EVIDENCE FOR INTERNAL TIDAL MIXING NEAR THE SHELF BREAK IN THE BAY OF BISCAY

The continental shelf break in the Bay of Biscay often corresponds to a region of high phytoplankton abundance and cool water near the surface. Numerical model results suggest that these phenomena may be due to near-surface mixing created, particularly near spring tides, by internal tidal motions that result from the interaction of the surface tide with bottom topography. This paper presents convincing field evidence that such mixing occurs in nature, and describes data from a range of instruments on a recent cruise. Of particular interest are observations made with the novel IOSDL towed thermistor spar, which allows an investigation of the small-scale temperature structures near the ocean surface. Data from other of the ship's instruments are also presented, and observational estimates of the gradient Richardson number are made. All the observations are set within the framework provided by a numerical model, and strongly reinforce the hypothesis that significant internal tidal mixing is occurring.