Seasonal Rainfall in Southwestern Australia and the General Circulation

Abstract

Seasonal rainfall data at 56 stations in southwestern Australia covering the period 1876?1970 are analysed. The square-root transformation is used. Principal component analysis is used for an exploratory study; the first eigenvector and coefficient are particularly informative. Rainfall patterns of recent years are then individually compared with features of the general circulation. In winter there is good spatial correlation over the region except in a small area near Hopetoun where reverse anomalies often occur. Two fundamental types of winter rain are postulated: continuous rains due to widespread uplift in mid-troposphere associated with winds from north of west, and showery rains associated with airstream instability and enhanced by topography and coastal convergence. Rainfall of early winter is mainly of the first type, closely related to the intensity of the westerlies and probably also to the quasi-biennial osculation of the equatorial stratosphere. Rainfall of late winter is more of the second type and is not similarly related to global circulation features. Rainfall at a given place is very sensitive to surface wind direction. Summer rain is mainly associated with ?meridional troughs? and is notably uniform spatially; tropical cyclones probably play an indirect part.