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.
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.
*this information came from a Canadian news source. As I know there is extreme heat in Australia right now, I would be very interested in getting responses from other regions about their weather.