Impacts of ENSO on the Bay of Bengal Summer Monsoon Onset via Modulating the Intraseasonal Oscillation

Abstract

AbstractThe Bay of Bengal (BoB) summer monsoon is often triggered by northward propagating intraseasonal oscillations (ISOs) in the eastern Indian Ocean. The timing of summer monsoon onset, with a mean date of 1 May and standard deviation of 13 days, is substantially advanced (delayed) following a cold (warm) ENSO event. In this study, the significant ENSO‐onset anomaly relationship is understood by investigating the ENSO modulations on the pivotal monsoon trigger. In the decaying spring of a cold (warm) ENSO event, pronounced easterly (westerly) vertical wind shear anomalies are induced in the northern Indian Ocean due to the enhanced (suppressed) convection anomalies in the northwestern Pacific Ocean. The above interannual vertical shear difference dominantly (more than 60%) contributes to a distinct ISO intensity contrast in the northern BoB. Consequently, the strengthened (weakened) ISOs associated with cold (warm) ENSO events lead to advanced (delayed) onsets of BoB summer monsoon.