Extreme rainfall deficit in southern coastal Australia signals a return to drought, low dam levels and declining stream flows

Milton S Speer, Lance Leslie

Southern coastal Australia is situated between 30o and 38oS and is the longest east-west mid-latitude coastline (4,300km) in the Southern Hemisphere. It includes over 35% of Australia’s total population. Approximately 75% of this coastline, west of Melbourne, has a Mediterranean climate dominated by cold frontal systems that produce cool season (April to October) rainfall. Historically, this area has been devastated by droughts, particularly since the early 1990s. Over the last 24 months since early 2023, extreme drought has returned with dam levels falling as stream flows plummett. Using observational and reanalysis data of the Southern Hemisphere atmosphere, we provide an explanation for the sustained southern Australian coastal drying since the 1990s, and particularly over the most recent decade. Significant changes in the atmospheric circulation are shown to be responsible for the observed rainfall decreases. Both the subtropical and polar jets have contracted poleward, reducing the frequency and intensity of the embedded rain-producing systems affecting Australia’s southern coastline. These atmospheric circulation changes have become highly noticeable in the current 2016-2025 decade and are attributed to an amplification of Australian climate drivers, from accelerated global and local atmospheric and ocean warming. Consequently, there are more frequent and intense droughts, with major reductions in stream flows and dam levels. The current extreme drought conditions are affecting water sustainability in cities and regional communities in southern coastal Australia, decreasing agricultural production, threatening ecosystem survival, contributing to more frequent and severe heatwaves, and increasing the frequency and length of bushfire seasons.