Decades of weather reports show a strong link between the polar blasts that have plunged the eastern United States in a deep freeze several times in the past few winters and the warming of the Arctic, where temperatures have been hitting unusual highs, a new study reports.
Weather in the Northern Hemisphere depends largely on the temperature difference between the temperate middle latitudes and the Arctic. But the Arctic has been warming twice as fast as the rest of the globe as heat-trapping carbon dioxide and industrial emissions build up the atmosphere.
How that Arctic amplification is affecting the weather further south is still a subject of hot debate. But since the trend started to show up around 1990, the Northern Hemisphere has seen more events like this February’s wild swings that drove temperatures above freezing north of Greenland while Europe froze.
“The warm Arctic itself is contributing to these cold temperatures,” climatologist Judah Cohen, the lead author of the new study, told Seeker.
The temperature differential between the cold of the far North and the warmer air of the temperate zones to the south fuel the jet stream, which steers weather patterns in the Northern Hemisphere. A large body of scientists suspect that when that differential narrows, the jet stream starts to curve like a whip being cracked, dipping further south and pulling icy air behind it.
Cohen is the director of seasonal forecasting at Atmospheric and Environmental Research, a Massachusetts-based firm that analyzes weather and climate risks. He and his co-authors looked at records from a dozen US cities, from Atlanta to Boston to Seattle, between 1950 and 2016. Then they compared them to Arctic weather records and found that when the Arctic was warm, cold snaps happened more often in the East — and the higher that Arctic warming reached into the atmosphere in mid to late winter, the more severe winter storms hit.
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The resulting paper, published March 13 in the research journal Nature, stops short of declaring the case closed. But Cohen said there was a “really robust” relationship between Arctic variability and wild swings in mid-latitude weather.
“We’re saying they’re connected, and certainly the fact that this warming of the Arctic is taking place up in the stratosphere, and that warming over the polar region is related to the increase in severe winter weather,” he said.
Those Arctic warm spells disrupt the cap of cold, low-pressure air that normally sits over the North Pole, known as the polar vortex. And when that happens, a burst of frigid air swings southward behind the jet stream, causing temperatures to plunge and sometimes bringing heavy snow with it. Cohen said that’s happened several times this winter already.
“The second one was in mid-December, and right after that was when we had that really severe period of winter weather in the eastern US,” he said. “Then there was even a more major disruption of the polar vortex in the second week of
February, and that was more hemispheric-wide. You saw this increase in severe winter weather in Europe, with historic cold and heavy snowfall in places like Rome and the French Riviera.”
Penn State climatologist Michael Mann, who was not part of the new study, called the findings “basically sound.” But he said there’s still a debate about how the rapid Arctic warming is affecting weather patterns further south.
The extreme cold snaps are taking place against a backdrop of generally warmer winters, and factors like a warming Atlantic Ocean have helped drive the powerful nor’easters and heavy snowfall seen on the US East Coast this year, Mann told Seeker.
“This is an entirely separate mechanism from Arctic warming,” he said. Computer models of the atmosphere’s behavior are important to capture those other potential drivers: “There isn’t just one factor at work, but many.”
But he added, “Uncertainty is not our friend when it comes to climate change. Here is yet another example that there are potential surprises — impacts that we didn’t foresee — in store. And typically, as in this case, they are not welcome surprises.”
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Cohen said computer models have been “all over the spectrum,” but haven’t shown a significant connection between Arctic warming and swings in the lower latitudes. He said this study tried to build out a stronger observational record by using daily weather data and found a “really robust” relationship.
He said getting a consensus from climate models would be more convincing to other researchers, but “I’m already there.”
“If there’s a consensus of the models that comes around to showing this influence, then I think pretty much the rest of the field will come along with this,” he said.