A Moist Entropy Budget View of the South Asian Summer Monsoon Onset

Using a high-resolution global model with explicit representation of convection, the physical processes involved in the abrupt onset of South Asian summer monsoon are investigated within a moist entropy budget framework. The monsoon onset is a two-stage transition. During the first stage, which starts around 2 months before onset, the source terms of column-integrated moist entropy gradually increase, while the export by the large-scale circulation slowly strengthens. The second stage is marked by a sudden increase of radiative heating and surface latent heat flux 10 days prior to the onset, followed by abrupt strengthening of large-scale export of moist entropy. When either cloud-radiative or wind-evaporation feedback is disabled in numerical experiments, the monsoon experiences much smoother and weaker onset. The evolution of the system in a gross moist stability plane demonstrates that these positive feedbacks destabilize the system and are responsible for the abruptness of the transition.

Duke Scholars

Author Ding Ma DKU Faculty