Indices of the Southern Hemisphere Zonal Wind

The time and spatial variation of the Southern Hemisphere 500 hPa zonal wind has been examined using spectral analysis, bandpass filtering, and empirical orthogonal function (EOF) analysis applied to a 15-year sample of Australian hemispheric analyses. Spectral analysis of the entire 15-year record of the 500 hPa zonal mean westerly wind shows significant peaks only on the seasonal and interannual time scales. The mean segmented spectra for each season failed to show any additional significant peaks at higher frequencies or any significant differences between seasons. The primary pattern of variation revealed by EOF analysis shows similar behavior on synoptic to seasonal time scales, with compensating departures occurring at intervals of 20° latitude. The first EOF has opposing peaks near 40° and 60°S and shows an equivalent barotropic pattern when correlated with individual grid point values at 500 hPa and mean sea level pressure. Correlations of the zonal pattern with regional anomalies reveal that at midlatitudes, the activity is concentrated in the Indian and western Pacific Oceans while the pattern is more zonally symmetric south of 60°S. The second EOF has opposing peaks at 30° and 50°S with differing correlation patterns at 500 hPa and mean sea level. The variance of the daily 500 hPa zonal wind anomalies is greatest for three “seasons” centered on April, August and December. Small but significant differences are observed in the corresponding hemispheric patterns for each season, and the July–September pattern has some feature related to those obtained in an earlier study for the Southern Hemisphere winter of 1979. Indices obtained from differencing the zonal wind anomalies at 40° and 60°S and at 30° and 50°S should be useful in characterizing variations in the Southern Hemisphere zonal circulation.