Tidal flow over three-dimensional topography generates out-of-forcing-plane harmonics

We conduct laboratory experiments and direct numerical simulations on internal gravity waves generated by tidal flow over a model three-dimensional Gaussian mountain on the sea floor. When 2 omega < N (where omega is the tidal frequency and N is the buoyancy frequency), the flow generates second harmonic internal waves that propagate perpendicular to the tidal forcing direction. These unexpected harmonics are observed only when the maximum slope of the bottom topography is greater than the slope of internal wave propagation. The out-of-plane harmonics saturate at a higher tidal forcing amplitude than the normal in-tidal-forcing-plane harmonics, and could lead to increased wave breaking and mixing near rough topography in the ocean. Citation: King, B., H. P. Zhang, and H. L. Swinney (2010), Tidal flow over three-dimensional topography generates out-of-forcing-plane harmonics, Geophys. Res. Lett., 37, L14606, doi:10.1029/2010GL043221.