On the interaction of internal tides over two adjacent sills in a fjord

The problem of to what extent two topographic features, namely, adjacent sills in a fjord, interact to modify the internal waves between the sills is considered using a two-dimensional vertical slice nonhydrostatic model. Motion is generated by forcing with a barotropic tide at the M2 frequency. Calculations are performed with a range of sill depths hs and sill separations L. Initially, a single sill is considered and a progressive internal tide, lee waves, and a baroclinic jet are formed in the region of the sill. When a second sill is introduced, the intensity of the sill jet is reduced and a standing internal tide is generated between the sills, with an associated increase in mixing and change in tidal energy flux. However, as the sill separation increases, the energy flux increases toward its single sill value. For higher harmonics of the tide, which have a wavelength shorter than the intersill separation, their magnitude is increased for certain sill separations L due to focusing with an associated broadband resonance. In essence, nonlinear interaction of waves between the sills increases mixing, which explains the observed enhanced mixing found in observations made in such regions