The Increasing Frequency of Heat Wave–Extreme Rainfall Whiplash Events across Australia

Abstract Heat wave-to-extreme rainfall sequences are a form of weather whiplash event (WWE), involving the abrupt transition from a “hot” to “wet” extreme. This type of subweekly temporal compounding can lead to an amplification of hazards associated with these extremes, including bushfire and flooding. Despite the potential for WWEs to have significant socioeconomic and environmental impacts on Australia, these sequences remain poorly understood. We present a custom definition to support the identification and analysis of subweekly hot-to-wet WWEs across Australia during extended summer periods (October–April) from 1910 to 2022. Using this definition, the highest WWE frequencies were observed over southeastern regions and southern Queensland. The frequency of WWEs has increased over the majority of Australia, with the largest increases in the northernmost areas. Particularly in the central and northern regions, this appears to be related to increases in heat wave frequency. Over southern Australia, the spatial structure and extent of extreme precipitation associated with WWEs are often analogous to that of a cold front. Atmospheric conditions support this relationship. In the northern areas of Australia, the spatial patterns and associated meteorological features of WWEs suggest smaller-scale convective processes dominated. The spatial coverage of a WWE has also become larger on average. It is likely that an ever-larger proportion of the Australian population is becoming increasingly exposed to these rapid hot-to-wet compound extremes. This work emphasizes a need to understand how WWEs will evolve in the future as global temperatures continue to rise and also how this will influence their associated hazards. Significance Statement Extreme weather events, like heat waves and heavy rainfall, can lead to fire and flooding and pose threats to human health, ecosystems, and infrastructure. In a warming world, extremes are occurring more frequently and often in combination with each other. Certain sequences of events, like sudden “whiplashes” between heat waves and extreme rainfall, remain understudied. This paper explores how “hot-to-wet” whiplashes vary across Australia and what atmospheric conditions may drive them. We find that these events have become more frequent over the last century, with southeastern Australia emerging as a hotspot. Heat waves are more often followed by extreme rainfall than we would expect by chance, underscoring the need to study extreme events in combination rather than in isolation.