Simulations of twentieth century atmospheric circulation changes over Australia

Abstract

We examine the performance of coupled models to simulate twentieth century winter circulation changes throughout the southern hemisphere, and particularly over Australia. A number of studies have recently shown that the early to mid-1970s was a time of major shifts in the structure of the large-scale circulation of both the northern and southern hemispheres. Over southern Australia there was a concurrent, dramatic and continuing reduction in the winter rainfall. In a recent study, the authors, using reanalyzed observations, have suggested 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 throughout most of the southern hemisphere, but in particular upstream and over Australia. These and other circulation changes, including changes in the Hadley circulation, and trends in the Southern Annular Mode, were shown to affect winter rainfall over southern Australia. Here, we examine the response of many of the CMIP3 IPCC climate models to observed anthropogenic forcing, including increasing greenhouse gases, from pre-industrial to the end of the twentieth century. Our interest here is on changes in the atmospheric circulation that affect wintertime cyclogenesis, and for that reason we focus on two diagnostics that can be used to evaluate the climate models in this regard. Thus, our focus is on the ability to simulate (a) the reduction in the strength of the wintertime subtropical 300hPa zonal wind upstream and over southern Australia, and a strengthening in the zonal wind further south, and (b) a reduction in the baroclinic instability of the subtropical SH circulation. We have considered these diagnostics in four cases; one involving the changes in the latter half of the twentieth century, and three involving changes between the end of the twentieth century and different base periods in pre-industrial simulations. The CMIP3 models display quite disparate abilities to simulate these two diagnostics. While the majority is able to simulate the former ((a)), especially when using the pre-industrial simulations, only about a third of the models capture the changes in the latter ((b)). Our analysis also suggests that there is a component of decadal variability in the model results that is dependent on the base period chosen in the pre-industrial runs. There are a number of models that consistently simulate changes in the two diagnostics that are in general agreement with results from the NCEP reanalysis. Projected changes in baroclinic instability from these models suggest that further large reductions in baroclinic instability are possible under SRESB1, SRESA1B and SRESA2 scenarios, especially over the Australian region. By implication, this suggests further reductions in the growth rates of SH cyclogenesis modes and further reductions in rainfall, over southern Australia.