The distribution of internal tidal energy around the North-west (NW) European Shelf edge is discussed with the aid to observations and model predictions. The largest signals occur at the edge of the Celtis Sea and probably also on the Wyville-Thomson Ridge. A significant internal tide is also found bordering the Rockall Trough, but energy levels are probably small at the northern edge of the North Sea. The internal tide is an important shelf-edge process, and it is intended to produce a detailed numerical study of its distribution.

INTRODUCTION

The motivation for improving our knowledge of the physical oceanography of the NW European Shelf edge has three main components. First, oceanographers want to understand and quantify exchanges processes between the shelf and the deep ocean which, in turn, have a bearing on biological productivity, climate and pollution levels. Secondly, oil explorers and developers need to know how the oceanographic climate of the shelf break regions differs from that of the North Sea and other areas of offshore oil production. Thirdly, the Ministry of Defence requires knowledge of how the shelf break affect acoustic travel paths and the dynamics of the open ocean. This chapter is a response to this motivation, and discusses the distribution of an important process, the internal tide, around the NW European Shelf edge.

The Oceanography of The Shelf Edge

The NW European Shelf is a large coastal shelf occupying an area of about 4.7 X 1010km2 (Fig 1). Most of the shelf has water less than 100 m deep, although there is a ribbon of deeper water (down to 200 m) along the 4500 km of shelf edge in the Rinne, to the west of Norway. At the edge of the shelf, bottom slopes increase dramatically below 200 m, descending to 1500 m in the Faeroe-Shetland Channel, and to 4500 m in the Bay of Biscay. The slope itself is very steep and indented in the south, becoming relatively shallow and smooth in the north, the shelf edge is interrupted at 60°N by the Wyville-Thomson Ridge, which runs NW at about 500 m deep and is an important oceanographic boundary (Gould, 1984).

Fig. 1 The NW European Shelf region., The location of current meter measurements; W-T R, Wyville-Thomson Ridge; F-S C, Faeroe-Shetland Channel. Contour depths are m metres. Shaded areas are below 2000 m (available in full paper)

The prominent oceanographic features of the shelf edge are the internal tide and the slope current. The latter flows northward along the entire length of the edge, to a depth of about 600 m, with typical speeds increasing from about 5 cm s-1m the Bay of Biscay to more than 30 cm sℒ1 in the Faeroe-Shetland Channel (Huthnance, 1986). Other phenomena include continental shelf waves, upwelling with northerly w d s and convective downwelling from the shelf in writer. In addition, there are the ubiquitous internal waves and (in summer) wind-driven surface inertial currents. From Infrared satellite images, the shelf break in the south appears as a band of cool water with eddies along the edge of the slope current.

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