Distortion of diffusion by non-breaking waves

Scalars in the upper ocean are transported by surface gravity waves. The mean transport is often attributed to the advective Stokes drift, however this ignores potential effects of waves on diffusion. In this talk, we show that non-breaking waves induce a mean distortion on diffusive transport through the unsteady straining of the wave orbital motion. Using a Lagrangian analysis, we show that scalars with a uniform and isotropic Eulerian diffusivity behave as if the diffusivity were non-uniform and anisotropic on average. This distortion effect varies with wave steepness and total water depth, and is strongest near the free surface. Closed-form expressions for the wave-averaged diffusivity are derived for monochromatic progressive waves, standing waves, and a wave spectrum respectively. We then contrast our results with numerical simulations and demonstrate that the wave motion itself causes buoyant scalars to concentrate closer to the free surface than they otherwise would.