Revisiting MJO, Kelvin Waves, and El Niño Relationships Using a Simple Ocean Model

Mechanisms governing interactions between MJO, Kelvin wave activity, and El Niño development are reexamined using the Zebiak - Cane (ZC) model of the Pacific basin. Prescribed wind stress from a free run of the Super-Parameterized Community Climate Model version 4 (SP-CCSM4) is used to force ZC and composites of strong (SCtl) and weak (WCtl) events based on an index measuring MJO influence on the ocean state reveal that SCtl shows more efficient coupling of the thermocline to the surface winds and earlier onset of an El Niño - like state of the thermocline. In contrast, WCtl events are dominated by a transient-like thermocline and relatively extensive periods of upwelling Kelvin wave activity. Sensitivity experiments performed to identify whether modifying MJO wind stress and oceanic Kelvin wave activity influences these features show that although MJO contributes to the development of these features, it is not the primary driver. The relative phasing between MJO and oceanic Kelvin wave activity seems to be the most important factor governing the influence of MJO on El Niño. When in phase and collocated with Kelvin wave activity, MJO westerly wind stress contributes to the amplification of preexisting downwelling Kelvin waves, leading to earlier onset and greater strength of resulting El Niño events. The out-of-phase interactions between MJO and oceanic Kelvin waves explain the lack of influence of MJO onto some El Niño events.