The decade of the 1990s over the Atlantic in comparison with longer instrumental and palaeoclimatic records

The North Atlantic Oscillation (NAO) is the principal mode of variability in surface pressure over the North Atlantic/European sector of the Northern Hemisphere (NH). The mode is particularly dominant during the winter season, explaining part of the surface temperature and precipitation variability over the region. Recent winter values of the NAO during the late 1980s and 1990s have been strongly positive, giving Northern Europe a succession of mild winters. Long instrumental records and palaeoclimatic reconstructions of the NAO indicate several earlier periods of comparable values over the past 500 years. The rise in NAO values from the 1960s to the 1990s. however, does appear unique in the long records. Although the NAO is capable of explaining some of the variability of surface temperature change during the 1990s in the winter season, it cannot explain the warming of the other seasons as the NAO influence is weak and the NAO has been less anomalous in these seasons.