Stratified flow over complex topography: A model study of the bottom drag and associated mixing

Abstract

The flow of stratified fluid over complex topography may lead to a significant drag on the fluid, exerted by the bottom obstacles. Using a 2-m resolution, three-dimensional, non hydrostatic numerical ocean model, the drag and associated mixing on a stratified flow over real, 1-m resolution topography (interpolated to model resolution) is studied. With a typical mountain height of 12 m in 174 m water and buoyancy frequencies ranging from 0.6×10-2 s-1 to 1.2×10-2 s-1, resolving the topographic features leads to extensive drag exerted on the flow manifested through three different processes: i) gravity wave drag, ii) aerodynamic or blocked flow drag, and iii) hydraulic drag. A parameterization of the internal wave drag based on linear, two-dimensional, hydrostatic wave solutions provides satisfactory results in terms of the turbulent kinetic energy levels. The depth of the layer where the vertical momentum flux is deposited, however, is underestimated, leading to an overestimated gravity wave drag in the layer.