Effect of Continental Geometry on the Timing of Monsoon Onset in a Warmer Climate

Abstract:

GCMs in the CMIP5 archive robustly project a delay in the timing of monsoon onset with warming. Several mechanisms have been proposed but their relative contributions remain debated. In this study, we explore the responsible mechanisms in a set of idealized aquaplanet simulations spanning a wide range of climates. We use an idealized aquaplanet model with a slab ocean, where land and ocean differ only by the mixed-layer depth (MLD). While some existing explanations of delayed monsoon onset emphasize the role of the differing response of surface latent heat flux over land and over ocean, aquaplanet simulations with uniform MLDs show similar phase delays. We analyze the angular momentum budget to focus on how warming may affect the rapidity and timing of the regime transition of the tropical circulation from an eddy-driven into a more angular momentum conserving Hadley cell. We also examine simulations with different zonally symmetric configurations of Northern Hemispheric land that extends poleward from southern boundaries at various latitudes and explore if and how the timing of monsoon transitions is affected by the presence and position of a continent.

CMIP models also predict an overall weakening of the tropical circulation. We hence analyze the atmospheric energy budget to connect the energetic and the angular momentum budget frameworks and to better understand how continental geometry in a warmer climate may also affect the Hadley cell strength. Through idealized warming simulations with various continental configurations, we hope to lay a strong foundation of the zonal mean response to improve predictions of how warming may affect the onset timing of various regional monsoon systems around the world.