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Turbulent energy dissipation in the Atlantic equatorial undercurrent Crawford, William Robert


A free-fall oceanographic instrument has been used to measure vertical microstructure scale gradients of horizontal velocity, temperature and electrical conductivity. The velocity gradients, or shears, were measured at scales between 3 and 40 cm by an airfoil shear probe whose specifications and calibration procedure are discussed. Data collected in the equatorial Atlantic in July 1974 indicated a consistent pattern of turbulence near the velocity core of the Atlantic Equatorial Undercurrent. (The velocity core is the region of maximum speed. ) The most intense turbulence was found above the velocity core of the undercurrent. Turbulence in the velocity core was weak and intermittently spaced. Below the core, near the base of the thermocline, moderately intense turbulence was found. The rate of viscous dissipation of turbulent energy has been estimated from the shear measurements, and typical values were 3x10 ⁻³ cm² sec ⁻³ above the velocity core. Spectra of the shears have been computed. At small wavelengths the measured spectral coefficients fall below the universal Kolmogoroff spectrum. This discrepancy between the two spectra is attributed to spatial averaging of velocity fluctuations by the shear probe. The estimates of viscous dissipation include a correction for this spatial averaging. An energy balance has been determined for the turbulent velocity fluctuations. Above and below the core the basic balance is local production of turbulent energy equals local dissipation, and this balance gives a vertical eddy viscosity of order 10 cm² sec ⁻¹ above the core. The equation of the energy balance of the average motion has been vertically integrated at the equator where meridional terms are assumed small. In the South Equatorial Current the rate of energy gain from the average zonal wind stress is balanced by the rate of energy loss to the zonal pressure gradient plus the rate of dissipation. In the undercurrent, above the core, the rate of energy gain from the zonal pressure gradient equals the rate of dissipation within the uncertainty of the measurements, and the advection term is small but not negligible.

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