Think winters are getting colder? Blame Arctic warming and, yes, the polar vortex
There is growing scientific support for one of the most provocative and counterintuitive ideas in climate change research, which holds that rapid Arctic warming may be causing colder winters across large swaths of the Northern Hemisphere.
A new study, to be published in the journal Bulletin of the American Meteorological Society, found that a weakening polar vortex, potentially set in motion by the rapidly warming and melting Arctic, has become more common during the past four decades. This results in colder winters across large regions of Europe and Russia, but also occasionally in the U.S. as well.
The study is the first to show that changes in winds in the stratosphere substantially contributed to a mysterious winter cooling trend in northern Europe and Asia, including a region already known for being frigid: Siberia.
The study, written by a group of European and American researchers, found that a weakening in the wintertime polar vortex can explain 60 percent of the observed cooling in Eurasia since 1990. That figure increases to 80 percent if other influences on winter weather, including El Niño events, are also included.
“In winter, the freezing Arctic air is normally ‘locked’ by strong circumpolar winds several tens of kilometers high in the atmosphere, known as the stratospheric polar vortex, so that the cold air is confined near the pole,” said study co-author Marlene Kretschmer from the Potsdam Institute of Climate Impacts Research in Germany, in a press release.
“We found that there’s a shift towards more-persistent weak states of the polar vortex. This allows frigid air to break out of the Arctic and threaten Russia and Europe with cold extremes."
Ok, but what the heck is the polar vortex?
The polar vortex probably isn't quite what you think it is.
It's not some sort of giant whirlpool in the sky. Nor is it something that can suck you in, like a massive, frigid tornado, rotating above the entire Arctic in a scene akin to the instant freeze of New York City during the film The Day After Tomorrow.
Instead, you can think of the polar vortex as a circular air current that exists at two levels of the atmosphere, one in the troposphere, where most weather occurs, and the other a bit higher up, in the stratosphere. The new study deals with the stratospheric polar vortex.
When the stratospheric polar vortex is strong, the study found, it tends to bottle up the cold air in the Arctic, making northern midlatitudes from Canada to Europe and Eurasia milder than average. A strong polar vortex means that the winds blowing from west to east at high altitudes across the Arctic are more powerful than when the vortex slackens and meanders.
When the vortex weakens, as it did during the infamous winter of 2012-2013, and several winters since, the ultra-cold air can spill out of the Arctic, as if someone opened the door to the planet's freezer. Not surprisingly, this can lead to cold snaps and snowstorms along the U.S. East Coast, in Western Europe, as well as Eurasia.
Meanwhile, during such episodes the Arctic can be comparatively mild, leading scientists to call such patterns, "warm Arctic, cool continents" setups. These patterns occurred last winter, when several storms brought such mild air to the center of the Arctic Ocean that the North Pole reached near or above freezing for short periods of time.
During the past several years, scientists have been finding that the shape and strength of the stratospheric polar vortex is hugely important for determining what kind of winter weather will dominate large regions of the northern midlatitudes, particularly in Eurasia, which has been seeing a wintertime cooling trend overall.
Paradoxically, global warming may be leading to this uptick in weak polar vortex episodes, and resulting frigid air outbreaks in highly populated areas of Europe, Asia, and North America. More specifically, some scientists, including the authors of the new research, think the increasingly open Arctic Ocean during the summer and fall may be helping to destabilize the Arctic climate system more broadly, including the polar vortex.
The Arctic is warming at about twice the rate of the rest of the world, and sea ice extent has been rapidly declining. Arctic sea ice ended the 2017 melt season at its eighth-lowest extent on record since 1979, after hitting its lowest-ever winter maximum earlier in the year.
As sea ice extent declines, the ocean in the Far North absorbs more heat, which it then releases into the atmosphere, adding to warming, which then melts more ice. This process, and other Arctic feedback loops, are known as Arctic amplification. Previous studies have shown that ice melt in parts of the Arctic Ocean can have a ripple effect well into the stratosphere, disrupting the polar vortex.
While scientists agree that this feedback loop and others are taking place, there are disagreements about whether cold air outbreaks in the midlatitudes are connected to Arctic warming or not. The new study clearly supports the idea that at least in some areas, such as large parts of Europe and Asia, this connection does exist.
“Our latest findings not only confirm the link between a weak polar vortex and severe winter weather, but also calculated how much of the observed cooling in regions like Russia and Scandinavia is linked to the weakening vortex. It turns out to be most,” said co-author Judah Cohen, a meteorologist at Atmospheric and Environmental Research, in a press release.
“Several types of weather extremes are on the rise with climate change, and our study adds evidence that this can also include cold spells, which is an unpleasant surprise for these regions.”
Not all climate scientists agree with the new study's findings, of course. Kevin Trenberth, a researcher at the National Center for Atmospheric Research in Boulder, Colo., said he has doubts about the claims made in the paper.
"This paper presents some interesting relationships in space and time that involve the stratosphere, that undoubtedly plays a key role in the evolution of Arctic climate anomalies," Trenberth wrote in an email. "However, the whole atmosphere is evolving in complex ways," he said, noting two cycles in particular, known as the Pacific Decadal Oscillation and the North Atlantic Oscillation. The study largely ignores both of these influences.
"The result is a number of quantities that are related to one another, but one can not say they are causal, as claimed," he said.
"On the contrary, there is good evidence of other influences that play a major causal role."
This story was updated on Sept. 26 to include comments from Kevin Trenberth.
Andrew Freedman is Mashable's Senior Editor for Science and Special Projects. Prior to working at Mashable, Freedman was a Senior Science writer for Climate Central. He has also worked as a reporter for Congressional Quarterly and Greenwire/E&E Daily. His writing has also appeared in the Washington Post, online at The Weather Channel, and washingtonpost.com, where he wrote a weekly climate science column for the "Capital Weather Gang" blog. He has provided commentary on climate science and policy for Sky News, CBC Radio, NPR, Al Jazeera, Sirius XM Radio, PBS NewsHour, and other national and international outlets. He holds a Masters in Climate and Society from Columbia University, and a Masters in Law and Diplomacy from The Fletcher School at Tufts University.
