Simulating vertical mixing in a shelf-break region: addition of a shear instability model, accounting for the overall effect of internal tides, on top of a one-dimensional turbulence closure mixed layer model

Abstract

A simple parameterization of eddy diffusivity is used to simulate the shear mixing from tidally induced internal waves generated in the continental slope region southwest of Brittany. Near the edge of the shelf, the seasonal thermocline oscillates under the forcing of the barotropic tide which propagates over the shelf break. A composite model is constructed to simulate the mixing of the upper ocean from both external (wind stress) and internal (internal waves) sources. A simple one-dimensional eddy kinetic energy model, which predicts the temperature profile from heat flux and wind stress inputs, is validated with respect to regional hydroelimatic conditions, then coupled with a two-layer model of non-linear internal waves, to simulate the mixing encountered in shelf break fronts submitted to tidal forcing. Numerical runs on a transect perpendicular to the shelf break show the formation of a spot of cool water over the edge of the continental margin. The one-dimensional eddy kinetic energy model has been validated over a decade with temperature profiles over the abyssal plain adjacent to the continental slope. An annual validation experiment has also been conducted for the combined models, beginning on 1st January 1985, as well as a short-term validation experiment, using a set of high-frequency temperature measurements at two stations near the shelf edge in September and early October 1985. The simulation has also been spatially validated against three sets of infrared satellite images. The one-dimensional model is calibrated for the minimum turbulent kinetic energy, whereas the best fit to the high-frequency measurements in autumn 1985 above the slope provides the optimum values for the initial thermal content and for the parameterization constant of internal wave diffusivity. The combined model reproduces successfully the seasonal and the high-frequency (neap-spring tidal cycle) variation of the temperature field in the upper ocean along a transect perpendicular to the shelf break. Horizontal advection and mesoscale turbulence somewhat limit the performance of the model at low-amplitude tides and over the shallower part of the shelf, but the satisfactory overall agreement between the results and the measurements is consistent with the formation of a shelf break front in the northern Bay of Biscay and southern Celtic Sea, mainly as a result of mixing enhanced by tidally induced internal waves.