A resonance mechanism leading to wind-forced motions with a 2f frequency.

This study revisits the mechanisms that spatially reorganize wind-forced inertial motions embedded in an oceanic mesoscale eddy field. Inertial motions are known to be affected by the eddy relative vorticity, being expelled from cyclonic structures and trapped within anticyclonic ones. Using shallow water equations (involving a single baroclinic mode), we show that nonlinear wave-wave interactions excite, through a resonance mechanism, motions with a 2f frequency (with f the inertial frequency) and a specific length scale. In a more general situation involving several baroclinic modes, this resonance is found to be reinforced and principally efficient for the lowest baroclinic modes. Such characteristics make the energy of the wind-forced motions potentially available to small-scale mixing through parametric subharmonic instability (not taken into account in the present study).