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,