What Happened To Winter? Just Wait

By Sora Song

El Nino, move over. The rising star on the meteorological scene, according to climatology insiders, is the Arctic Oscillation, or AO. It could be the reason, scientists argue, that people have been lunching outdoors in New York City, driving with the top down in Boston and tanning by the lake in Chicago--in December.

The Arctic Oscillation--sometimes called the North Atlantic Oscillation or the northern hemisphere annular mode--involves atmospheric-pressure changes over the northernmost part of the globe, from about 55[degrees] latitude on up, according to Colorado State University's David Thompson and the University of Washington's John M. Wallace. When the AO is in a positive phase--that is, pressure at the core of the region is low--a ring of wind that swirls around the North Pole increases in strength and keeps the frigid Arctic air from escaping southward out of the vortex. That means warmer than usual weather in most of the northern hemisphere, say researchers Thompson and Wallace, who coined the term Arctic Oscillation in 1998. The AO has been in a strong positive phase since late October, notes the National Weather Service (NWS), causing record-high temperatures nationwide. In fact, the AO seems to have tilted this way for many winters since the late '60s. No one knows why, but it could be related to greenhouse gases, ozone depletion or warming in the tropics.

Now for the flip side. When the AO swings negative, the circling Arctic winds weaken and cold air descends from the polar region, leading to below-normal temperatures across the hemisphere. The AO seesaws every winter, and appears to be drifting back to the negative now, at least momentarily. Combine that with an El Nino- and La Nina-free forecast, says the NWS, and the balmy fall just may have been a prelude to a frightful winter.

--By Sora Song. With reporting by J. Madeleine Nash

With reporting by J. MADELEINE NASH