What the heck is a polar vortex? And what does global warming have to do with this? » Yale Climate Connections

It’s that time again.

An influx of arctic air is blowing across the US, sending temperatures plummeting, snow falling, disrupting holiday travel plans and social media chatter about the polar vortex. .

But what exactly is the polar vortex? Where does the cold air come from? And is global warming more likely to cause cold waves like this? 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”, usually refers to the North Pole and we are talking about the winter weather of the Northern Hemisphere.

So this is an area of ​​low pressure in the stratosphere, usually centered near the North Pole. It can splash, stretch and break in two pieces.

There is also usually a vortex near the North Pole lower down, in the first few miles of the atmosphere, which we call the “weather layer” or troposphere. That’s where systems are moving fast around the world, coming and going, 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 trickle down to the surface.

So meteorologists are looking at both layers, the stratosphere and the troposphere, but when you hear “polar vortex,” it usually refers to that well-defined area of ​​low pressure that’s usually on or near the North Pole, between 10 and 30 miles above. the surface in the stratosphere.

Yale Climate Connections: How can the polar vortex influence temperatures here in the United States?

Henson: When it is dark 24 hours a day, at and near the North Pole, a large amount of cold air can accumulate very easily. Now sometimes that cold air stays close to the poles, and we can have a relatively mild climate in the United States, for example. But every once in a while, the jet stream will dip southward and pull some of that cold air toward the US. Usually, that’s when the stratospheric polar vortex is stretching downward from the poles toward lower latitudes, or a part of the vortex breaks and moves bodily toward the United States. Those are the circumstances in which the polar vortex can help bring cooler air into the US.

Yale Climate Connections: Do we have any idea why that happens and roughly how often it usually happens?

Henson: Over the course of a typical winter, you may have one or two episodes where the stratospheric polar vortex becomes dramatically distorted. Some winters go by without much happening. In other winters it can happen several times. But usually there’s an episode or two in a winter, and that often corresponds to what we think of as a cold snap in the US when we have a few days where a large part of the country gets very, very cold. cold.

Yale Climate Connections: Can you give some examples of times when there has been cold weather in the continental US due to this stretch?

Henson: Sure. In February 2021, Texas had its largest winter storm on record. In fact, it was the largest winter storm in US history: tens of billions of dollars in damage and more than 200 people killed directly or indirectly from this week of freezing temperatures, well below freezing. across much of Texas, affecting millions and millions of people.

The polar vortex stretched quite dramatically, and that allowed cold air to be funneled from the Arctic into the United States, all the way to Texas. And part of the problem was that the cold air was persistent, so it was up to a week or more below freezing in some of the largest cities in the state.

Yale Climate Connections: Some people have suggested that warming of the Arctic as a result of climate change is making some extreme cold snaps in the mid-latitudes more likely. For many people, that hypothesis may sound counterintuitive at first. So before I get into the evidence for and against this, why did some scientists suspect 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: They looked at the 30 years up to that point and found that behavior unusual in winter’s jet stream seemed to correspond to a warming Arctic. And 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 related to early snowfall in Russia and a lack of sea ice in northern Europe. . So those factors tended to cause snow accumulation and high pressure in Russia. That disrupted the jet stream, and in turn, that disruption leaked up to the stratospheric polar vortex and then leaked down to cause the disrupted vortex to affect winter weather. That’s a chain of events, and there are certainly cases where that chain of events seemed to be a strong correlation and a forcing mechanism.

Now the big question is: Is this a regular feature of our changing climate and the way weather and climate now operate? Or was it something that happened a lot over about 30 years, but maybe it was just a point in time? And I think that’s the crux of the debate.

When you extend the period that was analyzed beyond those 30 years, the connections are a little less strong. That tends to argue in favor of what we call natural variability. Sometimes the weather and climate can behave a certain way for several decades, depending on what happens in the oceans and the hot and cold areas in the oceans that can persist for a few years. And sometimes there are aspects of climate change that can emerge as the Earth warms and then perhaps disappear or change as the Earth warms even more.

We know that winters are generally getting warmer in mid-latitude places like the United States. In general, there is consistently less snowfall across the country. But that doesn’t mean 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 was exacerbated by how the energy network is managed over there. But it was not unprecedented meteorologically. There have been cold snaps on par with every 30 or 40 years.

So it’s important to consider: What are the types of natural variations that can occur? And what’s going on with the occasional extreme events, as opposed to the general heavy swings in winter that we see happening, which are basically warmer and less snowy, but mixed in with that, occasionally really bad snowstorms or waves? cold really bad?

So there’s been a very lively and vigorous debate in the research community, largely centered around the group of people who pioneered this research in the early 2010s, of the idea that the jet stream might be twisting more in the winter, and what we call “Weird weather” could be producing a more intense winter extremes

At the same time, we have other researchers showing that, in fact, winters have generally become milder in the United States and less snowy. And long-term global computer models indicate that we can expect them to continue to do so in the future.

From time to time there are these extremes that appear in what we are observing. It’s just not clear if they’re really caused by climate change or if they’re just variations that happen sometimes.

A demonstration of how to properly reposition a snowboard after the snow has been measured and removed. (Photo credit: CoCoRaHS and NOAA.)

The snow itself is somewhat difficult to measure. And sometimes there have been changes to how we measure snow that add to the mix. Many decades ago, official snow measurements were typically made only once a day. Today, snowfall is generally measured in what we call a snowboard every six hours. This is how the National Weather Service officially measures snow. Every six hours, check the snow depth, then remove the snowboard, and then add the six-hour totals for a 24-hour total. Now, if you measure the snow every hour, like a keen volunteer observer would, you’ll get a higher daily snow total because it doesn’t have time to compact and do the things that happen to snow while it’s sitting there for a while. . This ends up giving you daily totals that are unrealistically high, and such reports usually have to be leaked during official extreme event reviews.

Another complication is that you could say, “Well, the atmosphere gets wetter as it warms up, so it can produce more rain and more snow.” But snow is a threshold phenomenon. You can get more snow as temperatures climb into the low twenty degrees Fahrenheit and approach 32. But that amount of snow isn’t going to keep increasing. Eventually, you get to be above freezing, and the snowfall, instead of increasing, reaches almost zero.

And I think how all of 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 people in their lives who, when it’s really cold or when there’s a lot of snow, say “Climate change? Yeah, sure,” how should they respond to those people?

Henson: I would say that regardless of whether or how much climate change is involved, it is still winter. No one ever said that global warming would eliminate winter. We are still in the mid-latitudes of the northern hemisphere. We are far enough north to get really cold and snowy winter weather. So it may not be climate change so much as the continuation of climate variability that we’ve always had, perhaps intensified in some cases by this weird jet stream associated with warming of the Arctic.

I think what’s much more important in terms of climate change is what happens to our summers and what happens to heavy rains. The the biggest trend is towards heat in summer. and we know that heavy rainfall is becoming more intenseand those types of events are definitely driven by climate change.

Many people have referred to “weather-preparing” as the clothes you wear, and “weather-preparing” as what’s in your clothing closet. And we still have to keep those parkas in our coat closet for a while.

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