A Moist Entropy Budget View of South Asian Summer Monsoon Onset

The start of the South Asian summer monsoon is characterized by an abrupt dry-wet transition, known as monsoon onset, which is associated with a reversal of the large-scale circulation interacting with monsoon convection. Recent studies have advanced understanding of the abrupt onset of monsoons in general, and a few simple theories building on idealized simulations have been proposed. However, there is still a gap between these simple theories and the real South Asian monsoon onset.

To explore the physical processes involved in the onset of South Asian summer monsoon, this study analyzes the moist entropy budget over the monsoon region using high-resolution GCM simulations with explicit representation of convection. The results show that the onset of the South Asian summer monsoon is a two-stage transition. During the first stage, which occurs from early March to late May, the export of column-integrated moist entropy by the large-scale circulation slowly increases, while the source of column-integrated moist entropy (i.e., the sum of surface fluxes and radiative heating) gradually increases as the solar insolation shifts northward. The second stage corresponds to the short period during which the monsoon transitions from a winter to a summer state, and marks a sudden increase in the export of column-integrated moist entropy by the large-scale circulation, accompanied by a sudden decrease in radiative cooling and a rapid increase in latent heat flux. The radiative heating and latent heat flux evolve quickly compared to the seasonal progression of the solar forcing, indicating the important roles of cloud-radiation and wind-evaporation feedbacks in the abrupt monsoon onset. Cloud-radiation and/or wind-evaporation feedbacks are disabled in numerical experiments, in which the source of moist entropy over the monsoon region increases more slowly and peaks at lower values compared to the control run. Correspondingly, the monsoon region experiences much smoother increase of precipitation without these positive feedbacks, confirming the importance of cloud-radiation and wind-evaporation feedbacks to the abruptness of South Asian summer monsoon onset.