North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic low and the Azores high. Through fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic . It is part of the Arctic oscillation, and varies over time with no particular periodicity.

The NAO was discovered through several studies in the late 19th and early 20th centuries. Unlike the El Niño-Southern Oscillation 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 (AO) or Northern Annular Mode (NAM), but should not be confused with the Atlantic Multidecadal Oscillation (AMO).

The NAO has multiple possible definitions. The easiest to understand are those based on measuring the seasonal average air pressure difference between stations, such as:

These definitions all have in common the same northern point (because this is the only station in the region with a long record) in Iceland; and various southern points. All are attempting to capture the same pattern of variation, by choosing stations in the "eye" of the two stable pressure areas, the Azores high and the Icelandic low (shown in the graphic).

A more complex definition, only possible with more complete modern records generated by numerical weather prediction, is based on the principal empirical orthogonal function (EOF) of surface pressure. This definition has a high degree of correlation with the station-based definition. This then leads onto a debate as to whether the NAO is distinct from the AO/NAM, and if not, which of the two is to be considered the most physically based expression of atmospheric structure (as opposed to the one that most clearly falls out of mathematical expression).

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