MJO Initiation Triggered by Amplification of Upper-Tropospheric Dry Mixed Rossby-Gravity Waves

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). Plain Language Summary In the tropics, there exists a huge cluster of clouds and rainfall systems moving from the western Indian Ocean (WIO) to the western Pacific, called the Madden-Julian Oscillation (MJO). It is of great interest how and when MJO clouds are formed in the WIO because of their large impacts on global weather patterns, but we have not fully understood it yet. Using a simplified computer simulation and observational data, we find that the formation of MJO tall clouds can start from the "upper-sky" (similar to 10 km altitude) short-period wind variations. During the no-cloud period of MJO, energy of upper-sky wind variations can be input above the central Indian Ocean. Then, it is carried to the WIO and amplified there by upper-sky easterly winds and their convergence associated with the seasonal atmospheric circulation. Because the amplified upper-sky wind energy above the WIO is further dispersed downward, near-surface wind variations become active, which triggers MJO clouds. This new mechanism is theoretically plausible but has been confirmed only in a limited case. It thus should be evaluated for more observations. Key Points Amplification of upper-level mixed Rossby-gravity waves (MRGs) above the western Indian Ocean (WIO) can lead to Madden-Julian oscillation (MJO) initiation via low-level MRG-wave packet formation Upper-level MRGs propagating into the WIO are amplified by wave accumulation through the mean Walker circulation and then dispersed downward Upper-level circumnavigating Kelvin waves can assist MRG-induced MJO initiation by promoting MRG wave accumulation in the WIO