The Horizontal Diffusion of Tracers by Surface Waves

The horizontal dispersion of tracers in the presence of a random field of ocean surface waves is examined. Random fluctuations in the local Stokes-drift current cause a water particle to follow a random-walk path. The associated diffusion coefficients for individual particles, particle pairs and a continuous tracer patch can be calculated rigorously within the framework of perturbation analysis. For a fully developed Pierson-Moskowitz wave spectrum all diffusion coefficients scale as the third power of the wind speed and are typically of the order 10−2 m2 s−1 for a wind speed of 10 m s−1. The diffusion coefficients are strongly anisotropic and decrease approximately exponentially with depth below the sea surface.