North Atlantic oscillation

The North Atlantic oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea-level between the Icelandic Low and the Azores high. Through east-west oscillation motions of the Icelandic Low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is highly correlated with the Arctic oscillation, as it is a part of it.

The NAO was discovered in the 1920s by Sir Gilbert Walker. Unlike the El Niño phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.

The North Atlantic Oscillation is closely related to the Arctic oscillation but should not be confused with the Atlantic Multidecadal Oscillation.

Description

Yearly NAO indexes

Westerly winds blowing across the Atlantic bring 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 Iceland (the Icelandic Low) and a permanent high-pressure system over the Azores (the Azores High) 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 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 and wet winters in Central Europe and its Atlantic façade. In contrast, if the index is low (NAO-), westerlies are suppressed, these areas suffer cold winters and storms track southerly toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.

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

Although having a less direct influence than for Western Europe, the NAO is also believed to have an impact on the weather over much of eastern North America. During the winter, when the index is high (NAO+), the Icelandic low draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward. In combination with the El Niño, this effect can produce significantly warmer winters over much of the United States and southern Canada.

Ecological Effects

The NAO has been in an overall more positive regime since the late 1970s, bringing colder conditions to the North-West Atlantic, which has been linked with the thriving populations of Labrador Sea snow crabs, which have a low temperature optimum^[1]

The NAO+ warming of the North Sea reduces survival of cod larvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits.^[1] Though not the critical factor, the NAO+ peak in the early 1990s may have contributed to the collapse of the Newfoundland cod fishery.^[1]

On the East coast of the U.S.A. the NAO+ is causing warmer temperatures and increased rainfall, and thus warmer, less saline surface water. This prevents nutrient-rich upwelling which has reduced productivity. Georges Bank and the Gulf of Maine are affected by this reduced cod catch.^[1]

The strength of the NAO is also a determinant in the population fluctuations of the intensively studied Soay sheep.^[2]

Related material

• Anticyclone
• El Niño / Southern Oscillation Index
• Global warming
• Pacific decadal oscillation
• North Atlantic Current

External links

• Current NAO observations and forecasts
• UK's climatic research unit information sheet on the NAO
• Overview paper on the NAO from the USA's National Center for Atmospheric Research
• The North Atlantic Oscillation by Martin Visbeck
• Overview of Climate Indices

References