The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in precipitation, temperature, humidity, and zonal flow in the tropics. It is associated with positive anomalies precipitation that develop over large regions over the Indian Ocean or Western Pacific and propagate eastward. In addition to its direct impact on precipitation in equatorial regions, the MJO has been documented to modulate tropical cyclone activity throughout the globe. Also, its barotropic Rossby response causes changes to weather in mid-latitude regions. However, the mechanisms through which widespread convection in the MJO develops remain largely unexplained.
The Dynamics of the MJO (DYNAMO) field campaign was conducted in the central equatorial Indian Ocean in late 2011-early 2012 to address some hypotheses regarding MJO convective onset. From extensive rawinsonde and radar measurements obtained during DYNAMO, we have clarified the role that cumulus and moderately deep cumulonimbus clouds play in the eventual onset of deep, widespread convection. The cloud population appears to react to changes in the large-scale environment, particularly a reduction in large-scale subsidence, by growing deeper over a 3-7 day period prior to onset of widespread deep convection. The clouds moisten the troposphere between 500 and 850 mb, which makes the environment conducive to deep convection.
Some outstanding questions address the processes through which convection grows deeper during MJO events that occurred in previous years. The role of a circumnavigating eastward propagating Kelvin-wave like feature is also unclear for previous MJO convective events, especially those which were not immediately preceded by an obvious MJO convective outbreak.