Axisymmetric waves on a vertical vortex in a stratified fluid

Axisymmetric wave propagation along a vertical vortex core in a stably stratified fluid is considered theoretically. The fluid is assumed to be inviscid, incompressible, nondiffusive, and exponentially stratified. A linear analysis under the Boussinesq approximation shows that discrete inertial modes (bounded modes) are allowed in addition to continuous internal gravity waves (unbounded modes), when the stratification is not too strong. These inertial modes, whose eigenfunctions are confined to the vorticity region, disappear if the Brunt-Väisälä frequency N² exceeds the maximum value of the Rayleigh function. Concrete results are given for the Burgers vortex. A weakly nonlinear analysis indicates that inertial modes (if permitted) are generated through the resonant interactions between two internal gravity waves. The time evolution of its amplitude is described by a cubic nonlinear Schrödinger equation, which admits envelope soliton solutions for shorter carrier waves only, viz., the soliton window has a low wave-number cutoff.