Richard Melson

September 2006


North Atlantic Oscillation

The North Atlantic oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between Iceland and the Azores.

It controls the strength and direction of westerly winds and storm tracks across the North Atlantic.

It is related to the Arctic oscillation.

The NAO was discovered in the 1920s by Sir Gilbert Walker. Similar to the El Niņo phenomenon in the Pacific Ocean, the NAO is one of the most important drivers of climate fluctuations in the North Atlantic and surrounding continents.


Westerly winds blowing across the Atlantic, bring warm, moist air into Europe. In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heatwaves, deep freezes and reduced rainfall. A permanent low-pressure system over Greenland (the sub-arctic Low) and a permanent high-pressure system over the Azores control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the North Atlantic Oscillation (NAO). A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild & wet winters in Europe. In contrast, if the index is low (NAO-), westerlies are suppressed, European winters are cold and storms track southerly toward the Mediterranean Sea.

Especially during the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region; wind speed and wind direction changes, changes in temperature and moisture distribution and the intensity, number and track of storms.


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September 12, 2006