Heatwaves and compounding drought-heatwaves in Australia: historical changes, association with ENSO and IOD, and influence on bushfire fuels

Australia’s black summer of 2019-20 was characterised by ongoing drought, frequent heatwaves, and massive bushfires resulting in huge socio-economic and ecological losses. A better understanding of historical changes of Australian heatwaves and drought-heatwave compound events and their large-scale climate drivers can help improve their predictability. This thesis analyses the trends in Australia-wide heatwave metrics during the extended summers of 1951-2020. Results not only emphasise that heatwaves are becoming hotter, longer, and more frequent but also signify that they are occurring with excess heat, commencing much earlier, and expanding their season over many parts of Australia in recent decades. Investigation of heatwave and severe heatwave trends using three different observational products reveals that heatwave trends are noticeably different amongst the three datasets. Furthermore, the spatial characteristics of Australian heatwaves, particularly those occurring simultaneously in contiguous regions, and compound drought and heatwave (CDHW) metrics are analysed using observational gridded data. Results show that extremely large contiguous heatwaves cover significantly larger areas and get significantly longer during recent decades compared to the past. Moreover, CDHWs are also getting worse during recent decades, particularly in eastern Australia. The association of contiguous heatwaves and CDHWs in Australia with the combinations of the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) is then investigated using a single model initial-condition large ensemble. Results suggest that strong El Niño (Es) and strong El Niño co-occurring with strong IOD positive (EsIPs) seasons favour large contiguous heatwaves in Australia. Furthermore, results show that an extreme CDHW season in terms of frequency and severity occurs one out of every two seasons of Es (EsIPs) over Australia's northeast (northeast and southeast). Lastly, the relationship between heatwave characteristics and mean dead fine fuel moisture content and the influence of drought on their association is investigated over southeast Australia during the peak heat and fire seasons. This relationship varies across different heatwave characteristics and regions and is modulated by drought conditions. Overall, this research provides advanced insights into understanding the variability of heatwaves and CDHWs and their link with bushfires over Australia, which can help improve their seasonal predictability