What Controls the Frequency of Tropical Cyclones and Seeds

Abstract:

A framework is developed to explain the response of tropical cyclones (TCs) to climate in high-resolution global atmospheric models having different complexity of boundary conditions. The framework uses vortex dynamics to identify the large-scale controls on the evolution of TC precursors --- first convective clusters and then weakly rotating seeds. In experiments with perturbed sea surface temperature (SST) and CO₂ concentrations from the historical values, the response of TCs follows that of the seeds. The distribution of the seeds is explained by the distribution of the non-rotating convective clusters multiplied by a probability that they transition to seeds. The distribution of clusters is constrained by the large-scale vertical velocity and is verified in aquaplanet experiments with shifting Intertropical Convergence Zones. The probability of transition to seeds is constrained by the large-scale vorticity and its gradient via an analytical function. The function is derived from scale analysis of the vorticity equation and is verified in aquaplanet experiments with uniform SST. The consistency between seed and TC responses breaks down substantially when gradients of SST are significantly enhanced or reduced. In that case, the difference between the responses is explained by a change in the ventilation index, which influences the fraction of seeds that develop into TCs. The proposed TC-climate relationship serves as a framework to explain the diversity of TC projection across models and forcing scenarios.