Tiempo Climate Portal

North Atlantic Oscillation

The North Atlantic Oscillation is a major disturbance of the atmospheric circulation and climate of the North Atlantic-European region, linked to a waxing and waning of the dominant middle-latitude westerly wind flow during winter.

The North Atlantic Oscillation Index is based on the pressure difference between various stations to the north (Iceland) and south (Azores) of the middle latitude westerly flow. It is, therefore, a measure of the strength of these winds. Strictly, it should only be used for the winter period.

Discussion

When the North Atlantic Oscillation Index is positive, the westerly flow across the North Atlantic and western Europe is enhanced, along with the Iceland Low and the Azores High. During the winter half-year, the strengthened westerly winds bring warmer, maritime air over northwest Europe causing a rise in temperature. At the same time, enhanced northerly flow over the northeast Atlantic results in a drop in temperatures over the area.

When the Index is low, the opposite occurs with temperatures falling over northwest Europe and rising over northwest Atlantic.

The net result is a "sea-saw" in temperatures across the North Atlantic-European sector, as well as changes in other climate variables such as precipitation and sea-ice extent.

The North Atlantic Oscillation exerts a strong influence on year-to-year climate variability and there is evidence of longer-term trends in this phenomenon. It is related to the shorter-term shift between zonal and meridional circulation types that occurs on the day-to-day timescale and is known as the "index cycle".

Data availability

The North Atlantic Oscillation Index can be downloaded for the period since 1821 and a regular monthly update is available.

On the Web

An information sheet on the North Atlantic Oscillation is available.

Acknowledgements

The North Atlantic Oscillation Index is maintained by Phil Jones, Climatic Research Unit, University of East Anglia.

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