Influence of stratification and topography upon internal wave spectra in the region of sills

A non-hydrostatic model in vertical cross section form is used to examine the role of sill width and buoyancy upon the energy cascade into the internal tidal band and higher frequencies in sill regions. Power spectra analysis of computed time series show that as sill width is reduced the energy transfer from the baroclinic tide to its higher harmonics is reduced. However, there is an increase in energy at higher frequencies corresponding to lee wave periods. In addition internal mixing on the lee side of the sill increases. The addition of small scale topography on the sill slope produces a similar spectral change in the internal wave distribution and mixing. For a narrow sill the addition of a surface buoyant layer with a higher buoyancy frequency, significantly reduces the energy cascade to higher frequencies with a corresponding reduction in mixing. The associated spectra have characteristics resembling wide sill topography.