Nonlinear interaction between wind‐forced currents and near‐inertial oscillations and internal waves in nearshore frontal regions

Abstract

A cross‐sectional model with a coastal boundary and bottom front is used to examine the nonlinear interaction between wind‐forced currents and internal waves at the near‐inertial frequency (f) and those at super‐ and subinertial frequencies (ωf). In the frontal region, nonlinear effects associated with vorticity in the frontal jet and differences in eddy viscosity gives rise to Ekman pumping. This allows the wind's momentum at super‐ and subinertial frequencies to penetrate to a greater depth than that due to momentum diffusion. Internal waves produced in the frontal region are trapped on the western side of the front but can propagate on the eastern side, although some trapping occurs at depth. Away from the frontal region, nonlinear interaction gives rise to energy at the frequencies (ωf + f) and (∣ωf − f∣) primarily through vertical shear associated with the f frequency and vertical velocity at the ωf frequency. In the frontal region, where internal waves at the ωf and f frequency are trapped at depth by the sloping isotherms, the horizontal momentum advection term is comparable to the nonlinear term involving vertical velocity, and there is maximum nonlinear interaction. Internal waves at the (ωf + f) frequency are superinertial and can propagate away from their generation region, while those at (∣ωf − f∣) are subinertial. Calculations show appreciable spatial variability in the interaction processes, which will influence the spectra derived from observations. The spatial characteristics of the interaction change in the frontal region depending upon ωf being super‐ or subinertial.