Atmospheric Contributions to MJO Decay over the Maritime Continent

In this study, we examine two scenarios that lead to the decay of MJO convection over the Maritime Continent (MC). First, we find that roughly half of all MJO events encounter westward-propagating transient dry precursor (TDP) signals over the MC. These TDPs, which are external to the MJO circulation system, overwhelm the eastward-propagating MJO anomaly in the weakest MJO events, leading to an unfavorable environment for convection and MJO termination. MJO events with larger amplitude moist anomalies are better able to withstand TDP drying, and favorable convective conditions and MJO propagation are sustained.

Second, for events not affected by TDPs, MJO decay over the MC occurs because weak drying, rather than moistening, is observed at MC longitudes. For these decaying events, MC drying is the result of a westward, rather than eastward, zonal gradient of MC moisture over the Equatorial MC. Both MC-crossing and MC-blocked MJO events exhibit an eastward moisture gradient just south of the MC and north of Australia—a favored location for MJO convection in many MC-crossing events. However, in decaying events, the tendency for MJO convection to split into two off-Equator gyres in the Indian Ocean leads to strong westerly wind anomalies and drying in this region, which also contribute to MJO decay.

Propagation characteristics of the MJO and TDPs in the vicinity of the MC are both consistent with moistening by horizontal winds acting on background state moisture gradients. Understanding the time and space scales of processes that alter the background moisture distribution may advance our understanding of and ability to simulate the MJO.