Direct Experimental Observation of Wave‐Amplified Turbulent Mixing

Abstract Langmuir turbulence is a key mechanism to enhanced mixing in the upper ocean. Although often associated with wind, the theoretical framework proposed by Craik and Leibovich requires only vorticity and wave‐induced Stokes drift, regardless of their cause. In a laboratory flume, we make waves of varying steepness interact with a turbulent current to measure the vertical transport of a passive scalar directly. Once waves appear, the turbulent flow changes radically with long, intense Langmuir (like) vortices forming near the surface, aligned with the wave direction. Combining stereo particle‐image velocimetry and laser‐induced fluorescence, we simultaneously measure the turbulent velocity field and the concentration field of a surface‐injected dye, showing dramatic enhancement and deepening of the vertical scalar flux, increasing with wave steepness. The Langmuir enhancement factor for mixing is measured in the range 4–6. The results are consistent with three different variants of the most‐used parameterization of oceanic wave‐induced mixing.