Revisiting MJO, Kelvin waves, and El Niño relationships using a simple ocean model

Abstract

Mechanisms governing interactions between the Madden–Julian Oscillation (MJO), Kelvin wave activity, and El Nino development are reexamined using the oceanic component of the Zebiak–Cane (ZCocn) model of the Pacific basin. Prescribed wind stress from a free run of the super-parameterized Community Climate System Model version 4 (SP-CCSM4) is used to force ZCocn and the simulated El Nino events are analyzed with respect to their relationship with the MJO wind forcing. Composites of El Nino events strongly influenced by the MJO show the earlier onset of a flattened, El Nino-like state of the thermocline. In contrast, the composites of El Nino events not influenced by the MJO winds show a later onset, and are dominated by a transient-like thermocline along with 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 necessarily 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 Nino. 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 the resulting El Nino events. The out-of-phase interactions between MJO and oceanic Kelvin waves explain the observed lack of influence of MJO onto some El Nino events.