North Atlantic Oscillation

Known internationally by the acronym NAO (North Atlantic Oscillation) and identified for the first time, in 1924, by the British meteorologist Sir Gilbert Thomas Walker (1868-1958), it consists of a large-scale fluctuation in the air mass located between the Iceland region and the Azores region, both in the North Atlantic. This natural oscillation determines, to a large extent, the variability present in atmospheric circulation over Europe, mainly in winter. Its area of influence is not limited to the European continent, extending from North America to northern Asia, so it plays an important role in the behavior of climate throughout the Northern Hemisphere. The different sign and magnitude that the NAO can take on is calculated using a standardized numerical index (NAO index), defined as the difference in surface atmospheric pressure between two weather stations located, respectively, in the North Atlantic region where subtropical high pressure dominates and the low-pressure area of Iceland. Initially, the stations of Ponta Delgada, in the Azores islands (Portugal), and Stykkishólmur (Iceland) were chosen for its calculation. Over time, climatologists also began to use other reference stations, such as Reykjavik, Lisbon or Gibraltar. Calculating the NAO index for each year makes it possible to know whether the oscillation is positive (NAO+) or negative (NAO-), depending on whether its value is greater than +1 or less than –1, respectively. If the value falls between –1 and +1, we have neutral conditions. In the positive phase, the Azores High shows higher-than-usual pressure values, favoring blocking situations. In the negative phase, the aforementioned high is more weakened, allowing trains of low-pressure systems -with associated wind storms- to pass through lower latitudes.