The tropical intra-seasonal oscillation (TIO or MJO) is a dominant component of tropical intra-seasonal variability. In mid-latitudes baroclinic instability is the primary mechanism for interaction between synoptic scales and planetary scales. In the tropics there is a lack of understanding of the mechanisms behind the dominant source of intra-seasonal variability, the MJO. Since the MJO is a multi-scale low frequency mode of tropical atmospheric motion, a fundamental challenge is to understand the role of scale interaction in sustaining the MJO.

The present talk describes a new multi-scale model which focuses on the interaction of equatorial synoptic scale circulations induced by a wave train of superclusters with planetary scale circulation anomalies through upscale transfer of kinetic and potential energy. The theoretical framework for the TIO/MJO model is the intra-seasonal planetary equatorial synoptic dynamic (IPESD) model derived recently by Majda and Klein (JAS 2003). The IPESD model is a multi-scale balanced model, systematically derived from the primitive equations, and provides simplified equations for the upscale transfer of energy from a wave train of equatorial synoptic scale circulations to the planetary scale. The IPESD model has solutions which are readily processed through exact solution formulas and elementary numerics.

A new model for the MJO/TIO is developed by examining the upscale transfer of energy in the IPESD models where it is postulated that the wave train of equatorial synoptic circulation anomalies arise from specified heat sources due to the organized convection. It is shown that the resulting MJO model readily generates intra-seasonal low frequency variability and simultaneously predicts both the two prominent low frequency wave number spectral peaks along the MJO and equatorial Rossby bands as well as the unusual MJO dispersion relation, dƒçƒ}dƒnƒÛƒnƒƒnƒ~ƒzƒnas described by Wheeler and Kiladis (JAS 1999) and Wheeler, Kiladis, and Webster (JAS 2000) from their observational analysis. A simpler conceptual MJO model involving the planetary scale circulations generated by a traveling envelop of equatorial synoptic scale circulations is also discussed here through the IPESD framework. In an interesting fashion, the new multi-scale MJO models developed here require equatorial synoptic scale circulations associated with top heavy heating in order to generate a realistic planetary scale MJO response in the tropics. This feature of the models is also in agreement with recent analysis of observational data for the MJO.