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

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 0.6×10−2s−1 to 1.2×10−2s−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.

► Numerical simulations resolving topography lead to extensive drag on stratified flows.
► The drag is manifested through internal gravity wave, aerodynamic, and hydraulic drag.
► A parameterization for the gravity wave drag is proposed based on the linear theory.
► Parameterized drag yields satisfactory results compared to the resolved drag results.