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How Warmer Summers Cause Colder Winters
Snow covered the South last January. This season, the white stuff came earlier.
Credit: NASA/NOAA.

Counter to what logic might suggest, warm summers actually trigger cold winters, according to a new study.

The study, detailed in the Jan. 13 issue of the journal Environmental Research Letters,offers an explanation for the recent harsh winters in the Northern Hemisphere: Increasing temperatures and melting ice in the Arctic regions are creating more snowfall in the autumn months at lower latitudes, which, in turn, affects an atmospheric pattern that leads to colder winters.

The strongest winter cooling trends were observed in the eastern United States, southern Canada and much of northern Eurasia, which the researchers, from the University of Massachusetts and the University of Alaska, Fairbanks, believe cannot be entirely explained by the natural variability of the climate system.

Their results showed strong warming throughout July, August and September in the Arctic, which continued through the autumn and, according to their observational data, appeared to enhance the melting of sea ice.

This warmer atmosphere, combined with melting sea ice, allows the Arctic atmosphere to hold more moisture and increases the likelihood of precipitation over areas to the south, which, in the freezing temperatures, would fall as snow. Indeed, the researchers' observations showed that the average snow coverage in Eurasia has increased over the past two decades.

They believe the increased snow cover has an intricate effect on the Arctic Oscillation — an atmospheric pressure pattern in the mid- to high-latitudes — causing it to remain in the "negative phase."

In the "negative phase," a high pressure system resides over the Arctic region, pushing colder air into mid-latitude regions, such as the United States and northern Canada, and results in colder winters.

"In my mind, there is no doubt that the globe is getting warmer and this will favor warmer temperatures in all seasons and in all locations," said study leader Judah Cohen. "However, I do think that the increasing trend in snow cover has led to regional cooling as discussed in the paper and I see no reason why this won't continue into the near future."

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