Effects of Density Front on the Generation and Propagation of Internal Tides

Wen-Ssn Chuang Coastal Studies Institute, Louisiana State University, Baton Rouge, 70803

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Dong-Ping Wang Energy and Environmental System Division, Argonne National Laboratory, Argonne, IL 60439

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Abstract

A numerical model is developed to study the internal tidal motion on the continental margin. The system includes irregular bottom topography and a horizontal density stratification maintained by a mean geostrophic current. Both the propagation and generation processes are examined.

In the propagation process, the topographic effect alone will scatter a significant part of the incident wave energy into higher modes, resulting in a beamlike structure for the transmitted wave. For a density front over a flat bottom, energy in the transmitted waves remains largely in the original wave mode, though the wavelength, and hence wave amplitude, varies with local density stratification. When the density front is located over a sloping bottom, the topographic effect is reduced, whereas the frontal effect is not affected. Thus, scattering into higher modes becomes more restricted.

In the generation process, internal tides are produced in the upper and lower parts of the slope region due to the interaction between surface tides and bottom topography. With the presence of a density front, internal tide energy also can be derived from the surface frontal layer. In addition, the topographic effect will be modified by the front. The net effect is destructive (constructive) when the isopycnals tilt upward in the shoreward (seaward) direction.

These results indicate that on the continental margin, the density front has strong effects on the generation and propagation of internal tides, particularly when the density front is located above the continental slope. Since the internal tide provides an important energy source for mixing on the shelf, the density fronts will have a strong effect on the mixing processes.

Abstract

A numerical model is developed to study the internal tidal motion on the continental margin. The system includes irregular bottom topography and a horizontal density stratification maintained by a mean geostrophic current. Both the propagation and generation processes are examined.

In the propagation process, the topographic effect alone will scatter a significant part of the incident wave energy into higher modes, resulting in a beamlike structure for the transmitted wave. For a density front over a flat bottom, energy in the transmitted waves remains largely in the original wave mode, though the wavelength, and hence wave amplitude, varies with local density stratification. When the density front is located over a sloping bottom, the topographic effect is reduced, whereas the frontal effect is not affected. Thus, scattering into higher modes becomes more restricted.

In the generation process, internal tides are produced in the upper and lower parts of the slope region due to the interaction between surface tides and bottom topography. With the presence of a density front, internal tide energy also can be derived from the surface frontal layer. In addition, the topographic effect will be modified by the front. The net effect is destructive (constructive) when the isopycnals tilt upward in the shoreward (seaward) direction.

These results indicate that on the continental margin, the density front has strong effects on the generation and propagation of internal tides, particularly when the density front is located above the continental slope. Since the internal tide provides an important energy source for mixing on the shelf, the density fronts will have a strong effect on the mixing processes.

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