A possibly important dynamical process for the Madden–Julian oscillation (MJO) convective initiation is proposed. An MJO event during the “CINDY2011” field campaign is triggered by eastward-moving lower-tropospheric mixed Rossby-gravity (MRG) wave packets, and its leading precursor is predominance of upper-tropospheric MRGs in the Indian Ocean (IO). Simple three-dimensional model experiments reveal that the upper-tropospheric MRGs in the IO are amplified particularly in the western IO (WIO) by their westward advection and wave accumulation due to the upper-level convergence in mean easterlies of the Walker circulation. The model also predicts downward dispersion of the amplified upper-tropospheric MRGs and resultant lower-tropospheric MRG wave packet formation. This MRG evolution consistently explains the MJO initiation process during CINDY2011, which is further verified by ray tracing for MRGs. Upper-tropospheric circumnavigating Kelvin waves assist the proposed mechanism by promoting MRG-wave accumulation (advection) in their westerly (easterly) phases via enhanced zonal convergence and weakened easterlies (enhanced easterlies).

The eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model (NICAM), following the atmospheric model intercomparison project (AMIP) protocol, but with a slab ocean, was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when they are embedded within stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian ocean and eastern Pacific. However, the simulated MJO events displayed a slow bias and occurred disproportionately during El Niño events. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of MJO in long term simulations.