The atmospheric circulation over the Northern Hemisphere has exhibited anomalous behaviour over the past several decades. In particular, the dominant patterns of atmospheric variability in the winter half-year have tended to be strongly biased to one phase. Thus SLP has been lower than average over the mid- and high latitudes of the North Atlantic Ocean, as well as over much of the Arctic, while it has been higher than average over the sub-tropical oceans, especially the Atlantic. Moreover, in the past thirty years, changes in these leading patterns of natural atmospheric variability appear to be unusual in the context of the observational record.
Figure 2.30: December to March North Atlantic Oscillation (NAO) indices, 1864 to 2000, and Arctic Oscillation (AO) indices, 1900 to 2000, updated from Hurrell (1995) and updated from Thompson and Wallace (2000) and Thompson et al. (2000b), respectively. The indices were normalised using the means and standard deviations from their common period, 1900 to 2000, smoothed twice using a 21-point binomial filter where indicated and then plotted according to the years of their Januarys.
The dominant pattern of atmospheric circulation variability over the North Atlantic is known as the NAO, and its wintertime index is shown in Figure 2.30 (updated from Hurrell, 1995). As discussed in the SAR, positive values of the NAO give stronger than average westerlies over the mid-latitudes of the Atlantic with low SLP anomalies in the Icelandic region and over much of the Arctic and high SLP anomalies across the sub-tropical Atlantic and into southern Europe. The positive, enhanced westerly, phase of the NAO is associated with cold winters over the north-west Atlantic and warm winters over Europe, Siberia and eastern Asia (Thompson and Wallace, 2001) as well as wet conditions from Iceland to Scandinavia and dry winters over southern Europe. A sharp reversal is evident in the NAO index starting around 1970 from a negative towards a positive phase. Since about 1985, the NAO has tended to remain in a strong positive phase, though with substantial interannual variability. Hurrell (1996) and Thompson et al. (2000a) showed that the recent upward trend in the NAO accounts for much of the regional surface winter half-year warming over northern Europe and Asia north of about 40°N over the past thirty years, as well as the cooling over the north-west Atlantic (see Section 18.104.22.168). Moreover, when circulation changes over the North Pacific are also considered, much of the pattern of the Northern Hemisphere winter half-year surface temperature changes since the mid-1970s can be explained. This can be associated with changes in the NAO, and in the PNA atmospheric pattern related to ENSO or the PDO (Graf et al., 1995; Wallace et al., 1995; Shabbar et al., 1997; Thompson and Wallace, 1998, 2000).
The changes in atmospheric circulation over the Atlantic are also connected with much of the observed pressure fall over the Arctic in recent years (Walsh et al., 1996). Other features related to the circulation changes include the strengthening of sub-polar westerlies from the surface of the North Atlantic up, in winter as high as the lower stratosphere (Thompson et al., 2000a) and pronounced regional changes in precipitation patterns (Hurrell, 1995; Dai et al., 1997b; Hurrell and van Loon 1997; Section 22.214.171.124). Associated precipitation increases have resulted in the notable advance of some Scandinavian glaciers (Hagen et al., 1995), while decreases to the south of about 50oN have contributed to the further retreat of Alpine glaciers (Frank, 1997; see also Section 126.96.36.199)
The NAO is regarded (largely) by some as the regional expression of a zonally symmetrical hemispheric mode of variability characterised by a seesaw of atmospheric mass between the polar cap and the mid-latitudes in both the Atlantic and Pacific Ocean basins (Thompson and Wallace, 1998, 2001). This mode has been named the AO (Figure 2.30). The time-series of the NAO and AO are quite similar: the correlation of monthly anomalies of station data SLP series of NAO and AO is about 0.7 (depending on their exact definitions and epochs) while seasonal variations shown in Figure 2.30 have even higher correlations. The NAO and AO can be viewed as manifestations of the same basic phenomenon (Wallace, 2000).
Changes and decadal fluctuations in sea-ice cover in the Labrador and Greenland Seas, as well as over the Arctic, appear well correlated with the NAO (Chapman and Walsh, 1993; Maslanik et al., 1996; McPhee et al., 1998; Mysak and Venegas, 1998; Parkinson et al., 1999; Deser et al., 2000). The relationship between the SLP and ice anomaly fields is consistent with the idea that atmospheric circulation anomalies force the sea-ice variations (Prisenberg et al., 1997). Feedbacks or other influences of winter ice anomalies on the atmosphere have been more difficult to detect, although Deser et al. (2000) suggest that a local response of the atmospheric circulation to the reduced sea-ice cover east of Greenland in recent years is also apparent (see also Section 188.8.131.52).
A number of studies have placed the recent positive values of the NAO into a longer-term perspective (Jones et al., 1997a; Appenzeller et al., 1998; Cook et al., 1998; Luterbacher et al., 1999; Osborn et al., 1999) back to the 1700s. The recent strength of the positive phase of the NAO seems unusual from these reconstructions but, as in Figure 2.28, these proxy data reconstructions may underestimate variability. An extended positive phase occurred in the early 20th century (Figure 2.30), particularly pronounced in January (Parker and Folland, 1988), comparable in length to the recent positive phase. Higher-frequency variability of the NAO also appears to have varied. Hurrell and van Loon (1997) showed that quasi-decadal (6 to 10 year) variability has become more pronounced over the latter half of the 20th century, while quasi-biennial variability dominated in the early instrumental record.
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