Vortex Rossby waves in mesoscale dipoles

Abstract

Mesoscale oceanic vortex dipoles are stable coherent vortex structures formed by two closely packed regions of opposite sign vertical vorticity. The authors investigate periodic oscillations in the vortices that make barotropic and baroclinic dipoles depart from a complete steady state. These oscillations are a pair of vortex Rossby waves (VRWs) and are numerically simulated using a three‐dimensional, Boussinesq, and f‐plane model. The evolution of balanced (void of inertia‐gravity waves), static, and inertially stable dipoles is examined under different initial conditions. These initial conditions include the vortex potential vorticity (PV) geometry, initial distance between vortices, and PV extrema. The numerical results show that each VRW is an oscillation with azimuthal wave number 2 that amplifies preferentially at two vortex locations and has an angular phase speed of the same sign as the vortex vertical vorticity. The VRWs in the dipole (dipole VRWs) imply an oscillation in the dipole speed of displacement and, in baroclinic dipoles, interchange between kinetic and potential energy as well. In the absence of any external forcing, the amplitude, periodicity, and phase speed of the dipole VRWs depend on the initial conditions, especially on the PV extrema, vortex geometry, and initial distance between vortices. It is found that steady dipoles are possible but their steadiness is not robust in the sense that any small perturbation will cause the development of VRWs.