Upward trends in the power dissipation index (PDI) in the North Atlantic (NA) and western North Pacific (WNP) basins and increases in the number and proportion of intense hurricanes (categories 4 and 5) in all tropical cyclone basins have been reported in recent studies. These changes have been arguably viewed as the evidence of the responses of tropical cyclone intensity to the increasing tropical sea surface temperature (SST) over the past 30 years. However, our recent study shows that there are other factors responsible for climate change in tropical cyclone intensity.
Using the historical best track datasets from 1975 to 2004, we found that the lifetime and annual frequency of tropical cyclones significantly increased in the NA basin with no significant change in the average intensity. In the WNP basin the average intensity has significantly increased with no trends in lifetime and annual frequency over the past 30 years. Analyses of the long-term time series associated with the Saharan Air Layer activity, hurricane intensity and summer African Monsoon suggest that the SAL activities, which are associated with the summer African monsoon, play a dominant role in controlling the long-term trend in the basin-wide hurricane intensity in the Atlantic basin. While the decreased vertical wind shear and warming ocean surface may have allowed more storms to form and to form earlier or dissipate later, the SAL, which contains warm, dry and dusty air, inhibits tropical cyclone intensity.
We also found that the changes in the tropical cyclone formation locations and prevailing tracks may have contributed to the changes in the proportion of the intense hurricanes over the past 30 years. We suggest that the changes in the formation locations and prevailing tracks have a profound impact on the basin-wide tropical cyclone intensity. Thus, how the atmospheric circulation in the tropical cyclone basins responds to the global warming may be a critical factor to understanding the impacts of global warming on tropical cyclone intensity.