The inner shelf, the transition zone between the surf zone and the mid shelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from Sep.-Oct. 2017, conducted from the mid shelf, through the inner shelf and into the surf zone near Point Sal, CA. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the mid shelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.
In 2017, an ocean research team launched an unprecedented effort to understand what drives ocean currents in the overlap regions between surf zones and continental shelves.
Temperature and velocity measurements from 42 moorings were used to investigate the alongshore variability of nonlinear internal bores as they propagated across the central California inner shelf. Moorings were deployed September – October 2017 offshore of the Pt. Sal headland. Regional coverage was ~30 km alongshore and ~15 km across shore, spanning 9 – 100 m water depths. In addition to subtidal processes modulating regional stratification, internal bores generated complex spatiotemporal patterns of stratification variability. Internal bores were alongshore-continuous O(10s km) at the 50 m isobath, but the length scales of frontal continuity decreased to O(1 km) at the 25 m isobath. The depth-averaged, bandpass filtered (3-min – 16-hr) internal bore kinetic energy (KEIB¯) was found to be nonuniform along a bore front, even in the case of an alongshore-continuous bore. The pattern of along-bore KEIB¯ variability varied for each bore, but a 2-week average indicated that KEIB¯ was generally strongest around Pt. Sal. The stratification ahead of a bore influenced both the bore’s amplitude and cross-shore evolution. The data suggest that alongshore stratification gradients can cause a bore to evolve differently at various alongshore locations. Three potential bore fates were observed: 1) bores transiting intact to the 9 m isobath, 2) bores being overrun by faster, subsequent bores, leading to bore merging events, and 3) bores disappearing when the upstream pycnocline was near or below mid-depth. Maps of hourly stratification at each mooring and the estimated position of sequential bores demonstrated that an individual internal bore can significantly impact the waveguide of the subsequent bore.
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