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Turbulent energy dissipation over the continental shelf Dewey, Richard Kelvin


A free-falling instrument was used in coastal waters to measure turbulent velocity fluctuations, temperature, conductivity and pressure from the near surface to 15 cm above the bottom. A probe guard system has been developed that protects the delicate temperature and shear sensors from bottom sediments and minimizes instrument vibrations that would otherwise contaminate the shear signal. From the shear signal the viscous dissipation rate of turbulent kinetic energy is calculated. A new technique is presented for the analysis of shear spectra for dissipation rate calculations. The identification and elimination of noise, both at low and high frequencies, is accomplished by a positive feedback loop during analysis and insures more accurate estimates of the microstructure shear variance, [formula omitted]. The technique improves the confidence in the dissipation rate estimates and results in a noise level of 3.0 X 10⁻⁷ W m⁻³. This noise level is low, considering the structural modifications made to the profiler for near-bottom sampling. The microstructure instrument was used for 670 profiles over the continental shelf west of Vancouver Island in June, 1985. From the dissipation rates near the bottom, within the constant stress layer, values of the bottom stress are calculated. Variations in the bottom stress and the height of the turbulent bottom boundary layer correlate with the diurnal tidal currents that dominate the flow near the bottom. The height of the bottom well mixed layer was found to be nearly independent of the height of the turbulent bottom boundary layer. Over most of the shelf, vertical density variations are attributable to advection rather than local mixing. Near shore, in depths less than 100 m, the tidally driven turbulent bottom boundary layer extends throughout most of the water column during periods of maximum tidal current. Seaward of the 100 m depth contour the current and density measurements above the bottom boundary layer, 40 to 50 m above the bottom, reveal the mean structure of the Tully eddy. Contours of constant density show that the structure is an upwelling centre confined to a region over part of the Juan de Fuca Canyon system. Turbulent mixing within the core of the eddy was found to be weak. Oxygen samples indicate that the wind-induced upwelling brings slope water up the canyons to the shallow (<100 m) banks near shore. Nutrients in the slope water are mixed vertically by the tidally driven bottom boundary layer over these banks. Flux rates for NO₃ of 387 mmole s⁻¹ per metre of coastline are estimated during the strong upwelling conditions in June, 1985. From the turbulent dissipation rate measurements within the bottom boundary layer an estimated lower limit to the decay scale for the K₁ period shelf wave is roughly put at 1100 km. This is in good agreement with the model of Brink (1982a). From the dissipation rate measurements above the bottom boundary layer, a friction decay time scale for the Tully eddy is estimated to be 231 hours. This is supported by the observations of Freeland and Mcintosh (1987, personal communication) that show large, frequent fluctuations in the circulation at periods of ~ 20 to 330 hours. A global dissipation of 4.8 x 10¹⁰ W is estimated for the tides over all continental shelf regions, only 2.5% of the total tidal kinetic energy dissipated by friction in all the oceans and seas.

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