The 80 cyclones myth

For the past several decades there has been a persistent myth within the tropical cyclone community that the global number of tropical cyclones each year is highly stable when compared to the much larger variability observed within the individual cyclone basins. References to the myth usually imply that the stability of the number of global cyclones suggests important feedback relationships between these storms and the global climate. The myth is intriguing, and the lead author confesses that he is one of the many who has passed it on to a large number of students and other colleagues over the years. Alas, it also turns out to be false.

This study analyzes tropical cyclone interannual variability from 1984-2003. The variability of the total number of global storms with maximum winds greater than 17 m/s is found to be indistinguishable from the variability that would arise if the storms in each basin formed randomly in time with the variability observed in that basin. There is no observed tendency for above-normal cyclone activity in one basin to be compensated by net below-normal activity averaged over the remaining basins. There are, however, positive and negative correlations between annual tropical cyclone numbers and storm days in various basins, with the strongest factor apparently being related to ENSO. There are also non-ENSO-related inter-basin correlations, some of which are intensity dependent. There is also a suggestion that variability in the global number of strongest storms (categories 4 and 5) is actually greater than would be predicted from the individual basin variabilities, though this may largely reflect the increasing frequency of intense tropical cyclones noted in recent studies.