On breaking internal waves over the sill in Knight Inlet

Abstract

This paper describes a new series of numerical simulations of stratified flow over localized topography which has been designed to address issues arising from a recently published sequence of detailed observations from a coastal oceanographic setting. Results demonstrate that the numerically simulated flow is very similar to that which develops in Knight Inlet, British Columbia, a fjord which is subject to periodic tidal forcing, and that the detailed dynamical characteristics of this flow are also strikingly similar to those of severe downslope windstorms that often occur in the atmosphere. A typical sequence of events observed in such flows includes the ‘breaking‘ of a forced stationary internal wave induced by the topography, which results in irreversible mixing and the formation, through wave–mean flow interaction, of a decelerated mixed layer that extends downstream from the level of breaking. The formation of this mixed layer is a necessary precondition for transition of the flow into a supercritical hydraulic regime in which a low–level high–velocity jet develops in the lee of the topographic maximum. Simulations with both fixed inflow velocity and harmonically varying inflow velocity are performed and intercomparison of the results clearly demonstrates that flow evolution in the unsteady forcing case can be described, to reasonable approximation, by the results of the corresponding quasi–steady simulations, at least during the accelerating stage when inflow velocity is slowly increasing. At later times of flow evolution, however, the well–mixed fluid accumulates and the flow enters a statistically steady hydraulic–like regime which is characterized by a constant mean drag exerted by the topography on the flow even while the inflow velocity slowly decreases.