What is the polar vortex? And other cold weather questions

A neighborhood without power after the freeze in Austin, Texas on Feb. 16, 2021. (Tamir Kalifa/The New York Times)

The polar vortex is descending over the midsection of the United States, bringing extremely cold arctic air and causing temperatures to drop rapidly in many areas. The deep freeze will be accompanied by a major snowstorm which is expected to wreak havoc on travel.

The vortex is a large expanse of rotating cold air that generally circumnavigates 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 freezing air pushed deep into Texas, bringing temperatures up to 40 degrees Fahrenheit below normal.

This freeze killed at least 250 people 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 meanders south, two fundamental 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 even more to learn.

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

What is the polar vortex, exactly?

The vortex is a rotating air mass at high altitude which, as the term implies, occurs over the North Pole region. (There are actually two vortexes, one in the Arctic, the other in Antarctica, but only the northern one affects the weather in the Northern Hemisphere.)

The vortex strengthens and becomes much colder in winter, because with the northern hemisphere tilted away from the sun at this time of year, little or no sunlight reaches the Arctic to warm it.

If you looked at the North Pole, the air would appear to spin counter-clockwise. Some scientists have compared it to a spinning top. The vortex is encircled by the polar jet stream, a band of winds that blows from west to east around the planet.

What happens during freezing?

Under normal conditions, the vortex is stable and confined to the Arctic. But just as a spinning top can begin to wobble and drift if it hits something, the vortex can be disturbed. It is accompanied by changes in the jet stream, which develops a wavy snake-like pattern as it circles the globe.

Sometimes the vortex splits into several fragments that move south. Sometimes, as seems to be the case this week, it stretches, like a rubber band. In any case, the disturbance can have several major effects.

Temperatures in the atmosphere above the Arctic can increase, sometimes dramatically. At the same time, the freezing Arctic air is moving south.

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

How is the vortex disturbed?

For climatologists, this is the crux of the debate.

Some scientists say the warming Arctic is causing disturbances in the vortex, through changes in the polar jet stream. Others say the modeling suggests that naturally varying factors drive the disturbances instead, and that an increase in vortex disturbances that occurred before – including a noticeable increase in the 2000s – has not continued.

Judah Cohen, a climatologist with 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 warmer conditions create larger, more energetic atmospheric waves that cause the jet stream to ripple, with larger peaks and troughs. This affects the circulation of the polar vortex.

To use the spinning top analogy, “it’s like it’s starting to bump,” he says. “It loses its nice circular shape and in this case becomes more stretched out.” A lobe extends into Canada and the United States, causing an outbreak of cold weather.

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

Other scientists are not so certain. In a brief article 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 continue, short-term trends term extremes of cold, jet stream ripple and other climate-related measures in the 1990s and 2000s “have not continued over the past decade”, weakening the argument that rising temperatures in the Arctic was the cause.

Some experts suggest that instead of warming, other naturally variable elements of Earth’s climate could affect the vortex. Among these, said Ted Shepherd, a climatologist 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. which disturb the jet stream and the vortex.

Will this debate be resolved?

Scientists say questions about the role that 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 kinds of effects it is having, how severe they are, and whether they will get worse as warming continues.

Most scientists view this debate as an important debate that is still ongoing. Vavrus said some aspects “are on a pretty solid physical footing.” Among these, he said, is the idea that the warming of the Arctic, by reducing the temperature difference between the Arctic and the tropics, has weakened the winds of the jet stream. But other aspects, including if and where warming is wobbling the jet stream, “are the things we’ve really struggled with that remain uncertain,” he said.

“At first there was a lot of black and white thinking, including among people like me, on this issue,” Vavrus added. “As more and more evidence comes in, it’s clear there are many shades of gray.”

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