Frequency, duration and intensity of tropical cyclonic storms in a warming environment

All ocean basins in which hurricanes appear each year show a statistically significant trend in SST for at least the last four decades. A careful analysis of tropical cyclone statistics during the satellite era (1972-2004) shows that only in the North Atlantic have tropical cyclones possessed a statistically significant increase in number and in number of tropical cyclone days (duration). In all other basins the number of storms and duration have decreased substantially per year since the mid-1990's but have showed no significant statistical trend during the entire observational period. On the other hand, the intensity of tropical storms have increased substantially in all basins over the period with increases in category 4 and 5 storms increasing by 30-40 % between the 1970-1985 and 1990-2004 periods. While it is tempting to attribute the changes in the characteristics of tropical storms to changes in SST related to global warming, such a conclusion would be mere statistical extrapolation without understanding the role of tropical storms in the general circulation of the ocean-atmosphere system. For example, we do not know why there has been an average of 95 or so hurricanes per year globally almost constantly for the last 50 years. We test the hypothesis that tropical storms are integral components of the summer hemisphere circulation system and perform an important role in the transfer of heat between the equator and the summer higher latitudes perhaps by modulating the SST. We note that the decrease in the number of storms and duration in the last decade accompanies an increase of intensity and suggest that these covariations may be indicative of an integral constraint on the energy balance of the summer hemisphere.