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 reanalysis data and 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 stays largely constant, 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 outgoing longwave radiation and a rapid increase in latent heat flux. The outgoing longwave radiation and latent heat flux evolve quickly compared to the seasonal progression of the solar forcing, indicating the important roles of moisture-radiation and wind-evaporation feedbacks in the abrupt monsoon onset. The relevance of South Asian monsoon onset to the threshold behavior of the meridional circulation documented in previous studies is discussed.