New research shows that Arctic climate changes during the next few weeks may determine if and when the Eastern United States gets another extreme cold wave this coming winter.
Since the 1980s, Arctic sea ice extent has been dropping sharply, while the extent of land covered by snow in Siberia during autumn has increased. Those changes have combined to more frequently contort the “polar vortex” of high-altitude Arctic winds into a dumbbell shape that increases the potential for severe winter weather outbreaks like February’s Texas freeze, the study, published in Science Thursday, shows.
Arctic sea ice this year is once again near a record low, and medium range forecasts call for relatively cold and snowy conditions in Siberia, potentially setting the stage for renewed winter extremes at lower latitudes.
Disruptions of the polar vortex—a belt of strong, high altitude winds usually circling the central Arctic—have become more frequent in the last 40 years, the new research found. In the study, the researchers write that the lack of sea ice in the Barents-Kara Seas and heavier snowfall over Siberia combine to build a wave of high pressure in the atmosphere between Northern Europe and the Ural Mountains, along with low pressure over East Asia.
The extent, positioning and timing of sea ice and snow cover in the Arctic has changed since the region started warming rapidly, and the new pattern, shaped by global warming, intensifies the waves. If their rising energy is absorbed by the upper atmosphere, it’s more probable that the circulation above the Arctic will remain stable. But sometimes if that energy bounces downward off the polar vortex it distorts the vortex, pushing one end of the cold dumbbell over Siberia and the other toward the eastern half of the United States.
Why that energy is sometimes reflected downward remains one of the big climate mysteries, said study author Judah Cohen, a Boston-based climate researcher and director of seasonal forecasting with Atmospheric and Environmental Research (AER). But how the process can drive winter extremes was shown in February 2021, when bitter Arctic air swept southward across Texas and even into Mexico.
The freezing conditions across Texas killed 210 people and crippled regional transportation and power grids. The National Oceanic and Atmospheric Administration reported that the region’s prolonged exposure to Arctic air resulted in power outages affecting nearly 10 million people across the South. NOAA estimated the cost at more than $20 billion, making it the costliest winter extreme event on record in the United States. and the nation’s costliest climate-related extreme of the year so far.
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The extreme cold outbreak happened in a winter in the High North that illustrates the findings of the new research. “A record warm Arctic, low Arctic sea ice and deep Siberian snows,” led to a long polar vortex disruption and “most notably, the record breaking and possibly unprecedented combination of cold and snow in Texas,” Cohen said.
The Texas cold wave reignited a debate about the role of the changing Arctic in winter extremes elsewhere in the Northern Hemisphere, he added, and the new study’s findings indicate that severe winter weather outbreaks will keep happening, even as the world overall is warming rapidly.
In a 2019 study, Cohen and other scientists used a widely accepted winter weather index compiled from a network of weather stations spanning the continent from Seattle to Chicago, New York and Atlanta, to show how unusually severe winter weather in parts of the United States is more frequent when the Arctic is very warm.
The research provides new evidence that changes to stratospheric winds might play a role in extreme regional winter weather outbreaks, said Dim Coumou, who researches climate extremes at the Vrije Universiteit Amsterdam. Based on the winter weather index, the number of autumn and winter days almost doubled from 1980 to 2020, he said.
“Variability in the stratospheric polar vortex can have a strong effect on mid-latitude weather conditions,” Coumou wrote in a related article in the same issue of Science. When it’s disrupted as the study describes, “the vortex makes stronger southward excursions above the American and Eurasian continents.” Since the 1980s, he wrote, “the frequency of these stretched vortex states has increased.”
The new study shows that the influence of Arctic change on climate in lower latitudes is “likely to be rather subtle,” he said, and hard to separate from natural variability. But that doesn’t mean those effects should be ignored.
Relatively small changes “can have important consequences for extremes, with impacts on society,” he said. “The thermodynamics of global warming push toward milder winter weather, but changes to atmosphere dynamics could still present risks for society, as illustrated by the unusual events in Texas.”
“One of the big messages of the paper is, yes, there are fewer cold records being broken, overall. That’s expected, we see that happening,” said Woodwell Climate Institute researcher Jennifer Francis, who was not involved in the new study, “But, we can still get these very severe events occurring. And it’s also part of climate change, because it’s connected to these disruptions of the stratospheric polar vortex, which is happening more often, and now we think we know why.”
The study doesn’t look at annual averages, but focuses on seasonal details in the fall and winter, because the polar vortex dissipates during the warmer half of the year. It starts forming when the Arctic cools as sea ice reforms, starting in late September, and often circles tightly around the central Arctic.
But since the Arctic Ocean’s shield of sea ice started diminishing rapidly in the 1980s, more of its water is left uncovered. The dark water absorbs more heat than the white sea ice, and more of it is exposed for longer periods of time to the warmth of sunlight each year, especially in the Barents and Kara seas. In the fall, the water releases energy and moisture back into the atmosphere, where they can fuel early snowfall in Siberia.
Together, the warming over the ocean and the increasing autumn snow cover have shifted how the vertical waves of atmospheric energy move around the Northern Hemisphere in ways that more frequently stretch the polar vortex into the alignment that brings winter extremes to the Eastern United States, the researchers found.
It’s not everywhere across the Arctic, it’s regional, and that matters,” Cohen said. “This is the mechanism that’s been hiding in the shadows.”
Francis said the study shows “the Texas cold wave was not just a random, freak thing. This is going to happen again, and we think it’s maybe even going to happen more often because of what we are doing to the climate system.”.
She added, “Hopefully that will light a fire under some of the decision makers in regions where they are susceptible to extreme events like this. And I hope this helps clarify to the public, too, that this is something we have to expect more often in the future. This is one of the faces of climate change.”
A 2012 study by Francis on how declining Arctic sea ice might affect the jet stream helped spur wider research on how the changing polar region might affect climate extremes in the mid-latitudes of the Northern Hemisphere, including many major population centers and breadbaskets.
“No one believes that the Arctic could warm three times faster than the globe as a whole and have no impacts,” she said.
“Arctic amplification”—the driver of the polar region’s rapid warming—has been heavily researched in recent years. But different approaches to investigating how it affects climate extremes in the mid-latitudes have fueled debate. “It turns out sea-ice loss is only one of several factors that need to be taken into account to represent Arctic amplification realistically,” Francis said.
If a model only includes sea ice loss, but not some of the other related factors, it shows smaller atmospheric effects than those being observed in the real world. That “led to some conclusions that the Arctic’s role is no big deal,” she said.
Changes to the polar vortex could have other unexpected impacts, said Maine-based climate researcher Susan Conard, who retired from the U.S. Forest Service and now studies the links between climate change and wildfires and is the editor of the International Journal of Wildland Fire. Some of the changing patterns shown by the study, like increased precipitation in the fall or winter, have the potential to help put an earlier end to the summer fire season in the Northern Hemisphere, or to minimize the winter fire season in the southwestern United States,” said Conard, who was not involved in the new research.
If the changes increase seasonal snowpack regionally, she said, “They could delay the beginning of the fire season in the spring because of increased soil moisture and delays in the end of the period when there is snow cover.”
Evidence is mounting that the profound changes in the Arctic are affecting climate extremes in the densely populated and developed mid-latitudes of the Northern Hemisphere, including key agricultural regions, Francis said. There are likely other connections that are still largely unexplored, she added. It’s possible, for example, that amplified Arctic warming intensifies and perpetuates marine heatwaves in the North Pacific, like the one in 2013 known as the “blob,” which, in turn, are linked with long-term California droughts.
“The ice loss north of Alaska may reinforce the atmospheric ridge pattern set up by the warm blob,” she said. “I think the Arctic is an amplifier of these patterns, not a creator.”
The new research led by Cohen helps show how important it is to understand those connections, Francis said.
