Trends in global tropical cyclone numbers 1969-1998

In the analysis of recent global tropical cyclone (TC) activity uncertainty arises concerning the peak intensity of storms where intensity indications are recorded in the best track data only at intervals of twelve hours or more. By resampling complete track records to resemble the pattern of missing data in these incomplete tracks, a probability distribution for the annual number of storms of a given peak intensity in each basin is obtained. Using these distributions in Monte Carlo simulations of the annual TC number time series, limits on the possible trends in annual TC numbers are obtained.

We present the results of this trend analysis for each of the six individual regions of global TC activity, for each hemisphere and for the globe as a whole. It is found that while significant downward trends exist in total annual global TC numbers significant upward trends exist in the total annual global number of hurricane strength TCs. Upward trends also exist in the number of hurricane strength TCs making landfall.

It is found in general that while fewer TCs are forming, once formed they are increasingly likely to intensify and make landfall. In the regions where records permit analysis of intense hurricane strength storms (the North Atlantic and the North East and North West Pacific) the significant upward trend in total hurricane strength TC numbers is accounted for almost entirely by the trend in total intense hurricane strength TC numbers.

Further results are presented demonstrating the predominantly negative relationship between the interannual variability of large scale TC activity and the interannual variability in global and hemispheric temperatures. The potential role of enhanced greenhouse warming in the cause of the trends in TC numbers is briefly addressed.