Thermodynamic Constraints on Observed Global Tropical Cyclone Count

On average, approximately 90 tropical cyclones form globally each year. However, it is not well understood what processes govern this number. What controls the global annual rate of tropical cyclone formation? This work tackles this question by quantifying the extent to which tropical cyclone count in the present climate is limited simply by the geography of thermodynamic environments favorable for tropical cyclone formation and maintenance. We first combine reanalysis-based datasets of tropical cyclone potential intensity and environmental ventilation to define a spatiotemporal geography of environments capable of supporting tropical cyclones. We then merge this information with outer storm size data to define the packing density of storms within this geographic domain and, further, to quantify the thermodynamically-constrained upper-bound on tropical cyclone count in the present climate, including a simple estimate of how many more tropical cyclones could theoretically exist on Earth. The results provide a quantitative answer to a simple hypothesis for the controls on storm count amenable to application for both current and future climate states.