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When winter sets in, "polar vortex" becomes one of the most dreaded phrases in the Northern Hemisphere. It's enough to send shivers even before the first blast of bitter cold arrives.

New research shows that some northern regions have been getting hit with these extreme cold spells more frequently over the past four decades, even as the planet as a whole has warmed. While it may seem counterintuitive, the scientists believe these bitter cold snaps are connected to the warming of the Arctic and the effects that that warming is having on the winds of the stratospheric polar vortex, high above the Earth's surface.

Here's what scientists involved in the research think is happening: The evidence is clear that the Arctic has been warming faster than the rest of the planet. That warming is reducing the amount of Arctic sea ice, allowing more heat to escape from the ocean. The scientists think that the ocean energy that is being released is causing a weakening of the polar vortex winds over the Arctic, which normally keep cold air centered over the polar region. That weakening is then allowing cold polar air to slip southward more often.

The arrows show movement of the polar vortex winds. The size of the arrows reflects the strength of the flow. Credit: NASA

The polar vortex has always varied in strength, but the study found that the weaker phases are lasting longer and coinciding with cold winters in Northern Europe and Russia.

"The shift toward more persistent weaker states of the polar vortex lets Arctic air spill out and threaten Russia and Europe with extreme cold," said the study's lead author, Marlene Kretschmer, a climate scientist with the Potsdam Institute for Climate Impact Research. "The trend can explain most of the cooling of Eurasian winters since 1990."

Some other scientists aren't as sure that melting sea ice affects the polar vortex so strongly. They think other factors, like long-term variations in sea surface temperatures like El Niño, and changes in the tropics, might play bigger roles.

Primed for Longer Stretches of Extreme Cold

The research, published in the Bulletin of the American Meteorological Society, helps explain one way that rapid and intense Arctic warming affects climate extremes in the populated mid-latitudes of the Northern hemisphere.

Kretschmer and her colleagues focused on the region from Scandinavia through Siberia, where winter snow cover has increased and average winter temperatures have dropped since 1990. Co-author Judah Cohen, a climate researcher at MIT, said the results also provide new clues about how the Arctic affects cold extremes in the U.S.

The study tracked changes in the polar vortex in the months of December and January between 1979 and 2015. It concluded that the polar vortex is primed for extreme cold outbreaks for longer stretches—from 5.3 days during the first half of the study period to 14.1 days in the second half. During the same time, average winter surface temperatures in northern Eurasia declined.

"It's a piece of the whole puzzle which really helps us understand the linkages between Arctic changes and mid-latitude circulation changes," said Dörthe Handorf, a climate researcher with the Alfred Wegener Institute who was not involved in the study.

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Previous studies have also concluded that the changes in the stratosphere are important. "Without the stratospheric changes, we can't explain why we see an increase in cold days over Eurasia," Handorf said.

A Step Toward More Accurate Forecasts

Along with helping explain how melting sea ice affects the atmosphere, the new study is a step toward more accurate seasonal forecasts that can help prepare communities for extreme conditions, Cohen said.

Models used in forecasting don't currently anticipate these changes in the polar vortex, he said. Comparing polar vortex phases with temperatures in the study area and data on sea ice extent can potentially improve forecasts two to six weeks in advance, he said.

With that information, scientists soon may be able to say that, when the sea ice forms very late in the Arctic Ocean north of Russia, people living eastern Scandinavia and Siberia should prepare for harsh early winter conditions.

The picture is not as clear for North America, said Jim Overland, an oceanographer with the National Oceanic and Atmospheric Administration (NOAA), who was not involved in the study. Natural year-to-year variations in weather still masks the global warming signal to some degree, he said.

"You can take one view or another, but the research helps make people think about the effects and how to forecast them. What we know for sure is, the Arctic is warming and losing ice and the forcing is there," he said, referring to the potential effect of melting sea ice on weather patterns. Pinpointing the impacts on areas where millions of people live, he said, would pay off for those communities.