26th Conference on Hurricanes and Tropical Meteorology

Wednesday, 5 May 2004
Tropical cyclones in the Australian region: modeling future climate
Richelieu Room (Deauville Beach Resort)
Lance M. Leslie, University of Oklahoma, Norman, OK; and B. W. Buckley, M. Leplastrier, and M. S. Speer
The known limitations in the tropical cyclone records prior to 1970 (Buckley, Leslie and Speer, J. Climate, 2003) coupled with a recent and anomalously quite period of tropical cyclone activity in the populated Queensland coast make accurate assessment of insurance risk problematic. This presentation outlines an insurance-driven suite of experiments aimed at better understanding the current and future tropical cyclone risk in the Australian region.

This presentation first documents the number of tropical cyclones that have affected the coastline of Australia since records began. Then, the results of an extensive series of numerical climate modeling simulations are presented. These experiments examined various aspects of the behavior of tropical cyclones in the Australian region. The modeling utilized a state-of-the-science coupled climate model (CCM). It was decided that an ensemble approach was necessary as it also provides statistical measures of the validity of the results. Six ensemble members were generated for the period 1990 to 2050. For the future climate, chosen here as the period from 2001 to 2050, the ensemble modeling process was conducted twice with: (i) CO2 levels set at no increase from the 1990s mean values; and (ii) CO2 levels rising one per cent per annum. Therefore, a total of 12 simulations were made for the future climate estimation. The set of six model runs with no change in CO2 levels provided a set of reference simulations of the natural variability of the climate system. The other set, with increasing CO2 applied beyond 2000 at the rate of 1% per year, as recommended by the Intergovernmental Panel on Climate Change (IPCC) in their guidelines on greenhouse gas forcing scenarios (IS92), simulates the impact of global warming driven by increased greenhouse gas levels. There was a 20-year spin-up period from 1970 to 1990, with the following decade (the 1990s) used for verification.

Preliminary assessment of the future climate simulations generated in this study reveals a number of significant changes, relative to the no CO2 change climate simulation which is used to provide a reference measure of the natural variability in the climate modeling system for this region. The main results appear to be: a slight increase in the frequency of tropical cyclones in the region, from the present average of around 11 per annum to a mean of over 13 per annum; an increase in the longevity of tropical cyclones by an average of over one day; a greater number of intense tropical cyclones, that is, in the number of category 4 and 5 storms; and an enlargement in the tropical cyclone genesis area, which is seen to spread poleward by about 3 degrees of latitude.

In the presentation, the results of the future climate study will be presented in detail and will also include changes in landfalling tropical cyclones and associated destructive wind and flood impacts on coastal regions of Australia.

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