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

Revealing how the MaddenJulian oscillation (MJO) is initiated in the Indian Ocean (IO) is one of challenging tasks. Although many previous studies have focused on large-scale moisture variations as the preconditioning for MJO initiation, when MJO convection is triggered in the moist environment is still non-trivial. To fill this deficiency in thermodynamic-driven processes, we propose a possibly important dynamic process for MJO initiation, in which upper-tropospheric dry mixed Rossby-gravity waves (MRGs) are essentially involved. The observational data analysis of an MJO event during the CINDY2011 field campaign shows that upper-tropospheric MRG amplification in the western IO (WIO) is the robust precursor of the MJO initiation, and that the subsequently formed low-level MRG wave packets are associated with the MJO propagation in the IO. Here we hypothesize that the dry interaction between MRGs and the zonally-narrow downward branch of the Walker circulation above the WIO (referred to as the Wall) are the key to MJO initiation. To test our hypothesis, we perform numerical simulations using a simple dry model with three vertical modes. The simulations reveal that the upper-tropospheric MRGs in the IO are amplified particularly in WIO by their westward advection and wave accumulation due to the upper-level convergence in mean easterlies blowing into the Wall. The model also predicts downward dispersion of the amplified upper-tropospheric MRGs and resultant low-level MRG wave packet formation. This MRG evolution consistently explains the MJO initiation process during CINDY2011, which is further verified by ray tracing for MRGs. In addition, we present that 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). Our results support a notion that MJO initiation on the intraseasonal time scale is potentially supported by the cross-scale interaction between synoptic-scale dynamic variations and climatological Walker circulations.