A reply

Climate variability in the Southern Hemisphere is receiving increasing attention due to both the large greenhouse gas–forced response that many climate models exhibit in polar regions (Houghton et al. 2001), and observed recent trends in the southern annular mode (SAM) index (Thompson and Solomon 2002; Thompson et al. 2000). Hall and Visbeck (2002) described regional impacts of the SAM in a coarse-resolution coupled climate model. The overwhelming dominance of this mode in the interannual-to-interdecadal variability of the model’s atmosphere and ocean led us to the hypothesis that most of the interannual climate variability of the real world would also be associated with the SAM. This statement was contested in a recent commentary by White (2003), who claims that the Antarctic circumpolar wave (ACW) is the dominant mode of interannual climate variability in the Southern Hemisphere. He presented supporting evidence from an analysis of the sea level pressure anomaly data obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) atmospheric reanalysis. Modes of Southern Hemisphere atmospheric variability are often identified in terms of the first few empirical orthogonal functions (EOFs) of pressure or geopotential height data (Kidson 1988, 1999; Mo 2000; Renwick 2002). Most studies have used 500-hPa height anomalies, with fewer studies using pressure levels closer to the surface (850 hPa). However, the choice of level