Observations in the Bay of Biscay have shown that the shelf break mat often be a region of strong subsurface currents. It is postulated that these result from internal motions of semi-diurnal period, generated by the interaction of the surface tides with the shelf break topography.
A numerical model (after Prinsenberg and Rattray, 1975) has been used to study the topographic generation of the internal tides, and good agreement with available observations has been obtained. The location relative to the shelf break and the magnitude of the strongest currents are determined primarily by the temperature structure and, m particular, by the seasonal thermocline.
The model may be used to estimate the maximum current speeds, and also the times of low flow relative to local high water, as well as the effects of the spring-neap cycle and seasonal variability.
The Continental Shelf break in the Bay of Biscay is known to be an area of high barotropic tidal flow, often reaching on-off shelf speeds of 0.6-0.7 m s-1 at spring tides (Heathershaw et al., 1987.) When this surface tide flows over the steep shelf slope (a mean gradient of 1 in 11, see Fig.1), internal tidal motions may be expected to result. These internal tides add to the velocity field of the surface (barotropic) tide and may produce increases as much as 0.5 m s−1 both above the shelf (Heathershaw et al., 1987) and over the slope (R D. Pmgree, pers. comm.; see also Fig. 7).
In the Sulu Sea, Ape1 et al. (1985) have also discovered internal velocities of approximately 0.5 m s−1 near the ocean surface m deep (2000 m) water, while in the Rockall Trough, speeds close to 0.1 m s--1may be more appropriate, as described by deWitt et al. (1986). In order to produce internal motions of large amplitude, it is generally thought that a high surface tidal flow across the shelf break is necessary, together with a steep topographic slope. Nevertheless, in many areas of the world's oceans, significant increases in horizontal velocity may be expected from the internal tides, and Baines (1982) has attempted a categorisation of the most important generation sites, m which the Bay of Biscay figures prominently. Since the magnitudes and times of occurrence of these internal tidal velocities are of potential importance to the design and operation of offshore structures, as well as to shipping, this chapter provides a numerical description of the flow field.
Fig. 1 Chart of the Bay of Biscay, showing depth contours, the main survey area of PMN (broken lines), the positions of internal tidal troughs numbers 21, 22 and 23 as described by PMN, and the location of mooring 080 September 1985 (available in full paper)
Previous work by Pingree et al. (1983) and Pingree et al. (1986, to be referred to as PMN m the following), has indicated that, during the summer, the near-surface motion (down to 150 m, say) is dominated by internal tidal depressions of the thermocline, which appear to form close to the shelf break.