Changing compound rainfall events in Tasmania

While extreme weather and climate events have been studied for several decades, analysis of compound events has only begun in recent years. In this burgeoning field there are still many open questions around the optimal methodology and analysis tools for analysis. After consultation with state emergency services in Tasmania, Australia, we examined which compound events have the largest impacts on their organizations. Through this consultation process we found that many of the severe flooding events in the state do not coincide with the highest rainfall days. Flooding on intense rainfall days is well understood, but flooding can also occur on days where the rainfall is not particularly extreme, especially if catchments are already saturated. Using the Australian Gridded Climate Data and six dynamically downscaled, Representative Concentration Pathway 8.5, bias adjusted Coupled Model Intercomparison Project 5 models we developed a method to quantify such compounding events to examine how they are changing from 1961 to 2100. We optimized a pre-existing technique to estimate the antecedent conditions in catchments, combined with daily rainfall. We found that during 1961-2017, the number of compound rainfall events has been decreasing in the four Tasmanian catchments we studied, although the trend was statistically significant in only one case. The intensity of compound rainfall events was found to have increased significantly in some areas. Many future projections place Tasmania at the boundary of a drying trend to the west and wetting trend to the east and the position of this boundary varies between models leading to contrasting projected changes for parts of Tasmania. However, there is projected to be a decline in rainfall to 2100 associated with the southward shift in the storm-track. Compound rainfall events are projected to decline throughout Tasmania, except in the south which will remain stable to 2100. The intensities are projected to increase in the south and decrease in the west, related to the changing thermodynamics and dynamics of rainfall drivers in the region in a warmer climate.