The Birth and Death of the MJO: An Observational Study

The Madden Julian Oscillation (MJO), an eastward-propagating equatorial wave most active during the boreal winter, dominates atmospheric intraseasonal (10-100 day) variability in the warm-ocean Indian and Pacific Ocean areas. This phenomenon is characterized by cyclic periods of suppressed convection (dry phase) and intense rainfall (wet phase). In this study, we examine important physical mechanisms observed during the “birth” (wet phase approach) and “death” (wet phase departure) of the MJO.

Analyses of single events and event composites based on TRMM precipitation highlight cogent features of the MJO. Unlike previous studies, we base MJO events on hydrological activity due to its strong ties to latent heating, the primary driver of tropical circulations. Dynamical fields of mesoscale resolution are diagnosed from ECMWF reanalysis datasets.

Prior to the onset of intense rainfall, a slow increase in low-level temperature and moisture leads to greater instability. An enhancement of shallow cumulus activity, as inferred from the reanalysis data, is associated with increased moisture detrainment and an erosion of a mid-tropospheric dry layer. At this stage, vertical moisture advection is dominant over the horizontal component.

The “death” of the MJO involves immediate and delayed drying processes. Within five days after maximum rainfall, we observe anomalous low-level drying by horizontal advection during a time of weak moistening by vertical motions. This immediate drying has not been analyzed explicitly in previous composite studies. Subsidence drying is delayed, beginning and then peaking one and two weeks after intense precipitation, respectively.

Physical attributes of the composite results are compared to current wave instability theories. Our findings lend support to the discharge-recharge mechanism which involves a gradual, local build-up of instability.

Currently, no widely-accepted theory exists that can fully explain the MJO. Accurately diagnosing and modeling this phenomenon is of critical importance for weather and climate studies. It is our hope that this study contributes toward an improved understanding of the MJO and its depiction in atmospheric models.