Heat flux and mixing efficiency in the surface mixing layer

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 in depth. Stratified and convective turbulent regi mes 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 epsilon al lowed us to estimate the mixing efficiency Gamma (0) = F/F-epsilon, where F -epsilon was the heat flux estimate based on the average dissipation rate. In regions where the turbulence in the stratified boundary layer was sustai ned by shear instabilities 0.08 less than or equal to Gamma (0) less than o r equal to 1.38. The average was <(<Gamma>)over bar>(0) = 0.46, a value clo se to the maximum mixing efficiency predicted by classical scaling argument s and laboratory results. The measurements of w' were significantly influen ced by instrument motions and the orbital velocities induced by surface wav es. A motion correction algorithm made it possible to resolve overturning l ength 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 flu x-supporting scales were similar to 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, 0.5k(o).