Role of Intraseasonal Oscillation in Linking the Stepwise Onset of Summer Monsoon in the Northern Indian Ocean

ABSTRACT The summer monsoon onset in the northern Indian Ocean is crucial for the densely populated South Asia, as it ends the pre‐monsoon heatwave and kicks off the rainy agricultural season. Climatologically, this onset occurs in two distinct phases: first in the Bay of Bengal (BoB), followed by the Arabian Sea (AS). However, the possible mechanistic linkage between these phases remains unclear. Based on observational analysis, this study investigates how the BoB monsoon onset preconditions the subsequent AS monsoon onset, with particular focus on the pivotal trigger of the AS monsoon—the intraseasonal oscillation (ISO‐AS). We demonstrate that the BoB monsoon onset establishes an easterly vertical wind shear across the northern Indian Ocean. The shear environment interacts with the ascending motion of the ISO‐AS, which develops approximately 1 month later, enhancing cyclonic vorticity north of the convection center of the ISO‐AS. This process, in turn, promotes moisture convergence in the boundary layer and facilitates the northward propagation of the ISO‐AS, ultimately triggering the summer monsoon onset in the AS. By elucidating the stepwise nature of monsoon onset in the northern Indian Ocean, this work offers valuable insights for improving predictions of the Asian summer monsoon.