Analysis of an Internal Tide Observed on the Malin Shelf, North of Ireland

T. J. Sherwin Unit for Coastal and Estuarine Studies, Marine Science Laboratories, University College of North Wales, Anglesey, UK

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Abstract

Evidence of an internal tide on the eastern side of the Rockall Trough is derived from recording current meters and thermistor chains sited at the edge and near the center of the Malin Shelf. Although the barotropic tidal currents are small (M2 is 0.13 m s−1), temperature contour plots revealed significant semidiurnal oscillations at both sites. From an analysis of the data it appears that there was a linear, predominantly mode-1, internal tide which was coherent with the barotropic tide over the spring-neap cycle and which had a mean energy of 452 J m−2 at the shelf edge. The onshore flux (estimated to be 104 W m−1) was about twice that predicted by the Baines model for subcritical slopes but, in view of the simplifications that are made, the comparison is considered reasonable. For the model to be applicable, most of the energy must be generated in the deep water of the Rockall Trough, propagating eastward over a marginally subcritical slope and onto the shelf. The energy dissipated at about 10−3 W m−2 (with an e-fold decay length of about 85 km) between the two stations. A number of possible sinks are examined, and it is concluded that much of the energy must be lost through overturning due to local Richardson number instabilities. It is suggested that the dissipation rate is sufficient to cause internal mixing of the summer thermocline, and that the mixing power of the internal tide on the outer shelf is greater than that of the barotropic tide.

Abstract

Evidence of an internal tide on the eastern side of the Rockall Trough is derived from recording current meters and thermistor chains sited at the edge and near the center of the Malin Shelf. Although the barotropic tidal currents are small (M2 is 0.13 m s−1), temperature contour plots revealed significant semidiurnal oscillations at both sites. From an analysis of the data it appears that there was a linear, predominantly mode-1, internal tide which was coherent with the barotropic tide over the spring-neap cycle and which had a mean energy of 452 J m−2 at the shelf edge. The onshore flux (estimated to be 104 W m−1) was about twice that predicted by the Baines model for subcritical slopes but, in view of the simplifications that are made, the comparison is considered reasonable. For the model to be applicable, most of the energy must be generated in the deep water of the Rockall Trough, propagating eastward over a marginally subcritical slope and onto the shelf. The energy dissipated at about 10−3 W m−2 (with an e-fold decay length of about 85 km) between the two stations. A number of possible sinks are examined, and it is concluded that much of the energy must be lost through overturning due to local Richardson number instabilities. It is suggested that the dissipation rate is sufficient to cause internal mixing of the summer thermocline, and that the mixing power of the internal tide on the outer shelf is greater than that of the barotropic tide.

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