Understanding historical and future changes in mean and extreme rainfall in Australia

Abstract

Understanding anthropogenic changes in rainfall in Australia is difficult as Australia experiences one of the highest variable rainfall climates in the world. Firstly, this research aims to understand the observed historical changes in various characteristics of mean and extreme rainfall in Australia. Secondly, the research aims to understand the role of anthropogenic forcing and large-scale variability in driving changes in mean and extreme rainfall in Australia. Lastly, this research aims to identify the shortcomings of existing methods to study historical and future changes in rainfall and develop robust new approaches. 1. A comprehensive review of the persistent increase in rainfall in northwest Australia (NWA) since 1950, and a strong decrease in the southwest of Western Australia (SWWA) and southeast Australia (SEA), finds there are still significant knowledge gaps in our understanding of underlying mechanisms behind some of these changes. Extreme rainfall follows a similar trend to mean rainfall; however, very few regions show significant long-term historical changes in extreme rainfall. 2. Anthropogenic forcings play a significant role in modulating rainfall in NWA. Investigation using a set of CMIP5 models shows that the persistent increase in mean and extreme rainfall in NWA is better captured when aerosol forcings are incorporated. While aerosols lead to increased rainfall, greenhouse gases lead to a decrease in rainfall, resulting in an offsetting impact between aerosols and greenhouse gases. This study indicates the importance of correct representation of the interaction between natural drivers and all anthropogenic forcings (i.e. not just greenhouse gases) in climate models. 3. This research presented in this thesis indicates that the timing of extreme rainfall varies largely depending on the phases of ENSO and IPO and their interaction, particularly in SEA. Australia has a clear north and south distinction in the timing of extreme rainfall. Extreme rainfall in the north usually occurs in the summer, while in the south it typically occurs in late autumn/winter. The area of summer extreme rainfall extends southward (northward) during a negative phase of IPO (positive IPO). Variability in the timing of extreme rainfall is largest in SEA, indicating that extremes can occur at any time of the year in this region. During El Nino years, SEA receives extreme rainfall in late autumn/winter, and during La Nina years extremes usually occur in spring/summer months. Understanding these relationships have major implications on improving seasonal prediction of extremes. 4. The relationship between extreme rainfall and temperature, the scaling rate, has been used in previous studies to provide robust projections for extreme rainfall. However, there are large regional variations in scaling rates, with a rate higher than expected from the Clausius-Clapeyron (C-C) relationship found in the tropical north and south of Australia. Further decomposition into dynamic and thermodynamic drivers of extreme rainfall shows that dynamics play a key role in the high scaling rate in the north, while thermodynamic drivers of extreme rainfall play a crucial role in the south of Australia. Scaling rates in future simulations are higher than in historical simulations. This non-stationary nature of scaling rates makes it challenging to project extreme rainfall. 5. Lastly, a novel approach to detect long-term changes in rainfall characteristics is developed. By studying rainfall events (defined as consecutive n number of rain days), a continent-wide increase in the frequency and intensity of short duration (1-2 day) events is found and is evident across all seasons. In tropical north Australia (north of 20S), there is an increase in the frequency of long persistent (> 6 days) rainfall events, whereas the drought-prone regions in the south (below 20S) show a decrease in the frequency of rainfall events of duration > 2 days. Show more