Abstract

Large and continuing reductions in southern Australian winter rainfall are shown to be associated with major shifts in the Southern Hemisphere circulation since the mid-1970s. In particular, these changes have been linked to a reduction in winter storm formation with the growth rate of the leading storm track modes affecting southern Australia being more than 30% lower during the last three decades compared to the period between 1949 and 1968. These effects have become more pronounced with time. In this paper, we focus on the changes in southern Australian winter rainfall and relate them to circulation changes that are directly associated with storm formation. We employ a useful diagnostic of storm development which is the vertical shear in the atmospheric winds, commonly known as baroclinic instability, and is encapsulated in the Phillips (1954) criterion. The relationship between changes in the Phillips criterion and changes in rainfall during the twentieth century is discussed. We also consider projected changes and trends in rainfall and baroclinic instability in SRES scenarios using results from CMIP3 models. We elucidate the roles of anthropogenic forcing and internal variability. Our results show that the impact of further increases in anthropogenic CO₂ concentrations can lead to further large reductions in baroclinic instability, with model trends during the 21st century similar to those simulated during the second half of the 20th century. Associated reductions in modelled southern Australian rainfall can be as much as twice those seen at the end of the 20th century.