Upstream–propagating solitary waves and forced internal–wave breaking in stratified flow over a sill

We discuss numerical simulations of the flow of an inviscid, density stratified fluid over a broad obstacle of high elevation relative to the finite depth of the fluid layer. The density stratification and topography are chosen to be consistent with recent observations of the flow in Knight Inlet, a fjord in British Columbia, Canada. We find that under certain physically realized conditions the response of the fluid includes large–amplitude, resonantly generated internal solitary waves that propagate upstream, and in some cases break. The post–breaking structures consist of a nearly steady wave envelope and a highly unsteady core that is focused near the surface. The response of the fluid on the lee slope of the topography and further downstream can be understood in terms of the breaking of topographically forced internal waves. The qualitative features of the flow depend to a large degree on whether internal-wave breaking initially occurs in the strong near–surface pycnocline or in the underlying weakly stratified main part of the water column. We discuss the analogy between the oceanic flow and the extensively studied intense downslope windstorms that occur in the atmosphere when internal waves break over the topography that generates them.