Observed Variability of the Southern Hemisphere Atmospheric Circulation

DAVID J. KAROLY1

ABSTRACT

The dominant modes of interannual variation of the Southern Hemisphere (SH) circulation are described, using results from several studies of grid-point analyses of the SH troposphere. The two leading modes are primarily zonally symmetric, representing out-of-phase variations of height between middle and high latitudes in one case (called the high-latitude mode) and between the tropics and middle latitudes in the other (called the low-latitude mode). Wave-train patterns of anomalies are described also. These include a wave-number-three mode in winter with large amplitude at high latitudes, a wave-number-four mode in middle latitudes in summer, a meridional wave-train structure originating over Australia, and a monopole or a meridional wave train over the Pacific Ocean in winter. The two most important processes leading to interannual variations of the SH circulation appear to be the Southern Oscillation and the interaction between waves and mean flow in the SH storm track, which leads to the High Latitude mode.

Since upper-air data and grid-point analyses are available for only a short period, surface data must be used to describe decadal variability of the SH circulation. The barotropic vertical structure of low-frequency variations in the SH means that surface pressure variations are representative of variations throughout the troposphere. Relatively little attention has been paid to decadal variability in the SH. It appears from indices of the two dominant modes of interannual variations, determined using the surface data, that the modes show substantial decadal variability over the last 90 years. Few observational data exist that can be used to directly indicate climate variability on century time scales in the SH. The only possible observational evidence for this comes from proxy data, such as tree rings.

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Cooperative Research Centre for Southern Hemisphere Meteorology, Monash University, Clayton, Victoria, Australia



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