Relative Roles of Preconditioning Moistening and Global Circumnavigating Mode on the MJO Convective Initiation During DYNAMO

The relative importance of preconditioning moistening and global circumnavigating mode in the convective initiation of the October 2011 Madden-Julian Oscillation (MJO) event observed during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign is investigated using a series of convection-permitting regional model simulations. It is demonstrated that the MJO convective initiation is largely controlled by the global circumnavigating mode at the intraseasonal scales. Rapid moistening closely related to this eastward propagating mode a few days prior to the MJO active phase is crucial to the initiation of deep convection and enhanced rainfall. This moistening process nevertheless cannot be accurately described by the discharge-recharge hypothesis, which speculates the importance a gradual moisture buildup over an approximately 2-week period leading to the arrival of the active MJO phase. Plain Language summary The Madden-Julian Oscillation (MJO) is a dominant mode of intraseasonal (20-100 days) variability over the tropics. The convective initiation of the MJO corresponds to a transition from predominate shallow clouds to widespread deep convection over tropical oceans. The physical mechanisms related to the MJO convective initiation are still debated. This study uses a series of convection-permitting regional model simulations to show that the gradual buildup of moistening described by the discharge-recharge hypothesis cannot accurately describe the convective initiation of the October MJO event observed during the Dynamics of the Madden-Julian Oscillation field campaign. Rapid moistening in the middle troposphere a few days before the MJO active phase is important to the MJO convective initiation. A global eastward propagating circumnavigating mode at the intraseasonal time scales is primarily responsible for the middle-level preconditioning moistening and the eventual convective initiation and enhanced rainfall during the active MJO phase.