The coastal ocean, from the shoreline to the mid–continental shelf with water depths ranging from 0 to about O(100) m, spans regions with circulation patterns driven by distinct processes. Coastal ocean circulation regulates the transport of tracers like nutrients, pathogens, and pollutants critical to maintaining healthy ecosystems (e.g., Grant et al. 2005; Boehm et al. 2017), and controls lateral movement of heat, sediment, and entrained gases (e.g., Fewings and Lentz 2011; Sinnett and Feddersen 2019). Bottom sediment resuspension and the advection and mixing of particles, both organic and inorganic, contributes to variable optical clarity of coastal waters. Fluctuations in coastal ocean temperature modify the local stratification, sound speed, and shallow-water acoustics (e.g., Badiey et al. 2002).
Within the coastal ocean, the surfzone extends from the shoreline to the offshore extent of depth-limited wave breaking, while the midshelf region is categorized by nonoverlapping surface and bottom boundary layers separated by a distinct interior. The inner shelf (e.g., Lentz 1994; Lentz 1995b) is a transition region between the surfzone and the midshelf where the boundary layers can overlap. The dynamics within and immediately outside the inner shelf are complicated as surface waves, internal waves, wind, barotropic tidal processes, buoyancy, submesoscale eddies, and boundary layer–driven processes all contribute to changing the circulation pattern and local stratification on frictional, rotational, and longer time scales.
Previous studies targeting the inner shelf have well documented the wind-driven and surface gravity wave–driven dynamics on simple coastlines and bathymetry (e.g., Lentz and Fewings 2012), yet the role of complex, along-shelf-varying coastlines in modifying inner-shelf dynamics on subtidal and shorter time scales is not well understood. In addition, the importance of other physical mechanisms like the role of turbulence forced by high-frequency processes (e.g., nonlinear internal waves) is both poorly understood and undersampled. Moreover, prior studies generally treated processes like subtidal wind-driven circulation in isolation from other inner-shelf physical processes. Nonlinear interactions between wind, surface gravity waves, internal waves, surface heat fluxes, turbulence, and rip currents are yet to be quantified.
Consequently, in order to understand and predict the exchange of water properties (heat, gases, sediment, pollutants, biota) across the inner shelf over a range of temporal and spatial scales the Office of Naval Research Inner-Shelf Dynamics Departmental Research Initiative coordinated field observations (in situ and remote sensing) and numerical modeling efforts on a 50-km section of coast off of central California, in the vicinity of Point Sal, California. Hereinafter, we refer to this experiment as the Inner-Shelf Dynamics Experiment (ISDE).
The principal goals of the ISDE are to (i) diagnose interactions between physical processes in the time-varying inner-shelf circulation, (ii) quantify the importance of along-coastline variability in creating complex circulation patterns on length scales of order 1–10 km, (iii) determine the role of turbulence in mixing tracer fluxes at subtidal and shorter time scales and in constraining momentum and energy transports on all time scales, and (iv) improve the predictive capability of numerical ocean and wave propagation models to simulate regional dynamics.
Here we report on the major findings and data products from the ISDE. A background describing the major physical processes in the inner shelf is considered in the second section. The field experiment and numerical model applications are considered in the third and fourth sections, respectively. Various important physical processes observed during the field experiment, complemented by numerical modeling efforts, are presented in the fifth section. In the sixth section we discuss the spatial heterogeneity of inner-shelf processes observed during the experiment, and the nonlinear interaction between multiple dynamical drivers of circulation and mixing. Findings from this work and suggested future directions for investigation are summarized in the last section.