A Revised Real-time Multivariate MJO Index

This study investigates why OLR plays a small role in the Real-time Multivariate MJO (RMM) index and how to improve it. The RMM index consists of the first two leading principal components (PCs) of a covariance matrix, which is constructed by spatially combined daily anomalies of OLR and zonal winds at 850 hPa (U850) and 200 hPa (U200) in the tropics after being normalized with the globally averaged standard deviation of 14.7 W m^-2, 1.8 m s^-1, and 4.6 m s^-1, respectively. This covariance matrix is reasoned mathematically close to a correlation matrix. Both matrices suppress substantially the overall contribution of OLR and make the index more dynamical and nearly transparent to the initiation of MJO convection. A true covariance matrix, without normalizing the anomalies, leads to the other extreme where OLR plays a dominant role while U850 and U200 are minor. Numerous tests indicate that a simple (2 W m^-2, 1 m s^-1, 1 m s^-1) to scale the anomalies can make the contributions more balanced. The revised PCs substantially enhance OLR over the Eastern Indian Ocean - Western Pacific while change it less notably in other locations and they only reduce U850 and U200 slightly. Their contributions to the total MJO become 85-95% in OLR and 80-85% in U850 and U200. Analyses of power spectrum, standard deviation, intensity and location of strong MJO convective centers as well as several MJO events demonstrate improvements of the revised RMM index.