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 find that the strength of the monsoonal, cross-equatorial circulation changes non-monotonically, achieving its maximum at a climate slightly colder than present-day Earth's climate. We also find that as the climate is warmed, the onset of the monsoon is progressively delayed to later pentads in the summer season, while the end of the monsoon varies less strongly with climate. This results in a progressive shortening of the overall monsoon season as the climate is warmed. The energy and zonal momentum budgets are analyzed to account for the simulated changes. Comparisons with comprehensive GCM simulations in aquaplanet configuration and with trends from observed data are also presented.