The response of hurricane frequency to climate change in 50-km resolution aquaplanet GCM simulations

The response of hurricane frequency to climate changes in an aquaplanet conguration of a 50-km resolution aquaplanet general circulation model is examined. The lower boundary condition is an energetically consistent slab ocean with a prescribed time-independent cross-equatorial ocean heat flux, which breaks the hemispheric symmetry. In this idealized conguration, the global hurricane frequency increases in response to warming forced by increases in the solar constant or by increases in the carbon dioxide concentration. This increase in hurricane frequency is consistent with a more poleward position of the intertropical convergence zone and a sensitive dependence of hurricane genesis on the convergence zone location. Varying the amplitude of the cross-equatorial ocean heat flux to isolate the changes in convergence zone latitude confirms the interpretation that changes in convergence zone location are important in determining the global hurricane frequency in this modelling framework. If the convergence zone location is fixed by simultaneously perturbing the radiative balance and ocean heat flux amplitude, the simulated global hurricane frequency decreases under warmed conditions, which is consistent with comprehensive projections of twenty-first century global tropical cyclone frequency. The simulation results allow the dependence of tropical cyclogenesis on mean vorticity, a predictor in "genesis indices", to be quantified.