Variations in Apparent Mixing Efficiency in the North Atlantic Central Water

Microstructure data from the North Atlantic Tracer Release Experiment (NATRE) are presented, providing detailed profiles of the thermal variance χ in the upper 360 m of the Canary Basin for the fall and spring seasons. The Osborn–Cox model is used to compute the diffusivity KT. The diffusivity for the depth range 240–340 m is found to be 1.0(±0.04) × 10−5 m2 s−1 in the fall and 2.2(±0.1) × 10−5 m2 s−1 in the spring, in good agreement with dye-inferred diffusivities at similar depths. Measured turbulent kinetic energy (TKE) dissipation rates were found to be contaminated by hydrodynamic noise, so the Osborn dissipation method was not used to compute Kρ. However, data segments for which the TKE dissipation rate (ε) was large enough to be unaffected by noise were used to compute the “apparent mixing efficiency” Γd. The computed Γd values are used to investigate variations in apparent mixing efficiency with respect to density ratio (Rρ) and turbulence Reynolds number [ε/(νN2)], in an attempt to elucidate the underlying mechanisms of mixing in the NATRE region. Observed variations of Γd are compared with existing theoretical models of mixing due to: salt fingers, a combination of salt fingers and turbulence, “conventional” high Reynolds number turbulence, and low Reynolds number buoyancy-modified turbulence. Significant variations of Γd with respect to both Rρ and ε/(νN2) are found. Although Monte Carlo tests show that some of the observed variations could be noise-induced, a substantial portion of the systematic variations the authors observed were not reproduced by Monte Carlo simulations. These trends are found to be statistically significant, and the authors conclude that they represent real variations in the apparent mixing efficiency. The authors find that Γd is an increasing function of ε/(νN2) and a decreasing function of Rρ; these variations are not fully consistent with any of the available mixing models.