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. An alternative explanation invokes energetic arguments, in which elevated latent energy demand in the hemisphere warming up seasonally, as dictated by the Clausius–Clapeyron relation, drives a shift of tropical rainfall towards the opposite hemisphere, manifesting itself as a seasonal delay in the onset of the rainy season. 

 

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 earlier work, we have in fact shown how monsoons with rapid circulation and precipitation changes at the beginning of the warm season can be simulated even without any land-sea contrast, provided that the lower boundary has sufficiently low thermal inertia. As the climate is warmed, we find that 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 indeed result from increases in the effective heat capacity of the atmospheric column with warming, through changes in its latent energy component.