Abstract

The boundary layer near a sloping bottom may have a major influence on the ocean's interior density structure (due to “boundary mixing”) and on its circulation (because of the arrest of the Ekman layer by buoyancy forces). As a first attempt to measure eddy fluxes of momentum and buoyancy, in order to quantify the mixing in this region, we have carried out a 5-day pilot experiment on a sloping side of Emerald Basin on the Scotian Shelf. A moored upward-looking 1.2 MHz ADCP and a thermistor chain mounted along its vertical axis returned analyzable data between 8 and 17 m above the bottom at one-minute intervals. An extensive set of microstructure profiles was also obtained. The predominantly tidal flow regime causes the bottom boundary layer thickness to vary between 3 < z < 30 m, with most high frequency activity during the upslope phase. A bottom-normal momentum flux significantly different from zero is found in the cross-isobath direction only. The main contribution comes from a band near the buoyancy frequency N, possibly indicative of advective or Kelvin-Helmholtz instability. When cast in terms of mean-flow shear, the stress yields an eddy viscosity A ≈ 9 × 10−3 m2 s−1 within the boundary layer and twice this value at z = 15 m, the average height of the pycnocline that caps the boundary layer. The buoyancy flux also seems to be dominated by fluctuating signals near N, but is countergradient and only significantly different from zero at a height of about 15 m. The associated restratification occurs in short periods of approximately one hour when isotherms rise rapidly. Indirect evidence for the importance of the tertiary circulation within the boundary layer is found from the gradient of stress divergence and the mean bottom-normal velocity. An approximate turbulent kinetic energy balance has been investigated, with the currents split into three parts (mean, tidal, and the high frequency part of the internal waveband (“turbulence”)). Production balances viscous dissipation within a factor of 2. Turbulent kinetic energy production by interaction between the turbulent Reynolds stress and the mean flow shear and tidal shear are of the same order of magnitude, but the buoyancy term appears to be of equal importance at the pycnocline.

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