Why Are There Approximately 100 Tropical Cyclones Annually on Earth?

Despite a strong fluctuation in the number of tropical cyclones (TCs) over all ocean basins, there are on average about 80-100 TCs globally every year. While this number has been long documented, the physical mechanisms underlying such a specific range of the global TC count are however still elusive. Using idealized simulations for a tropical channel domain to maximize the favorable conditions for TC formation, it is shown in this study that the TC formation is particularly sensitive to the temperature gradient in the 5⁰-10⁰N tropical band. As the temperature gradient in this tropical band increases above some threshold, a band of TCs form and subsequently migrate to higher latitudes. The formation of TCs subsequently reduces the temperature gradient in this narrow tropical band, thus delaying the formation of the next band of storms. We estimate a 15 day timescale for recovery of this temperature gradient above the necessary threshold to initiative a new band of TC formation. Combining this timescale with an estimate for overall storm size yields a simple explanation for why the Earth can support a global count of approximately 100 TCs annually.