Controls on the Width of Tropical Precipitation and Its Contraction Under Global Warming

Abstract

Climate models robustly and unanimously simulate narrowing of the intense tropical precipitation under greenhouse gas forcing. We argue that the meridional width of tropical precipitation is controlled by the seasonal meridional range of the Intertropical Convergence Zone (ITCZ). The contraction of tropical precipitation under greenhouse forcing results from a reduced seasonal range of ITCZ migration. An energetic theory—similar to the energetic theory for ITCZ shifts based on the hemispheric contrast of energy input to the atmosphere—is developed. The meridional width of tropical precipitation is proportional to the seasonal range of the interhemispheric contrast in atmospheric heating divided by the efficiency of atmospheric cross-equatorial heat transport. Climate models are biased toward overly expansive tropical precipitation resulting from an exaggerated seasonal atmospheric heating. The robust contraction of tropical precipitation under global warming results from increased efficiency of interhemispheric energy transport consistent with enhanced gross moist stability of the tropical atmosphere.