5.2.2 The North Atlantic Oscillation

The large-scale atmospheric circulation in the North Atlantic at mid-latitudes is characterised by westerlies driven by the sea-level pressure difference between the Azores high and the Icelandic low (Fig. 1.5). As described in section 5.2.1 for the Walker circulation, there are irregular changes in the intensity and location of the maximum of these westerlies. This is associated with a North-South oscillation of the pressure, and thus of the atmospheric mass (Fig. 5.6), known as the North Atlantic Oscillation (NAO). The intensity of this mode of variability is measured by the normalised SLP difference between meteorological stations in the Azores and in Iceland:

where SLP'_Azores and SLP'_Iceland are the SLP anomalies in the Azores and Iceland and ${\sigma }_{\mathrm{Azores}}$ and ${\sigma }_{\mathrm{Iceland}}$ the standard deviations of these anomalies. Because of the longer time series available, the station in the Azores is sometimes replaced by one in Portugal. When the NAO index is high, the westerlies are stronger than average while they are weaker than the mean when the NAO index is negative.

The NAO can be observed in all seasons but its amplitude is greater in winter when the atmosphere is more dynamically active. When the winter NAO index is positive, the stronger westerly winds transport warm and moist oceanic air towards Europe. This leads to warming and increased precipitation at mid and high latitudes in Europe as well as in large parts of northern Asia, the Greenland Sea and the Barents Sea (Fig. 5.7). In the Barents Sea, the warming is associated with a decrease of the sea ice extent.

By contrast, the anomalous circulation when the NAO index is high brings cold air to the Labrador Sea inducing a cooling (Fig. 5.7) and an increase in the sea ice extent there. Further southward, the stronger flow around the subtropical high leads to a drop of temperature over Turkey and North Africa and a rise in the eastern US.

A tripole is associated with positive NAO index over the Atlantic Ocean: the temperature anomaly is positive around 30^o-40^oN while it is negative southward and northward of this latitude band (Fig. 5.7). The dominant cause of this pattern appears to be the air-sea interactions. Indeed, the SSTs tend to be lower in areas where the wind speed is higher, leading to higher evaporation rates and heat losses from the ocean to the atmosphere.

In contrast to the ENSO, which is a coupled ocean-atmosphere mode, the NAO appears to be mainly an intrinsic mode of variability of the atmosphere. It has been found in many types of atmospheric models, whether or not they are coupled to an oceanic layer. The mechanisms governing its existence are related to interactions between the mean and the transient circulation (in particular transient cyclones and anticyclones). However, its amplitude can also be influenced by, for instance, changes in sea-surface temperatures and by the external forcing (see section 5.5).

Although interesting in its own right, the NAO is sometimes considered as a regional manifestation of a larger scale oscillation of the pressure between subtropical areas and high latitudes. As this nearly-hemispherical mode shows a high degree of zonal symmetry, it is referred to as the Northern Annular Mode (NAM, but is also called sometimes the Arctic Oscillation). Like the NAO (with which it is highly correlated), the NAM is associated with changes in the intensity of the westerlies at mid-latitudes.