Every year, weather enthusiasts eagerly watch and wait for signs that the polar vortex, a mass of cold air spinning around the Arctic, might meander south, sending cold and snow into the lower latitudes.
Their wait might soon be over—and if you’re not a meteorologist, you may be surprised to learn that it’s because of a recent spike in Arctic temperatures.
Specifically, temperatures high in the stratosphere above Siberia. In the first week of January, they increased from about minus 92 degrees Fahrenheit to 8 degrees Fahrenheit. While these “sudden stratospheric warming” events happen to some extent every year, this one is categorized as a major event and is less common.
The mass of extremely warm air threw the freezing polar vortex out of balance, shoving it off its North Pole axis so forcefully that it in effect split in two, as if growing a pair of legs: one over North America and one over Europe.
The result of this disruption could mean frigid winter weather pummelling the U.S. Midwest and Northeast and the mid-latitude regions of Europe. It’s expected to arrive in the next week or two and could last, in fits and spurts, into February.
What is a polar vortex and how does it work?
When “polar vortex” appears in the news, it can refer to one of two different, but related, weather patterns.
In the lowest layer of the atmosphere, the troposphere—where weather occurs—there spins a polar vortex that circles the globe year round, a jet stream that’s also sometimes referred to as the circumpolar vortex. It’s large, often dipping into the mid-latitudes, the region above the tropics and below the Arctic—in North America, think central Mexico to northern Canada—and moving west to east.
In the atmospheric layer above it, about 10 to 30 miles up, is the stratospheric polar vortex, where every winter a sunlight-starved Arctic spins up a mass of cold air that, ultimately, dissipates in the spring. It’s much smaller than the vortex below it, and typically spins west to east above the North Pole.
While both systems can influence our weather, it’s a disruption to the vortex in the stratosphere that may now send winter weather our way. That polar vortex requires a stable temperature difference with the one below it to keep it in its lane above the North Pole. If the temperatures become too close—and a sharp warming will do that—it starts to wander off its path and head south, pushing the lower vortex ahead of it.
On January 4, scientists detected the sudden stratospheric warming over Siberia. Such stratospheric warming events are common, but major disruptions like that one typically only happen about every other year, says Judah Cohen, the director of seasonal forecasting at Atmospheric and Environmental Research, a company that advises government agencies and the private sector on weather risks.
Stratospheric warming events themselves still aren’t that well understood, says Michael Ventrice, a meteorologist for The Weather Company, a forecasting firm. After a cold snap sent demand for heat skyrocketing and caused a natural gas shortage in the U.K. in 2018, he began researching the origin of these Arctic disruptions to make better long-term predictions earlier in the season. He found that it’s typically over Siberia or the North Atlantic that “zones” of warm air funnel up into the stratosphere.
The warm air in the lower atmosphere, as well as cooler air, block the straight-flowing winds of the jet stream, like atmospheric bumper cars. That creates “kinks” in the winds, which get batted up another level into the stratospheric polar vortex and disturb that circular flow, too.
“These kinks send wave energy upward into the stratosphere, and when it's strong enough and lasts long enough, that wave energy can cause the usually nearly-circular stratospheric polar vortex to become disrupted,” says Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center.
The interaction between disruption to the stratosphere and weather in the troposphere is still not precisely understood. But when the vortex in the stratosphere is disrupted—split, displaced, or elongated—it can push the jet stream below it south, bringing Arctic air into cities in the U.S., Europe, and Asia. That’s why, in 2019, Chicago was briefly colder than the North Pole.
And that’s also why forecasts recently adjusted to account for the polar vortex disruption show the northeastern and midwestern U.S. could feel more extreme winter weather in the coming weeks.
Eye to the future
The role of climate change is just one of the complicated and mysterious factors that make it a challenge to predict what the spinning gyre nearly 30 miles above the Arctic means for our winters.
In the past 30 years, the Arctic has warmed about twice as fast as the rest of the world, a phenomenon known as arctic amplification. The warming has led to retreating glaciers and a loss of sea ice in the region, and it may also make the stratospheric polar vortex less stable, though that connection isn’t yet clear to scientists.
Francis points out that kinks in the jet stream that destabilized the polar vortex may also be made stronger by less Arctic sea ice.
“Losing all that ice allowed a great deal of extra heat from the sun to warm the Arctic waters, which is now being released back into the atmosphere, creating bulges of warm air in those key regions,” she says. “These bulges can make northward swings in the jet stream larger, stronger, and more persistent, which in turn can disrupt the polar vortex.”
2020 was tied with 2016 as one of the hottest years on record, capping off what was the hottest decade to date. There was also record low sea ice cover over the Arctic. Still, more research is needed to understand exactly how warming is changing the weather patterns that flow down from the Arctic every year.