Instability and Nonlinear Evolution of the MJO in a Model with Vertically Varying Convective Adjustment

Tropical variability exhibits several pronounced features, including a red background spectrum and a variety of spatiotemporal scales including those of the Madden-Julian oscillation (MJO) and convectively coupled equatorial waves (CCEWs). Understanding and successfully modeling the source of these features with a single model remains a challenging task with open questions. For example, is the MJO an unstable mode or a forced stable mode? In a recently proposed linear, two-vertical-level stochastic model of tropical rainfall and waves, convective adjustment was shown to produce rainfall variability corresponding to both the MJO and CCEWs and a red background spectrum. Due to the model's linearity, it was necessary to treat the MJO as a forced stable mode. During this talk, it will be shown that a 3-D nonlinear version of this model produces a linearly unstable MJO that has realistic nonlinear evolution and structure. In particular, the linearly unstable MJO has reasonable growth rate and frequency, and as this mode grows, nonlinearities slow the wave growth until the wave saturates with reasonable amplitude, structure, speed, and dynamics, particularly for such a simple model. Despite its relative simplicity, this model is also able to produce a rich variety of linear and nonlinear dynamics, including variability associated with both the MJO and with CCEWs in some parameter regimes. The gross features of the model's results are generally robust, though changes in the background moisture profile can significantly affect both the linear and nonlinear MJO. Potential uses for the convective adjustment scheme beyond the current study will be briefly discussed.