DETAIL AND SCALING OF TURBULENT OVERTURNS IN THE PACIFIC EQUATORIAL UNDERCURRENT

We analyze turbulent overturns in the high-shear, low Richardson numbe r flow of the upper 350 m at 0 degrees/140 degrees W. Profiles of shea r and stratification combined from fine- and microscale sensors resolv e the vertical wavenumber spectrum from large-scale to dissipation sca les. We compute a turbulent length scale l from Thorpe-sorting potenti al temperature, while using potential density to avoid thermohaline in trusions. Pragmatically, we consider scales smaller than I turbulent a nd scales large than l nonturbulent. From local fine-scale velocity sp ectra, we extract the horizontal turbulent kinetic energy at scales sm aller than I and thus estimate the turbulent velocity q, a parameter c haracteristic of the energetic eddies. The independently observed visc ous dissipation rate epsilon, q, and I follow Taylor scaling, epsilon = C-q q(3)/l, with C-q approximate to 4. Similarly, the measured therm al dissipation rate chi arid the turbulent temperature fluctuation T', also estimated by spectral extraction at scales smaller than l, follo w similarity scaling, chi = c(t) T'(2)q/l, with c(t) approximate to 7. From q, l, buoyancy frequency N, and kinematic viscosity nu, we estim ate turbulent Reynolds numbers Re-t = ql/nu and turbulent Froude numbe rs Fr-t = q/(Nl). The more energetic overturns of vertical thickness e xceeding 1 m have 0.1 less than or similar to Fr-t less than or simila r to 3 and 250 less than or similar to Re-t less than or similar to 10 (5). Ozmidov scales follow overturning scales as described by Dillon ( 1982) only on average, but not in individual overturns. Richardson num bers Ri of overturns show large scatter around a median of Ri = 0.23 a nd virtually no correlation with epsilon. The mixing efficiency shows a weak increase with increasing Ri and a weak decrease with increasing Fr-t. Turbulence parameters are briefly compared with formulations in turbulence closure models. A few individual mixing events are analyze d in detail, with focus on possible forcing mechanisms. In such identi fiable events, enhanced turbulence is paralleled by enhanced fine-scal e 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 dissipa tion rates indistinguishable from the surroundings.