The Arctic Oscillation has been linked to wide-ranging climate effects in the Northern Hemisphere, but new evidence shows that in recent decades it has been the key in preventing freezing temperatures from extending as far south as they had previously.

"Public perceptions that winters are becoming less wintry appear to be as much or more due to the change in the Arctic Oscillation as to global warming," said David Thompson, an assistant atmospheric science professor at Colorado State University.

The Arctic Oscillation -- also referred to as the North Atlantic Oscillation or the annular mode -- is a climate pattern defined by winds circulating counterclockwise around the Arctic at about 55 degrees north latitude (about even with Moscow; Belfast, Northern Ireland; and Ketchikan, Alaska). Its effects on weather patterns appear to be as far-reaching as those triggered by El Niño in the South Pacific.

Thompson, who began his research while a doctoral student at the University of Washington, and John M. Wallace, a UW atmospheric sciences professor, examined daily January-through-March weather data from specific stations for each year from 1958 through 1997. In the July 6 edition of the journal Science, the researchers report finding a strong correlation between the Arctic Oscillation's negative phase and near-record cold days and snow storms over a much broader region of the hemisphere than was previously thought.

In its negative phase, the Arctic Oscillation's ring of air spins more slowly and is more easily disturbed, allowing cold arctic air to spill out of the far-north regions and into midlatitudes. In the positive phase, the ring of air spins faster and acts much as a dam that impedes frigid air moving south.

"The Arctic Oscillation flips back and forth a lot between positive and negative phases within a winter," Thompson said. "These changes affect weather throughout much of the hemisphere."

Thompson and Wallace found that days on which the Arctic Oscillation is in its negative phase are on average several degrees colder than normal over most of the United States, Northern Europe, Russia, China and Japan. Cities with normally mild winters, such as Seattle, Dallas, Paris and Tokyo, experience most of their subfreezing temperatures and snow and ice storms on those days.

Positive-phase days show a greater frequency of high winds over northern Europe and North America's Pacific Northwest. Negative-phase days bring to New England a greater likelihood of strong coastal storms known as Nor'easters.

In the 1980s and '90s, the Arctic Oscillation has spent most of the time in the positive phase, the researchers said. That reduced the number and frequency of days with subzero temperatures or substantial snowfall in the midlatitudes.
"It is conceivable that this change in the behavior of the Arctic Oscillation could be linked to the buildup of greenhouse gases in the atmosphere," Thompson said.

The research by Thompson and Wallace was paid for by grants from the National Science Foundation and the National Aeronautics and Space Administration.

The new information suggests that forecasts of the Arctic Oscillation would have widespread practical applications, Wallace said. For example, if oil companies knew in advance that a negative-phase winter was on the way, they could plan to increase supplies to their distributors.

Wallace expects the Arctic Oscillation to continue, at least for now, the tendency it has shown in recent decades.

"If this trend of the last 30 years is human induced and if it continues, snow in Seattle or Dallas or Tokyo will become an even rarer event than it is now," he said. "But if that trend reverses, all bets are off."

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For more information, contact Thompson at (970) 491-3338 or davet@atmos.colostate.edu, or Wallace at (206) 543-7390 or wallace@atmos.washington.edu.