Tropical Cyclone Frequency and ITCZ Dynamics in Zonally Asymmetric Aquaplanet Simulations

Global tropical cyclone (TC) frequency and intertropical convergence zone (ITCZ) dynamics are investigated using a suite of simulations performed with a 50-km-resolution aquaplanet configuration of the GFDL Atmosphere Model 4.0, forced by fixed sea surface temperature (SST) distributions. In simulations with zonally symmetric SST profiles, TC frequency per unit area is found to be proportional to the Coriolis parameter at the ITCZ, as defined by the latitude of maximum precipitation. When the same scaling is applied to observed precipitation and SST data, to predict TC frequency in each ocean basin and month, it captures a substantial fraction of observed TCs. Additional simulations are performed with uniform warming (+2°C) and cooling (-2°C) of the zonally symmetric SST profiles, which produces modest effects on the TC frequency and the latitude, width, and strength of the ITCZ. Finally, a series of simulations is performed in which the central latitude of the SST distribution is sinusoidally perturbed by successively larger amplitudes, yielding a zonally asymmetric SST distribution with an increasingly pronounced wave-1 pattern. The resulting precipitation distribution is highly zonally asymmetric in both latitude and strength, with a strong precipitation maximum on the eastward flank of the region where the warm SSTs extend poleward. The TC frequency also is highest in this region, and the number of TCs no longer responds so strongly to the latitude of the ITCZ alone. Overall, these results suggest that in addition to the Coriolis parameter at the ITCZ, which appears to be a determinant of the TC frequency in a zonally symmetric sense, the ITCZ width, strength, or other factors also play a role in determining TC frequency in zonally asymmetric configurations.