Australian winter circulation and rainfall changes and projections

Purpose - Major shifts in the southern hemisphere circulation occurred during the mid-1970s concurrent with large reductions in southern Australian winter rainfall associated with decreased cyclogenesis. The purpose of this paper is to study changes in cyclogenesis over southern Australia during the last 50 years, examine the ability of coupled climate models to simulate the winter circulation changes, and discuss projected changes in winter circulation and rainfall under different climate change scenarios. Design/methodology/approach. - Three dimensional instability theory is used to study changes in cyclogenesis. The response of 22 coupled model intercomparison project three (CMIP3) IPCC climate models to observed increases in greenhouse gases, from pre-industrial to the end of the twentieth century, is examined. The authors focus on two diagnostics closely related to the changes in cyclogenesis: changes in the 300 hPa zonal wind strength; and changes in the baroclinic instability. Projected changes in baroclinic instability and rainfall are investigated in SRESB1, SRESA1B and SRESA2 scenarios. Findings - There has been a 30 per cent reduction in the growth rate of the leading storm track mode crossing southern Australia in the 1975-1994 period, and a 37 per cent reduction in the 1997-2006 period, when compared to 1949-1968. Most of the CMIP3 models capture the changes in the zonal wind; only about a third the changes in baroclinic instability. Projected changes in baroclinic instability and rainfall suggest further reductions in the growth rate of storm track modes and further large reductions in rainfall over southern Australia. Originality/value - The paper addresses a major cause of the reduction in Australian winter rainfall, and provides guidance on future-projected changes.