Onset of the Bay of Bengal summer monsoon and the seasonal timing of ENSO's decay phase

Abstract

ABSTRACTBased on multiple sources of atmospheric and oceanic data, this study demonstrates a close relationship between the onset of the Bay of Bengal (BOB) summer monsoon (BOBSM) and the seasonal timing of El Niño–Southern Oscillation (ENSO)'s decay phase. Through distinguishing ‘later‐decay’ and ‘normal‐decay’ ENSO events, it is found that a later/earlier onset of the BOBSM following El Niño/La Niña is mainly caused by later‐decay ENSO events, while no significant changes in BOBSM onset can be identified between normal‐decay El Niño and normal‐decay La Niña events. Diagnosis of the related dynamic and thermodynamic processes further confirms that, for later‐decay ENSO events that remain active until mid‐April, persistent ENSO‐induced ‘atmospheric‐bridge’ processes can yield a stronger vertical coupling of circulation between the upper and lower troposphere, and thus an anomalously earlier (later) BOBSM onset following a later‐decay La Niña (El Niño). In the lower troposphere, the persisting zonal sea surface temperature (SST) gradient between the Indian Ocean and the western Pacific following later‐decay ENSO events can significantly modulate the lower tropospheric barotropic instability over the northern BOB, which in turn overwhelms the local SST conditions to dominate the development of monsoon convections. In the upper troposphere, later‐decay ENSO alters the position of the South Asian high and the upper atmospheric divergence‐pumping through the anomalous Walker circulation. In contrast, due to the earlier damping of normal‐decay ENSO, the BOBSM onset processes are barely modulated.