Although a growing number of studies have explored the Madden-Julian Oscillation (MJO) and its sub-seasonal influences in the EP- and CP-type of El Niños, whether and how the MJO and its remote weather-climate impacts vary under the two types of La Niña have not been investigated yet. Using observations and reanalysis products, we have performed a comprehensive study to understand the contrasts of MJO intensity and propagation under the EP and CP La Niñas. Compared to the CP La Niña, the MJO convection and circulation are more energetic to the west of the dateline under the EP La Niña. In addition, the MJO under the EP La Niña can propagate further reaching east of the dateline with a faster speed even comparable to that under the El Niño, corresponding well to the relatively wider warm waters over the Indo-Pacific warm pool. Faster MJO with a larger zonal scale under the EP La Niña is associated with a deep convection-circulation coupled system involving more vigorous ascending motions in the boundary layer and stronger easterly wind anomalies leading the MJO deep convection over the Indian Ocean. Moreover, the faster MJO under the EP La Niña features a wetter boundary layer and stronger moisture tendency in front of the deep convection center. The contrasts of MJO propagation under the two types of La Niña in the “moisture mode” context are mainly contributed by the zonal advection process, which is dominated by the advection of background mean moisture due to zonal wind anomalies and also the advection of anomalous moisture due to background mean zonal winds. The meridional advection process, as one of the fundamental causes of MJO eastward propagation, does largely source the lower-tropospheric moistening ahead of MJO deep convection but has comparable amplitudes under the two types of La Niña. The invigoration of the zonal moisture advection mainly arises from the zonal shift of the SST cooling center between EP and CP La Niña, which causes a net zonal gradient of background mean moisture over the western Pacific. Besides, the further penetrating low-frequency background westerly and stronger MJO winds and humidity over the western Pacific also support the invigoration of zonal advection under the EP La Niña. Observational results are further confirmed by the full AGCM experiments.

Madden-Julian Oscillation,El Nino/Southern Oscillation,Tropical-Extratropical Teleconnection,La Nina diversity,CESM