Abrupt onset and slow seasonal evolution of summer monsoon in an idealized GCM simulation

The world's greatest monsoons and deserts co-exist at the same latitudes on the subtropical Afro-Eurasian Continent. To investigate the mechanisms for these diverse subtropical climates, we conduct atmospheric general circulation model experiments under an idealized land-sea distribution featuring an Asian continent with a straight south coast at 17 degrees N. Sea surface temperature and land surface parameters are all set zonally uniform to avoid a priori longitudinal preference for rainfall. The model with hat land surface produces a realistic zonal distribution of precipitation, with arid (wet) conditions over the western (eastern) part of subtropical Afro-Eurasia in association with a gigantic high pressure system over the ocean surface. The key to maintaining the western deserts is the slow precipitation-soil moisture interaction, which keeps the northward migration of the monsoon rainbelt to lag behind the sun. This is demonstrated by conducting a perpetual summer integration, where the Sahara and Arabia are soaked in heavy rain. The model monsoon starts abruptly in late June, three months after the spring equinox when a northward temperature gradient is established near the ground on the south coast. The onset is associated with explosive growth of a westward-traveling moist baroclinic wave. Further analysis suggests a Geostrophic Monsoon model: The geostrophy resists and delays the formation of a thermally direct circulation, until baroclinic instability triggers the rapid onset.