By Carolyn Gramling

S

ea ice may have grabbed the headlines, but for nearly 2 decades a team led by climate scientist Judah Cohen of the Massachusetts Institute of Technology in Cambridge has been quietly and persistently chasing an alternative link between the changing Arctic and midlatitude weather: how much snow falls over Siberia each October. Snow cover in Eurasia from year to year can vary dramatically, Cohen says, by as much as a factor of three. Like the retreat of sea ice, more snow cover would ultimately lead to a wavier jet stream, allowing cold polar air to invade midlatitudes. But whereas sea ice proponents suggest that a rapidly warming Arctic leads directly to a slow, more meandering jet stream, Cohen envisions a series of causal links from Siberian snow via the lower atmosphere to large-amplitude planetary waves that reach into the stratosphere and shape weather patterns in the midlatitudes. His “six-step” cycle goes like this: With broader snow cover blanketing more of Siberia in October, there is more cold, dense air in the lower atmosphere, and the high-pressure center known as the Siberian high becomes bigger and stronger. By November to December, these atmospheric transformations increase the flow of energy upward, from the troposphere into the stratosphere. The stratosphere suddenly warms, and the stratospheric polar vortex—the mass of cold air that is normally confined to the Arctic—breaks down, creating a more meandering jet stream and allowing cold polar air to penetrate farther south.

In a 2014 paper, Cohen brought sea ice into the picture as well. “We tried to argue how they can all work together,” he said. Although melting sea ice warms the lower atmosphere over the Barents and Kara seas, and melting snow cools it to the east, over Siberia, the two effects can actually work in tandem to force the same planetary wave pattern in the atmosphere that promotes the weakening of the polar vortex in wintertime. But Cohen’s hypothesis is far from widely accepted. “His hypothesis that all these things are connected … there are so many steps in that kind of chain” and there’s a lot of uncertainty in the data, says James Screen, a climate modeler at the University of Exeter in the United Kingdom. For example, Screen says, Cohen “reports increasing snow cover in Eurasia in October,” but other researchers haven’t seen that trend. Cohen, who directs seasonal forecasting efforts at the firm Atmospheric and Environmental Research, a unit of Verisk Climate, has been working not only to understand these links but also to use them to predict changes in the polar vortex and anticipate storms. (They’ve made their predictions public at https://www.aer.com/science-research/climate-weather/arcticoscillation.) One recent success: In late November 2014, the team predicted—on the basis of extensive Siberian snow cover last October—that the stratospheric polar vortex would weaken in early January 2015. In mid-January, a blizzard pounded the northeastern United States. “This is relevant to the whole argument of whether the Arctic can have any influence on midlatitude weather, or [whether] it is all just random noise,” Cohen says. “If all we can discern in the system is noise, then it should be impossible to make an accurate prediction of a stratospheric warming 6 weeks in advance.” ■

Increased autumn snow in Siberia

Jet stream pushes south

Polar vortex

Polar vortex

Jet stream

Colder winter in eastern United States and Europe

Str at Tro os po s

St ra Tro t p

SORTING OUT THOSE “known knowns”

is exactly what researchers should be doing, Barnes says. “In my mind, we’ve put the cart before the horse a bit” by focusing on weather extremes, she says. “We’re missing the theory, the dynamics, the mechanistic understanding.” She suggests that the community go back to basics to determine how Arctic warming could influence the jet stream dynamics. “It’s certainly not sexy like telling you that

Jet stream

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Jet stream

Stratosph ere Troposph ere

ILLUSTRATIONS: ADAPTED FROM NSF BY G. GRULLÓN/SCIENCE

Polar vortex

we’re going to have more extreme cold events—but in terms of the science, it’s the way forward.” One key to success, she says, is teasing out the effects of the Arctic on the jet stream from those of other atmospheric powerhouses, such as the chaotic atmospheric flow at midlatitudes and the ENSO pattern in the tropics. Indeed, researchers need to look beyond the potential impacts of Arctic change, Overland says, and aim

SCIENCE sciencemag.org

for a better picture of atmospheric dynamics as a whole. “We’re just getting into that time where people get interested,” Barnes says. “If you look over the literature in the last 10 years, there’s a lot of focus over how tropical warming will influence the jet stream, but not a lot on how Arctic warming will influence it. As a community our eyes were turned to the south, and now we’re looking north. That, to me, is what’s exciting.” ■ 20 FEBRUARY 2015 • VOL 347 ISSUE 6224

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The Siberian snow connection