Analysis of Unstable Solution with an MJO Structure Using a Three-Dimensional Spectral Linear Baroclinic Model

Abstract

In this study, we conducted a linear stability analysis by using a linear baroclinic model (LBM) derived from a three-dimensional spectral primitive equation model with only gravity modes. According to the stability analysis for a non-zonal basic state subject to an inviscid adiabatic condition, some unstable modes appear in a low-frequency range, and one of these unstable modes has an MJO structure. The MJO mode has a life cycle of ∼12.5 days. The dynamical field of the mode has a structure with zonal wavenumber 1 near the equator, and its vertical structure indicates an opposite sign in the upper and lower troposphere. This feature is confirmed by the energy spectra in the zonal, vertical, and meridional directions. It is found from the basic properties of the LBM, that the MJO mode arises from an up-scale energy interaction with the non-zonal basic state.