The role of internal variability and external forcing on southwestern Australian rainfall: prospects for very wet or dry years.

The cool-season (May to October) rainfall decline in southwestern Australia deepened during 2001-2020 to become 20.5% less than the 1901-1960 reference period average, with a complete absence of very wet years (i.e., rainfall > 90th percentile). CMIP5 and CMIP6 climate model simulations suggest that approximately 43% of the observed multi-decadal decline was externally-forced. However, the observed 20-year rainfall anomaly in 2001-2020 is outside the range of both preindustrial control and historical simulations of almost all climate models used in this study. This, and the fact that the models generally appear to simulate realistic levels of decadal variability, suggests that 43% might be an underestimate. A large ensemble from one model exhibits drying similar to the observations in 10% of simulations and suggests that the external forcing contribution is indeed larger (66%). The majority of models project further drying over the twenty-first century, even under strong cuts to greenhouse gas emissions. Under the two warmest scenarios, over 70% of the late twenty-first century years are projected to be drier than the driest year simulated during the 1901-1960 period. Our results suggest that few, if any, very wet years will occur during 2023-2100, even if strong cuts to global emissions are made.