Interdecadal changes in southern hemisphere winter storm track modes

The interdecadal changes in southern hemisphere (SH) winter cyclogenesis have been studied using a global two-level 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 degrees 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 over SWWA in the period after 1975 is the reduction of the intensity of cyclogenesis and the southward deflection of some storms.