Propagation characteristics of Madden-Julian Oscillations (MJOs) are significantly influenced by the phase of the El Niño-Southern Oscillation (ENSO). However, it is unclear how far eastward the MJO enters the western Pacific during different ENSO phases. The western Pacific remains a poorly simulated region for the MJO propagation as well. In this study, we discover the MJO propagates further eastward during La Niña compared to that during El Niño, even though warm water expands much further toward the east. In addition, we identify a distinct MJO southward shift during La Niña. The southward shift can be as far as to 25°S. As a result, the MJO reaches the South Pacific Convergence Zone (SPCZ). To reveal the underlying mechanism, the moist static energy (MSE) budget is diagnosed using reanalysis products. It is found that the eastward propagating MJO in the SPCZ is mainly controlled by the increase in MSE due to zonal advection on intraseasonal and synoptic scales. This indicates that differences in sea surface temperature (SST) in the western Pacific during ENSO do not affect how far eastward the MJO travels. During La Niña, enhanced low-level northwesterly and westerly winds associated with cross-equatorial northly surge allow moisture convergence to intensify over the southern Maritime Continent, leading to a southward detour in the MJO eastward propagation. The results of the study deepen our understanding of the mechanisms of MJO propagation in the western Pacific and highlight the need to properly consider the role of SST in the different stages of MJO propagation.

Madden-Julian Oscillation,Western Tropical Pacific,El Nino/Southern Oscillation