What is the polar vortex? And more cold weather questions

    A neighborhood that had no electricity after the freeze in Austin, Texas, February 16, 2021. (Tamir Kalifa/The New York Times)

A neighborhood that had no electricity after the freeze in Austin, Texas, February 16, 2021. (Tamir Kalifa/The New York Times)

The polar vortex is descending on the central United States, bringing frigid arctic air and rapidly dropping temperatures in many areas. The deep freeze will be accompanied by a major snowstorm which is expected to cause travel havoc.

The vortex is a large, rotating expanse of cold air that generally surrounds the Arctic, but occasionally moves south from the pole. Vortex-related cold snaps occur regularly in the United States. One of the most damaging occurred in February 2021, when frigid air reached deep into Texas, causing temperatures up to 40 degrees Fahrenheit below normal.

That freeze left at least 250 dead and caused extensive damage to the state’s electrical infrastructure.

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As global emissions of heat-trapping carbon dioxide continue, the Arctic is warming nearly four times faster than other parts of the planet, according to the latest analysis, and the region’s sea ice cover is shrinking. So, as the vortex snakes its way south, two key questions arise. What role, if any, does climate change play? And will extreme frosts increase as warming continues?

The short answer: Scientists aren’t sure yet. There are clues, but there’s still a lot to learn.

“I wish I had a straight answer,” said Steve Vavrus, a climate scientist at the University of Wisconsin. With Jennifer Francis, now at the Woodwell Climate Research Center in Massachusetts, Vavrus wrote a seminal 2012 paper that presented the idea that arctic warming was affecting the polar vortex. “Unfortunately the state of affairs is still ambiguous,” he said.

What exactly is the polar vortex?

Vortex is a high altitude rotating mass of air which, as the term suggests, occurs over the North Pole region. (There are actually two eddies, one in the Arctic, the other in Antarctica, but only the northern one affects the climate in the Northern Hemisphere.)

The gyre strengthens and gets much colder in the winter, because with the Northern Hemisphere tilted away from the sun at that time of year, little or no sunlight reaches the Arctic to warm it.

If you were looking at the North Pole, the air would appear to rotate counterclockwise. Some scientists have compared it to a spinning top. The vortex is surrounded by the polar jet stream, a belt of winds that blow from west to east around the planet.

What happens during a deep freeze?

Under normal conditions, the vortex is stable and confined to the Arctic. But just as a top can start to wobble and drift if it bumps into something, the vortex can be broken. It is accompanied by changes to the jet stream, which develops a snake-like undulating pattern as it circles the globe.

Sometimes the vortex splits into several fragments that move towards the south. Sometimes, as seems to be the case this week, it stretches, like a rubber band. Either way, disruption can have several important effects.

Temperatures in the atmosphere over the Arctic can rise, sometimes dramatically. At the same time, the frigid Arctic air moves south.

If the movement is rapid enough, temperatures in areas exposed to the cold air mass can drop by tens of degrees within hours and can remain extremely low for days or even weeks until the vortex stabilizes again in the North Pole region. .

How is the vortex broken?

For climate scientists, this is the crux of the debate.

Some scientists say Arctic warming is causing disruptions in the vortex through changes in the polar jet stream. Others say the modeling suggests that naturally variable factors are instead driving the outages, and that an increase in vortex outages that have occurred previously, including a large increase in the 2000s, has not continued.

Judah Cohen, a climate scientist at Atmospheric and Environmental Research, a weather risk assessment firm in Lexington, Massachusetts, authored a paper this year that linked the 2021 Texas freeze to Arctic warming . He sees the same thing happening now.

The basic idea, he said, is that hotter conditions create bigger, more energetic atmospheric waves that make the jet stream more wavy, with bigger peaks and troughs. This affects the circulation of the polar vortex.

To use the top analogy, “it’s like it starts bumping into things,” she said. “It loses its beautiful circular shape and in this case becomes more elongated.” A lobe extends into Canada and the United States, bringing a cold epidemic.

Cohen said he has been studying the subject since 2005 and is more hopeful than ever about the link to changes in the Arctic. “The evidence is only growing,” he said.

Other scientists aren’t so sure. In a short paper in the journal Nature Climate Change in 2020, two researchers from the University of Exeter in England wrote that although Arctic warming and sea ice loss continued, short-term trends in cold extremes, Jet stream ripple and other climate-related measurements in the 1990s and 2000s “didn’t continue over the past decade,” weakening the argument that rising Arctic temperatures were the culprit.

Some experts suggest that instead of warming, other naturally variable elements of Earth’s climate could be influencing the vortex. Among them, said Ted Shepherd, a climate scientist at the University of Reading in England, are sea surface temperatures in the tropical Pacific Ocean, which can lead to changes in air masses in the Arctic that disrupt the current at jet and the vortex.

Will this debate resolve itself?

Scientists say questions about the role a warming Arctic may play in extreme cold spells are an example of the kind of healthy debates about climate change taking place now. It’s not about whether climate change is real — that question has been answered — but what kind of effects it has, how severe they are, and whether they will get worse as warming continues.

Most scientists consider this an important debate that is still ongoing. Vavrus stated that some aspects “have a pretty solid physical foundation”. Among them, he said, is the idea that Arctic warming, by narrowing the temperature difference between the Arctic and the tropics, has weakened the jet stream winds. But other aspects, including whether and where warming is causing the jet stream to ripple, “are the things we’ve really struggled with and remain uncertain,” he said.

“There were a lot of black and white thoughts, even among people like me, about this question at first,” added Vavrus. “As more and more evidence has come in, it’s clear there are many shades of gray.”

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