The Role of Moist Entropy on Termination of Madden-Julian Oscillation Events

Abstract

The Madden-Julian oscillation (MJO) is the leading mode of intraseasonal variability in the tropics. Although many studies have examined the MJO, few have considered its termination. Multiple indices exist to track and forecast the MJO although the details of each index to MJO decay have not been documented. This study presents an intercomparison of several MJO indices for a 34-year period to examine the evolution of MJO events and comprise an index-specific climatology for MJO termination. We present an analysis of common indices including the OLR-based MJO Index (OMI) and filtered OMI (FMO) in addition to the Real-time Multivariate MJO (RMM) and velocity potential MJO (VPM) index. Normalized EOF comparisons reveal only minor longitudinal and time shifts in the corresponding convection and circulation anomalies between indices. The absolute and fractional daily rate-of-change (ROC) of each index is derived and the RMM and VPM indices experience nearly twice the daily amplitude changes than the univariate indices. Results from the ROC analysis are implemented in an MJO event identification algorithm to study the sensitivity of MJO termination to changes in the temporary minimum amplitude and threshold used to define MJO events. The corresponding counts of primary, continuing, and circumnavigating events were recorded with primary events being insensitive to buffer changes, generally less than 5% among indices. Finally, an MJO climatology is created that presents MJO termination as a function of phase for each index. Recent studies have also examined moist entropy (ME) as a proxy for moist static energy (MSE) and the relative role of the underlying processes responsible for changes in ME that potentially affect MJO propagation in GCMs. We present an analysis of the intraseasonally varying (ISV) ME throughout the lifetime of MJO events. A climatology of primary, continuing, and terminating MJO events is created from the four indices used earlier. ME composites for all indices show a break in the wavenumber-1 oscillation at Day 0 for terminating events in nearly all domains except RMM Phase-6 and 7. Statistical significance between the mean ISV ME for continuing and terminating events is identified on average 10 days prior to MJO termination and 20 days prior in some cases. ME tendency is decomposed into horizontal and vertical advection, sensible and latent heat fluxes, and vertically integrated shortwave and longwave heating using reanalysis. The relative role for eastward propagation of the MJO for each of these processes is calculated. For continuing events over the RMM Phase-2 domain, the total ME tendency is attributed mostly to horizontal ME advection although differences in the relative roles for vertical ME advection and radiative fluxes appear as early as 15 days prior to MJO termination. The analysis suggests processes other than horizontal advection may inhibit further eastward propagation and lead to MJO decay over this domain.