Just six months ago residents of the eastern U.S. were shoveling themselves out of the snowiest winter ever—weather that prompted mockery of global warming among some people. Now, scientists have a new explanation for why such anomalous snowstorms can coexist with global warming: The storms were kicked up by the convergence of two natural, large-scale weather patterns.

In order to better understand possible triggers of last year's media-dubbed "snowmaggedon," a team of scientists from Columbia University's Lamont–Doherty Earth Observatory analyzed more than 50 years of snow data as well as measurements of atmospheric pressure and sea-surface temperatures. They found that a combination of El Niño (periodic sea-surface warming in the tropical Pacific Ocean) with an unusual period of decreased variability in atmospheric pressure across the North Atlantic (known as the North Atlantic oscillation, or NAO) frequently results in a pile-up of snow in the mid-Atlantic region.

The NAO is a fluctuating pattern of air-pressure, extending from an area of low pressure that hovers over Iceland to a high-pressure region near the Azores, which helps to control the strength and direction of winds and storms across the Atlantic. When the NAO enters a negative phase, as it did last winter, the Icelandic low and the Azores high do not reach their respective pressure extremes. This causes freezing Arctic air to descend along North America's east coast. The researchers suggest that when those colder temperatures combined with the increased moisture delivered by El Niño, a greater amount of the precipitation in the area fell as snow.

The researchers were also able to use the atmospheric data to compute expected snowfalls for the 2009–10 winter, and found that their calculations matched well with what actually occurred.

A record-breaking 141.9 centimeters of snow buried Washington, D.C., last year, compared with the long-term average of 38.6 centimeters for the region.* Parts of Baltimore and Philadelphia were blanketed with more than 175 centimeters. Northwestern Europe was also cold and particularly snowy.

"What happened this past winter has nothing to do with climate change," says Richard Seager, senior research scientist and lead author of the study, which was published online July 26 in Geophysical Research Letters. He explains that although he does not doubt that human activity is altering the climate, individual weather events like last winter's blizzards cannot be used to make arguments for or against the reality of climate change.

Mark Serreze, director of the National Snow and Ice Data Center at the University of Colorado at Boulder who was not involved in the study, agrees. "What we saw last winter was just weather. The current heat wave is also weather," he says. He explains that although we have global warming, superimposed on the long-term warming trend are fluctuations in short-term weather patterns. "The NAO naturally varies between positive and negative phases, but last year it got stuck in a negative phase for a good part of the winter," Serreze says. "That was part of why it was so unusual."

Unfortunately, scientists will not be able to use this information to anticipate when massive snowstorms are likely to occur. Seager explains that El Niño events are predictable up to year in advance, because they are related to slowly evolving sea-surface conditions over the Pacific Ocean. By contrast, the NAO is primarily an atmospheric phenomenon, and is only predictable to the same extent as weather forecasting.

"Snowy winters like this will just remain a seasonal surprise that you can't predict," Seager says. "It's a complicated system out there. We have to expect the unexpected."

*Correction (7/30/10): This sentence was edited after publication to correct an error.