The Role of Convection–Circulation Coupling in Abrupt South Asian Monsoon Onset: Insights From SP‐CAM

Abstract This study investigates the effects of convection‐circulation coupling on the onset of the South Asian monsoon from a sectorial zonal‐mean perspective, using the Super‐parameterized Community Atmospheric Model (SP‐CAM), which couples Community Earth System Model version 1 (CESM1) with a cloud resolving model. The SP‐CAM simulations shows an equatorially symmetric meridional circulation during spring, representing the eddy‐driven regime, and a cross‐equatorial circulation during summer, indicating the angular momentum‐conserving regime. Focusing on the onset period, the SP‐CAM simulations exhibit much more abrupt onset, compared with onset in CESM1 without coupling with a cloud resolving model. Our analysis suggests that the shallow convection over the northern Indian Ocean 20 days before onset contributes to the abruptness of the South Asian monsoon onset. Based on a boundary‐layer‐dominated perspective from the convective quasi‐equilibrium (CQE) framework, we propose that the pre‐onset shallow convection influences the South Asian monsoon onset through its distinct characteristics of energy transport: The pre‐onset shallow circulation transports the boundary‐layer moist static energy (MSE) into the northern Indian Ocean and causes its accumulation near 15°N. The associated increased gradient of boundary‐layer MSE subsequently triggers the development of thermal direct circulation, representing monsoon onset. Our study highlights the role of pre‐onset shallow convection and the associated circulation on the South Asian monsoon onset, suggesting the importance of convective variability in South Asian climate.