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Free Content Internal wave band eddy fluxes above a continental slope

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Three weeks of velocity and temperature measurements from the bottom 45 m above the continental slope in the Bay of Biscay are used to evaluate the role of the internal wave band in boundary mixing near a sloping bottom. Utilizing acoustic Doppler current profilers and thermistor strings, internal wave band eddy fluxes of momentum and heat are estimated. The instrumentation is specifically designed to resolve internal wave band processes. Due to unresolved Doppler shifting, this wave band may include turbulence as well as internal waves. A very energetic and highly variable near-bottom environment is found. Periods of mixing and restratification alternate at the M2 tidal frequency. Interpreting the observations in an Ekman sense, the three-week mean current is downwelling-favorable, which is consistent with existing boundary layer theories. However, a bi-directional flow associated with sloping boundary mixing is not found in the near-bottom layer, possibly due to observed strong stratification all the way to the bottom.

We evaluated boundary layer dynamics and the effect of internal wave-band fluxes from two frequency ranges ( ≥ 15 cpd and  ≥ 1.9 cpd, including tides) on the three-week mean flow. The high-frequency range ( ≥ 15 cpd) of the internal wave band supports significant momentum and buoyancy fluxes while the low-frequency range ( ≥ 1.9 cpd) only supports significant momentum fluxes. Mean bottom-normal eddy diffusivities associated with anisotropic, nonlinear internal waves, are negative and O (-10-2 m2s-1). Interpreting these negative eddy diffusivities as indication of a restratification process, high mixing efficiencies are expected throughout the mixing layer, which extends typically 20 m above the bottom. Mean eddy viscosities are positive in cross-slope direction and negative in alongslope direction, implying a strong anisotropy in the interaction between internal wave band eddies and the mean flow. Alongslope momentum is transferred from the internal tide to the mean flow. Buoyancy and pressure gradient forces, which we could not determine directly, may generate a buoyancy-driven secondary flow. The buoyancy equation is dominated by advection, possibly balanced by divergence of cross-slope and alongslope internal wave band fluxes.

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Document Type: Research Article

Publication date: March 1, 2002

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