Possible mechanisms behind summertime temporally compounding dry-hot to wet extreme events in southeastern Australia

Compound weather extremes in southeastern Australia are increasing in frequency and complexity, yet the mechanisms driving their interactions remain poorly understood. This study examines how compound drought and heatwave (CDH) events in Victoria transition into heavy rainfall and flooding (CDH_R) during extended austral summers (November–March) from 1980–81 to 2018–19. Using threshold-based criteria and joint indices, we identify 28 CDH_R events and compare them with 34 CDH events not followed by extreme rainfall (CDH_noR) at the regional scale. Composite analysis reveals that eastern Victoria is particularly susceptible to rapid transitions from CDH to wet extremes. CDH_R events are typically preceded by shorter, more humid heatwaves during drought, in contrast to the longer, drier CDH_noR events that may exacerbate water stress. Two key synoptic-scale mechanisms are found to drive CDH_R transitions: (i) equatorward intrusion of a transient midlatitude trough via Rossby wave breaking, which enhances tropical disturbances prior to CDH onset; and (ii) poleward transport of tropical moisture into Victoria during peak CDH conditions under a persistent high-pressure anomaly, which helps sustain the dry and hot regime. This moisture influx is facilitated by an active Madden–Julian Oscillation phase and enhanced rainfall in the tropical north, followed by eastward propagation of a midlatitude low-pressure system that promotes southward moisture advection. At CDH termination, the interaction between a midlatitude cold front and warm, moist air over Victoria triggers high atmospheric instability and dynamic uplift, supported by a persistent upper-level anticyclonic circulation over the Tasman Sea. These findings provide new insights into the interconnected drivers of compound extremes, valuable for improving the predictability of hydroclimate risks and enhancing early warning and preparedness.