Understanding the dynamics of Madden–Julian Oscillation (MJO) is crucial for enhancing subseasonal–seasonal prediction of global extremes. This study has explored the modulation of Indian Ocean Basin Mode (IOBM) on the MJO during May–August, with minimal concurrent influences of El Niño-Southern Oscillation. Results show that the zonal phase speed of the MJO under the warm IOBM (WI) phase is reduced by approximately 26% than that under the cold IOBM (CI) phase. Additionally, compared to the CI phase, the WI phase yields more weakened convective activity of MJO over the Maritime Continent (MC). These various MJO characteristics under the IOBM modulation are understood by examining dynamical-wave and moisture-convection feedbacks. For the MJO under the WI phase, the stronger Rossby westerly and weaker Kelvin easterly jointly slowdown the eastward propagation of the MJO, while the opposite holds for the accelerated MJO under the CI phase. Furthermore, the easier propagation of CI MJO over the MC than WI MJO is related to a stronger pre-moistening over the western Pacific, which is mainly contributed by the high-frequency eddy moisture transport and secondly by the meridional advection of anomalous moisture under IOBM-related low-frequency background winds. Besides these two feedback processes, we also discuss contrasts of WI and CI MJOs in their initiation location and northward propagation over the IO under the IOBM modulation. Observational results are partly reproduced by climate model experiments. These findings should advance our basic understanding of multiscale interactions among high-frequency transients, MJO and low-frequency background mean state in the tropical atmosphere.
Wang et al. (Thu,) studied this question.