Prolonged Droughts over Southern Australia: Causes and Future Projections

Since the early to mid-1970s, there has been a dramatic and continuing reduction in rainfall over southern Australia, especially during May to October. These changes were first noticed over the southwest of Western Australia where the inflow into Perth dams has steadily reduced from a long term mean of 338 gigalitres to an average of 64 gigalitres (2006 - 2012). Since about 1997 there has also been an ongoing downward trend in rainfall over southeastern Australia including over the Murray-Darling Basin, a major food growing area.

During this period, we show that large changes in the seasonal cycle of the Southern Hemisphere circulation have impacted on the properties of weather systems associated with mid-latitude storms and consequently on reductions in rainfall particularly in the southern Australian region. In particular, there have been significant negative trends in the baroclinic instability of the mid-latitude atmospheric circulation resulting in a reduction in storm formation at these latitudes, while increases in baroclinicity further poleward has led to increased storm development. These effects have become more pronounced with time.

Here, we consider the observed changes in the baroclinicity of the Southern Hemisphere circulation in all months and examine the corresponding changes in the growth rate and structure of the dynamical modes of variability with emphasis on the mid-latitude storm track modes, in the four seasons. We focus on the periods 1949-1968, 1975-1994 and 1997-2006, to highlight the inter-decadal changes in these modes. We relate the continuing reduction in the rainfall in the southwest of Western Australia since the mid-1970s and in southeastern Australia since the mid-1990s to changes in the growth rate and structures of the leading storm track modes. These changes in the stormtracks are also shown to have an impact on rainfall over the entire hemisphere.

Future projections using CMIP3 and CMIP5 models under different climate change scenarios show similar trend patterns in baroclinicity and associated reductions in southern Australian rainfall, and hemispheric reductions in rainfall in a zonal band between about 20S-40S and increases further south. Our results suggest that these trends are related to increasing anthropogenic greenhouse gas forcing.