Fast and Slow MJO Modes Modulated by ENSO in Boreal Winter

This study investigates modulation of the El Niño-Southern Oscillation (ENSO) on the Madden-Julian Oscillation (MJO) propagation during boreal winter. Results show that the spatial-temporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Niño, while the MJO during La Niña 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 Niño. Based on the dynamics-oriented diagnostics, it is found that, during El Niño, 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 pre-moistening 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 Niña. 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 Niño, as opposed to the westward and poleward gradients during La Niña, induces moist advection over the equatorial eastern MC-EWP due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. Role of nonlinear advection by synoptic-scale Kelvin waves is also non-negligible 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 the ENSO.