What is the polar vortex? And Other Cold Weather Weather Questions

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

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

The polar vortex is descending in the midsection of the United States, bringing very cold arctic air and causing temperatures to drop rapidly in many areas. The deep freeze will be accompanied by a major snowstorm that is expected to cause travel chaos.

The vortex is a large, rotating expanse of cold air that usually encircles the Arctic, but occasionally drifts 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, resulting in temperatures up to 40 degrees Fahrenheit below normal.

That freeze caused at least 250 deaths and caused extensive 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 when the vortex meanders south, two basic 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 is still more to learn.

“I wish I had a clear 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 put forward the idea that warming in the Arctic was affecting the polar vortex. “Unfortunately, the state of affairs remains ambiguous,” he said.

What exactly is the polar vortex?

A vortex is a high-altitude rotating air mass that, as the term suggests, occurs over the North Pole region. (There are actually two vortices, one in the Arctic and the other in the Antarctic, but only the northern one affects weather in the Northern Hemisphere.)

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

If you were looking toward the North Pole, the air would appear to rotate counterclockwise. Some scientists have compared it to a top. The vortex is surrounded by the polar jet stream, a band of winds that blows 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 hits something, the vortex can be interrupted. It is accompanied by changes in the jet stream, which develops a wavy, snake-like pattern as it encircles the globe.

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

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

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

How is the vortex interrupted?

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

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

Judah Cohen, a climate scientist at Atmospheric and Environmental Research, a climate risk assessment firm in Lexington, Massachusetts, is the author of a paper this year linking 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 make the jet stream more wavy, with larger peaks and troughs. That affects the circulation of the polar vortex.

To use the analogy of the top, “it’s like it starts hitting things,” he said. “It loses its nice circular shape and in this case it becomes more stretched out.” One lobe extends to Canada and the United States, causing a bout of cold weather.

Cohen said he has been studying the issue 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 aren’t so sure. In a short article published in the journal Nature Climate Change in 2020, two researchers from the University of Exeter in England wrote that while Arctic warming and sea ice loss continued, short-term trends in cold extremes, the Ripple of jet streams and other climate-related measurements in the 1990s and 2000s “have not continued over the past decade,” weakening the argument that rising temperatures in the Arctic were to blame.

Some experts suggest that, rather than warming, other naturally variable elements of Earth’s climate may be affecting the vortex. Among these, 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 jet stream. and the vortex.

Will this debate be resolved?

Scientists say that questions about what role warming of the Arctic may play in waves of extreme cold is an example of the kind of healthy debates about climate change that are happening now. The question is not 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 view this debate as an important one that is still ongoing. Vavrus said that some aspects “have a pretty solid physical foundation.” Among these, he said, is the idea that warming of the Arctic, by reducing the temperature difference between the Arctic and the tropics, has weakened jet winds. But other issues, including whether and where the warming is making the jet more wavy, “are the things we’ve really been struggling with and remain uncertain,” he said.

“In the early days, there was a lot of black and white thinking, even among people like me, on this question,” Vavrus added. “As more and more evidence has come in, it’s clear that there are many shades of grey.”

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