DECADAL VARIATIONS OF THE SOUTHERN-HEMISPHERE CIRCULATION

A data set of monthly mean surface and sea-level pressure observations from a number of stations in the Southern Hemisphere (SH) for the per iod since 1901 has been used to investigate interannual and interdecad al variations of the SH circulation. A fairly uniform network of 62 st ations was selected with data for the period 1955-1985, with a reduced network of 35 stations having data for the longer period 1901-1985. T he sparse network has stations mainly on the SH tropical and mid-latit ude land masses, with few islands or high-latitude stations. A rotated principal component analysis of the annual mean station pressure anom alies has been used to describe the dominant modes of interannual and interdecadal variations of the SH circulation that can be resolved wit h the available station network. We show that the sparse station netwo rk is able to capture the dominant modes of variability found in the p eriod 1955-1985 using the denser network. The leading mode is associat ed with the El Nino-Southern Oscillation and is well resolved by the s parse network. This is the leading mode for interannual and interdecad al variations throughout the last century, but there are periods when it has reduced variance and a somewhat different structure, notably 19 16-1935. Other modes of interannual variations show marked changes in importance on decadal or longer time-scales.