Mechanisms of Delayed Monsoon Onset with Warming

Comprehensive general circulation models (GCMs) in the CMIP3/CMIP5 archive project a delay in the timing of monsoon onset, as the climate warms in response to greenhouse gas (GHG) concentration increases. It has been argued that surface latent heat flux, and its differing response to GHG perturbations over land and over ocean, plays an important role in the redistribution of rainfall from early to late in the warm season in monsoon regions. However, similar phase delays in tropical precipitation have been shown to occur even in warming aquaplanet simulations forced by sea surface temperature perturbations. In this study, we explore mechanisms of delayed monsoon onset with warming in aquaplanet simulations with an idealized GCM spanning a wide range of climates. In these simulations, as the climate is warmed, the onset of the monsoon is progressively delayed to later pentads in the summer season, in agreement with results from the comprehensive GCMs. However, the end of the monsoon season varies less strongly with climate, resulting in a progressive shortening of the overall monsoon season as the climate is warmed. The atmospheric energy balance is examined to separate possible influences of changes in surface fluxes, atmospheric energy storage and gross moist stability on the circulation's seasonality. Radiative-convective equilibrium experiments with the same GCM are also examined, to explore if and to what extent the delayed monsoon onset can result from increases in the effective heat capacity of the atmospheric column with warming, and hence an increasingly lagged response of tropical precipitation to the seasonal cycle of insolation.