North Atlantic Oscillation (NAO)

The north atlantic oscillation (nao) refers to a large-scale oscillation of atmospheric mass observed across the North Atlantic Ocean. The NAO was first identified in the 1920s by Sir Gilbert Walker. This oscillation is between the two prominent North Atlantic pressure centers: the Azores high, centered in the North Atlantic, and the Icelandic Low, centered between Greenland and Iceland.

It is now firmly established that fluctuations in the NAO influence weather and climate variations around the world, especially in the North Atlantic region. The NAO is a major controlling factor in basic meteorological variables such as surface wind, temperature, and precipitation, which have large socioeconomic impacts on energy, agriculture, industry, and human health across Europe and eastern North America. Changes in the NAO also significantly impact North Atlantic ecosystems through its influence on oceanic convection, deep-water formation, food availability, plant phenology, and the distribution patterns of fish and shellfish. Although this pattern is evident throughout the year, it is most pronounced during the winter months.

It is difficult to assess whether positive NAO index values since 1980 reflect human influence or random variations.

The phase of the NAO can be determined using a simple index, defined as the difference in sea level pressure between Lisbon, Portugal, and Stykkisholmur, Iceland. Instrumental station pressure, temperature, and precipitation measurements along with paleoclimate indicators have been used to construct a monthly times series of the NAO index dating back several hundred years. Depending on the direction of the pressure intensification, a NAO event is considered either positive or negative. Positive values of the index indicate a stronger-than-average Icelandic Low and Azores High, while negative index values signify weaker-thanaverage pressure centers.

Positive values of the NAO index are associated with stronger-than-average westerly winds across the middle latitudes of the Atlantic onto Europe, with anomalous southerly flow over eastern North America and anomalous northerly flow across western Greenland, the Canadian Arctic, and the Mediterranean. The positive phase of the NAO reflects belownormal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States, and Western Europe. During times of a high NAO index, the axis of maximum moisture transport shifts to a more southwest-to-northeast orientation across the Atlantic and extends much [Page 1264]farther to the north and east onto northern Europe and Scandinavia.

Strong positive phases of the NAO tend to be associated with above-average temperatures in the eastern United States and across northern Europe, while northern Canada, Greenland, and oftentimes southern Europe and the Middle East experience below-average temperatures. Storm systems travel on a more northerly track across the North Atlantic, resulting in drier-than-normal conditions over much of central and southern Europe, the northern Mediterranean countries, and west North Africa. At the same time stronger westerlies result in wetter-than-normal conditions from Iceland to Scandinavia. Wetter winters in the southeastern United States and more frequent cold, dry air outbreaks from the Labrador Sea have also been found to be associated with the positive phase of the NAO.

When the NAO index is negative the westerly winds are weaker than normal, resulting in fewer and weaker winter storms crossing the North Atlantic on a more west-east pathway. North Atlantic sea surface temperature anomalies are above average while steering winds shift southward during low NAO index phases. More frequent and longer-lasting atmospheric blocking patterns across the North Atlantic are associated with low NAO index values.

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