The role of tropical modes of variability in recent rainfall deficits across the Murray-Darling Basin

Southeastern Australia has been experiencing ongoing rainfall deficits since 1997. The spatial extent of the current drought closely corresponds to those regions where rainfall is influenced by interannual variations in the intensity of the subtropical ridge. An upward trend in the intensity of the subtropical ridge thus provides an explanation for the recent rainfall deficit. The possibility that slow variations in tropical modes of variability that are known to influence rainfall in the region, such as the El Nino-Southern Oscillation, may have contributed to southeastern Australia rainfall deficits is investigated using a tripole sea surface temperature index. This index takes into account key influences on Australian climate from variations in both the Indian and Pacific Oceans. The tripole index correlates well with rainfall variations across southeastern Australia on interannual to decadal time scales in winter and spring but not in autumn when most of the recent rainfall deficits have occurred. The influence of tropical sea surface temperature (SST) variations associated with the tripole on southeastern Australian rainfall is explained in terms of Rossby wave propagation, which helps us to understand why the impact is stronger in the southern half of the Murray-Darling Basin and why there is no relationship in autumn. On the basis of the linear relationship between the tripole SST variation and rainfall across the Murray-Darling Basin, we conclude that tropical SSTs have not contributed to the observed rainfall deficiencies in southeast Australia since 1997 but did contribute to the 1935-1945 rainfall deficits.