Long-term natural variability of tropical cyclones in Australia

Numerous late Holocene records of tropical cyclones have been collected from tropical northern Australia. They are in the form of multiple shore parallel sedimentary ridges deposited over the past 6,000 years and an 800 year long annual resolution oxygen isotope record from a calcium carbonate cave stalagmite. The sedimentary ridges are composed of coral fragments, or shell and sand or pure sand. Numerical models relating surge height and tropical cyclone central pressure were used to determine the intensity of the tropical cyclone responsible for deposition of the ridges at each site. The results suggest that in the majority of cases these features were deposited by very high magnitude events. The results suggest that extrapolation from short instrumental and historical records, which is the method commonly used to assess risk from this hazard, substantially underestimate the risk from this hazard. This is confirmed for the Cairns region by the 800 year long high resolution isotope record which suggests that tropical cyclone activity in northeast Queensland has been in a phase of quiescence since before European settlement of the region in approximately AD 1870. Comparisons between the short and long-term records suggest that non-stationarity may be an inherent feature of the long-term natural variability of tropical cyclones in this region