Developing a better understanding of the Australian monsoon and wet season onset climatology

By some estimates, about 40-60% of the world's population lives within a monsoonal climate. For all of these people, the timing of the monsoon onset is an annual event that is critical for sustainable agriculture, fire management, water management, travel and tourism, and so much more. A late monsoon onset can create serious issues in ways that are similar to drought conditions in higher latitudes but they may onset faster and last only a few weeks. The topic of this thesis focuses on the Australian monsoon, a singular monsoon region in a global weather pattern, which experiences high wet season rainfall variability, including in the timing of the precipitation, which can cause short-term, rapid-onset droughts. For example, by most definitions the Australia monsoon onset has a standard deviation of more than ±2 weeks and a range of onset dates of nearly two months from the earliest to the latest. This research has the following two objectives: 1. Determine which monsoon onset definitions provide the most predictability at seasonal time scales, and which seasonal-scale climate drivers provide the strongest influence on onset timing. 2. Investigate the frequency of 'false onsets'—when an onset criterion is met, but follow-up rainfall is not received—and if these lead to 'flash drought' conditions over northern Australia. These objectives were accomplished by, first performing a systematic literature review of Australian monsoon onset definitions. Second, recreating 11 dynamical monsoon onset datasets and extending them to the same time period to test their seasonal predictability through correlations with large-scale seasonal climate drivers. And, third, when considering a standard wet season rainfall onset criterion, the date after 1 September that 50 mm of precipitation is accumulated, quantify the frequency of occurrence of false onsets as a physical characteristic of the north Australian climate, rapid soil moisture declines and drought development. Results presented in this thesis from the first research objective demonstrate that while the wet season rainfall onset (first rainfall of the season, usually mesoscale features and not the global monsoon) is highly predictable on a seasonal time scale, the dynamical monsoon onset (i.e. the global-scale weather pattern) is not easily predictable at these timescales by traditional seasonal climate influences. Only a strong (