Just when we might be expecting the influence of unusually high Pacific ocean temperatures to warm us up — or for global warming to bring relief — along comes another wave of incredibly cold storms. How the season finally turns out is still up in the air, so to speak, but clearly, that weather patterns that are typical of El Niño have not taken hold across the United States.
And it's easy to forget that global warming is a long-term climate trend that has little to do with individual seasons in one part of the world or another. In fact, it might be hard to appreciate just now, but the year just ended — 2009 is a single data point — actually came in a little warmer than the two years before and is fairly close to the middle range of model simulations of the long-term trend that has provoked international scientific concern about global warming.
Shorter-term, naturally variable patterns such as El Niño account for seasonal differences — making one winter warmer or colder than another. But it takes a strong El Niño to dominate the pattern of a U.S. winter with unusually warm and dry conditions across the northern tier of the country, and cooler and wetter weather across the south, and the current El Niño is not strong.
Winter patterns — shown here Tuesday in a NOAA satellite image showing cold clear skies over much of the country and another wave of storminess entering the Pacific Northwest — are marching to the beat of other drummers in the band.
Among the more striking this year is a pattern of atmospheric pressure known to climate researchers as the Arctic Oscillation, or the North Atlantic Oscillation. The pressure pattern drives a ring of winds that blows counter-clockwise at about 55 degrees north, over places like Moscow and Belfast and Ketchikan, Alaska.
When the pattern is positive, the winds are strong, and the power of the vortex holds the storms in its embrace. When the pattern is negative, the winds are weak, winter storms slide farther south and their sub-freezing temperatures grip much deeper into the Northern Hemisphere. As Andrew Revkin at the New York Times has pointed out recently, the Arctic Oscillation is more deeply negative this year than it has been since the 1980s.
In 2001, after analyzing its impact on Northern Hemisphere winters, University of Washington researchers suggested that effects of the Arctic Oscillation on weather patterns "appear to be as far-reaching as those triggered by El Niño in the South Pacific."