Atmospheric-Oceanic Processes over the Pacific Involved in the Effects of the Indian Summer Monsoon on ENSO

This study investigates the physical mechanisms responsible for the impacts of the Indian summer monsoon (ISM) on the evolution of El Nino-Southern Oscillation (ENSO), with a focus on understanding the monsoon-induced Pacific air-sea interactive processes. An observational analysis displays that a weaker-than-normal ISM can strengthen an ongoing El Nino event and weaken a La Nina event, and conversely for a stronger-than-normal ISM. A 1000-yr 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 ISM can cause a cyclonic circulation over the western North Pacific via stimulating atmospheric cold Kelvin waves, and conversely for a strong ISM. 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 sensitivity experiments in which additional monsoon heating or cooling anomalies are imposed over the Indian region during the developing summer of an ENSO event.

Automated summary

This study investigates the physical mechanisms through which the Indian summer monsoon (ISM) affects the evolution of El Nino-Southern Oscillation (ENSO) by analyzing observations and using a climate model. The analysis reveals that a weaker ISM can enhance an ongoing El Nino event and weaken a La Nina event, and vice versa for a stronger ISM. The climate model simulations show that the ISM influences ENSO by inducing cyclonic circulations and generating wind and current anomalies, which in turn affect oceanic advection and thermocline feedback.