It’s that time again.
An influx of Arctic air is blasting across the U.S., sending temperatures plunging, dropping snow, disrupting Christmas travel plans, and setting social media atwitter about the polar vortex.
But what exactly is the polar vortex? Where does the cold air come from? And is global warming making cold snaps like this one more likely? Yale Climate Connections meteorologist Bob Henson has answers.
This interview has been lightly edited.
Yale Climate Connections: Could you start by defining the polar vortex?
Bob Henson: When you hear the phrase “polar vortex,” it’s usually referring to the North Pole and we’re talking about Northern Hemisphere winter weather.
So this is an area of low pressure up in the stratosphere, usually centered near the North Pole. It can slosh around, it can stretch, and it can break into two pieces.
There is also often a vortex near the North Pole lower down, in the first few miles of the atmosphere, which we call the “weather layer,” or the troposphere. That’s where systems hustle around the globe, come and go, cold fronts and warm fronts. Above that, in the stratosphere, is the much more defined vortex over the North Pole, and that one can have effects that percolate down to the surface.
So meteorologists are watching both of those layers, the stratosphere and the troposphere, but when you hear “polar vortex,” it’s usually referring to that well defined area of low pressure that’s typically over or near the North Pole, about 10 to 30 miles above the surface in the stratosphere.
Yale Climate Connections: How can the polar vortex influence temperatures all the way down here in the United States?
Henson: When it’s dark 24 hours a day, in and near the North Pole, you can build up a lot of cold air really easily. Now, at times, that cold air stays near the poles, and we can have relatively mild weather in the United States, for example. But every so often, the jet stream will dip southward and yank some of that cold air down toward the U.S. Typically, that’s when the stratospheric polar vortex is either stretched downward from the poles toward lower latitudes, or a piece of the vortex breaks off and moves bodily toward the United States. So those are the circumstances where the polar vortex can help bring colder air to the US.
Yale Climate Connections: Do we have a sense of why that happens and roughly how often it typically happens?
Henson: In the course of a typical winter, you might get an episode or two where the stratospheric polar vortex is dramatically distorted. Some winters go by without it happening much at all. In other winters it might happen several times. But usually, there’s an episode or two in a winter, and that often corresponds to what we think of a cold wave in the U.S. when we get a few days where a large part of the country gets very, very cold.
Yale Climate Connections: Can you give some examples of times when there has been cold weather in the continental U.S. due to this stretching?
Henson: Sure. In February 2021, Texas had its highest-impact winter storm on record. In fact, it was the most extensive winter storm in U.S. history — tens of billions of dollars in damage and more than 200 people killed either directly or indirectly by this week of frigid temperatures, well below freezing over large parts of Texas, affecting millions and millions of people.
The polar vortex stretched pretty dramatically, and that allowed cold air to be funneled from the Arctic down into the United States, well into Texas. And part of the problem was that the cold air was persistent, so you had as much as a week or more below freezing in some of the biggest cities in the state.
Yale Climate Connections: Some people have suggested that Arctic warming as a result of climate change is making some extreme cold snaps in the mid-latitudes more likely. To many people, that hypothesis might sound counterintuitive at first. So, before we get into the evidence for and against this, why did some scientists suspect that this might be the case?
Henson: In the early 2010s, a couple of scientists including Jennifer Francis, then at Rutgers, and Stephen Vavrus at the University of Wisconsin, collaborated on work that was pretty groundbreaking — looking at the about 30 years up to that point and finding that unusual behavior in the wintertime jet stream seemed to correspond to a warming Arctic. And at around the same time, Judah Cohen at a private company, Atmospheric and Environmental Research, Inc., was looking at how some of these processes seemed to be connected to early season snowfall in Russia and lack of sea ice north of Europe. So those factors tended to cause snow and high pressure to build up in Russia. That disrupted the jet stream, and in turn, that disruption percolated up to the stratospheric polar vortex and then percolated back down to cause the disrupted vortex to affect winter weather. So that’s one chain of events, and there’s certainly cases where that chain of events seemed to be a solid correlation and a forcing mechanism.
Now the big question is: Is this a regular feature of our changing climate and the way climate and weather operate now? Or was it something that happened a lot for about 30 years but maybe was just a point in time? And I think that’s the crux of the debate.
When you stretch out the period that was looked at beyond those 30 years, the connections are a little less solid. So that tends to argue for what we call natural variability. Sometimes climate and weather can behave in a certain way for several decades, based on what’s happening in the oceans and warm and cold patches in the oceans that can persist for a few years. And sometimes there are aspects of climate change that may crop up as the Earth warms and then perhaps fade or change as the Earth warms even more.
We know that winters in general are getting warmer in mid-latitude places like the United States. Overall, there’s less snowfall consistently around the country. But that doesn’t mean that we can’t have occasional extremes. And I think the Texas event in 2021 is a great example. That was a bitter, intense, cold wave. And it was exacerbated by how the energy grid is managed there. But it wasn’t unprecedented meteorologically. There have been cold waves on par with that every 30 or 40 years.
So it’s important to factor out: What are the kinds of natural variations that can happen? And what’s happening to occasional extreme events, versus just the overall lumbering changes in winter that we see happening, which is basically warmer and less snowy — but mixed in with that, occasionally the really bad snowstorm or the really bad cold wave?
So there has been quite a lively and vigorous debate in the research community, largely centered around the group of folks who pioneered this research in the early 2010s, of the idea that the jet stream might be torquing more in the winter, and what we call “weather weirding” might be producing more intense winter extremes.
At the same time, we have other researchers showing that, in fact, winters have generally been getting milder in the United States and less snowy. And long-term global computer models indicate we can expect them to continue doing so going forward.
Occasionally there are these extremes that do show up in what we’re observing. It’s just not clear whether those are really being caused by climate change or whether they’re just variations that happen at times.
Snow itself is a difficult thing to measure. And sometimes there have been changes in how we measure snow that are tossed into the mix. Many decades ago, official snow measurements were typically done just once a day. Today, snowfall is generally measured on what we call a snowboard every six hours. This is how the National Weather Service officially measures snow. Every six hours, you check the depth of the snow, then you clear the snowboard off, and then you add the six-hour totals to give you a 24-hour total. Now if you measure snow every hour, like an enthusiastic volunteer observer might do, you’re going to get a higher daily snow total because it doesn’t have time to compact and do the things that happen to snow as it sits there a while. This ends up giving you daily totals that are unrealistically high, and typically such reports need to be filtered out during official reviews of extreme events.
Another complication is that you might say, “Well, the atmosphere is getting more moist as it warms up, so you can produce more rain and more snow.” But snow is a threshold phenomenon. You can get more snow as the temperatures warm up into the twenties Fahrenheit and get close to 32. But that snowfall amount is not going to keep increasing. Eventually, you get above freezing, and the snowfall instead of increasing goes to near zero
And I think how all this intersects with the Arctic is still an outstanding research question.
Yale Climate Connections: So the jury is still out. But for readers who have folks in their lives who, when it gets very cold or when there is heavy snow, say “Climate change? Yeah, right,” how should they respond to those folks?
Henson: I would say that regardless of whether or how much climate change is involved, it’s still wintertime. Nobody ever said global warming would eliminate winter. We’re still in the mid-latitudes of the Northern Hemisphere. We’re far enough north to get really cold and snowy winter weather. So it may not be climate change so much as just the continuation of the climate variability we’ve always had, perhaps intensified in a few cases by this jet stream weirding being associated with the Arctic warming.
I think what’s much more important in terms of climate change is what happens to our summers and what happens to heavy rainfall. The bigger trend is toward heat in the summer. And we know that intense precipitation is getting more intense, and those kinds of events are definitely climate-change-driven.
A lot of people have referred to “preparing for weather” as the clothes you wear, and “preparing for climate” as what’s in your clothes closet. And we still have to keep those parkas in our coat closet for a while.
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