Variability of tropical cyclones over the southwest Pacific Ocean using a high-resolution climate model

Knowledge of the variability in tropical cyclone (TC) frequency and distribution is essential in determining the possible impact of natural or human-induced climate change. This variability can be investigated using the available TC data bases and by carrying out long-term climate model simulations for both past and future climates. A coupled ocean-atmosphere climate model (referred to here as the OU-CGCM) is described and applied with a higher resolution (50km) nested domain in the southwest Pacific region. Six-member ensembles of simulations with the OU-CGCM have been run for 80 years, from 1970 to 2050. During the period 1970-2000, the OU-CGCM runs were compared with the observed TC data base. For the period 2000-2050, two ensembles of simulations were performed, one with constant greenhouse gas concentrations and the second with increasing greenhouse gases. The OU-CGCM simulated well the observed TC frequency and distribution in the southwest Pacific during the period 1970-2000. It also produced clear interannual and interdecadal TC variability in both the fixed and enhanced greenhouse gas simulations during the period 2000-2050. The variability in TC frequencies was associated with the typical atmospheric and SST anomaly patterns that occur in periods of quiet and active TC frequencies. The main findings from the enhanced greenhouse gas scenario for the period 2000-2050 are: no change in the mean decadal number of TCs relative to the control run, but a marked increase of about 15% in the mean decadal number of TCs in the most severe WMO categories 4 and 5; the likelihood of TCs during the next 50-year period that are more intense than ever previously experienced in the Australian region; a poleward extension of TC tracks; and a poleward shift of over 2 degrees of latitude in the TC genesis region.