From CNN (Alan Duke):
Americans in two dozen states from the Midwest to the Southeast and Northeast are shivering this week courtesy of a distorted polar vortex. The rush of cold air it’s sending southward is the biggest visitor from the North Pole since Santa Claus. The gifts it brings, however, are chilling and generally unwelcome. Much of the United States has plunged into a deep freeze from record low temperatures.
CNN International senior meteorologist Brandon Miller answers a few pressing questions about this phenomenon.
What is a polar vortex? What distinguishes it?
The polar vortex, as it sounds, is circulation of strong, upper-level winds that normally surround the northern pole in a counterclockwise direction — a polar low-pressure system. These winds tend to keep the bitter cold air locked in the Arctic regions of the Northern Hemisphere. It is not a single storm. On occasion, this vortex can become distorted and dip much farther south than you would normally find it, allowing cold air to spill southward.
How frequently does this polar vortex distortion occur?
The upper-level winds that make up the polar vortex change in intensity from time to time. When those winds decrease significantly, it can allow the vortex to become distorted, and the result is a jet stream that plunges deep into southern latitudes, bringing the cold, dense Arctic air spilling down with it. This oscillation is known as the Arctic Oscillation and it can switch from a positive phase to negative phase a few times per year. This oscillation — namely the negative phase where the polar winds are weaker — tends to lead to major cold air outbreaks in one or more regions of the planet.
Where on Earth can this happen?
The polar vortex can lead to major cold air outbreaks in any portion of the Northern Hemisphere — North America, Europe and Asia. This will lead to cold snaps in multiple locations, though not always…
Is it a side effect of global warming and should we expect more events like this?
This is a hotly researched topic. In short, yes, it could be. It seems counterintuitive that global warming could cause significant cold snaps like this one, but some research shows that it could. We know that different types of extreme weather can result from the overall warming of the planet, melting of the Arctic Sea ice, etc. This includes extreme distortions of the jet stream, which can cause heat waves in summer and cold snaps in winter.
From NBCNews.com (Erik Ortiz):
So what exactly is a polar vortex?
A polar vortex is basically a great swirling pool of extremely cold air located tens of thousands of feet in the atmosphere, said Frank Giannasca, senior meteorologist with The Weather Channel.
Basically an arctic cyclone, it ordinarily spins counterclockwise around the north and south poles.
While it tends to dip over northeastern Canada, it’s catching everyone’s attention because it has moved southward over such a large population — as many as 140 million Americans are feeling the freeze.
Why has it traveled so far south?
There’s a variety of reasons why a chunk of cold air over Canada would break off our way.
Chiefly, warmer air builds up over areas such as Greenland or Alaska, and that air forces the colder, denser air southward.
Also, weather patterns can create the right conditions for the polar vortex to point south.
But in this case, “this very well just may be one of those anomalies where it forces itself southward,” Giannasca said.