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Climate Patterns in the Atmosphere

North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is usually defined through the regional sea-level pressure (SLP) field, although it is readily apparent in mid-tropospheric height fields. Its influence extends across much of the North Atlantic and well into Europe (Figure 3-1a). Like other patterns to be discussed here, it has a basically fixed spatial structure. The NAO' s amplitude and phase vary over a range of time scales from intraseasonal (van Loon and Rogers, 1978) to interdecadal (Wallace et al., 1992); the largest amplitudes typically occur in winter. Figure 3-1b shows more than 100 years of NAO variability.

The NAO is often indexed by the difference in SLP between Iceland, representing the strength of the Icelandic (or Newfoundland) climatological low, and the Azores or Lisbon, near the central ridge of the Azores high. Correlation of the NAO index with surface air temperature and sea surface temperature (SST) further reveals the extent of the atmospheric connection between the North Atlantic and the northern portion of Europe, and part of northern Asia (Hurrell and van Loon, 1996; Hurrell, 1995). Typically,

Figure 3-1
(a) Differences between sea-level pressures in high and low NAO-index years, showing the region of 
NAO influence. (b) Variation in the NAO (December-March) index since 1864; the heavy line represents
 a filtered version of the data. (Both figures from Hurrell, 1995; reprinted with permission of the American 
Association for the Advancement of Science.)