3.6.5 The Southern Hemisphere and Southern Annular Mode
The principal mode of variability of the atmospheric circulation in the SH extratropics is now known as the SAM (see Figure 3.32). It is essentially a zonally symmetric structure, but with a zonal wave three pattern superimposed. It is associated with synchronous pressure or height anomalies of opposite sign in mid- and high-latitudes, and therefore reflects changes in the main belt of subpolar westerly winds. Enhanced Southern Ocean westerlies occur in the positive phase of the SAM. The SAM contributes a significant proportion of SH mid-latitude circulation variability on many time scales (Hartmann and Lo, 1998; Kidson, 1999; Thompson and Wallace, 2000; Baldwin, 2001). Trenberth et al. (2005b) showed that the SAM is the leading mode in an EOF analysis of monthly mean global atmospheric mass, accounting for around 10% of total global variance. As with the NAM, the structure and variability of the SAM results mainly from the internal dynamics of the atmosphere and the SAM is an expression of storm track and jet stream variability (e.g., Hartmann and Lo, 1998; Limpasuvan and Hartmann, 2000; Box 3.3). Poleward eddy momentum fluxes interact with the zonal mean flow to sustain latitudinal displacements of the mid-latitude westerlies (Limpasuvan and Hartmann, 2000; Rashid and Simmonds, 2004, 2005).
Gridded reanalysis data sets have been utilised to derive time series of the SAM, particularly the NRA (e.g., Gong and Wang, 1999; Thompson et al., 2000) and more recently ERA-40 (Renwick, 2004; Trenberth et al., 2005b). However, a declining positive bias in pressure at high southern latitudes in both reanalyses before 1979 (Hines et al., 2000; Trenberth and Smith, 2005) means that derived trends in the SAM are too large. Marshall (2003) produced a SAM index based on appropriately located station observations. His index reveals a general increase in the SAM index beginning in the 1960s (Figure 3.32) consistent with a strengthening of the circum-polar vortex and intensification of the circumpolar westerlies, as observed in northern Antarctic Peninsula radiosonde data (Marshall, 2002).
The observed SAM trend has been related to stratospheric ozone depletion (Sexton, 2001; Thompson and Solomon, 2002; Gillett and Thompson, 2003) and to greenhouse gas increases (Hartmann et al., 2000; Marshall et al., 2004; see also Section 126.96.36.199). Jones and Widmann (2004) reconstructed century-scale records based on proxies of the SAM that indicate that the magnitude of the recent trend may not be unprecedented, even during the 20th century. There is also recent evidence that ENSO variability can influence the SAM in the southern summer (e.g., L’Heureux and Thompson, 2006).
The trend in the SAM, which is statistically significant annually and in summer and autumn (Marshall et al., 2004), has contributed to antarctic temperature trends (Kwok and Comiso, 2002b; Thompson and Solomon, 2002; van den Broeke and van Lipzig, 2003; Schneider et al., 2004); specifically a strong summer warming in the Peninsula region and little change or cooling over much of the rest of the continent (Turner et al., 2005; see Figure 3.32). Through the wave component, the positive SAM is associated with low pressure west of the Peninsula (e.g., Lefebvre et al., 2004) leading to increased poleward flow, warming and reduced sea ice in the region (Liu et al., 2004b). Orr et al. (2004) proposed that this scenario yields a higher frequency of warmer maritime air masses passing over the Peninsula, leading to the marked northeast Peninsula warming observed in summer and autumn (December–May). The positive trend in the SAM has led to more cyclones in the circumpolar trough (Sinclair et al., 1997) and hence a greater contribution to antarctic precipitation from these near-coastal systems that is reflected in δ18O levels in the snow (Noone and Simmonds, 2002). The SAM also affects spatial patterns of precipitation variability in Antarctica (Genthon et al., 2003) and southern South America (Silvestri and Vera, 2003).
The imprint of SAM variability on the Southern Ocean system is observed as a coherent sea level response around Antarctica (Aoki, 2002; Hughes et al., 2003) and by its regulation of Antarctic Circumpolar Current flow through the Drake Passage (Meredith et al., 2004). Changes in oceanic circulation directly alter the THC (Oke and England, 2004) and may explain recent patterns of observed temperature change at SH high latitudes described by Gille (2002).
Figure 3.32. (Bottom) Seasonal values of the SAM index calculated from station data (updated from Marshall, 2003). The smooth black curve shows decadal variations (see Appendix 3.A). (Top) The SAM geopotential height pattern as a regression based on the SAM time series for seasonal anomalies at 850 hPa (see also Thompson and Wallace, 2000). (Middle) The regression of changes in surface temperature (°C) over the 23-year period (1982 to 2004) corresponding to a unit change in the SAM index, plotted south of 60°S. Values exceeding about 0.4°C in magnitude are significant at the 1% significance level (adapted from Kwok and Comiso, 2002b).