Response of aquaplanet monsoons to changes in longwave radiation

In idealized aquaplanet general circulation model (GCM) simulations with an Earth-like mean climate, the Hadley circulation undergoes rapid transitions over the course of the seasonal cycle between an equinox regime, in which the divergence of angular momentum by large-scale extratropical eddies strongly influences the circulation strength, and a monsoon regime, in which the eddy momentum flux divergence is weak and the circulation approaches the angular momentum-conserving limit. The rapid transition in the idealized GCM has circulation and precipitation changes similar to those occurring at the onset and end of the South Asian monsoon and results from feedbacks between the large-scale eddies and the overturning circulation that can occur even in the absence of surface inhomogeneities, if the surface has sufficiently low thermal inertia (Bordoni and Schneider 2008). In this study, we explore the dynamics of the aquaplanet monsoon transitions in a wide range of climates in an idealized GCM. The climates are simulated by perturbing the optical thickness of the atmosphere's longwave absorber, in analogy to changes in greenhouse gas concentrations. We focus on how the timing of the onset and decay of the simulated monsoons changes as the climate is warmed or cooled. Changes in monsoonal circulation strength, precipitation intensity and location will also be examined. The relevance of the dynamical and/or thermodynamical mechanisms diagnosed in the idealized simulations for projected changes in the Asian monsoon will be assessed in state-of-the-art GCM simulations in the CMIP5 archive.