The North Atlantic Oscillation or NAO is an atmospheric circulation pattern located in the North Atlantic Ocean and characterized by the cyclical fluctuation of the sea level pressure difference between Iceland and the Azores.
Through the east-west oscillation motion of the depression of Iceland and of the anticyclone of the Azores, it determines the strength and direction of the western zonal flow and the direction of perturbations along the northern Atlantic.
Unlike El Niño, the NAO is a strictly atmospheric mode of variability and is one of the most important manifestations of weather-climate fluctuation in the North Atlantic. Discovered in the 1920s by Sir Gilbert Thomas Walker, it is strongly correlated with the Arctic Oscillation; the NAO should not be confused with the AMO, the multidecadal oscillation of the surface waters (SST) of the central-northern Atlantic, which is instead a strictly oceanic teleconnection moreover with a different oscillation period.
A semi-permanent low pressure system over Iceland (Iceland Depression) and a semi-permanent high pressure center over the Azores (Azores anticyclone) control the direction and strength of westerly winds over Europe. Relative strength and position of these two systems vary from year to year and this variation is known as the NAO.
Referring to an average atmospheric circulation situation, a large pressure difference between the points described (High NAO, NAO+ phase) leads to an increase in westerly winds and consequently to cool summers and mild and rainy winters in Central Europe and along the Atlantic coast, contrasted with low rainfall in the Mediterranean regions.
On the contrary, if the index is low (Low NAO, NAO- phase) the westerly winds are reduced, the circulation is mainly anti-zonal and these areas suffer from cold winters with the trajectory of the perturbations moving southwards or towards the Mediterranean.
This leads to increased disturbed activity, and with it rainfall, in Southern Europe and North Africa. The period of the oscillation is not regular: the NAO index can weaken and invert within 10-20 days and also with a seasonal cadence throughout the year thus showing a certain meteorological variability.
However, non-regular multi-year average fluctuations of a climatic nature are highlighted, as shown above in the figure. The oscillation of the NAO does not seem to be accurately predictable either by seasonal climate models or by global climate models, evidently due to the chaotic nature of the atmospheric system, while meteorological models are able to highlight its dynamics quite correctly up to a maximum of 15 days.