Surface Wave–Turbulence Interactions. Scaling ϵ(z) near the Sea Surface

Abstract

A freely rising profiler was used to collect vertical microstructure profiles in the upper oceanic boundary layer under various atmospheric and sea conditions. Near the sea surface, the rate of viscous dissipation of turbulence kinetic energy, ε, exhibited a range of behaviors under different forcing conditions. Sometimes, ε was closely balanced by the wind stress production of turbulence kinetic energy. At other times, ε was greatly enhanced relative to wind stress production and exhibited an exponential depth deny. In these instances, simple scaling laws predicted for turbulence near a solid surface severely underestimate turbulent mixing near the ocean surface. Plausible explanations for enhanced ε(z) near the sea surface will have to address the effects of wave-turbulence interactions. The authors propose two different mechanisms to explain the behavior of ε near the surface, leading to two scaling schemes. The first mechanism requires high levels of turbulence kinetic energy, created by wave breaking at the surface, to be transported downward away from the surface by the motion of the swell. This transport is then locally balanced by ε. The second mechanism requires a rotational wave field and significant wave stresses that balance the turbulence Reynolds stresses. Energy drawn from the wave field to the mean flow, via the wave stresses, is in turn drawn from the mean flow by the turbulence production term, which is balanced by ε.