Changes in Australian rainfall and their links to large scale drivers of variability
Creators: name: Jones, Matthew
Publisher: name: University of Wollongong
Year: 2025
Subjects: classificationCode: 370202
subject: Climatology

description: Large scale changes in rainfall in Australia have been the subject of extensive previous study. Significant trends have been observed in seasonal rainfall around the country, such as increasing summer rainfall in the tropics and decreasing winter rainfall in southwest Western Australia. These changes are linked to anthropogenic factors such as land clearing, aerosol concentrations and increased greenhouse gas concentration. They may also be linked to changes in three drivers of internal variability affecting Australia. These drivers are the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM). In this thesis, the observed changes in rainfall and their links to changes in these drivers are investigated. Throughout this work, station level rainfall data from the High Quality Daily and Monthly Rainfall datasets and gridded rainfall data from the Australian Water Availability Project are used in conjunction with the most commonly used indices for the climate drivers. Trend analysis performed on seasonal rainfall agrees well with prior work, particularly noting the strong declines in winter rainfall in the southwest, autumn rainfall in the southeast, and the strong increase in summer rainfall in the north. Rainfall variability during autumn and winter in the southwest is also declining, with fewer very wet seasons occurring, resulting in a decline in annual rainfall variability. Elsewhere, the seasonal rainfall variability is mostly increasing, particularly during summer in the north. The impact of the Millennium drought on these trends appears to have reduces an observed positive trend in rainfall prior to the drought in New South Wales. Positive trends are observed in each of the three climate drivers, with quantile regression analysis also indicating that the upper quantiles for ENSO and the IOD are increasing at a greater rate than the lower quantiles. The trends observed for these drivers do not appear to correlate well with observed trends in rainfall. Change point analysis performed on seasonal rainfall totals detects structural breaks in the time series of winter rainfall in southwest Western Australia (SWWA), with a very consistent time of change detected across the region. This change is a major factor in the observed negative decline in winter rainfall in the region. Changes are also detected in other parts of the country, in eastern Australia during spring and in northern Australia during summer, both contributing to increases in mean seasonal rainfall. Performing the change point analysis on the drivers of internal variability results in few significant change points being detected with meaningful links to the observed changes in rainfall. Some changes during summer for ENSO occur around the same time as some changes in rainfall in the north, but the increasing trend in ENSO since the change would imply a reduction in rainfall rather than the observed increase. A change in the IOD around the same time as those changes in SWWA is also detected, but the correlations between the IOD and winter rainfall in SWWA are very low. An analysis of changes in rain events indicates that extended periods of rainfall are expected to decline in length and rainfall volume across most of the country, with a notable exception being increases in the tropics during the monsoon season. Correlating these rain events with the drivers of variability supports this observation, especially when considering the observed positive trends in all three indices, though these correlations are weak and these drivers are therefore unlikely to be key drivers of observed change. The difference in how the SAM influences rainfall in southern Australia and areas further north is clear during winter, but less so during the rest of the year. These results are important in giving a clearer picture of Australian rainfall changes when it comes to drought or flood mitigation, but could be brought into a greater context if the analyses are repeated with rainfall on shorter time scales, such as sub-daily totals which are linked more with flash flooding. The changes observed here may also be limited by the relatively short length of the instrumental record in Australia, which extends back to the late 19th century. The use of an ENSO index which measures the temperature gradient across the Pacific may also allow for some conclusions to be drawn on changes in El Niño and La Niña cycles. The changes observed in this study may be part of a larger cycle on a much greater time scale, and so it is important to continue to build a complete record of historical Australian rainfall data, upon which these analyses could be repeated.