Scattering of inertial waves by an ocean front - Comments

Abstract

Rubenstein and Roberts (1986, hereafter RR86) have studied the scattering of inertial waves by a geostrophic jet. The most remarkable effect of the jet is to concentrate near-inertial energy in the negative vorticity region. This effect is due to the horizontal dispersion of near-inertial waves. It is demonstrated by RR86 Using a stratified linearized model. A similar result has been obtained by Kunze (1985) using a ray-tracing approach and bas been confirmed by observations (Kunze and Stanford 1984). Another result of RR86 concems the mixed layer depth variations. Their Figs. 3 and 8 show a systematic downwelling in the negative vorticity region of the jet and systematic upwelling in the positive vorticity region. Although large upwellings and downwellings occur in the presence of a wind stress perpendicular to the front (Niiler 1969), it is not clear how such systematic effects can result from unforced inertial oscillations. RR86 find this behavior of the mixed layer depth in a shallow-water model where horizontal dispersion is neglected (their Fig. 3 ) and therefore suggest it is due to the nonlinearity of the mass equation. Our failure to reproduce the results of RR86 with a fully nonlinear shallow-water mode1led us to investigate more carefully the role of nonlinear terms, using both direct numerical simulation and asymptotic analysis. In the present note we show that the systematic upweIIings and downwellin~ are spurious effects due to the neglect ofsome (but not all) nonlinear terms in RR86's mixed layer equations.