Assessing propagation characteristics and predictability of the MJO using multiple MJO indices

Understanding of the convection associated with the Madden-Julian oscillation (MJO) and its multiscale variability remains a challenge. There is need for simple diagnostics applicable to both observations and model output to improve understanding and aid with model evaluation. Here, a diagnostic based on a two dimensional linear damped oscillator with white noise stochastic forcing and observed power spectra is presented to diagnose decay time, period and stochastic forcing statistics for the MJO.

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 (PC_1,2) 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.