Initiation of the MJO vs. non-MJO convective events

Forecasting convective initiation of the Madden-Julian Oscillation (MJO) faces two challenges. The first is the timing of a transition from convectively suppressed to active period over the tropical Indian Ocean, which marks the initiation of a large-scale convective event. The second is the timing when large-scale convective envelope of the MJO starts to move eastward and thus marks the convective initiation of the MJO. Only about 50% of large-scale convective events over the tropical Indian Ocean become MJO events. Others cease to exist without any eastward movement. This study attempts to distinguish convective events that become MJO events and those that do not (“failed events”). Criteria are first determined for selection of large-scale convective events and then the RMM index is used to confirm its eastward motion or a lack of it. Composites are made before and after the local (averaged over the tropical Indian Ocean) convection maximum for both MJO events and failed events. Variables that are thought crucial to initiation of both convective and MJO events are used in the composites. These variables include moisture, winds, divergence, diabatic heating, potential vorticity and its generation. Preliminary diagnostics suggest that increases in low-level moisture take place in both MJO and failed events, suggesting any interpretation of such increases, such as those in the context of “recharge-discharge) paradigm, is applicable for initiation of general convective events, but not necessarily for convective initiation of the MJO. It appears that the horizontal and vertical structure of diabatic heating may hold the key to distinguish the MJO from failed convective events.