Impact of ENSO on MJO Pattern Evolution over the Maritime Continent

The modulation of Madden–Julian oscillation (MJO) pattern evolution over the Maritime Continent (MC) by El Niño–Southern Oscillation (ENSO) was investigated through a combined observational and modeling study. MJO convective branches shifted south of the equator over the MC during eastern Pacific (EP) El Niño winters, while it became relatively symmetric about the equator during La Niña winters. The impact of central Pacific (CP) El Niños to MJO pattern, on the other hand, is not statistically significant. The cause of the distinctive MJO pattern evolutions is likely attributed to the ENSO-induced changes of the background moisture and vertical shear over the MC. Idealized numerical experiments with a 2.5-layer model were carried out, and the result revealed that the background moisture change played a dominant role. An observational diagnosis of column-integrated moist static energy (MSE) budgets was further conducted. The result indicated that the MJO pattern difference was attributed to the MSE tendency asymmetry in front of MJO convection between EP El Niño and La Niña, caused by the advection of the mean MSE by anomalous meridional wind. The difference in the MJO-scale anomalous meridional wind was ultimately controlled by the change of the background meridional moisture gradient associated with EP El Niño and La Niña.