Convection Organization by the MJO during CINDY/DYNAMO Revealed by the Spatio-Temporal Wavelet Transform

This is a companion paper of Kiladis et al. (2016) in the conference and is concerned mainly with the MJO during the CINDY/DYNAMO based on the results of the spatio-temporal wavelet transform (STWT). The STWT is a two-dimensional version of the wavelet transform and is able to localize the space-time spectral characteristics of signals in both space and time, i.e., providing local, instantaneous power distributions as a function of zonal wavenumber and frequency. When averaged over space and time, the STWT spectra yield consistent results with a conventional Fourier-based approach. We investigate how the MJO convective envelopes were made up of synoptic-scale disturbances of particular space-time scales, namely in terms of convectively coupled equatorial waves (CCEWs). As in previous studies and in other approaches, it is found that the types of CCEWs that were enhanced within the MJO envelope vary from one event to another and from one location to another, suggesting that MJOs are diverse in terms of the internal structure. Some MJOs display a rather clear hierarchical structure composed of a number of Kelvin waves, each of which is composed mainly of a number of n=1 westward inertio-gravity waves, reminiscent of the schematic model of Nakazawa (1988), while others are not. Among the salient features is the appearance of slow Kelvin waves with phase speed of ~9 m s^-1 (i.e., ~8 m equivalent depth) in all MJO events, whose signals do not stand out in the space-time averaged STWT (or Fourier-based) spectra and tend to have been overlooked.