Escape of near-inertial waves trapped in strong fronts through wave-wave interactions

Abstract Wind-generated inertial motions are refracted by geostrophic vorticity gradients to propagate into the stratified ocean interior as near-inertial waves that can be trapped in anticyclonic regions of ocean fronts and eddies as subinertial waves. Here, we explore a nonlinear wave-wave interaction mechanism by which subinertial near-inertial waves trapped in a sharp front (vorticity Rossby number Ro∼−0.85, front gradient Froude number Fr∼0.8) can escape as superinertial near-inertial waves. Inferences are drawn from spectral and cross-bispectral analyses of numerical solutions of a process-study ocean model configured to represent a two-dimensional baroclinic front. The model is forced with a wind impulse. The resulting near-inertial wave fields are analyzed over the ensuing five inertial periods as the waves radiate downward. Resonant wave-wave interactions provide a pathway for trapped subinertial wave energy to radiate out of the front as free superinertial waves. Nonlinear interactions represent a significant component of the near-inertial wave energy budget, only a factor-of-two smaller than dissipation. Escaping waves only occur for either large front vorticity Rossby number or large front gradient Froude number in the band constrained by 0 < 1+Ro–Fr 2 < 0.36.