New Findings on a Key Player in Winter Climate
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Researcher Jim Hurrell has analyzed decadal trends in the NAO. The NAO is one of the primary factors in the earth's yearto-year atmospheric variability. A north-south oscillation in atmospheric mass, the NAO's centers of action are near the semipermanent Icelandic low and Azores high. When the NAO is in a "positive phase," pressures are higher than normal across the North Atlantic, and lower than normal across the Arctic. The resulting westerly flow brings maritime warmth to Europe during the winter, and allows the northwest Atlantic to cool below normal.
Using a data set for the years 1899-1993, Hurrell compared surface pressures with sea-surface and continental temperatures. The comparison revealed a strong correlation between the NAO and the presence of European warmth and northwest Atlantic cold. The highest positive values of the NAO index in this century have occurred since 1980. Hurrell then linked variations in circulation patterns to changes in atmospheric moisture that agree with variations in precipitation. The message from these and other analyses performed by Hurrell is that the NAO is largely responsible for the unusually warm and wet winters observed across much of Europe over the past 15 years.
What might be causing these unusually intense positive phases of the NAO? Hurrell notes that the large, warm wintertime anomalies over Europe resemble some results obtained by computer models when they couple atmospheric and oceanic processes -- and incorporate steadily increasing greenhouse gases. However, the models do not tend to project the cold anomalies observed over the North Atlantic. One reason may be the absence of sulfate aerosols in the models.
To answer this question, NCAR's David Erickson has put the effects of sulfate aerosols into a global climate model. With researchers Robert Oglesby (Purdue University) and Susan Marshall (University of North Carolina at Charlotte), Erickson used a function of cloud reflectivity that varies with industrial sulfate emissions.
Sulfate emissions are mainly confined to the industrial centers of Europe, China, and eastern North America. Cooling might be expected over these regions due to the increased reflection of sunlight by the sulfate aerosols, say scientists. However, Erickson and colleagues found otherwise. The model's results are remarkably similar to the NAO pattern analyzed by Hurrell: wintertime warming over Europe and cooling across the northwest Atlantic and eastern United States.
"The model results suggest that it is possible that the way the North American continent responds to sulfates is fundamentally different than how Europe's climate evolves under the same conditions," says Erickson. He notes that the influences of greenhouse gases and sulfates may be producing a persistent change in the hemispheric "wave train" of storm systems.
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