Led by Judah Cohen, a climate scientist at Atmospheric and Environment Research, the new study is the latest salvo in a decade-long debate over how Arctic warming may be driving some winter extremes in the mid-latitudes, paradoxically leading to intense cold spells in a warming climate.
The stratospheric polar vortex is a semi-permanent pool of cold air over the poles about 10 to 30 miles high, encircled by strong winds. When the vortex is split in two, or stretched out, associated shifts in the jet stream at lower altitudes can push frigid surface air into the mid-latitudes, including the United States.
The stretched-polar-vortex concept was first brought into the dialogue with a 2018 paper in the Bulletin of the American Meteorological Society by Marlene Kretschmer of the University of Potsdam; Cohen was a co-author. A stretched vortex can pull a frigid air mass from high latitudes and drive it toward low latitudes even more effectively than when the vortex is weakened and split and a piece of it moves to lower latitudes, the mode that has been studied more closely.
The new study uses observations to identify the stretched vortex as an increasingly prevalent mode since 1980, and one that is especially likely to be related to intense mid-latitude cold outbreaks. The authors then use climate modeling to show that changes in Arctic sea ice and Eurasian snow cover may be fostering the stretched mode and contributing to high-impact winter weather extremes.
This year’s Texas cold wave, which the study identifies as related to vortex stretching, caused close to 150 deaths and at least $20 billion in damages. Houston was below freezing for nearly 48 hours, and millions across the state lost power. The impacts were widely considered to be a result of the state’s main power grid operator, ERCOT, being unprepared for such high-end events in a fast-growing region.
“Last winter following the Texas cold wave, many debated for and against the contribution of climate change to the event,” Cohen said in an email. “However, there were no studies supporting or refuting the link between climate change and the dynamical mechanism behind the Texas cold wave until our study.”
Overall, the most extreme winter cold is on the decrease globally and nationally, consistent with a warming climate. Yet some winter extremes, such as the Texas cold blast, have been particularly impactful. It’s been argued that rising temperatures and reduced sea ice in the Arctic are driving a chain of events behind some of the worst recent mid-latitude cold waves in North America and Eurasia.
Cutting-edge models probe the polar vortex
A recent set of experiments using newly comprehensive climate models, the Polar Amplification Model Intercomparison Project (PAMIP), has found only a weak connection between Arctic warming and changes in mid-latitude winter circulation.
The new study involved a more idealized model that allows Arctic sea ice and Eurasian snow cover to be easily manipulated to tease out their effects.
According to the chain of events set forth by Cohen and colleagues, increased October snow cover in Eurasia and reduced ice cover in the Barents and Kara seas north of Eurasia could both contribute to an increase in polar-vortex stretching events. The impetus is a cold surface high-pressure zone that develops atop snow cover in Siberia, with the autumn snow facilitated by prolonged periods of open water over the Barents and Kara seas.
As west-to-east flow hits the Siberian high, wave energy can propagate up to the stratosphere, distorting and ultimately stretching the vortex.
In the idealized model, snow and ice changes combine to produce an atmospheric response that is weaker than the observed changes but still stronger than in most Earth system model simulations to date.
Jennifer Francis, senior scientist and acting deputy director of the Woodwell Climate Research Center, co-wrote the landmark paper that kicked off the debate in 2012, along with Stephen Vavrus of the University of Wisconsin at Madison. She was not involved with the new study, but she sees it as a key contribution, especially the parsing of the stretched-polar-vortex component.
“This study highlights the underappreciated impacts of the stretching-mode type,” Francis said in an email. “The finding that this mode is occurring more often — while strong, circular vortex states are decreasing — is sobering because a stretched vortex is known to cause extreme winter weather events of various types, not only cold spells.”
Still an open question: Is climate change at work here?
Lantao Sun, a research scientist at Colorado State University who has studied Arctic impacts on global circulation, praised the new study. However, he isn’t confident that it proves a climate-change link to the observed increase in polar-vortex stretching and the resulting weather impacts in the troposphere, the lowest layer of the atmosphere.
“I think this paper advances our understanding on stratosphere-troposphere coupling by studying stretching of the [vortex],” Sun said in an email. “However, I am not convinced on the climate change part.”
Sun believes the observed increase in stretching modes could still be more a function of natural variability than of long-term climate change.
He also noted the limits of the model used in the Cohen study. “To quantitatively evaluate the impact of observed Eurasian snow cover and sea ice loss on atmospheric circulation, they might need to use comprehensive Earth system models,” Sun said.
Even in the most sophisticated models, it can be difficult to pluck out natural variability, given that only the last several decades — a short period by climatological standards — are scrutinized closely.
A study published earlier this year in the Journal of Climate led by Yannick Peings of the University of California at Irvine found that even a 100-member ensemble of climate model runs may not be enough to fully isolate how the atmosphere responds to Arctic sea ice loss beyond natural variability.
Cohen and colleagues assert in their paper that state-of-the-art Earth system models such as those in PAMIP, which can track how Arctic sea ice interacts with the atmosphere, may not be not well-suited to pinning the theorized chain of events to real-world weather events: “The key advantage is that [our model] allows for isolating the role of sea ice vs. snow changes in a single modeling framework, as opposed to free-running … models where both change together and are interdependent.”
Francis sees the new study as a natural extension of a hypothesis that is still a work in progress. “It’s clear that the hypothesis described and supported in our first paper … was an oversimplification (but not wrong),” she said. “As many groups around the world have dug into the gory details of these linkages, we now know that the story is much more variable and complicated.”
One strength of the new study, according to an accompanying essay by Dim Coumou at Vrije University in Amsterdam, is its analysis of observations and modeling, which he says is crucial for disentangling the forces at work.
“Thus far, observational studies tend to support a role for Arctic amplification in spurring winter cold spells, whereas most model experiments show little or no connections between Arctic sea ice or snow cover and mid-latitude winter extremes,” Coumou wrote.
“The effects of sea ice and snow cover changes on the stratosphere are likely small relative to internal variability. Yet, given the marked changes in Arctic sea ice and snow cover, even small effects can have important consequences for the stratospheric polar vortex.”
Coumou added: “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 extraordinary events in Texas.”