During the positive phase of the North Atlantic Oscillation (NAO), a large pressure gradient across the North Atlantic creates strong winds that drive winter storms across the Atlantic and into Northern Europe. During the negative phase, there is only a small pressure gradient. Southern Europe and Africa receive weak winter storms while Northern Europe and the eastern United States are cold and dry.
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Courtesy of UCAR

The North Atlantic Oscillation

Will there be lots of snow in New York this winter? Will it be warmer than usual in France? Will the Mediterranean get rainy weather? That will depend on the North Atlantic Oscillation.

Winter weather in the Northern Hemisphere is affected by changes in atmospheric pressure over the North Atlantic called the North Atlantic Oscillation. During the winter, a low pressure system over Iceland and a high pressure system over the Atlantic become stronger or weaker. The changes in pressure cause changes in the amount of wind and the number of winter storms that cross the Atlantic. This affects the weather in North America, Europe, and North Africa.

When the difference in pressure between the high and low is large, strong winds cross the North Atlantic, bringing wet winter storms from eastern North America to northern Europe. This is the positive phase of the North Atlantic Oscillation.

When the difference in pressure is small, there is less wind over the ocean. Eastern North America and northern Europe have fewer winter storms. The weather is rainy in southern Europe and North Africa. This is the negative phase of the North Atlantic Oscillation.

There is evidence of more negative phases during the 1960s and 1970s and more positive phases during the 1980s and 1990s. Scientists are using computer models to figure out whether this change is due to global warming .

The changes in atmospheric pressure associated with the NAO are connected with other patterns of atmospheric pressure including the Arctic Oscillation and the El Nino-Southern Oscillation.

Last modified September 18, 2008 by Lisa Gardiner.

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