9.5.3.3 Southern Annular Mode

The SAM is more zonally symmetric than its NH counterpart (Thompson and Wallace, 2000; Section 3.6.5). It too has exhibited a pronounced upward trend over the past 30 years, corresponding to a decrease in surface pressure over the Antarctic and an increase over the southern mid-latitudes (Figure 9.16), although the mean SAM index since 2000 has been below the mean in the late 1990s, but above the long term mean (Figure 3.32). An upward trend in the SAM has occurred in all seasons, but the largest trend has been observed during the southern summer (Thompson et al., 2000; Marshall, 2003). Marshall et al. (2004) show that observed trends in the SAM are not consistent with simulated internal variability in HadCM3, suggesting an external cause. On the other hand, Jones and Widmann (2004) develop a 95-year reconstruction of the summer SAM index based largely on mid-latitude pressure measurements, and find that their reconstructed SAM index was as high in the early 1960s as in the late 1990s. However, a more reliable reconstruction from 1958, using more Antarctic data and a different method, indicates that the summer SAM index was higher at the end of the 1990s than at any other time in the observed record (Marshall et al., 2004).

Based on an analysis of the structure and seasonality of the observed trends in SH circulation, Thompson and Solomon (2002) suggest that they have been largely induced by stratospheric ozone depletion. Several modelling studies simulate an upward trend in the SAM in response to stratospheric ozone depletion (Sexton, 2001; Gillett and Thompson, 2003; Marshall et al., 2004; Shindell and Schmidt, 2004; Arblaster and Meehl, 2006; Miller et al., 2006), particularly in the southern summer. Stratospheric ozone depletion cools and strengthens the antarctic stratospheric vortex in spring, and observations and models indicate that this strengthening of the stratospheric westerlies can be communicated downwards into the troposphere (Thompson and Solomon, 2002; Gillett and Thompson, 2003). While ozone depletion may be the dominant cause of the trends, other studies have indicated that greenhouse gas increases have also likely contributed (Fyfe et al., 1999; Kushner et al., 2001; Stone et al., 2001; Cai et al., 2003; Marshall et al., 2004; Shindell and Schmidt, 2004; Stone and Fyfe, 2005; Arblaster and Meehl, 2006). During the southern summer, the trend in the SAM has been associated with the observed increase of about 3 m s^–1 in the circumpolar westerly winds over the Southern Ocean. This circulation change is estimated to explain most of the summer surface cooling over the Antarctic Plateau, and about one-third to one-half of the warming of the Antarctic Peninsula (Thompson and Solomon, 2002; Carril et al., 2005; Section 3.6.5), with the largest influence on the eastern side of the Peninsula (Marshall et al., 2006), although other factors are also likely to have contributed to this warming (Vaughan et al., 2001).