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1. The polar vortex is pretty much always there in winter. The strong winds circumnavigating the Arctic -- or Antarctic -- help keep the North/South pole cold and middle latitudes...less cold (as a person who is happiest in the tropics, I can't bring myself to say "warm").
2. The cold weather in the UK and extreme cold in parts of North America right now are due to the polar vortex being disrupted, so that the winds no longer blow from west to east but instead in a wavy pattern: the dip from northwest to southeast, then back up to the northeast, bringing the cold arctic air with them.
3. Breakdowns of the polar vortex are often a consequence of sudden and rapid warming in the stratosphere (which can happen for a number of reasons, including, say, shifts in the location of tropical rainfall due to El Nino -- the atmosphere is complicated). It takes a few weeks for a Sudden Stratospheric Warming (SSW) to lead to a collapse in the polar vortex.
4. This extreme cold event was remarkably well-forecasted because the polar vortex breakdown was preceeded by an SSW: see, e.g., this blog post from late December. The UK Met Office, in early January, wrote in their outlook that late January would be very cold, on the basis of the SSW that was occurring at the time. January proceeded to be anomalously warm, until the end of the month, when temperatures dropped just like they said it would (take that, Daily Mail!).
5. The link between disruptions of the polar vortex and global warming is tenuous, and an area of active research. Some climate scientists argue for a mechanism that would make it more likely, because the poles are warming faster than the rest of the planet. But the evidence for that mechanism has not been fully shown.
6. In the absense of that mechanism, it's more likely that extreme cold events will become less common with global warming*. It might well be that the cold spells that only come once -- or less than once -- per year in the UK used to happen a couple of times per year, and so now when they happen they seem remarkable and we (those of us who aren't global warming deniers) are inclined to attribue them to climate change. (I'm not applying this argument to what's happening in parts of North America, because my impression is that that's a more rare cold event.)
7. Because SSWs and their associated polar vortex disruptions last for several weeks, there can't be more than two or three of them per winter, so even if global warming does make them more likely, it will probably be hard to show that with any statistical rigor for a long time.
Source: I'm an atmospheric scientist, and a co-author on a peer-reviewed paper on this topic. (But it's not my usual area of research; I joined the project to bring my tropical meteorology expertise.)
*but extreme heat, drought, and flooding are all definitely becoming more common -- I'm not saying that global warming isn't a problem!
2. The cold weather in the UK and extreme cold in parts of North America right now are due to the polar vortex being disrupted, so that the winds no longer blow from west to east but instead in a wavy pattern: the dip from northwest to southeast, then back up to the northeast, bringing the cold arctic air with them.
3. Breakdowns of the polar vortex are often a consequence of sudden and rapid warming in the stratosphere (which can happen for a number of reasons, including, say, shifts in the location of tropical rainfall due to El Nino -- the atmosphere is complicated). It takes a few weeks for a Sudden Stratospheric Warming (SSW) to lead to a collapse in the polar vortex.
4. This extreme cold event was remarkably well-forecasted because the polar vortex breakdown was preceeded by an SSW: see, e.g., this blog post from late December. The UK Met Office, in early January, wrote in their outlook that late January would be very cold, on the basis of the SSW that was occurring at the time. January proceeded to be anomalously warm, until the end of the month, when temperatures dropped just like they said it would (take that, Daily Mail!).
5. The link between disruptions of the polar vortex and global warming is tenuous, and an area of active research. Some climate scientists argue for a mechanism that would make it more likely, because the poles are warming faster than the rest of the planet. But the evidence for that mechanism has not been fully shown.
6. In the absense of that mechanism, it's more likely that extreme cold events will become less common with global warming*. It might well be that the cold spells that only come once -- or less than once -- per year in the UK used to happen a couple of times per year, and so now when they happen they seem remarkable and we (those of us who aren't global warming deniers) are inclined to attribue them to climate change. (I'm not applying this argument to what's happening in parts of North America, because my impression is that that's a more rare cold event.)
7. Because SSWs and their associated polar vortex disruptions last for several weeks, there can't be more than two or three of them per winter, so even if global warming does make them more likely, it will probably be hard to show that with any statistical rigor for a long time.
Source: I'm an atmospheric scientist, and a co-author on a peer-reviewed paper on this topic. (But it's not my usual area of research; I joined the project to bring my tropical meteorology expertise.)
*but extreme heat, drought, and flooding are all definitely becoming more common -- I'm not saying that global warming isn't a problem!
