Propagation Characteristics of BSISO Indices

The tropical intraseaonal oscillation propagates eastward in boreal winter and northward/eastward in boreal summer. A number of MJO/BSISO indices have been developed to characterize its propagation and amplitude for the purpose of monitoring and prediction. Nevertheless, it is unclear if these indices can faithfully represent the propagation. In this study, we undertake this task and assess several popular BSISO/MJO indices in representing propagating characteristics of OLR diagnosed with these different indices. It is shown that the spatial structure and propagation of tropical intraseasonal convection anomalies diagnosed with the OLR‐based MJO index (OMI, Kiladis et al. 2014) represents both northward and eastward propagation in summer and eastward propagation in winter in a manner consistent with MJO and BSISO propagation as diagnosed in many previous studies. The OMI and three other widely used indices for tracking the BSISO are then compared in their lag correlation structure over selected reference areas, cross correlation coefficients of the two principal component time series, and time‐dependent phase angle composites. The OLR anomalies from these different indices propagate differently according to these diagnostics. One of them exhibits little propagation at all, even though one would expect good propagation based on composites of the phases separately. This illustrates the general point that while composites of individual phases, presented in a sequence, are generally taken to imply that the phases tend to occur in that sequence in time, that need not be the case. It is suggested that propagation characteristics are relevant to the application of the indices and that smoother propagation is desirable.