‘Polar vortex’ hitting Canada, U.S. with brutal, bone-chilling cold

Srirupa Chatterjee holds her hood as she crosses a street Sunday, Jan. 5, 2014, in St. Louis. Heavy snow continues to fall Sunday with forecasters calling for up to a foot (30 centimeters) in eastern Missouri and parts of central Illinois followed by bitter cold. (AP Photo/Jeff Roberson)

Brutal cold weather is gripping much of Canada and the U.S. today, as a 'polar vortex' treks down from the north, and while it may seem contradictory, global warming is playing a role in the spread of this bitter cold.

Even with Canada and the U.S. experiencing a more 'normal' winter this year, after two very mild winters thanks to the El Nino-La Nina pattern in the Pacific, this kind of extreme weather is still not what we usually see. We're certainly used to cold, snowy weather, but this shot of bone-chilling cold is coming at us from what's known as a 'polar vortex'.

This may sound like something out of the disaster movie, The Day After Tomorrow, or maybe a made-up name to make headlines sound more urgent and scary. However, the polar vortex is very real. In the northern hemisphere, these strong circulations of frigid air typically hang out over the north pole, trapping most of the extreme cold over the Arctic. The jet stream — that wavy 'ribbon' of extremely strong winds that wraps itself around the world — is the southern edge of where the polar vortex is spinning. The strength of the vortex depends on there being a big difference in temperatures between the Arctic and the equatorial regions.

[ Related: Winter weather blasts Central, Atlantic Canada ]

If the polar vortex remains strong, it keeps the jet stream (and thus all those frigid temperatures) further north and the waves of the 'ribbon' are very shallow. The waves can still dip down to bring Arctic chills to Canada and the northern U.S. from time to time, of course. That's just part of the natural variations in the jet stream pattern due to the weather. However, as long as the vortex stays strong, the jet stream waves (and thus the chills) typically don't reach very far south.

If the polar vortex weakens, though, the jet stream winds don't blow as fast. The stream 'loosens up' a bit more, the normally shallow waves in the stream get deeper (in some cases, a lot deeper), and the entire pattern of weather the stream usually drives along moves a lot slower. This spreads bitter Arctic chills far to the south, even into the southern United States, like we're seeing happen right now, and it can keep them there for a long time. At the same time, the eastern edge of these waves pull warmer temperatures up from the south, so that we get things like the pre-Christmas ice storm, and the shot of freezing rain that passed through Ontario and Quebec, and is currently making its way through Atlantic Canada.

While this shifting back and forth of the polar vortices does happen naturally from time to time, and scientists track this by looking at the Arctic Oscillation, what's going on now isn't all natural. Global warming is playing a role.

[ More Geekquinox: Severe winter weather sweeps through much of Canada ]

It may seem contradictory that a warming world could produce colder temperatures, but it's due to something called Arctic amplification. Now, it's winter in the Arctic, so temperatures are extremely cold there. That's not likely to change very much, even with global warming (remember, we'll likely be seeing a rise of around 4 degrees C by 2100, which can't really compete with -40 to -50 degrees C winter temperatures in the Arctic).

However, even though Arctic sea ice extent didn't reach the extreme low it did in 2012, this past year's extent was still one of the lowest on record (and no, it wasn't a 'recovery'), and large areas of the Arctic tundra have lost their snow and ice cover over the past seasons. Both sea ice, and snow and ice cover on land, are slowly building back through this winter, but the loss of it during the spring and summer means there was a lot of open water and bare land to absorb energy while the sun was shining in the Arctic (unlike in years past, when there was a lot more ice there year-round, to reflect more of that energy back into space).

Now, as the ground and water freeze again, that freezing is releasing the energy that's been trapped there. The released energy is going into the air, which is making the difference in temperatures between the poles and the equator smaller, and that's weakened the jet stream and the polar vortex.

As the winter continues, and thus sea ice builds in the Arctic and more snow and ice accumulate over land, this may strengthen the vortex again and pull the more extreme cold back to the north. However, each hit of these bitter chills further south also serves to weaken the temperature difference between the poles and the equator. Also, the peak of Arctic sea ice growth doesn't happen until late February or early March. So, hopefully things will improve soon, but hold on to your hats and scarves, as it may be awhile before we can pull out of this pattern.

(Photo courtesy: The Canadian Press/Wikipedia Commons)

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