LAUSR

This study investigates the physical mechanisms responsible for the impacts of the Indian summer monsoon (ISM) on the evolution of the El Niño–Southern Oscillation (ENSO), with a focus on understanding the monsoon-induced Pacific air-sea interactive processes. An observational analysis displays that a weaker (stronger)-than-normal ISM can strengthen (weaken) an ongoing El Niño event and weaken (strengthen) a La Niña event. A 1,000-year output from the Community Earth System Model version 2 is capable of reproducing this observed feature and is therefore used to explore the responsible ocean-atmosphere interactive processes involved. Results show that a weak (strong) ISM can cause a cyclonic (an anticyclonic) circulation over the western North Pacific via stimulating atmospheric cold (warm) Kelvin waves. The westerly (easterly) wind anomalies on the southern flank of the anomalous cyclone (anticyclone) generate eastward (westward) current anomalies in the mixed layer and thus induce anomalous warm (cold) zonal advection. Furthermore, the wind anomalies excite oceanic downwelling (upwelling) Kelvin waves, which deepen (shoal) the thermocline in the equatorial eastern Pacific and result in anomalous warm (cold) vertical advection. A quantitative mixed-layer heat budget analysis demonstrates that the influence of the monsoon-induced Pacific wind anomalies on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback. This result is confirmed by model sensitive experiments in which additional monsoon heating or cooling anomalies are imposed over the Indian region during the developing summer of an ENSO event.