LAUSR

Based on observational data analyses of a Madden‐Julian Oscillation (MJO) event during the field campaign of the Years of the Maritime Continent Project, we propose a new mechanism for MJO convective initiation and propagation to which mixed Rossby‐gravity waves (MRGs) contribute significantly. The intensive observation captured midtropospheric wind variations associated with MRGs, and those signals propagated westward over the Indian Ocean in the MJO‐suppressed phase. The transition to the MJO active phase follows tight MRG‐convection coupling, which is enhanced when the wavelength becomes shorter due to weak midlevel convergence in the western Indian Ocean. Basin‐scale anomalous midtropospheric moistening caused by MRG shallow circulations also contributes. These processes lead to the lower‐tropospheric MRG wave packet formation and successive triggering of MJO convection, with the eastward MRG group velocity corresponding to MJO propagation. Additional analysis for other MJO cases implicates potential for robustness of this MJO‐MRG relationship.