A Suite of Skeleton Models for the MJO with Refined Vertical Structure

Abstract: TheMadden-Julian oscillation (MJO) is the dominantmode of variability in the tropical atmosphere on intraseasonal timescales and planetary spatial scales. The skeleton model is a minimal dynamical model that recovers robustly the most fundamental MJO features of (I) a slow eastward speed of roughly 5 ms−1, (II) a peculiar dispersion relation with dω/dk ≈ 0, and (III) a horizontal quadrupole vortex structure. This model depicts theMJO as a neutrally-stable atmospheric wave that involves a simplemultiscale interaction between planetary dry dynamics, planetary lower-troposphericmoisture and the planetary envelope of synoptic-scale activity. Here we propose and analyze a suite of skeleton models that qualitatively reproduce the refined vertical structure of the MJO in nature. This vertical structure consists of a planetary envelope of convective activity transitioning from the congestus to the deep to the stratiform type, in addition to a front-to-rear (i.e. tilted) structure of heating, moisture, winds and temperature. A first example of skeleton model achieving this goal has been considered recently in work by the authors. The construction of such a model satisfies an energy conservation principle, such that its solutions at the intraseasonal-planetary scale remain neutrally stable. Here, additional classes of skeleton models are constructed based on the same principle. In particular, those new models are more realistic then the former one as they consider fully coupled interactions between the planetary dry dynamics of the first and second baroclinic mode and the details of the vertical structure of moisture and convective activity. All models reproduce qualitatively the refined vertical structure of the MJO. In addition, when consideredwith a simple stochastic parametrization for the unresolved details of synopticscale activity, all models show intermittent initiation, propagation and shut down of MJO wave trains, as in previous studies.