The role of external forcing and natural processes on past and future changes in rainfall over sub-regions of Victoria, Australia

Rainfall across the state of Victoria in Australia exhibits a strong climatological gradient from north to south and from east to west. In general, the highland areas in eastern Victoria receive the highest rainfall, followed by the southern coastal regions and substantially less rainfall occurs over the north-west of Victoria. The latter two regions show a pronounced annual cycle with winter maxima while rainfall over eastern Victoria is more uniform throughout the year. These distinct variations in rainfall arise from the fact that the different regions of Victoria are influenced by different weather systems and large-scale climate drivers and they also respond differently to external forcing. Many of the previous studies on rainfall changes were derived using all-Victoria area-averaged rainfall. However, there is a strong interest of stakeholders, both within and beyond the climate science community in Australia, about the role of climate change on the decline in local rainfall since the beginning of the Millennium Drought in 1997.

In this study, we used both observations and climate models to provide comprehensive and robust information on past and future rainfall changes on three sub-regions of Victoria (Murray Basin Victoria: MBVic, southeast Victoria: SEVic, and southwest Victoria: SWVic) during the cool season (April – October) due to climate change. Our results show that the percentage decline of rainfall for the 1997-2018 period, relative to the 1900-1959 period average, is more pronounced over the MBVic and SEVic regions of Victoria and least pronounced over the SWVic region. However, the fractional contribution of external forcing is estimated to be about 30% to the observed drying over SWVic which is about 1.5 times higher than the other two regions. Equivalently, the external forcing contribution to the observed trend for the 1900-2018 period are 56%, 17% and 24% for SWVic, MBVic and SEVic, respectively. These numbers suggest that the recent drying across Victoria, while primarily driven by internal rainfall variability, was reinforced by external forcing.  We are currently investigating the time of emergence of the external forcing signal beyond its pre-industrial and historical range and the expected combined impact of both external forcing and internal variability on rainfall over different sub-regions of Victoria for coming decades.