Measurements of Lagrangian trajectories in the upper ocean boundary layer: Implications for mixing parameterizations.

Most upper ocean boundary layer parameterizations invoke an eddy diffusivity to parameterize fluxes and/or other high order covariances. To be valid, this requires that the eddy size is significantly smaller than the boundary layer scale. We test this hypothesis by finding the top and bottom turning points of measured Lagrangian trajectories in a variety of ocean boundary layer forced by wind, wind-wave, convection and symmetric instability. Downward trajectories mostly start near the surface and end within the boundary layer with a nearly equal probability at all depths. Upward trajectories return to the surface. This is inconsistent with eddy diffusivity models. However, the trajectory statistics scaled by the layer depth are similar regardless of the forcing mechanism, while the energetics varies. This suggests that a nonlocal mixing model with prescribed geometry and variable energetics might be both efficient and accurate. Progress toward such a model based on the analyis of measured oceanic trajectories and the variations of properties along these trajectories will be presented.