High quality historical temperature and rainfall data sets have been used to produce time series of annual rainfall and temperature, averaged over Australia. The relationships between these series, and with the Southern Oscillation Index, have been examined. A change in the relationships between the variables appeared in the early 1970s. Since then, for any value of the SOI or rainfall, maximum temperature has tended to be higher than previously. Likewise, rainfall, for any value of the SOI, has tended to be greater than would have been expected for such an SOI value in previous years. Artificial changes, such as changes in instrumentation, seem unlikely to account for these observed changes in relationships. Model experiments have duplicated some of the changes in the relationships. Increased Indian Ocean temperatures may be the causal factor underlying these changes.

A B S T R A C TThe 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 over SWWA in the period after 1975 is the reduction of the intensity of cyclogenesis and the southward deflection of some storms.

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