Observed Characteristics of the MJO Relative to Maximum Rainfall

Abstract

This study examines various dynamical and thermodynamical processes that characterize the Madden–Julian oscillation (MJO). Episodes of deep convection related to the MJO based on rainfall data from the Tropical Rainfall Measuring Mission (TRMM) satellite and the Global Precipitation Climatology Project (GPCP) are identified. Although broad convective envelopes are located utilizing spectrally filtered precipitation, analyses of the features within the envelopes are carried out using unfiltered rainfall and 40-yr ECMWF Re-Analysis (ERA-40) fields. The events are composited and categorized based on geographic location and relative intensity. The composited fields illustrate that, prior to the onset of deep convection, shallow cumulus and cumulus congestus clouds are actively involved in vertical convective transport of heat and moisture. Drying, first accomplished immediately following deep convection in the lower troposphere, is associated with an enhanced horizontal (westerly) advective component and may be related to mesoscale processes. Drying related to deep-layer subsidence is delayed until one to two weeks following intense rainfall. The importance of gradual lower-tropospheric heating and moistening and the vertical transport of energy and moisture are shown in a comparison of vigorous and weak MJO events. Additionally, a comparison of the composite fields to proposed wave instability theories suggests that certain theories are effective in explaining specific phases of the disturbance, but no single theory can yet explain all aspects of the MJO. The discharge–recharge and frictional moisture convergence mechanisms are most relevant for explaining many of the observed features of MJO evolution.