Heat flux and mixing efficiency in the surface mixing layer

Abstract

Fluctuations of vertical velocity and temperature, w′ and T′, were measured with a horizontal profiler that was towed at night in the oceanic boundary layer between 15 and 25 m depth. Stratified and convective turbulent regimes were encountered along the tow path. A direct estimate of the turbulent heat flux F was computed from the correlation of w′ and T′. The concurrent measurement of the dissipation rate of turbulent kinetic energy ε allowed us to estimate the mixing efficiency Γ0 = F/Fε, where Fε was the heat flux estimate based on the average dissipation rate. In regions where the turbulence in the stratified boundary layer was sustained by shear instabilities 0.08 ≤ Γ0 ≤ 1.38. The average was Γ̄0 = 0.46, a value close to the maximum mixing efficiency predicted by classical scaling arguments and laboratory results. The measurements of w′ were significantly influenced by instrument motions and the orbital velocities induced by surface waves. A motion correction algorithm made it possible to resolve overturning length scales up to 33 m and thus to capture all scales that contributed to the heat flux F. For the surface mixing layer reported here the largest flux-supporting scales were ∼14 m. For the stably stratified regimes in particular the peak of the heat flux cospectrum was at one half of the Ozmidov wave number, 0.5ko. Copyright 2001 by the American Geophysical Union.