Modelling of changes in Southern Hemisphere weather systems during the 20 th century

The changes in Southern Hemisphere autumn and winter storms have been studied using a global two-level primitive equation instability model with reanalysed observed May and July basic states during the 20th century. For July, storm track instability modes growing on the subtropical jet show a dramatic reduction in growth rate post-1975. This reduction in the intensity of cyclogenesis has continued to the present time for storm track modes that cross Australia and is associated with the observed decrease in rainfall in southern Australia. For May, the strength of the subtropical storm track crossing Australia has decreased while the polar storm track has increased. This again is associated with a decrease in the strength of the divergence field and rainfall across southern Australia. These effects have become more pronounced with time. We find for both autumn and winter that the rainfall reduction is also associated with a decrease in the vertical mean latitudinal temperature gradient and in the peak upper tropospheric jet-stream zonal winds near 30° south throughout most of the Southern Hemisphere. We have examined the ability of a number of climate models with anthropogenic forcing including increasing greenhouse gas concentrations to replicate the changes in transient weather systems during the 20th century. Here, we focus on Australian models including the CSIRO Mark 3.0 and 3.5 coupled models and the ACCESS atmospheric model with prescribed SSTs; we also consider the UKMO HadGem1 coupled model that has a similar atmospheric component to the ACCESS model. The coupled models are not able to capture the storm track changes. However, the leading storm track mode crossing Australia from the ACCESS model simulations with prescribed SSTs has a structure that is remarkably similar to those based on July reanalyses and has reasonably good growth rate.