Seasonality in atmospheric circulation patterns leading to wet and dry seasons in southeast Australia and implications for droughts

Abstract

Southeastern Australia has experienced two major droughts in recent decades: the Millennium Drought that started in 1997 and lasted for more than a decade and the Tinderbox Drought (2017–2019) that was shorter but more intense. As has been reported by previous studies, the termination of droughts in semi-arid regions effectively happens after very wet events at monthly-to-seasonal time scales. Building on this finding, the paper reviews the drivers of very wet events in southeastern Australia across different seasons. From the start of autumn till the end of spring, the rainfall in southeast Australia is significantly correlated with the moisture over the Maritime continent, which may be modulated by the Indian Ocean Dipole (IOD) in winter and spring, or other processes such as the Madden-Julian Oscillation in the warmer months. It is shown that tropical convection interacts with extratropical Rossby waves enhancing low-pressure anomalies that help transport moisture into southeastern Australia, thereby contributing to increased rainfall from April to November. Warm season rainfall anomalies show no direct link to large-scale drivers. By considering these relationships, the study explores the large-scale conditions during the Millennium and Tinderbox Droughts to understand why wet events at subseasonal to seasonal time scales were absent during those periods. It is proposed that the Millennium Drought was promoted by the positive phase of the Atlantic Multidecadal Oscillation, which may affect low-frequency rainfall variability in southeast Australia by modulating the relationship between the IOD and the moisture over the Maritime continent, especially during the early cool season. This approach doesn't fully elucidate the severity of rainfall anomalies; rather, it focuses on understanding the termination of droughts.