Understanding the Driving Mechanisms of two Extreme Precipitation and Drought Events in Australia from a Moisture Source Perspective

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Moisture availability is a fundamental prerequisite for precipitation. Within the water cycle, moisture contributing to precipitation originates from evapotranspiration (ET) in both local and remote regions. This moisture is transported through the atmosphere and may be progressively depleted during transit through precipitation. Consequently, the moisture supply to a region can vary in response to changes in evapotranspiration, atmospheric circulation, and environmental conditions that influence moisture transport and precipitation efficiency. Here we use a Lagrangian moisture source identification model BTrIMS1.1, in combination with analysis of weather systems, ET, and convective environment to understand the mechanisms of precipitation variability during two extreme events that lead to drought and floods in Australia.The Tinderbox Drought (January 2017–December 2019) in Australia severely threatened urban water supplies including Sydney, caused substantial agricultural losses and contributed to the devastating Black Summer bushfires. This drought was associated with a ~50% reduction in precipitation compared with climatology. In stark contrast, the following triple La Niña period (September 2020– August 2023) brought persistent heavy precipitation to eastern Australia, resulting in widespread flooding and storm-related damage. Despite their opposite hydrological impacts, both events were characterized by pronounced precipitation anomalies.We focus on the Murray-Darling Basin, Australia, because of its critical importance to agricultural production. Our analysis indicates that oceanic moisture contributions were substantially reduced during the Tinderbox Drought, driven primarily by changes in atmospheric circulation. Altered weather systems diverted climatological moisture sources away from the Basin, shifting dominant moisture sources towards regions with lower ET. This shift resulted in a pronounced moisture deficit, which was further exacerbated by reduced local ET.In contrast, during the triple La Niña period, there was an increased occurrence of slow-moving cyclone and anticyclone pairs, enhancing easterly flow and oceanic moisture transport towards eastern Australia. In addition, moisture contribution from inland Australia increased, driven by a substantially higher land ET during this period. By the third year, precipitation was further amplified by enhanced local moisture recycling due to wetter land surfaces. The persistence of slow-moving low-pressure systems also provided a more favourable environment for precipitation over extended periods, consistent with the higher mean convective available potential energy observed during triple La Niña period. Together, circulation anomalies and enhanced convective conditions combined to produce anomalously high precipitation and widespread flooding during this period. Key words: precipitation, moisture sources, Lagrangian, weather systems, evapotranspiration, ENSO, extreme events

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