A Moisture Mode Analysis of Counterintuitive Madden-Julian Oscillation Events

The Madden-Julian oscillation (MJO) is often considered a “moisture mode” where the MJO is driven by convective anomalies. The importance of this is observed in previous work where continuing (i.e., propagating) events have a higher amplitude of horizontal and vertical moist entropy (ME) advection to the east of the MJO than terminating (i.e., non-propagating) events. Preliminary work shows some events will continue when they are otherwise expected to terminate (e.g., an event continues despite large-scale drying in the upstream environment) given the distributions of ME for propagating and non-propagating MJOs. Further analysis of these cases may reveal the presence of other processes at work or predictors for these apparent outlier events.

This study presents a climatology of continuing and terminating MJO events from the RMM index using an event identification algorithm used in previous work. We analyze and sort these events by the zonal gradient of ME tendency over the phase-2 (Indian Ocean) domain about the convective center of the MJO. ERA-5 reanalyses is used for variables at a daily timescale in tandem with the RMM daily amplitude. We generate box and whisker plots for continuing and terminating events to document the variation among events and the climatological mean background state of ME.

Terminating events with favorable conditions that are greater than the median ME gradient for continuing events (and vice versa) are identified as “counterintuitive” events. Preliminary work using ERA-Interim data to analyze ME budget terms shows insufficient vertical advection of ME east of the convective center for events that terminate despite a favorable mean ME background. Planned results will include other domains such as the Maritime Continent, which is often hostile for MJO maintenance, and over the Pacific where the MJO convection and circulation anomalies tends to decouple. Outside the Indian Ocean domain where moisture mode theory may not be as applicable, we consider alternative explanations for observed counterintuitive events such as observing wave interactions for gravity-wave theory.