Long-term variations in winter rainfall of southwest Australia and the African monsoon

The rainfall in southwest Western Australia (SWWA) mostly falls in May to September, and predominantly in June, July and August (JJA). Records show that, prior to the 1960s, this area had the most reliable rainfall in the country, but in the late 1960s a decrease of approximately 20 per cent (from approximately 3.8 mm/day to 3 mm/day for JJA) occurred in the longterm mean (IOCI 2002), which persists to this day and is largely unexplained (Fig. 1). So far, explanations for this long-term change have been sought from processes in the equatorial Pacific, from sea-surface temperatures in the Indian Ocean, and from greenhouse warming (IOCI 2002). This rainfall record may be compared with the rainfall in the African Sahel (Fig. 2). Here a decrease in excess of 0.4 mm/day Aust. Met. Mag. 54 (2005) 91-102