The risk of hazards associated with tropical cyclones (TCs) is correlated with TC frequency. However, the controls on global annual TC frequency are poorly understood. Furthermore, future change in TC frequency is highly uncertain. This paper reviews the state of the science regarding what is known about TC frequency.
This review paper highlights promising tools and research directions to better understand the controls on global tropical cyclone frequency, including high-resolution and idealized numerical models and study of tropical cyclone precursor disturbances or “seeds.”
The frequency with which tropical cyclones (TCs) occur controls all other aspects of tropical cyclone risk since a storm that does not occur can do no harm. Yet this frequency is poorly understood. There is no accepted theory that explains the average number of TCs that occur each year on the Earth, nor how that number will change with global warming. Arguments based on global budgets of heat or moisture do not yet appear helpful, nor does a detailed understanding of the physical processes of TC genesis. Empirical indices that predict TC frequency as a function of large-scale environmental variables can explain some of its relative variations in space and time, but not its absolute value. Global numerical models with horizontal grid spacings on the order of 25–50 km have allowed much improved simulations of TC activity, however. Many such models project a decrease in frequency with warming, but some project an increase. Idealized simulations, including those at higher resolutions, offer promise by allowing a systematic, deductive investigation of the roles of individual environmental factors. In addition to the larger-scale environmental modulation of genesis likelihood, precursor disturbances, or “seeds”, may exert an independent influence on TC frequency.