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

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 (<-1 standard deviation) La Niña pattern shows a statistically significant correlation with an early onset of the dynamical monsoon. A weak La Niña, ENSO neutral, and a weak or strong El Niño pattern has only a weak or non-statistically significant correlation and should not be used to make monsoon onset predictions. A negative and neutral Indian Ocean Dipole (IOD) do not have a statistically significant correlation with onset dates, but a strong positive IOD correlates with a delayed monsoon onset and could be used in monsoon onset predictions.

The outcomes of the second objective show that false wet season onsets are relatively common across northern Australia; 30% to 50% of wet seasons experience a false onset. False onsets are more common during La Nina and negative IOD events. False onsets do not always coincide with a 'flash drought' (investigated here as a rapid drop in soil moisture). These rapid drops in soil moisture are relatively common across northern Australia in the wet season, occurring on average at least once within about 25% of seasons. These rapid drops in soil moisture are common enough that they probably should not be a considered a drought (i.e. a climatological extreme).

The findings presented in this thesis significantly advance our knowledge of Australian monsoon temporal variability. This includes: A systematic and comprehensive assessment of the literature on Australian monsoon onset definitions and timing; An analysis of monsoon onset dates and the correlations of onset timing with climate drivers; A study of wet season onset variability, false onsets and flash drought.

The significance of this work extends well beyond the Australian monsoon. Similar analysis could be applied to other monsoon regions. It is also very likely that, given the variability of global monsoon patterns, other monsoon regions may experience seasons with false onsets. Investigation of the frequency of occurrence of false
onsets would give residents of other monsoon regions an understanding of their climatological propensity toward drought.

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