North Atlantic Oscillation

Definition of 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 . This is the common definition for North Atlantic Oscillation, other definitions can be discussed in the article

Fig. 1. The NAO index obtained by projecting the NAO loading pattern to the daily anomaly 500 millibar height field over 0-90°N. The NAO loading pattern has been chosen as the first mode of a Rotated Empirical Orthogonal Function (EOF) analysis using monthly mean 500 millibar height anomaly data from 1950 to 2000 over 0-90°N latitude .

Through fluctuations in the strength of the Icelandic low and the Azores high, the North Atlantic Oscillation controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic. A permanent low-pressure system over Iceland (the Icelandic Low) and a permanent high-pressure system over the Azores (the Azores High) control the direction and strength of westerly winds into Europe. A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, northern European areas suffer cold dry winters and storms track are directed southwards toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa .

The NAO index exhibits considerable interseasonal and interannual variability, and prolonged periods (several months) of both positive and negative phases of the pattern are common (Fig. 1). The wintertime NAO exhibits significant multi-decadal variability (Fig.2). This figure also shows that the winter temperature of the North Sea is correlated with the wintertime NAO. The atmospheric pressure anomaly typical for positive and negative NAO phases is shown in Fig. 3.

Fig. 2. Winter (December through March) index of the NAO based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland since 1864. The SLP values at each station were normalized by removing the long-term mean and by dividing by the long-term standard deviation. Both the long-term means and standard deviations are based on the period 1864-1983. Normalization is used to avoid the series being dominated by the greater variability of the northern station . Green inserted graph: North Sea winter temperature (after Alheit et al., 2005 ). The North Sea winter temperature correlates with the NAO index.

Fig. 3. Location of atmospheric high and low pressure zones during winter, typical for positive (left panel) and negative (right panel) NAO phases. The corresponding distribution of warm (yellow) and cold (blue) water masses over the North Atlantic is also indicated in the figure, together with the dominant storm tracks (black arrow). Adapted from (Wanner et al., 2001).