Detail and scaling of turbulent overturns in the Pacific Equatorial Undercurrent

Abstract

We analyze turbulent overturns in the high‐shear, low Richardson number flow of the upper 350 m at 0°/140°W. Profiles of shear and stratification combined from fine‐ and microscale sensors resolve the vertical wavenumber spectrum from large‐scale to dissipation scales. We compute a turbulent length scale l from Thorpe‐sorting potential temperature, while using potential density to avoid thermohaline intrusions. Pragmatically, we consider scales smaller than l turbulent and scales large than l nonturbulent. From local fine‐scale velocity spectra, we extract the horizontal turbulent kinetic energy at scales smaller than l and thus estimate the turbulent velocity q, a parameter characteristic of the energetic eddies. The independently observed viscous dissipation rate ε, q, and l follow Taylor scaling, ε = cq q3/l, with cq ≈ 4. Similarly, the measured thermal dissipation rate χ and the turbulent temperature fluctuation T′, also estimated by spectral extraction at scales smaller than l, follow similarity scaling, χ = ctT′2q/l, with ct ≈ 7. From q, l, buoyancy frequency N, and kinematic viscosity ν, we estimate turbulent Reynolds numbers Ret = ql/ν and turbulent Froude numbers Frt = q /(Nl). The more energetic overturns of vertical thickness exceeding 1 m have 0.1 ≲ Frt ≲ 3 and 250 ≲ Ret ≲ 105. Ozmidov scales follow overturning scales as described by Dillon (1982) only on average, but not in individual overturns. Richardson numbers Ri of overturns show large scatter around a median of Ri = 0.23 and virtually no correlation with ε. The mixing efficiency shows a weak increase with increasing Ri and a weak decrease with increasing Frt. Turbulence parameters are briefly compared with formulations in turbulence closure models. A few individual mixing events are analyzed in detail, with focus on possible forcing mechanisms. In such identifiable events, enhanced turbulence is paralleled by enhanced fine‐scale variance of velocity, shear, and temperature. One mixing event shows signatures of critical layer absorption. A very large overturn in the nighttime turbulent layer near the surface surprisingly shows dissipation rates indistinguishable from the surroundings.