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Natural Climate Variability on Decade-to-Century Time Scales
anomalies are a common low-frequency pattern in the SH, but that they do not have a strongly preferred phase. Mo and van Loon (1984) describe a change in the amplitude of wave-number-three on long time scales.
As well as the zonal wave-number-three pattern in winter, two versions of a zonal wave-number-four pattern were found in summer that are phase-shifted with respect to each other. Together these modes explain about 13 percent of the summer height variance. They both have centers of action around 45°S and show a continent-ocean contrast. MW identified a teleconnection pattern in the SH summer associated with the continent-ocean thermal contrast; this pattern appears to be related to this zonal wave-number-four pattern found in the upper troposphere.
A meridionally oriented standing-wave pattern resembling a wave train has been found in winter. The pattern for this winter wave train is shown in Figure 3a. Five centers of action can be seen extending from eastern Australia southeastward to high latitudes and equatorward into the southern Atlantic. This mode, which explains about 6 percent of the variance, is not clearly evident in the lower-level variables, where only parts of the wave-train structure are reproduced. Using low-pass filtered ECMWF analyses, however, K88 has identified a similar wave-train pattern of height variations.
The third principal component of 300 hPa height in winter explained a relatively large fraction of the variance, but was not stable; it displayed different structures when different analysis methods were used. The pattern for this mode, shown in Figure 3b, essentially represents monopole height variations over the Pacific Ocean, although there is some indication of opposite variations further equatorward and poleward, suggesting a meridional wave train. Although the feature over the central Pacific was relatively stable, the weaker features in this pattern were quite variable.
Although the above studies have described low-frequency variations in the SH, much less attention has been paid to the mechanisms associated with these observed variations. Karoly (1989a,b) and SK have shown that the interannual variations of the low-latitude mode in summer are are related to the Southern Oscillation and the associated variations of the temperature of the tropical troposphere. In winter, the Southern Oscillation is associated with a pattern of large-amplitude anomalies over the Pacific Ocean, as shown in Figure 3b. Hence the Southern Oscillation is associated with two of the dominant modes of interannual variability in the SH.
The other common interannual variations in the SH are linked to the high-latitude mode. K88, Karoly (1990), and Shiotani (1990) have shown that the zonally symmetric variations of the high-latitude mode are associated with
Rotated PC patterns for 300 hPa height in winter for the two meridional wave-train modes.
changes in the observed transient-eddy activity and momentum flux in the SH storm track. Trenberth (1984) showed that the large circulation anomalies during the SH winter of 1979, during the Global Weather Experiment, were related to mean flow and eddy variations associated with this high-latitude mode. A general-circulation modeling study of Zwiers (1987) found that the largest interannual variations of the model SH circulation were zonally symmetric opposite variations between high and middle latitudes, very similar to those of the observed high-latitude mode. Simplified