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.