Near-linear response of mean monsoon strength to thermal forcings

Previous studies have asserted that monsoons may contain "tipping points", in which the seasonal mean circulation shifts between a moist and a dry stable state once the thermal forcing reaches a critical value. Here it is shown that the theoretical models on which such assertions were based omitted dominant terms in the equations of motion, and that a corrected model of a monsoon exhibits a continuous and nearly linear response to changes in the forcing. An ensemble of comprehensive climate model simulations is also conducted with varying South Asian land surface albedo, and time-mean monsoon precipitation is found to scale nearly linearly with the albedo-controlled thermal forcing. Thus, neither physically sound theoretical models nor comprehensive climate models support the idea that seasonal mean monsoon strength will undergo large shifts as the strength of the mean summer forcing is varied. The relevance of nonlinear dynamical theories for monsoon onset to this scaling of mean summer monsoon strength is discussed. Additionally, several climate model ensembles are used to assess how seasonal mean monsoon strength scales with a wide range of greenhouse gas and aerosol forcings.