Abstract

Intensive in-situ observations of nonlinear internal waves on the Portuguese shelf were made in August 1994, including measurements of the same wavepacket at three locations as it propagated on-shelf. The waves were characterized by sudden isotherm depressions of up to 45 m lasting 10-35 minutes, accompanied by current surges of up to 0.45 m s-1 and shears of up to 0.7 m s-1 over 60 m. The waves propagated away from the shelf break with an estimated phase speed of 0.57 m s-1. The amplitude of the waves was comparable to the theoretical maximum. The kinetic and potential energies of the waves were calculated directly from the observed current and density structures. Individual waves were associated with a total energy of up to approximately 3.0 M J m-1 per unit crest length. The depth integrated on-shelf internal wave energy flux approached 2000 W m-1 per unit crest length near the shelf break and decreased on-shore at a depth integrated rate of approximately 7.7 × 10-2 W m-2. The internal waves provided an important source of vertical mixing where they occurred on the Portuguese shelf throughout most of August 1994, represented by a sustained vertical eddy diffusivity profile which peaked at Kz ≈ 2.2 × 10-3 m2 s-1 at 30 m depth. Internal mixing would have to be parameterized by such a Kz profile in any model of the Portuguese shelf in which the stratification were to be accurately represented.

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