INSTABILITY OF WIND-FORCED INERTIAL OSCILLATIONS

An instability mechanism that can amplify wind-forced inertial oscilla tions in the upper ocean is investigated. This forced instability happ ens because of the phase relationship between the mixed-layer depth an d the surface current. It allows the inertial oscillations propagating against the wind to extract energy from it and amplify. The key ingre dients for the instability to work are (a) a non-zero mean wind stress , (b) a spatial variability of the oscillations in the direction of th e wind stress. The amplification is demonstrated using a simple shallo w-water model in a few situations: the dispersion of a localized distu rbance with steady and time-varying wind forcing, generation of inerti al waves at a coast, and spatial variability induced by mesoscale eddi es. Estimates of the growth rate are provided for both dissipative and nondissipative cases.