A model intercomparison of the tropical precipitation response to a CO2 doubling in aquaplanet simulations

Abstract

In the present-day climate, the mean Intertropical Convergence Zone (ITCZ) is north of the equator. We investigate changes in the ITCZ latitude under global warming, using multiple atmospheric models coupled to an aquaplanet slab ocean. The reference climate, with a warmer north from prescribed ocean heating, is perturbed by doubling CO2. Most models exhibit a northward ITCZ shift, but the shift cannot be accounted for by the response of energy flux equator where the atmospheric energy transport (F-A) vanishes. The energetics of the simulated circulation shifts are subtle: changes in the efficiency with which the Hadley circulation transports energy, the total gross moist stability (m), dominate over mass flux changes in determining F-A. Even when F-A approximate to 0, the ITCZ can shift significantly due to changes in m, which have often been neglected previously. The dependence of ITCZ responses on m calls for improved understanding of the physics determining the tropical Delta m.