Model uncertainty in tropical rainfall projection: The crucial role of ocean warming pattern and atmospheric circulation coupling

Abstract:

Model uncertainty in projected tropical precipitation change is studied by using 26 models from Phase 5 of the Coupled Model Intercomparison Project. Intermodel spread in projected rainfall change reaches maximum in the tropics where the rainfall response to global warming is large. Moisture budget analyses reveal that much of the model uncertainty in tropical rainfall change originates from intermodel discrepancies in the dynamical contribution due to the atmospheric circulation change. Intermodel Singular Value Decomposition (SVD) analyses further show a tight coupling between the intermodel variations in tropical SST warming pattern and circulation change. Organized into two robust modes: one is a interhemispheric asymmetric mode and the other is an equatorial peak mode. It is also found that these two uncertainty modes exert a strong control on the spread of changes in tropical cloud cover and cloud radiative effect among models, implying the importance of studying underlying mechanisms for the intermodel variability in tropical circulation change. Indeed, the intermodel differences in interhemispheric asymmetry of extratropical cloud radiative flux change at the top of the atmosphere are suggested to contribute to the tropical interhemispheric asymmetric mode. The Hadley circulation change and wind-evaporation-SST feedback may aid the tropical adjustments to extratropical radiative forcing.