Point Sal Inner Shelf Experiment (PSIEX): Turbulent Mixing in a Wave-Current Bottom Boundary Layer

Wind driven cross-shelf circulation on the inner shelf is highly sensitive to the vertical distribution of the eddy viscosity. Spatial variability in turbulence may be affected by a diverse range of external forcings, including ocean swell, surface wind waves, internal waves, shear from wind-driven and tidal currents, variable roughness of the bottom boundary, and suppression from stratification. In the bottom boundary layer, the basic model estimates mixing through mean current generated shear. To account for the impact of wave orbital velocities on current generated mixing, models typically adjust the effective bottom roughness. However, these models were generated in a laboratory environment, where the wave and currents aligned and the Reynolds numbers were relatively low. Using observational data from the California inner shelf, we tested the ability of empirical models to predict turbulent viscosity.

During June and July 2015, we observed turbulent mixing in the bottom boundary layer from an instrumented quadpod deployed in 25 meters water near Pt. Sal, CA. We directly measured turbulent mixing through the eddy viscosity, ν_T, computed as ν_T = - <u'w'>/∂U/∂z. Co-located acoustic Doppler velocimeters and acoustic Doppler current profilers yielded the Reynolds stress, <u'w'>, and mean horizontal shear, ∂U/∂z, respectively. We applied wave-turbulence decomposition to compute the Reynolds stress and accounted for instrument noise in the computed covariances. Results demonstrate that models based on mean current only over-predict the eddy viscosity by an order of magnitude. We will present comparisons of the measured eddy viscosity with external forcings, including surface wave direction and magnitude, internal waves, and vertical stratification.