Interaction of near-inertial waves with an anticyclonic vortex

TL;DR

This paper investigates how near-inertial waves interact with anticyclonic vortices, showing that vortices trap and modify these waves, with theoretical models aligning well with numerical simulations.

Contribution

It introduces a wave-averaged model capturing the nonlinear feedback of near-inertial waves on vortices, validated against 3D Boussinesq simulations.

Findings

Vortices trap near-inertial waves, elevating energy within the core.

Eigenmode frequencies and structures are predicted accurately by the model.

Nonlinear feedback increases modal frequency proportionally to wave energy.

Abstract

Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initial wave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.