Northward Propagation of the First Monsoon Intraseasonal Oscillation in the Arabian Sea: A Moisture Mode Perspective

ABSTRACT The summer monsoon onset in the Arabian Sea (AS) is closely linked to the northward propagation of the first monsoon intraseasonal oscillation (FMISO). Based on the moisture mode theory, this study investigates the underlying mechanism driving the FMISO's northward movement. Originating in the equatorial western Indian Ocean, the FMISO evolves eastward and northward across the AS, manifesting as a southeast‐northwest‐oriented convective belt that ultimately triggers the AS summer monsoon. The FMISO's northward propagation is governed by meridional contrast in moist static energy (MSE), with accumulation occurring north of the convection center and dissipation to the south. MSE budget analysis reveals that the zonal MSE advection, dominated by the moisture component, serves as the primary driver of MSE tendency. Physically, during the monsoon transitional period, the seasonal moisture evolution in the northern Indian Ocean has formed a mean eastward moisture gradient in the AS. Its interactions with FMISO‐induced easterlies (north of the convection center) and westerlies (south of the convection center) enhance moisture to the north and reduce it to the south, thereby propelling the FMISO northward. During the FMISO's northward propagation, notable positive sea surface temperature anomalies appear to the north of the convection center. By altering the air‐sea turbulent heat fluxes, these anomalies account for approximately 35%–40% of the positive MSE tendency, though their signal is obscured in net turbulent heat flux measurements due to complete cancellation by the pronounced negative effects of surface wind speed. This detailed exploration of FMISO dynamics potentially advances our understanding of the monsoon onset process in the AS.