A Review of Drivers of Cool Season Rainfall in Southwest Western Australia

ABSTRACT Southwest Western Australia (SWWA), with its Mediterranean climate, has undergone a persistent drying trend since the 1970s. As such, it is often referred to as the “canary in the coal mine” of climate change. This review examines drivers of SWWA rainfall and rainfall decline, focusing on the cool season (April to October) when most rainfall occurs, and includes the influence of weather systems, modes of natural climate variability, and land‐use change. While paleo‐climate evidence shows that similar declines have occurred in the past, the current trend is at the upper end of natural climate variability, indicating an increasing influence from anthropogenic climate change. The reasons for this drying trend are complex, with research linking decreasing SWWA rainfall to a strengthening subtropical ridge and more frequent positive phases of the Southern Annular Mode resulting in fewer winter fronts reaching the region. Decreasing baroclinity has been associated with a decrease in cold front rainfall amounts and a possible shift toward increased post‐frontal showery activity. While the El Niño Southern Oscillation and Indian Ocean Dipole (IOD) have shown weak historical links to SWWA rainfall, recent trends toward more frequent positive IOD may change this association. Finally, the influence of 20th–21st century land use changes has also been identified as a contributing factor to the rainfall decline. Given the complex interplay of these drivers and the increasing influence of anthropogenic climate change on the region's rainfall, a holistic approach is becoming crucial. We provide multiple avenues for further research. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change