Energy accumulation and emanation at low latitudes. Part III: forward and backward accumulation

Abstract

Using ray-tracing techniques and nonlinear numerical models, the impact of a zonally varying basic state on the characteristics of equatorial modes is investigated. The original low-latitude energy accumulation zone theory of Webster and Chang is extended to include the complete family of tropical waves. In equatorial regions, the majority of Rossby waves are longitudinally trapped in regions where the stretching deformation of the background flow is negative. The results of this study suggest that the ubiquity of the longwave approximation for equatorial modes should be questioned. As the basic state modifies the scale of the mode such that initially very long modes may evolve to shorter scales during propagation through the longitudinally varying flow (and vice versa for initially short modes), the approximation appears questionable. -from Authors