Southwest Western Australian winter rainfall and its association with Indian Ocean climate variability

Abstract

Southwest Western Australia (SWWA) has experienced a significant decrease in winter rainfall since the late 1960s. This decrease is unexplained and the resultant problem of reduced water storage has been compounded by the lack of any useful predictive skill at the seasonal time scale. This study uses recent gridded, historical data and simple linear correlation in order to evaluate the importance of links between rainfall and both mean sea level pressure (MSLP) and sea-surface temperature (SST) patterns over the Indian Ocean. The decrease in rainfall is linked to decreases in the density of low-pressure systems in the region and to increases in both MSLP and SST over the southern Indian Ocean. Warmer SSTs and increases in MSLP are associated with the observed long-term changes, but changes in these variables do not explain a great deal of the observed interannual variability. Greenhouse-induced climate change is not regarded as a likely explanation for the observed decrease, however, the existence of links with both MSLP and SSTs suggests the existence of coupled air–sea interactions over the southern Indian Ocean which may be relevant at decadal or multi-decadal timescales. A major difficulty with defining any such processes is the relative sparseness of data at high latitudes in the Southern Hemisphere. This should be partly alleviated as more recent high quality data becomes available over time. Copyright © 2000 Royal Meteorological Society