Modulation of ENSO on Fast and Slow MJO Modes during Boreal Winter

This study investigates modulation of El Nino-Southern Oscillation (ENSO) on the Madden-Julian oscillation (MJO) propagation during boreal winter. Results show that the spatiotemporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Nino, whereas the MJO during La Nina is very slow and tends to frequently detour via the southern Maritime Continent (MC). The westward group velocity of the MJO is also more significant during El Nino. Based on the dynamics-oriented diagnostics, it is found that, during El Nino, the much stronger leading suppressed convection over the EWP excites a significant front Walker cell, which further triggers a larger Kelvin wave easterly wind anomaly and premoistening and heating effects to the east. However, the equatorial Rossby wave to the west tends to decouple with the MJO convection. Both effects can result in fast MJO propagation. The opposite holds during La Nina. A column-integrated moisture budget analysis reveals that the sea surface temperature anomaly driving both the eastward and equatorward gradients of the low-frequency moisture anomaly during El Nino, as opposed to the westward and poleward gradients during La Nina, induces moist advection over the equatorial eastern MC-EWP region due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. The role of nonlinear advection by synoptic-scale Kelvin waves is also nonnegligible in distinguishing fast and slow MJO modes. This study emphasizes the crucial roles of dynamical wave feedback and moisture-convection feedback in modulating the MJO propagation by ENSO.