Multiplicative MJO Forcing of ENSO

The Madden–Julian oscillation (MJO) is parameterized to study the role of the feedback it receives from sea surface temperature (SST) in its influence on El Niño–Southern Oscillation (ENSO). The parameterization describes MJO surface westerlies in terms of a few basic parameters that include amplitude, zonal propagation extent, propagation speed, and the interval between adjacent events. It is used to drive a coupled ocean–atmosphere model of intermediate complexity tuned to a marginally stable regime. The MJO parameters acquire values either additively (i.e., based on observed estimates of most probable value and stochasticity) or multiplicatively (i.e., modulated by an evolving model ENSO SST, albeit with some stochasticity). Simulations reveal that ENSO variance increases with the stochasticity of MJO amplitude but is insensitive to the stochasticity of zonal extent and speed; except that ENSO vanishes completely when the propagation speed is zero. Likewise, ENSO strengthens linearly with the SSTmodulation of MJO amplitude, but not of speed and zonal extent—even though the two are known to be significantly influenced by SST. Ensemble comparisons between simulations with and without SST feedback demonstrate that SST feedback to the MJO acting in a stable regime can be responsible for the observed ENSO variance. The multiplicative case has a larger ensemble spread than the additive case, whichmanifests in a larger interdecadal variability of ENSO. The results emphasize that ENSO reproduction in coupled models depends upon correctly representing the MJO, especially its amplitude and SST feedback.