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A Study of Internal Wave Propagation in the Strait of Gibraltar Using Shore-Based Marine Radar Images

G. WatsonDepartment of Oceanography, The University of Southampton, England

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I. S. RobinsonDepartment of Oceanography, The University of Southampton, England

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Abstract

An X-band marine radar on Windmill Hill, Gibraltar, was used to monitor the propagation of internal waves in the Strait of Gibraltar during March, April, and June 1986. Surface roughness features of the waves were observed out to a range of ∼15 km. From photographic images of the radar screen, the positions of image features were measured manually using a digitizing table, and these data were then computer processed to give wave arrival times and phase speeds.

During most tidal cycles, an internal undular bore generated at Camarinal Sill was observed to pass eastward (A-waves). These occurred throughout the experiments although less frequently near neap tides. The time of travel to Gibraltar varied significantly, so that the bore arrived between four and nine hours after high water. Arrival times were found to be closely related to the amplitude of the upper layer tidal current. This current had a strong diurnal component, which was manifested as an inequality between the arrival times of successive wave packets. The inequality varied between zero and five hours, with an average period of half a tropical month (13.7 days), so that the arrival time cycle drifted gradually relative to the semidiurnal spring-neap cycle.

Wave phase speed was observed to be dependent on propagation direction and on water depth in qualitative agreement with theory. The dependence on direction was used to infer that the flow near Gibraltar was predominantly supercritical during this experiment. This suggests that the method has some potential as a means of long-term monitoring of the exchange flow in the strait.

Other results included the observation of many wave packets propagating northward across the eastern Strait. Thew were named “O-waves&rdquo, and it is suggested that they were generated at prominent feature on the Moroccan gaining large amplitudes as shock waves in the supercritical flow.

Abstract

An X-band marine radar on Windmill Hill, Gibraltar, was used to monitor the propagation of internal waves in the Strait of Gibraltar during March, April, and June 1986. Surface roughness features of the waves were observed out to a range of ∼15 km. From photographic images of the radar screen, the positions of image features were measured manually using a digitizing table, and these data were then computer processed to give wave arrival times and phase speeds.

During most tidal cycles, an internal undular bore generated at Camarinal Sill was observed to pass eastward (A-waves). These occurred throughout the experiments although less frequently near neap tides. The time of travel to Gibraltar varied significantly, so that the bore arrived between four and nine hours after high water. Arrival times were found to be closely related to the amplitude of the upper layer tidal current. This current had a strong diurnal component, which was manifested as an inequality between the arrival times of successive wave packets. The inequality varied between zero and five hours, with an average period of half a tropical month (13.7 days), so that the arrival time cycle drifted gradually relative to the semidiurnal spring-neap cycle.

Wave phase speed was observed to be dependent on propagation direction and on water depth in qualitative agreement with theory. The dependence on direction was used to infer that the flow near Gibraltar was predominantly supercritical during this experiment. This suggests that the method has some potential as a means of long-term monitoring of the exchange flow in the strait.

Other results included the observation of many wave packets propagating northward across the eastern Strait. Thew were named “O-waves&rdquo, and it is suggested that they were generated at prominent feature on the Moroccan gaining large amplitudes as shock waves in the supercritical flow.

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