Multiscale Mixing Variability on the Inner Shelf

On the inner continental shelf, shoaling internal waves produce energetic stratified turbulence that is essential for the vertical mixing of heat, momentum, and biological tracers. Using shipboard microstructure data collected as part of the Inner-Shelf Dynamics Experiment in 2017, we demonstrate that midcolumn turbulence and diapycnal mixing off the coast of central California are highly variable, with bulk dissipation rates dominated by less than 15% of the measurements that are consistently collocated with shoreward-propagating internal bore features. Locally, the variability in values of the mixing efficiency is predicted well by a model based on the ratio between the Thorpe and Ozmidov length scales. Importantly, however, we also observe variability on larger temporal scales up to multiple days whereby the shapes and locations of the distributions of local mixing statistics taken over successive time windows change significantly. Bulk measurements of efficiency decrease by more than a factor of 2 over the observational period. We discuss how observed variations in the dynamical properties of the system might act to modify turbulent energy pathways, with important consequences for parameterizations of internal-wave-driven transport of energy, momentum, and tracers across the nearshore region.