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The Effect of a Bottom Shelf Front upon the Generation and Propagation of Near-Inertial Internal Waves in the Coastal Ocean

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  • 1 Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, United Kingdom
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Abstract

A three-dimensional nonlinear baroclinic model in cross-sectional form is used to study the generation and propagation of wind-forced near-inertial internal waves in a coastal region in the presence of a bottom front. Initially calculations are performed with the front in an infinite domain region. By this means coastal effects are removed. The initial response is in terms of inertial oscillations in the surface layer. However, in the frontal area these are modified by interaction through the nonlinear momentum terms with regions of positive and negative vorticity associated with the alongfront flow. This leads to a change in amplitude, phase, and frequency of the inertial current, and a resulting Ekman pumping that drives near-inertial internal waves in the frontal region. On the positive vorticity side of the front these waves are at the superinertial frequency and rapidly propagate away. On the negative side they are at the subinertial frequency and are trapped and inertial energy leaks to depth. Calculations with a coastal boundary and no front show that the offshore propagation of near-inertial waves is similar to that found on the positive vorticity side of the front. This shows that in terms of superinertial internal wave generation and propagation the front acts in a similar manner to a coastal boundary. However, with a coastal boundary, inertial currents below the thermocline are phase shifted by 180° from those above. This phase shift is only found in the frontal case when the front is adjacent to a coast and is due to the no-normal-flow condition at the coast. For the case of a stratified region between the coast and the front, near-inertial energy is trapped in this region and is dissipated at a rate depending upon the local value of vertical eddy viscosity. The offshore spatial distribution of near-inertial internal waves is found to be independent of whether the coastal region is represented by a vertical wall or a more realistic sloping seabed. However, in the case of a weak coastal front, the near-frontal distribution of near-inertial energy is influenced by the horizontal spatial variability of stratification within the front.

Corresponding author address: Dr. Alan M. Davies, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside CH43 7RA, United Kingdom. Email: amd@pol.ac.uk

Abstract

A three-dimensional nonlinear baroclinic model in cross-sectional form is used to study the generation and propagation of wind-forced near-inertial internal waves in a coastal region in the presence of a bottom front. Initially calculations are performed with the front in an infinite domain region. By this means coastal effects are removed. The initial response is in terms of inertial oscillations in the surface layer. However, in the frontal area these are modified by interaction through the nonlinear momentum terms with regions of positive and negative vorticity associated with the alongfront flow. This leads to a change in amplitude, phase, and frequency of the inertial current, and a resulting Ekman pumping that drives near-inertial internal waves in the frontal region. On the positive vorticity side of the front these waves are at the superinertial frequency and rapidly propagate away. On the negative side they are at the subinertial frequency and are trapped and inertial energy leaks to depth. Calculations with a coastal boundary and no front show that the offshore propagation of near-inertial waves is similar to that found on the positive vorticity side of the front. This shows that in terms of superinertial internal wave generation and propagation the front acts in a similar manner to a coastal boundary. However, with a coastal boundary, inertial currents below the thermocline are phase shifted by 180° from those above. This phase shift is only found in the frontal case when the front is adjacent to a coast and is due to the no-normal-flow condition at the coast. For the case of a stratified region between the coast and the front, near-inertial energy is trapped in this region and is dissipated at a rate depending upon the local value of vertical eddy viscosity. The offshore spatial distribution of near-inertial internal waves is found to be independent of whether the coastal region is represented by a vertical wall or a more realistic sloping seabed. However, in the case of a weak coastal front, the near-frontal distribution of near-inertial energy is influenced by the horizontal spatial variability of stratification within the front.

Corresponding author address: Dr. Alan M. Davies, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside CH43 7RA, United Kingdom. Email: amd@pol.ac.uk

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