Control of the Intertropical Convergence Zone on Tropical Cyclones during Early and Late Stages of Genesis

The development of tropical cyclones (TC) is investigated in a 50km-grid-spacing global aquaplanet model. With zonally symmetric forcing, the genesis of TCs is primarily controlled by the inter-tropical convergence zone in this model, and robust statistics of TC tracks are collected. The early and late stages of the genesis process are controlled by different environmental variables. The early stage associated with convective organization depends on the partition between the resolved and parameterized precipitation, which is sensitive to the detail of the convection scheme. On the other hand, the late stage depends on the relative importance of equatorial Rossby waves, which is less sensitive to model details. This dependence is summarized by a ratio between the characteristic scale of TCs and the equatorial Rhines scale expressed in terms of the environmental vorticity and surface fluxes. The combination of those two effects explains the response of TC frequency and spatial distribution to different types of forcing, including changing sea surface temperature and CO2 concentrations.