The dryness control of MJO emergence

Despite decades of researches into the Madden–Julian Oscillation (MJO) dynamics, the mechanisms of the MJO initiation from the eastern Africa–western Indian Ocean remain a major barrier to subseasonal predictions. Here, we identify a unique controlling effect on the MJO emergence by the preceding suppressed convection over the Indian Ocean (IOSC) and western Pacific (WPSC) that likely originates from Southern-hemisphere wave trains. A topographic pathway is tactfully revealed: East-African highlands force southerly transitions via the mechanical blocking of equatorial westerlies responding to an anomalous convective pattern. Consequently, moist air is advected from equatorial Africa to fuel the MJO emergence. Ensemble high-resolution simulations confirm this hidden topographic link. Moreover, the IOSC flattens the background moisture gradient over the Indian Ocean, while the WPSC introduces anomalous westerlies due to its westward propagation, both lowering the efficiency of the horizontal moisture advection in initiating the MJO. These findings advance our understanding of MJO genesis pathways from perspectives of topographic-link and moisture sources over equatorial Africa.