Polar Vortex Basics
Typical Polar Vortex Position
One of several semi-permanent weather systems over the Earth, the polar vortex is an area of low pressure in the upper atmosphere, primarily in the stratosphere, the layer of the atmosphere above which most of our sensible weather occurs (known as the troposphere).
To emphasize, this vortex is semi-permanent. It is often in place near the poles. It is nothing new.
The Northern Hemisphere polar vortex frequently, but not always, has centers in two main areas: near Canada’s Baffin Island, and over northeast Siberia. There is a Southern Hemispheric version of the polar vortex, as well, within which depletion of the upper-atmospheric ozone layer occurs.
The vortex is strongest in winter, thanks to an increased temperature contrast between the polar regions and the mid-latitudes, including the United States.
Occasionally, pieces of the larger spin can break off and sweep toward southern Canada, helping to drive Arctic cold plunges into the U.S.
Early January 2014
What Happened: Polar Vortex Remained Anchored
In the case of this outbreak, the main circulation of the polar vortex in the stratosphere and upper reaches of the troposphere remained in place over northern Greenland and near Baffin Island. If you think of the vortex as a spinning wheel, one of its “spokes” did extend southward into the U.S. as January began.
Meanwhile, lower in the atmosphere at jet stream level, a pair of upper-level disturbances, one from the northeast Pacific Ocean and another rotating southward out of Canada’s Northwest Territories, merged to help dig a sharp southward plunge in the jet stream, unleashing the Arctic blast into the nation’s Midwest, South and East.
When this pattern sets up, the central, southern and eastern U.S. can plunge into a deep freeze, with subzero cold over large swaths of the Midwest, interior Northeast and parts of New England. Freezing temperatures can plunge deep into Florida.
Depiction of the Greenland block pattern sometimes responsible for locking cold into the East and Midwest.
Sometimes, ironically, a cold outbreak in the eastern U.S. results when the polar vortex in northeast Canada weakens.
Instead of a whirling dervish of low pressure aloft, high pressure aloft near Greenland, referred to as a Greenland block, effectively keeps cold, Canadian air from sweeping eastward into the north Atlantic Ocean. Instead, it is forced south into the U.S., often plunging through Florida and sometimes parts of the northern Caribbean.
In this pattern, referred to by meteorologists as the negative phase of the North Atlantic oscillation (-NAO), persistent cold can lock in for a week, or more.
As is the case with this cold snap, there is no Greenland block, so the Arctic outbreak, while dangerously cold, will not persist long as the polar vortex returns home to near Baffin Island, Canada after its southern vacation later in the week.[Full story]