8.5.3 Tropical Cyclones

The spatial resolution of the coupled ocean-atmosphere models used in the IPCC assessment is generally not high enough to resolve tropical cyclones, and especially to simulate their intensity. A common approach to investigate the effects of global warming on tropical cyclones has been to utilise the SST boundary conditions from a global change scenario run to force a high-resolution AGCM. That model run is then compared with a control run using the high-resolution AGCM forced with specified observed SST for the current climate (Sugi et al., 2002; Camargo et al., 2005; McDonald et al., 2005; Bengtsson et al., 2006; Oouchi et al., 2006; Yoshimura et al., 2006). There are also several idealised model experiments in which a high-resolution AGCM is integrated with and without a fixed global warming or cooling of SST. Another method is to embed a high-resolution regional model in the lower-resolution climate model (Knutson and Tuleya, 1999; Walsh et al., 2004). Projections using these methods are discussed in Chapter 10.

Bengtsson et al. (2006) showed that the global metrics of tropical cyclones (tropical or hemispheric averages) are broadly reproduced by the ECHAM5 model, even as a function of intensity. However, varying degrees of errors (in some cases substantial) in simulated tropical storm frequency and intensity have been noted in some models (e.g., GFDL GAMDT, 2004; Knutson and Tuleya, 2004; Camargo et al., 2005). The tropical cyclone simulation has been shown to be sensitive to the choice of convection parametrization in some cases.

Oouchi et al. (2006) used one of the highest-resolution (20 km) atmospheric models to simulate the frequency, distribution and intensity of tropical cyclones in the current climate. Although there were some deficiencies in simulating the geographical distribution of tropical cyclones (over-prediction of tropical cyclones between 0° to 10°S in the Indian Ocean, and under-prediction between 0° to 10°N in the western Pacific), the overall simulation of geographical distribution and frequency was remarkably good. The model could not simulate the strongest observed maximum wind speeds, and central pressures were not as low as observed, suggesting that even higher resolution may be required to simulate the most intense tropical cyclones.