Aquaplanet monsoons and their response to climate changes
Abstract:In idealized aquaplanet general circulation model (GCM) simulations with an Earth-like mean climate, the tropical 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.
Here, we explore the dynamics of these 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, which can be understood through the perturbation energy budget and changes in the moist energy stratification. We also find that as the climate is warmed, the monsoon onset is progressively delayed to later pentads in the warm season, while the monsoon end 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.