Winter Weather and the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States. In Atlanta, for example, there are on average five times as many days each year when trace snow falls occur during negative phases, versus positive phases. Over the last thirty years, the index has been predominately positive. The index is currently positive, has been mostly positive this winter.

(Sources: Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293, 85 (2001) and National Oceanic and Atmospheric Administration: Climate Prediction Center. North Atlantic Oscillation. Accessed Online 19 February 2008 )