Unusual Winter Cold

How Arctic air plunges southward

After all, the surging greenhouse effect as humans burn more fossil fuels is triggering three times as many observations of record high temperatures than record lows. Each of the past two years were significantly hotter, on average, than any other period in human history.

The poles will always be the coldest spots on the planet. That’s because for half the year, Earth’s tilted axis means they receive little to no sunlight.

Even through the summer, they have historically remained ice-covered — though that has dramatically diminished in recent decades. Arctic ice has seen what scientists found to be “irreversible” thinning since 2007; Antarctic sea ice hit a record low, by far, in 2023.

During many memorable episodes over the past decade or so, the cold air has shifted far from where it normally swirls.

Cold outbreaks — including an episode in January 2014 that brought the coldest winter in the Midwest in decades — introduced much of the public to the polar vortex. The phrase refers to a naturally occurring column of rapidly circulating air over the poles in the stratosphere, from 10 to 50 miles above Earth’s surface. That air swirls well above the troposphere, which is the layer closest to the ground, where weather happens.

The vortex can sometimes shift away from the pole, stretch or even split in a way that sends bitter cold plunging into midlatitudes.

Those disruptions to the circulation are happening more often as global warming has weakened the polar vortex. What’s less clear is whether those events are also leading to bouts of cold on the ground.

A strong polar vortex — and one that stays swirling tightly above the pole — depends on a dramatic contrast between frigid polar temperatures and milder conditions across midlatitudes. As rapid Arctic warming decreases that contrast, the vortex often isn’t spinning as tightly and is more prone to send its cold air spilling away from the pole.

“You need really cold air in the pole to have a very fast-spinning polar vortex,” said Mostafa Hamouda, a postdoctoral researcher at the Massachusetts Institute of Technology. Any warming “slows the whole circulation down.”

That warming and slowing can allow the polar vortex to become increasingly stretched out — oblong, instead of tightly circular — and can spread its chill into latitudes with typically balmy climates.

Sometimes, lobes of the polar vortex can separate and get stuck well to the south. Multiple studies have strengthened scientists’ understanding of the phenomena as a consequence of Arctic warming and climate change.

Even if these disruptions in the vortex are not causing cold outbreaks to become more frequent or intense in the United States or other parts of the world, they may be translating into outbreaks of cold weather that are more widespread and last longer, Francis said. Her research suggests Arctic cold is also extending farther south into areas that are not well prepared for such conditions.

Newer studies also back up the idea that warming of the Arctic and the oceans may be encouraging weather patterns that allow polar air to spill southward for longer, more stubborn stretches of cold.

Those patterns — called blocking patterns — have always developed from time to time. They occur when areas of high pressure get in the way of the jet stream, a river in the upper atmosphere that steers the weather. And, as their name suggests, they can allow certain weather patterns to remain stubbornly in place — including undulating jet-stream patterns that allow Arctic cold to flow south into the United States, for example.

One recent study by Hamouda suggested warmth in the North Pacific is causing those blocking patterns to form more often and encouraging bouts of cold weather across the eastern United States.

Other research is inconclusive.

A ‘more complicated’ connection

One 2023 study found “no detectable trend” in midlatitude cold extremes tied to Arctic warming, including across the eastern United States, northeast Asia and Europe. Another from 2020 found that analyses and observations “continue to obfuscate a clear understanding” of how Arctic warming may be affecting weather at lower latitudes.

Simple physics suggests changes to polar vortex and jet-stream patterns are logical responses to the dramatic changes humans have caused to Earth’s natural systems, said Andrew Dessler, director of the Texas Center for Climate Studies at Texas A&M University. “I don’t think there’s a lot of disagreement there,” he said.

The question is whether those larger atmospheric changes are translating into significantly colder temperatures on the ground, he said.

“I think that is the missing link,” Dessler said, adding, “Lots of things happen in the stratosphere that don’t make it down to the surface.”

Francis agreed the connection between global warming and cold-weather outbreaks is “much more complicated” than the well-established links to heat waves, droughts, sea-level rise, extreme precipitation and intensifying storms.

Still, she added, “the evidence supporting the link is more solid now.”

Some, like Dessler, will remain skeptical until research establishes a clearer link. But that does not diminish the dramatic changes happening to weather and climate systems or rule anything out.

“Nothing is the same in the climate as it was,” he said.