Atmospheric Mechanisms for MJO Decay Over the Maritime Continent

Eastward propagating Madden‐Julian oscillation (MJO) events that develop in the Indian Ocean from November to April are separated into events whose convective anomalies do and do not propagate across the Maritime Continent (MC). Propagating (P) events are divided into strong (sP) and weak (wP) subsets based on the initial amplitudes of their convective anomalies. Eastward decaying (ED) MJO events have initial amplitudes similar to those of wP events. 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 weak, eastward propagating moist anomaly in ED events, leading to MJO termination. In wP events, the MJO moist anomaly is stronger than the TDP dry anomaly, and the MJO propagates beyond the MC. ED MJO events that do not encounter TDPs decay because of insufficient moistening in the southern MC region. The background states of TDP‐ and non‐TDP‐affected MJO events resemble La Niña and El Niño conditions, respectively. The regional regulation of TDP activity and background moisture gradients by El Niño–Southern Oscillation can both be understood in terms of the zonal shift and the meridional expansion or contraction of Warm Pool moisture in response to El Niño–Southern Oscillation sea surface temperature anomalies. Our results demonstrate that modulation of TDPs by the background state moisture can affect the MJO in a manner that reinforces its propagation or decay via the interaction of MJO circulation anomalies and background moisture gradients.