The dominant pattern of atmospheric circulation variability over the North Atlantic is known as the North Atlantic Oscillation (NAO), and its wintertime index is shown in Figure 7.3.2a (updated from Hurrell, 1995a). The NAO, which is linked to changes in the surface westerlies across the Atlantic onto Europe, refers to a meridional oscillation in atmospheric mass with centres of action near Iceland and over the subtropical Atlantic from the Azores across the Iberian Peninsula. Positive values of the NAO index indicate stronger than average westerlies over the middle latitudes with low-pressure anomalies in the Icelandic region and high-pressure anomalies across the subtropical Atlantic. This phase of the oscillation is also associated with cold winters over the northwest Atlantic and warm winters over Europe, as well as wet conditions from Iceland through Scandinavia and winter dry conditions over southern Europe (Hurrell, 1995a).
A sharp reversal is evident in the index over the past 30 years and, since 1980, the NAO has tended to remain in a highly positive phase with associated negative SLP anomalies over the subpolar North Atlantic and most of the Arctic and positive SLP anomalies over the subtropical Atlantic (Fig. 7.3.1a). The most pronounced anomalies have occurred since the winter of 1988-89 (Hurrell, 1995a; Walsh et al, 1996; Thompson and Wallace, 1998; Watanabe and Nitta, 1999) when the index registered its most positive values in the instrumental record (>170 years). While the signature of the NAO is strongest during winter, it is the dominant mode of atmospheric behaviour throughout the year (Barnston and Livezey, 1987; Rogers, 1990).
Thompson and Wallace (1998, 2000) and Thompson et al. (2000) have recently drawn attention to the fact that the NAO may be the regional manifestation of a hemispheric mode of variability characterized by exchanges of atmospheric mass between the polar cap and the middle latitudes, as shown by the leading pattern of variability in the hemispheric pressure field. They name this mode
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