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June 19, 2013

Predicting Winter

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Want the most reliable forecast of how severe this winter will be? One scientist says just watch Siberia's weather come October. This ScienCentral News video explains.

Siberia-cast

If you're wondering if this winter is likely to feel like last year's--with summer temperatures in January on the East Coast-- or more like the snows of Siberia, here's a hint: check how much it really does snow in Siberia, in October.

Writing in the Journal of Climate, Judah Cohen of Atmospheric and Environmental Research, Inc. has reported verification of a simple equation he's tested for several years.

"If there was above normal snow cover in Siberia in October, there tended to be colder temperatures in the Eastern U.S. in the wintertime," Cohen says.

While conventional models look to the oceans as controlling our weather with changes such as El Nino, Cohen says land features like the vast Siberian snow fields have a bigger impact on North American winters than previously thought. The increased cold and reflected heat of heavy autumn snows in Siberia affect a less well-known pattern called the Arctic Oscillation, the circulation of wind around the North Pole, which pushes high pressure and cold southward.
Cohen showed that his model outperforms today's commonly-used forecast models in predicting forecasts for the last 35 years of weather. But he says actual predictions are what make or break a model--or a reputation.

"We've been making real time forecasts for seven years," says Cohen. "In my mind, the best test of a model, seasonal forecast model, is through a real time forecast."

map forecasted

image: Atmospheric and Environmental Research

For example, while others predicted a mild winter in 2002, Cohen called for cold for the blue areas on this map.

map actual

image: Atmospheric and Environmental Research

This map shows he was right.

Cohen's real-time winter forecast for Europe for 2003-04, was also more accurate than those issued using conventional models.

"I think before my research, the mainstream, or most of the people, felt winter weather kind of just happened," Cohen says. "It just happened spontaneously, randomly, and had these large-scale shifts, these large-scale patterns that happened really abruptly and without any kind of warning. But I felt differently about climate, particularly winter climate, about how it works.That it's something that evolves over time.

"You can actually follow what's going on," he says. "It's not just some random or haphazard change."
Cohen says he'd next like to discover the factors that contribute to Siberian snowfall, so he can tell us even sooner if we have to break out the snow shovels or the sun screen.

PUBLICATIONS: Journal of Climate, August, 2007

RESEARCH FUNDED BY: National Science Foundation, Canadian Foundation for Climate and Atmospheric Sciences

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by Joyce Gramza

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