On the interaction between internal tides and wind-induced near-inertial currents at the shelf edge

Numerical calculations are performed at the shelf edge to examine the role of nonlinear processes in transferring energy from the M-2 tide- and wind-induced currents close to the inertial frequency f into a wave at the sum of their frequencies, termed the fM(2) frequency. A numerical model of the Hebrides shelf edge ( represented by a cross section), initially with idealized topography and subsequently with realistic topography is used in these calculations. Results show that in the near-coastal ocean, currents at the fM(2) frequency are primarily due to coupling between wind-induced inertial oscillations and the M-2 internal tide. A major source is associated with vertical shear in the inertial oscillations and the vertical velocity due to the internal tide. A secondary source is due to the nonlinear momentum advection term. In the case in which eddy viscosity is computed from a turbulence energy model, shear across the thermocline is larger than when a constant viscosity is used. The reduction in shear with a constant viscosity reduces the role of the nonlinear term involving vertical shear, and hence the magnitude of the fM(2) current in the region of the thermocline. Increasing the wind stress leads to a deeper thermocline, and hence the location of maximum fM(2) current in the water column. Changing the vertical stratification influences the intensity of the inertial oscillations in the surface layer and the distribution of the internal tide, and hence changes the pattern of fM(2) currents. Results with realistic topography confirm the major conclusions.