How the Polar Vortex Can Bring Arctic Blasts to the U.S.

By Meghan Bartels edited by Andrea Thompson

Winter is supposed to be cold—and indeed, in the absence of global warming, the season would be colder than it typically has been in the U.S. in recent years. Nevertheless, when freezing temperatures and nipping winds team up for days on end, it can be difficult not to wonder why conditions must be quite so cold. But if you can keep your brain thawed enough to be curious, the answer offers intriguing glimpses into the weather systems that govern our planet.

“Thinking about how the central part of the country or even the Gulf Coast states get cold air isn’t just thinking about what’s happening locally,” says Andrea Lopez Lang, an atmospheric scientist at the University of Wisconsin–Madison. “You have to really zoom out and take a big-picture perspective.”

That big-picture perspective is centered on the Arctic and driven by two atmospheric phenomena. The first is the polar jet stream, a huge current of air that circles the globe between 50 and 60 degrees north latitude within the low-altitude troposphere, the same layer of atmosphere where most weather unfolds. North of the polar jet stream is colder air; south of it is warmer air.

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When the polar jet stream zips more or less evenly around the Arctic, the continental U.S. and other midlatitude locations remain relatively warm because it keeps cold air trapped to the north. But when the jet stream meanders, drifting north and south as it circles Earth, teeth-chattering air can sneak farther south, causing local temperatures to drop sharply in what’s sometimes known as an “Arctic blast.”

The polar jet stream exists year-round and can cause winter cold snaps on its own. But during the winter, it is joined by a second atmospheric phenomenon dubbed the polar vortex. This swirl of frigid air, located more than 10 miles above Earth’s surface, strengthens each year during the Northern Hemisphere’s coldest months. And even though polar vortex antics aren’t involved in all cold air outbursts, the two atmospheric phenomena can interact in ways that cause particularly frigid spells.

In normal conditions, the polar vortex spins tightly over the Arctic, allowing the polar jet stream—which blows by beneath and south of the polar vortex—to remain fairly smooth. “If the polar vortex is left alone, nothing’s bothering it, it’s a nice fast, quiet rotation—[and] the cold air is close to the center of that rotation,” says Judah Cohen, a climate scientist at the company Atmospheric and Environmental Research.

But sometimes the polar vortex gets messy—slowing or wandering or even breaking in two—and that destabilized air mass can cause the polar jet stream to develop particularly large meanders along its path, allowing brutally cold weather to sneak farther south than usual. This was a factor in February 2021, for example, when freezing temperatures in Texas lasted more than eight days, causing widespread power outages and killing more than 200 people.

Regardless of whether the polar vortex contributes to a specific cold air outbreak or that event is the work of the polar jet stream alone, these frigid spells are only sometimes joined by winter precipitation. “There’s a lot of ingredients that need to be in place to get a really big snowstorm,” Lopez Lang says. One of those factors is cold, of course, but another is water vapor that is available to freeze into snow.

Intriguingly, the Southern Hemisphere is also home to a polar vortex, but that system doesn’t undergo the disruptions that characterize the northern vortex. “These events never happen over the Antarctic,” says Aditi Sheshadri, an atmospheric scientist at Stanford University.

And the absence of these polar vortex disturbances in the Southern Hemisphere, which has its own polar vortex and polar jet stream, can point toward an explanation of what’s causing the phenomenon in the Northern Hemisphere. The South Pole is located in the continent of Antarctica, surrounded by the Southern Ocean. In contrast, in the north Arctic sea ice is surrounded by alternating land and ocean: North America, the Atlantic, Eurasia, the Pacific. Earth’s atmosphere behaves differently over land and ocean, creating large-scale atmospheric waves in the Northern Hemisphere that, when circumstances align, have the power to upset the polar vortex, triggering more extreme cold air outbreaks.

Just as it affects everything else on Earth, climate change is contributing to cold air outbreaks in some clear ways and in some ways that scientists are still working to understand. What’s certain, researchers say, is that winters are becoming more mild and cold extremes are becoming less common than they were in the middle of the 20th century.

Models aren’t clear on whether climate change is making polar vortex disruptions more common. Cohen notes that cold extremes have become more frequent since 2000, when the Arctic’s warming rate picked up steam—and he believes they are linked to the way melting ice is shaping the atmosphere. “In this period of Arctic change, extreme winter weather and severe winter weather have been surprisingly resilient,” he says.