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Warming Arctic May Be Causing Cooler Winters in Eastern U.S., Europe | Climate Central
A look at weather extremes and the big-picture climate connections.

Warming Arctic May Be Causing Cooler Winters in Eastern U.S., Europe

The past few winters have featured unusually cold and snowy conditions in the Eastern U.S. and parts of Europe, causing many to question whether global warming exists at all. In the wake of several Mid-Atlantic and Northeast blizzards, which were given social media-driven monikers such as "Snowpocalypse" and "Snowmageddon," journalists and scientists went to great lengths to explain that heavy snowstorms are actually consistent with global warming. In fact, warming sea and air temperatures are putting more water vapor into the air, which supplies more energy for storms to work with and drives precipitation extremes.

Arctic sea ice extent at the end of the 2011 melt season was far below average. Credit: NASA.

That explains the snow, but not the cold. Last winter, however, I wrote several stories about a tantalizing explanation for the snow and cold, which I called the "Arctic Paradox." In short, it holds that as the Arctic warms up — it's generally warming twice as fast as the rest of the globe — the loss of sea ice and changes in how heat moves between the ocean and the atmosphere is rearranging weather patterns. The result, paradoxically, favors colder and snowier conditions outside the Arctic. In other words, the Arctic gets warmer, while winters in Boston, London, and Paris turn colder and snowier.

Now, two new studies have recently been published that advance this concept. The lead author of both studies, Judah Cohen of Atmospheric and Environmental Research, a private forecasting firm based in Lexington, Mass., has spent years trying to find patterns that would improve seasonal climate forecasts.

For winter forecasts, he thinks he has found the answer in Siberia, of all places.

Cohen and his colleagues have published a new paper in the journal Environmental Research Letters that ties Arctic warming to cooler winters in the eastern U.S. and northern Europe and Asia, via changes in Siberian snowfall.

According to the study, as the Arctic warms and loses more sea ice during the summer, more moisture can evaporate from the Arctic Ocean. That extra moisture becomes available for rain and snowstorms during the fall in northern Eurasia and other high-latitude regions. When it falls as snow, the added snow cover can set off a series of events in the lower and upper atmosphere that eventually leads to cooler and snowier conditions in the eastern U.S. and parts of Europe. 

Temperature departures from average during December 2009, when the Arctic Oscillation was in a negative phase. Credit: NASA.

Late last year, Cohen and his colleagues published another study that found a statistical link between the buildup of snowfall in Siberia during October and the so-called Arctic Oscillation, a weather pattern that affects the East Coast and Europe during the winter.

The Arctic Oscillation describes the pattern of air pressure that lies between the Arctic and the North Atlantic. When it is in a negative phase, cold air is more easily pushed down from the Arctic and into the United States. Most of the East Coast's snowiest winters — including the epic winters of 2009-10 and 2010-11 — occurred when the Arctic Oscillation was in the negative phase.

As I wrote in the Washington Post in late November, forecasters have long regarded the Arctic Oscillation as virtually unpredictable beyond about two weeks in the future. 

Cohen's work suggests that by analyzing Siberian snowfall patterns during October, forecasters can get a good idea how the Arctic Oscillation will behave during the winter months. As I reported for the Post:

His study... shows a statistically significant link between the rate of change in Siberian snow cover during October and the dominant phase of the Arctic Oscillation during the following winter. A rapid advance of Siberian snow cover, Cohen and his colleague Justin Jones found, is linked with the negative phase. A slow advance, by contrast, is linked with a positive oscillation, which brings milder winter weather to Washington.

Cohen claims that rapidly advancing snow cover in Siberia can set off a chain of events from Earth’s surface to the stratosphere. The quick expansion can lead to a large dome of cold high pressure over Siberia. That dome, in turn, perturbs the jet stream so it flows more north to south in addition to west to east, resulting in more intense cold-air outbreaks in eastern North America and western Europe, which often breed snowstorms.

You may be wondering: "How does this explain what's going on this winter?" Well, it does and it doesn't. Siberian snowfall was about average during October 2011, indicating that the Arctic Oscillation would probably wind up close to neutral for the winter season (assuming Cohen's study is correct). So far, however, it has been in positive territory, favoring mild conditions in the East.

As I reported on January 10th, there are signs that the early winter weather pattern is changing, but it's unclear what that will mean for the East Coast and Europe.

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