Relative Contribution of Anthropogenic Forcing and Natural Processes to Rainfall Variability over Victoria, Australia

Victoria is the second-most populated state in Australia with a population exceeding 6.5 million, 5 million of whom live in greater Melbourne. Victoria it is also a major region for agriculture and tourism and is the second largest economy in Australia, accounting for a quarter of Australia's Gross Domestic Product. The long-term average annual rainfall (1900-2018) across Victoria is approximately 655 mm year^-1, with about two-thirds of the annual rain falling during the cool season (April to October). Cool season rainfall is very important for many crops and for replenishing reservoirs. While Victoria's rainfall varies considerably from year to year and decade to decade, cool season rainfall over Victoria has been 12% below the 20^th century average since the beginning of the Millennium Drought in 1997, causing major challenges to the farming community and water managers. Here we examine the extent to which this reduction in cool season rainfall is driven by external forcing, and the prospects for future rainfall, taking both external forcing and internal natural climate variability into account. We analyse simulations from 40 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under preindustrial and historical forcing, as well as three scenarios for the 21^st century: Representative Concentration Pathways, RCP2.6, RCP4.5 and RCP8.5, which vary markedly in the amount of greenhouse gas emitted over the coming century.

We show that the 1997-2018 average rainfall for cool season is below the preindustrial average in more than two-thirds of models under historical forcing to 2005 and RCP forcing during 2006-2018, regardless of scenario. However, the magnitude of the externally-forced drying is very small, with a median decline of only 2.5% with an interquartile range around -5% to +1%. The model ensemble results suggest that external forcing contributed only 20% (interquartile range -41% to 4%) to the drying observed in 1997-2018, relative to 1900-1959. These results suggest that the observed drying was dominated by natural, internal rainfall variability. While the multi-model median is below average from 1997-2018 onward, the externally-forced drying only becomes clear from 2010-2029, when the proportion of models exhibiting drying increases to over 90% under all three RCP scenarios. This agreement reflects an increase in the magnitude of the externally-forced drying. We estimate that there is a 12% chance that internal rainfall variability will completely offset the externally-forced drying averaged over 2018-2037, regardless of scenario. By the late 21^st century the externally forced change under RCP8.5 is – according to the models - so large that drying appears inevitable even after taking internally variability into account.

Confidence in the model projections is lowered, however, because models have difficulty in simulating the magnitude of the observed decline in rainfall. Some of this difficulty appears to arise because most models seem to underestimate the magnitude of multidecadal rainfall variability. Other reasons why this apparent difficulty might arise will be discussed.