Nonlinear interactions of nearly non-dispersive equatorial shallow-water waves

Abstract

Abstract This paper is concerned with nonlinear interactions of fundamental equatorial modes. In order to understand the mechanism of large-scale atmospheric motions in the near equator regime—especially the observed wavenumber-frequency spectrum—we develop novel models describing interactions among Kelvin, Yanai and Poincaré waves. Based on the methods of multiple scales and Galerkin projection, the primitive equations can be simplified to model equations which reduce the complexity and cost of computation significantly. Subsequently, the detailed numerical results indicate that wave interactions between the aforementioned modes in the non-dispersive regime depends on initial amplitude and relative phase and that the eastward Yanai wave can be generated from the second Poincaré mode. We also compare the simplified models to an advanced finite element approximation for the primitive equations. The fact that results of the latter are in good agreement, at least qualitatively, with those of the simplified models, indicates that reduced models capture most of the wave interaction mechanisms in the nearly non-dispersive regime.