Interdecadal changes in southern hemisphere winter storm track modes

The interdecadal changes in southern hemisphere (SH) winter cyclogenesis have been studied using a global twolevel primitive equation instability-model with reanalysed observed July three-dimensional basic states for the periods 1949–1968 and 1975–1994. The early to mid-1970s were a time of quite dramatic reduction in the winter rainfall in the southwest of western Australia (SWWA). We find that the rainfall reduction is associated with a decrease in the vertical mean meridional temperature gradient and in the peak upper tropospheric jet-stream zonal winds near 30◦ south throughout most of the SH. These changes are reflected in the properties of the leading SH cyclogenesis modes: for 1975–1994 both the fastest growing mode, and on average the 10 leading SH cyclogenesis modes that cross Australia, have growth rates which are around 30% smaller than for the corresponding modes for 1949–1968. The sensitivity of our results, to the strengths of physical parametrizations and to the choice of basic states based on different data sets, is examined. Our results suggest that a primary cause of the rainfall reduction overSWWA in the period after 1975 is the reduction of the intensity of cyclogenesis and the southward deflection of some storms