Influence of multiple sills upon internal wave generation and the implications for mixing

A cross sectional non-hydrostatic model of a fjord is used to examine to what extent the internal tide is modified by two closely spaced sills. The model is forced with an M2 barotropic tide and internal tides, unsteady lee waves and a jet are generated at the sills. In contrast to an isolated sill, calculations with two closely spaced sills show that the presence of the second sill leads to standing internal wave generation with associated changes in wave spectrum and Richardson number in the inter sill region. Associated with these there is a cascade of energy towards short waves leading to enhanced mixing between the sills. This increase in mixing is consistent with enhanced mixing between closely spaced sills found in observations. It is also relevant to enhanced oceanic and lake mixing, and suggest that future mixing measurements in both oceans and lakes should focus on these regions.