Relevance of the vertical internal mode decomposition approach for the study of the MJO

Some recent studies of the Madden-Julian Oscillation (MJO) have used vertical internal modes to analyze the structure of perturbations in temperature, divergence and other variables. The underlying motivation for using vertical mode decomposition is the hope that the first few leading modes physically well separate various (notably deep and shallow) convective perturbations associated with the MJO.

In order to critically examine this assumption, the vertical mode decomposition is applied to a series of MJO events. These MJO events are detected and characterized by performing a Local Mode Analysis (LMA) of the OLR (proxy for deep convection) time series. The OLR temporal spectrum of each MJO event is then used to extract the associated perturbation fields of other atmospheric variables (ERA40 data set). The LMA gives thus a simple mathematical representation of the 3D atmosphere for each event that is then projected on the vertical modes. This extraction method is valid under the assumption that the vertical modes do indeed represent the effects of convective processes, since in this case perturbations in the OLR and the other physical fields must share a common temporal spectrum.

Our analysis shows that the vertical structure of the perturbations associated with MJO varies between the Indian Ocean, Maritime Continent and Pacific Ocean regions. Qualitatively, the phase relation between perturbations in the upper and lower troposphere is significantly different from one region to the other. Simple metrics are developed to quantify these differences. The results call into question the physical validity of using vertical mode decomposition to analyze MJO events. Our analysis demonstrates that such techniques could conceal a possible heterogeneity of physical processes that would give rise to the complicated nature of the Madden-Julian Oscillation.