the following:
- Weakening. One of the main reasons for weakening of the polar vortex is warmth in the Arctic. If sea ice extent has been low, the oceans are able to absorb more of the sun's heat; in turn, the warm ocean pumps more heat into the atmosphere, which reduces temperature contrast at the pole, and thus, weakens vortex strength.
Polar vortex weakening was responsible for the unusually cold winter of 2009-2010 (commonly known as "Snowmageddon").
- Southward Forcing. If the part of the polar jet stream flowing over the western or eastern U.S. is oriented far north, the jet responds by dipping far south. Because the jet lives at the boundary of the polar vortex air, this air too will dig farther south. (If an upper level high is parked near Greenland in a configuration called a "Greenland block," it can have this same effect.)
This type of setup usually results in prolonged periods of cold in the eastern U.S., since both the jet stream and the high pressure can stall out at their locations for weeks at a time.
- Splitting. Sometimes, a piece of the polar vortex circulation will randomly detach. When this happens, the resulting cold snap is usually short-lived -- it only lasts as long as it takes the 'escapee' polar low to exit off of the Atlantic coast.