Internal waves in the Strait of Gibraltar : a study using radar imagery

Watson, Gary (1988) Internal waves in the Strait of Gibraltar : a study using radar imagery. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A shore-based radar was used to monitor the propagation of internal waves in the Strait of Gibraltar. As a background to the experiment, the present understanding of the mechanism by which internal waves are imaged by radar, via modulations of the surface wave spectrum, is reviewed. The results of other work in Strait are summarised, including the background stratification and shear, and previous observations of the internal waves. Some aspects of the theory of internal waves in stratified shear flows are reviewed, emphasising the nonlinear theory which would be required for a full treatment of these waves. Suitable simplifications are suggested. The method consisted of an X-band marine radar on Gibraltar, with an automated camera recording the image, during March, April and June, 1986. Roughness features were observed out to a range of ∼ 15km. Direct evidence that the image features were associated with internal waves was provided by a ship's echosounder trace. A semi-automatic method of measuring wave arrival times and phase speeds was developed; the positions of image features were measured manually using a digitising table, and these data were then computer-processed. During most tidal cycles, an undular bore generated at Camarinal Sill was observed to pass eastward (`A-waves'). Its arrival time was found to be predominantly controlled by the tidal current amplitude, and hence exhibited a strong diurnal variation. Wave phase speed was observed to be dependent on propagation direction and water depth, in qualitative agreement with theory. The dependence on direction was used to infer that the flow near Gibraltar was predominantly supercritical during the experiment. The refraction of the wavefronts as they passed Gibraltar was simulated using a two-layer ray-tracing model, based on data from the `Gibraltar Experiment'. Wavefront shape was found to be primarily determined by a combination of interface slope and horizontal shear, with bottom topography also contributing near the shore. After the bore, a second packet of waves (`O-waves') was observed to propagate northward, on most tidal cycles. It is suggested that these were generated at prominent features on the Moroccan coast, gaining large amplitudes as shock waves in the supercritical flow. Finally, the oceanographic potential of marine radar is discussed, together with ideas for future experiments in the Strait. (DX86651)

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