An Analysis of the MJO Guided by TRMM Rainfall Data

The Madden-Julian Oscillation (MJO), an eastward-propagating equatorial disturbance most active during the boreal winter, dominates atmospheric intraseasonal variability in the tropical Eastern Hemisphere. The MJO involves cyclic patterns of suppressed convection and intense rainfall, has a period ranging from 20-100 days, and is manifested in numerous atmospheric and oceanic variables. This study examines various dynamical and thermodynamical processes that characterize the MJO. We identify episodes of deep convection related to the MJO based on rainfall data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Although broad convective envelopes are located utilizing spectrally-filtered TRMM precipitation, analyses of the features within the envelope are carried out using unfiltered versions of TRMM rainfall and ECMWF 40-year reanalysis (ERA40) dynamical 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 as discussed in Houze et al. (2000). Deep-layer subsidence drying 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. A tiered vertical structure in moisture seen in intense MJO events indicates the possible importance of stable layers and their involvement in the growing disturbance. Additionally, a comparison of the composite fields to proposed wave instability theories suggests that the localized destabilization and discharge-recharge mechanisms are highly relevant for explaining many of the observed features of MJO evolution. 2 structure in moisture seen in intense MJO events indicates the possible importance of stable layers and their involvement in the growing disturbance. Additionally, a comparison of the composite fields to proposed wave instability theories suggests that the localized destabilization and discharge-recharge mechanisms are highly relevant for explaining many of the observed features of MJO evolution.