Direct Measurements of Turbulence in the Pacific Equatorial Undercurrent

Abstract

Measurements of small-scale fluctuations of temperature are used to estimate turbulent parameters such as viscous dissipation rate (ϵ), dissipation rate of temperature variance (χ), and turbulent diffusion coefficients of momentum (ν_T) and temperature (D_T). Results from two locations are reported: one in the center of the undercurrent (ON 150W) and one toward the northern edge (IN 150W); both at depths of about 100 m where high vertical shear and high vertical stability are found. Universal similarity and local isotropy assumptions were used to determine the dissipation rates from measured spectra. While ϵ(∼0.08 cm² sec⁻³) was about the same at both locations, χ at ON [7 × 10⁻⁵ (°C)² sec⁻¹] was larger by a factor of 9. Even greater differences were found in D_T: 27 cm² sec⁻¹ at ON vs 0.52 cm² sec⁻¹ at 1N indicating large vertical mixing at the equator. From two independent methods ν_T yielded about the same results within 15%: 12 cm² sec⁻¹ at 1N and 25 cm² sec⁻¹ at ON.