Semidiurnal internal tide in JASIN: Observations and simulation

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Abstract
Temperature was observed in the upper 80 m by moored thermistor chains at three locations in Rockall Channel west of Scotland. Isotherms were interpolated, and a 1‐week period of exceptionally energetic tidal oscillations was analyzed. The moored array (horizontal separations ranging from 6 to 20 km) was used as an antenna to determine the dominant horizontal wavelength and direction of propagation of the internal tide within the array. Rockall Bank, 100 km to the southeast, was identified as the source of the internal tide. The semidiurnal internal tide generated by the interaction of the surface tide with Rockall Bank was simulated by use of a model due to Prinsenberg and Rattray. The model predicts generation of the internal tide at the shelf break and propagation seaward as energetic beams which lie along internal wave characteristics. Some energy is trapped near the surface in association with the pycnocline. There is substantial structure in the velocity and vertical displacement fields. Most of the total energy of the internal tide is in the first vertical mode. However, at particular depths, vertical modes as high as 4 (horizontal wavelengths of 25 km) dominate. The high degree of spatial variability in the modeled internal tide illustrates the potential for error when basing a description of the tide on sparse observations. There is good agreement between the modeled vertical displacements and the 1‐week period of energetic oscillations of isotherm depth observed at the moorings. In addition, there is good agreement between the model and the tidal velocity variance measured during the same week at depths ranging from 10 to 1000 m and distances ranging from 50 to 130 km from Rockall Bank. Richardson numbers associated with the vertical shear of the modeled internal tide range down to values less than 2.

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