9.5.3.6 Tropical Cyclones

Several recent events, including the active North Atlantic hurricane seasons of 2004 and 2005, the unusual development of a cyclonic system in the subtropical South Atlantic that hit the coast of southern Brazil in March 2004 (e.g., Pezza and Simmonds, 2005) and a hurricane close to the Iberian Peninsula in October 2005, have raised public and media interest in the possible effects of climate change on tropical cyclone activity. The TAR concluded that there was ‘no compelling evidence to indicate that the characteristics of tropical and extratropical storms have changed’, but that an increase in tropical peak wind intensities was likely to occur in some areas with an enhanced greenhouse effect (see also Box 3.5 and Trenberth, 2005). The spatial resolution of most climate models limits their ability to realistically simulate tropical cyclones (Section 8.5.3), therefore, most studies of projected changes in hurricanes have either used time slice experiments with high-resolution atmosphere models and prescribed SSTs, or embedded hurricane models in lower-resolution General Circulation Models (GCMs) (Section 10.3.6.3). While results vary somewhat, these studies generally indicate a reduced frequency of tropical cyclones in response to enhanced greenhouse gas forcing, but an increase in the intensity of the most intense cyclones (Section 10.3.6.3). It has been suggested that the simulated frequency reduction may result from a decrease in radiative cooling associated with increased CO₂ concentration (Sugi and Yoshimura, 2004; Yoshimura and Sugi, 2005; Section 10.3.6.3; Box 3.5), while the enhanced atmospheric water vapour concentration under greenhouse warming increases available potential energy and thus cyclone intensity (Trenberth, 2005).

There continues to be little evidence of any trend in the observed total frequency of global tropical cyclones, at least up until the late 1990s (e.g., Solow and Moore, 2002; Elsner et al., 2004; Pielke et al., 2005; Webster et al., 2005). However, there is some evidence that tropical cyclone intensity may have increased. Globally, Webster et al. (2005) find a strong increase in the number and proportion of the most intense tropical cyclones over the past 35 years. Emanuel (2005) reports a marked increase since the mid-1970s in the Power Dissipation Index (PDI), an index of the destructiveness of tropical cyclones (essentially an integral, over the lifetime of the cyclone, of the cube of the maximum wind speed), in the western North Pacific and North Atlantic, reflecting the apparent increases in both the duration of cyclones and their peak intensity. Several studies have shown that tropical cyclone activity was also high in the 1950 to 1970 period in the North Atlantic (Landsea, 2005) and North Pacific (Chan, 2006), although recent values of the PDI may be higher than those recorded previously (Emanuel, 2005; Section 3.8.3). Emanuel (2005) and Elsner et al. (2006) report a strong correlation between the PDI and tropical Atlantic SSTs, although Chan and Liu (2004) find no analogous relationship in the western North Pacific. While changes in Atlantic SSTs have been linked in part to the AMO, the recent warming appears to be mainly associated with increasing global temperatures (Section 3.8.3.2; Mann and Emanuel, 2006; Trenberth and Shea, 2006). Tropical cyclone development is also strongly influenced by vertical wind shear and static stability (Box 3.5). While increasing greenhouse gas concentrations have likely contributed to a warming of SSTs, effects on static stability and wind shear may have partly opposed this influence on tropical cyclone formation (Box 3.5). Thus, detection and attribution of observed changes in hurricane intensity or frequency due to external influences remains difficult because of deficiencies in theoretical understanding of tropical cyclones, their modelling and their long-term monitoring (e.g., Emanuel, 2005; Landsea, 2005; Pielke, 2005). These deficiencies preclude a stronger conclusion than an assessment that anthropogenic factors more likely than not have contributed to an increase in tropical cyclone intensity.