Background
The author writes that "two recent papers (Emanuel, 2005; Webster et al., 2005) have claimed that the recent increase in sea-surface temperatures (SSTs) as a result of global warming is likely to be responsible [for] the concomitant increase in either the power dissipation (Emanuel, 2005) or the number of intense hurricanes and typhoons (Webster et al., 2005) through a direct forcing of enhanced thermodynamic energy supply," but he notes that "many papers in the past have failed to identify such relationships," citing the studies of Evans (1993), Wang and Chan (2002), Chan and Liu (2004) and Michaels et al. (2006).

What was done
To further explore this important issue, Chan searched for "possible physical causes responsible for the interannual variations of the activity of intense typhoons in the WNP [Western North Pacific] (here defined as the region 0-40°N, 120-180°E)."

What was learned
The City University of Hong Kong researcher reports that "in years with a high frequency of occurrence of intense typhoons, both the dynamic (relative vorticity in both the lower and upper troposphere as well as the vertical wind shear) and thermodynamic (as represented by the moist static energy in the low to mid troposphere) conditions in the atmosphere, especially in the eastern part of the WNP, are favorable for the formation of TCs [tropical cyclones]," and that "once formed, these TCs tend to have longer lifetimes over the ocean, and therefore have a high chance to become more intense." In addition, he notes that the factors responsible for increasing the number of strong TCs are "also significantly correlated with the Niņo3.4 SST anomalies." Consequently, as Chan describes it, "the frequency of occurrence of intense typhoons in this region is not [our italics] likely determined by the average SST over the region," which is what would be expected to increase in response to greenhouse gas-induced global warming.

What it means
Chan's primary finding, i.e., that "interannual variations of intense typhoons in the WNP are likely caused to a large extent by changes in the planetary-scale atmospheric circulation and thermodynamic structure associated with the El Niņo phenomenon," provides no support for the contentions of either Emanuel (2005) or Webster et al. (2005). In fact, it tends to argue against them.

References
Chan, J.C.L. and Liu, K.S. 2004. Global warming and western North Pacific typhoon activity from an observational perspective. Journal of Climate 17: 4590-4602.

Emanuel, K.A. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.

Evans, J.L. 1993. Sensitivity of tropical cyclone intensity to sea surface temperature. Journal of Climate 6: 1133-1140.

Michaels, P.J., Knappenberger, P.C. and Davis, R.E. 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters 33: 10.1029/2006GL025757.

Wang, B. and Chan, J.C.L. 2002. How strong ENSO events affect tropical storm activity over the western North Pacific. Journal of Climate 15: 1643-1658.

Webster, P.J., Holland, G.J., Curry, J.A. and Chang, H.-R. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846.