Tropical Cyclone Frequency under Varying SSTs in Aquaplanet Simulations

Abstract:

Tropical cyclone (TC) activity is investigated in a 50-km-resolution aquaplanet configuration of the GFDL Atmosphere Model 4.0, forced by zonally symmetric sea surface temperature (SST). As the latitude of maximum SST is shifted by 5° increments from the equator to the pole in successive model experiments, TC frequency exhibits a nonmonotonic dependence, increasing as the SST maximum moves northward from the equator and then decreasing as the SST maximum moves northward past 25°N. Through linear regression, this nonmonotonic trend is modeled as a function of the Coriolis parameter at the ITCZ and the area of the latitude band of maximum SST. As the SST maximum is shifted northward from the equator to 25°N, the ITCZ also shifts northward, and the increase in TC frequency is proportional to the Coriolis parameter at the ITCZ, supporting the idea that TC genesis occurs via precursors originating as instabilities at the ITCZ. As the SST maximum is shifted northward from 25°N, the ITCZ remains stationary, and the TC frequency per unit area is relatively constant. High-energy, long-lived TCs form even when the SST maximum is placed in the midlatitudes. Global precipitation and energy budgets provide additional context for the TC activity in each simulation. Results suggest that future changes in TC activity will be modulated by changes in the large-scale circulation, and in particular that the location of the ITCZ is an important determinant of the number of TCs.