The polar vortex descends on the northeastern United States, bringing bitterly cold air to the Arctic and causing temperatures to drop rapidly.
A vortex is a large rotating expanse of cold air that generally circles the Arctic but occasionally shifts south from the pole. Colds associated with the vortex occur regularly around the world. One of the most devastating occurred in February 2021, when freezing air reached deep into Texas, resulting in temperatures that were up to 40 degrees Fahrenheit below normal.
In January, the polar vortex brought arctic air to Central Asia that moved slowly eastward, causing an extreme cold in East Asia that lasted for much of the month. Forecasters expect about a dozen daily record cold temperatures in the Northeast to be broken over the next few days, mostly Saturday morning.
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 to the 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 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, Dr. Vavrus wrote an important paper in 2012 that presented the idea that Arctic warming was affecting the polar vortex. “Unfortunately the state of affairs is still unclear,” he said.
What exactly is the polar vortex?
A vortex is a mass of rotating air at high altitude that, as the term suggests, occurs over the North Pole region. (There are actually two eddies, one in the Arctic, one in the Antarctic, but only the northern one affects the weather in the Northern Hemisphere.)
The vortex strengthens and becomes 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 down at the North Pole, the air would look like it was spinning counterclockwise. Some scientists have compared it to a whirlwind. The vortex is surrounded by the polar jet, 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 spinning top can start to wobble and drift if it hits something, the vortex can be disrupted. It is accompanied by changes in the jet stream, which develops a wavy, serpentine pattern as it circles the globe.
Sometimes the vortex splits into several fragments that move southward. Sometimes, as seems to be the case this week, it becomes stretched, like a rubber band. Either way, the disorder can have many significant effects.
Temperatures in the atmosphere above the Arctic can rise, sometimes dramatically. At the same time, cold Arctic air is moving south.
If the movement is fast enough, temperatures in areas exposed to the cold air mass can drop by tens of degrees within hours and can remain extremely cold for days or even weeks until the vortex becomes stable again in the North Pole region .
How is the vortex disrupted?
For climate scientists, this is the crux of the debate.
Some scientists say that Arctic warming is causing disruptions in the vortex, through changes in the polar jet stream. Others say the modeling suggests that physically variable factors are causing disturbances, and that the increase in eddy disturbances that occurred previously — including a notable spike in the 2000s — has not continued.
Judah Cohen, a climate scientist at Atmospheric and Environmental Research, a risk assessment firm in Lexington, Mass., is the author of 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 and more energetic atmospheric waves that make the jet stream more wavy, with larger crests and troughs. This affects the circulation of the polar vortex.
To use the spinning top analogy, “it's like it started hitting things,” he said. “It loses its nice circular shape and in that case it becomes more stretched.” One lobe extends across Canada and the United States, bringing a burst of cold weather.
Dr 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 aren't so sure. In a short paper in the journal Nature Climate Change in 2020, two researchers at the University of Exeter in England wrote that although Arctic warming and sea ice loss continued, short-term trends in cold extremes, jet-stream undulations and other Climate-related measurements in the 1990s and 2000s “did not continue over the last decade,” weakening the argument that Arctic warming was the culprit.
Some experts suggest that instead of warming, other naturally variable elements of the earth's climate may 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 jet stream and the gyre. .
Will this debate be resolved?
Scientists say questions about the role a warming Arctic might play in extreme crises are an example of the kind of healthy discussions about climate change happening 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 debate to be an important one that is still ongoing. Dr Vavrus said some aspects “are on a fairly solid physical basis”. Among them, he said, is the idea that Arctic warming, by reducing the temperature difference between the Arctic and the tropics, has weakened jet winds. But other aspects, including whether and where warming makes the jet stream more wavy, “are the things we really struggle with and remain uncertain,” he said.
“In the early days there was a lot of black-and-white thinking, including among people like myself, about this question,” Dr Vavrus added. “As more and more evidence comes in, it's clear that there are many shades of gray.”
