What is the polar vortex? And other questions about climate in cold weather

    A neighborhood that lost power after the freeze in Austin, Texas on February 16, 2021.  (Tamir Kalifa/The New York Times)

A neighborhood that lost power after the freeze in Austin, Texas on February 16, 2021. (Tamir Kalifa/The New York Times)

The polar vortex descends on the central United States, bringing bitterly cold Arctic air and rapidly dropping temperatures in many areas. The deep frost will be accompanied by a major snowstorm that is expected to cause travel chaos.

The gyre is a large rotating sheet of cold air that generally orbits the Arctic but occasionally shifts southward from the pole. Vortex-induced cold snaps occur regularly in the United States. One of the most damaging occurred in February 2021, when the frigid air penetrated deep into Texas, resulting in temperatures as much as 40 degrees Fahrenheit below normal.

This freeze resulted in at least 250 deaths and caused significant damage to the state’s power 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 winds its way south, two fundamental questions arise. What role, if any, does climate change play? And will extreme freezing increase as warming progresses?

The short answer: Scientists aren’t sure yet. There are clues, but there is more to learn.

“I wish I had a clear answer,” said Steve Vavrus, a climate scientist at the University of Wisconsin. Along 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 is affecting the polar vortex. “Unfortunately, the state of affairs is not yet clear,” he said.

What exactly is the polar vortex?

The vortex is a high-altitude mass of rotating air that, as the term suggests, occurs over the north polar region. (There are actually two vortices, one in the Arctic and one in Antarctica, but only the northern one affects northern hemisphere weather.)

The vortex intensifies and becomes much colder in winter because the Northern Hemisphere is tilted away from the Sun at this time of year and little or no sunlight reaches the Arctic to warm it.

Looking down at the North Pole, the air would appear to be rotating counter-clockwise. Some scientists have compared it to a spinning top. The gyre is orbited by the polar jet stream, a band of winds that blow from west to east around the planet.

What happens when you freeze?

Under normal conditions, the vortex is stable and confined to the Arctic. But just like a top can start wobbling and drifting when it hits something, the vortex can be disrupted. It is accompanied by changes in the jet stream, which develops a wavy, snake-like pattern as it orbits the earth.

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

The temperatures in the atmosphere above the Arctic can sometimes increase dramatically. At the same time, the frigid arctic air is moving south.

If the movement is fast enough, temperatures in the areas exposed to the cold air mass can drop tens of degrees in a matter of hours and remain extremely low for days or even weeks until the gyre in the north polar region becomes stable again.

How is the vortex disturbed?

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

Some scientists say that Arctic warming is causing perturbations in the vortex due to changes in the polar jet stream. Others say the modeling suggests that naturally variable factors are driving disturbances instead, and that increases in vertebral disturbances that have occurred before — including a noticeable spike in the 2000s — have not continued.

Judah Cohen, a climate scientist at Atmospheric and Environmental Research, a weather risk assessment company in Lexington, Massachusetts, is the author of this year’s article that linked the 2021 Texas freeze to Arctic warming. He now sees the same thing happening.

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

To use the spinning top analogy, “It’s like it started hitting things,” he said. “It loses its nice rounded shape and in this case becomes more elongate.” A lobe extends to Canada and the United States, bringing with it a burst of cold weather.

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

Other scientists are not so sure. In a short 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 continued, short-term trends in extreme cold, jet stream ripple and other climate-related measurements in the 1990s and 2000s “have not continued for the past decade,” belying the argument that rising temperatures in the Arctic were the culprit.

Some experts suggest that instead of warming, other naturally variable elements in Earth’s climate could be affecting the vortex. These include sea surface temperatures in the tropical Pacific, which can lead to changes in air masses in the Arctic that disrupt jet streams and eddies, according to Ted Shepherd, a climate scientist at the University of Reading in England.

Will this debate be resolved?

Scientists say asking what role Arctic warming might play in extreme cold snaps is an example of the kind of healthy debates about climate change that are happening now. The issue is not whether climate change is real – that question has been answered – but what its effects are, how severe they are and whether they will worsen as warming continues.

Most scholars regard this debate as an important one that is still ongoing. Vavrus said some aspects “stand on pretty solid physical feet.” Among them, he said, is the idea that Arctic warming, by reducing the temperature difference between the Arctic and the tropics, has weakened jet stream winds. But other aspects, including whether and where warming is making the jet stream more rippled, “are the things that we’ve really wrestled with that remain uncertain,” he said.

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

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