Understanding nonlinear tropical precipitation responses to CO2 forcing



[1] Using a traceable framework of idealized General Circulation Model experiments, a nonlinear dependence of tropical precipitation pattern change on CO2 forcing is identified. These nonlinearities are relatively large and widespread throughout the tropics and so should not be neglected in projections of future precipitation change. This has implications for the use of pattern scaling and simple climate models to produce precipitation projections and for physical understanding of precipitation change across forcing scenarios. The nonlinearities can be understood by considering that processes which cause precipitation change, such as increasing moisture, a weakening circulation, and convergence zone shifts, interact in a nonlinear manner even when individual processes are quasi-linear. Three driver interactions are identified: “warm-shift”, “warm-weak”, and “shift-weak”. Combined with Clausius-Clapeyron nonlinearity in moisture increase, these interactions drive the nonlinear pattern change. A strong convergence feedback response substantially amplifies the nonlinearity. This analysis is limited to ocean regions, as mechanisms are simpler than over land.