January 15, 2025
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How the polar vortex brings an arctic blast to the US
Widespread weather patterns can transport frigid air far from the poles. Here’s how it works:
A woman walks on a pedestrian bridge with her face covered to protect herself from the cold on January 31, 2019 in New York City.
Johannes Eisel/AFP via Getty Images
Winter is supposed to be cold, and in fact, absent global warming, this season would be colder than recent years in the United States. Nevertheless, after days of sub-zero temperatures and cold winds, you can’t help but wonder why things have to be this way. very It’s very cold. But if you can unfreeze your brain enough to be curious, the answers can provide a fascinating glimpse into the weather systems that govern our planet.
“Thinking about how cold air occurs in the central part of the country and even in the Gulf states is simply a way of thinking about what’s happening locally,” said Andrea López Lang, an atmospheric scientist at the University of Wisconsin-Madison. It’s not just about thinking.” “You really have to zoom out and look at the big picture.”
Its big picture focus is on the North Pole and is driven by two atmospheric phenomena. The first is the polar jet stream, a huge air current that circles the Earth between 50 degrees and 60 degrees north latitude in the low-altitude troposphere, the same atmospheric layer where most weather occurs. There is cold air north of the polar jet stream. To the south there is warm air.
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When the polar jet stream flows roughly evenly around the North Pole, the continental United States and other mid-latitude regions remain relatively warm because cold air is trapped to the north. But as the jet stream snakes around the Earth, drifting north and south, the crunching air can sneak further south, causing local temperatures to plummet in a phenomenon known as an “Arctic blast.”
The polar jet stream exists year-round and can itself cause winter cold waves. However, during the winter, a second atmospheric phenomenon is added called the polar vortex. This frigid vortex of air sits more than 10 miles above the Earth’s surface and intensifies each year during the northern hemisphere’s coldest months. And while polar vortex anomalies aren’t responsible for all cold air outbursts, the two atmospheric phenomena can interact in ways that cause particularly frigid periods.
Under normal conditions, the polar vortex rotates tightly over the North Pole, and the polar jet stream that blows beneath and south of the polar vortex remains fairly smooth. “If the polar vortex is left alone, there’s nothing wrong with it. It’s a nice, fast, quiet rotation (and) the cold air is close to the center of the rotation,” says Atmospheric and Environmental. says Judah Cohen, climate scientist at Mental. the study.
But the polar vortex can sometimes become disorganized by slowing, wandering, or even splitting in two, and its unstable air masses can cause the polar jet stream to meander especially hard along its path, causing extremely cold temperatures. weather could creep further south than normal. For example, this was a factor in February 2021, when more than eight days of subzero temperatures in Texas caused widespread power outages and killed more than 200 people.
Regardless of whether the polar vortex contributes to a particular episode of cold air, or whether the phenomenon is solely a function of the polar jet stream, winter precipitation only rarely adds to these frigid periods. “There are a lot of factors that need to be in place for a really big snowstorm to occur,” López-Lang said. Of course, one of those factors is the cold, but another is water vapor that freezes into snow.
Interestingly, a polar vortex also exists in the Southern Hemisphere, but its star systems do not experience the disruption characteristic of a polar vortex. “These events would never happen in Antarctica,” says Aditi Sheshadri, an atmospheric scientist at Stanford University.
And the absence of such polar vortex disturbances in the Southern Hemisphere, which has its own polar vortex and polar jet stream, could help explain what causes this phenomenon in the Northern Hemisphere. The South Pole is located in Antarctica, surrounded by the Southern Ocean. In contrast, Arctic sea ice is surrounded by alternating land and ocean regions: North America, the Atlantic Ocean, Eurasia, and the Pacific Ocean. Earth’s atmosphere behaves differently on land and in the oceans, causing large atmospheric waves in the Northern Hemisphere that, when conditions align, can have the power to disrupt the polar vortex, causing more extreme cold air events.
Just as it affects everything else on Earth, climate change contributes to cold weather in several distinct ways, and some that scientists are still struggling to understand. Masu. What is certain, researchers say, is that winters are milder and less extreme than they were in the mid-20th century.
Models are not clear on whether climate change is making polar vortex disruptions more common. Cohen points out that extreme cold events have become more frequent since 2000, when the rate of warming in the Arctic accelerated, and believes that the extreme cold events are related to the formation of the atmosphere from melting ice. . “Extreme and severe winter weather is surprisingly resilient during this period of Arctic change,” he says.
