Abstract

Vortex stretching has been proposed as a possible mechanism by which internal waves can extract energy from the mean flow. Relationships between the slowly varying (ω < 0.25 cpd) and rapidly varying (0.13 cph < ω < 0.5 cph) components of the flow have been examined in a 4-month-long data set taken off the coast of British Columbia. The rapidly varying component of horizontal velocity generally rotates clockwise, and is in rough agreement with internal wave dynamics. It is horizontally incoherent within a distance of 10 km and is vertically coherent across the water column with a nearly 180° phase change. Scatter plots show that the wavefield is anisotropic, with the Reynolds stresses generally obeying uv < 0, vw < 0 and uw > 0, where (u, v, w) are the fluctuating velocity components in the (onshore, alongshore, upward) directions. Instances have been found in which time variations of uv and the mean horizontal shear rate Vx are negatively correlated, with an implied horizontal viscosity of VH ∼ (3 ± 2) × 105 cm2/s. No correlation of vw and the mean vertical shear Vz is found. It is suggested that nonlocal behavior is important because propagation times in the vertical are smaller than the interaction times of the wave packets. With the observed shear rates and the wave energy levels, the vertical viscosity is unlikely to be beyond the range ± 20 cm2/s.

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