Stratified turbulence in the nearshore coastal ocean: Dynamics and evolution in the presence of internal bores

High-frequency measurements of stratified turbulence throughout the water column were collected over a 2 week period in the nearshore environment of southern Monterey Bay, CA, using a cabled observatory system and an underwater turbulence flux tower. The tower contained a vertical array of acoustic Doppler velocimeters and fast-response conductivity-temperature sensors, providing a nearly continuous data set of turbulent velocity and density fluctuations and a unique look into the stratified turbulence field. The evolution of various turbulence quantities and direct measurements of the vertical turbulent diffusivity is examined in the presence of nearshore internal bores, both in the near-bed region and in the stratified interior. We show that individual bores can drive substantial changes in local turbulence and mixing dynamics, with considerable differences between the leading and trailing edges of the bores. Using direct observations of the flux Richardson number, our measurements confirm previous observations that show the highest mixing efficiencies (Γ) occurring in regions of buoyancy-controlled turbulence. Parameterizations of the flux Richardson number as a function of the turbulence activity number are also presented. Finally, we demonstrate that the commonly used assumption of a constant mixing efficiency (Γ = 0.2) for calculating turbulent diffusivities leads to significant overestimates compared to diffusivity values calculated using the directly measured mixing efficiency. Implications of the results are discussed