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

A simple parameterization of eddy diffusivity is used to simulate the shear mixing from tidally induced internal waves generated in the continental sl ope region southwest of Brittany. Near the edge of the shelf, the seasonal thermocline oscillates under the forcing of the barotropic tide which propa gates over the shelf break. A composite model is constructed to simulate th e 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 inp uts, is validated with respect to regional hydroelimatic conditions, then c oupled with a two-layer model of non-linear internal waves, to simulate the mixing encountered in shelf break fronts submitted to tidal forcing. Numer ical 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-d imensional eddy kinetic energy model has been validated over a decade with temperature profiles over the abyssal plain adjacent to the continental slo pe. An annual validation experiment has also been conducted for the combine d models, beginning on Ist 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 simul ation has also been spatially validated against three sets of infrared sate llite images. The one-dimensional model is calibrated for the minimum turbu lent kinetic energy, whereas the best fit to the high-frequency measurement s in autumn 1985 above the slope provides the optimum values for the initia l thermal content and for the parameterization constant of internal wave di ffusivity. 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. Hori zontal 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 measure ments is consistent with the formation of a shelf break front in the northe rn Bay of Biscay and southern Celtic Sea, mainly as a result of mixing enha nced by tidally induced internal waves. (C) 2001 Elsevier Science Ltd. All rights reserved.