Multiscale mixing dynamics on the California Inner Shelf

The inner shelf (the transition between the surf zone and midshelf) is home to a wide range of physical processes, including surface and internal waves, boundary layers, submesoscale features, and vortices arising from interactions with complex coastal topography. The interplay between these different processes sets the circulation, stratification, and transport across this region, leading to complex dynamics over a variety of length and time scales. Here, using data from the Inner Shelf Dynamics Experiment field campaign (Waterhouse et al., 2020), we explore the details of the turbulent mixing off the coast of central California over an intensive sampling period in September 2017. Using microstructure turbulence measurements, we show that mixing coefficients are well-predicted by the ratio of the Thorpe and Ozmidov scale and that the overall mixing is dominated by a small number of intense events. By looking at detailed timeseries, we find that these events are associated with elevated dissipation at the front of shoaling internal bores, with different bore structures arising over the course of the observational period. We finally compare our results with numerical studies of nonlinear internal waves and gravity currents propagating in stratified ambients.