Impact of the Indian Ocean high pressure system on winter precipitation over western and southwestern Australia

Previous studies have linked ongoing winter drought in southwest Western Australia (SWWA) to changes in local, as well as large-area, sea-level pressure. Because the area of SWWA occupies only a few grid points of a typical global climate model (GCM), it is difficult to infer the possible role of global climate change in the drought in SWWA. Since the Indian Ocean High is a robust and large scale feature of atmospheric general circulation, it is simulated qualitatively in all GCMs. A quantitative comparison of the simulated past changes in its pressure and position in a GCM with those found in reanalysis data call be used to verify its suitability ill projecting future changes in winter rainfall in southwest Western Australia. This paper examines the linkages between changes in the Indian Ocean High and rainfall in SWWA, as well as a larger section of Western Australia. By introducing objective indices for area-weighted pressure, the area-weighted latitude and longitude positions of the High, this analysis shows that winter rainfall in a large section of Western Australia and in SWWA is significantly correlated with longitudinal displacement of the Indian Ocean High. Rainfall in SWWA is also correlated with the area-averaged pressure of the High. A regression model of May to August rainfall in SWWA using the pressure and longitude of the Indian Ocean High as independent variables explains 52 per cent of the rainfall variance during 1951-2008.