Assessing propagation characteristics and predictability of the MJO using multiple MJO indices
To assess the MJO predictability two different indices are used. The OLR-based MJO index (OMI) uses outgoing longwave radiation (OLR) exclusively to focus on the convective signal associated with the MJO, while the Real-Time Multivariate MJO (RMM) index has been shown to capture primarily the large scale circulation.
Spectral analysis of the two principal components (PC1,2) for both indices reveals a quadrature relation between these. A diagnostic based on this quadrature relation is introduced. The diagnostic yields comparable results for both indices with a period of 40 days and a decay time of 10-13 days, which is close to previous estimates of the prediction limits for the MJO, although slightly on the low side. In addition, seasons with longer (shorter) periods tend to be associated with shorter (longer) decorrelation times.
There is less forcing variance on the PC2 time series as estimated by the diagnostic. This can be related to the spatial pattern associated with PC2. The main loading of EOF2 is located between 90°E - 150°E in the western Pacific warm pool, where generally the warmest sea surface temperatures (SSTs) are found. Due to this convection in this region can be initiated by small perturbations on the warm SST background state.