Storm surge risk from transitioning tropical cyclones in Australia

When tropical cyclones come ashore strong winds can set up storm surges that devastate coastal communities through erosion and inundation of low lying areas. In Australia, storm surge risk from tropical cyclones is generally restricted to warmer northern regions whilst in the southern regions damaging storm surges tend to arise from extratropical low pressure systems. Often, tropical cyclones weaken as they move into higher latitudes passing over cooler waters and experiencing higher wind shear. However, in some cases tropical cyclones can interact with mid-latitude weather systems causing the storms to lose tropical characteristics and become extratropical in nature, often intensifying and accelerating- this is known as Extratropical Transition (ET). Storms that undergo ET pose a serious threat to life and property by extending tropical cyclone-like conditions over a larger area in latitudes that do not normally experience such events. Here we aim to describe the storm surge risk from tropical cyclones transitioning into the higher latitudes around Australia using observations, atmospheric reanalyses, and a coupled storm surge-wave model. We highlight modelling challenges and examine two contrasting case studies: (1) the worst case scenario of Cyclone Alby (1978) that underwent ET causing large waves, flooding, erosion and $50 million in damage in southwest Australia; and, (2) a 'near miss' event from Cyclone Bianca (2011) that weakened just before making landfall in the same region. The simulated surge from Alby peaked at 1.1 m in Busselton, with approximately 10% of the height resulting from wave effects included in the model. Predicted wave heights exceeded 10 m offshore. Cyclone Bianca weakened dramatically before landfall and major damage did not occur. We propose that tropical cyclones moving into higher latitudes need to be given special consideration due to their unpredictable nature and the potential for damaging storm surges.