The cross-shore exchange of tracers (for example, pollutants, larvae, nutrients, and plankton) across the surfzone (SZ; region of depth limited surface gravity wave breaking) and the inner shelf (IS; region offshore of surfzone to ~15-m water depth) is integral to coastal human and ecosystem health. Broadly, many different mechanisms are known to contribute to cross-inner-shelf exchange of tracers. An inner-shelf (IS) dye plume that formed following a 3.84-h early morning surfzone (SZ) dye release off of Imperial Beach, California, is analyzed with in situ and aerial remotely sensed observations. Midmorning, 5 h after release start, the IS plume extended 800 m offshore (or 8Lsz, where Lsz is the surfzone width) and was surface intensified. Over the next 2 h, the IS plume deformed (narrowed) cross-shore with the offshore front progressing onshore at 5 cm/s, deepened by up to 3 m, and elongated alongshore at 4.5 cm/(s km) (at ~2.5Lsz). Coincident with IS plume deformation and deepening, IS isotherms also deepened, with relatively stable IS plume joint dye and temperature statistics. Offshore tracer transport and subsequent IS plume deformation and deepening likely resulted from two phases of the diurnal internal tide (DIT). During and after deformation, the IS plume did not reenter the warm surfzone, which potentially acted as a thermal barrier. High-frequency internal waves (HF IWs) propagated through the IS plume at 9 cm/s and dissipated onshore of 4Lsz. Surface HFIW signal was elevated in the plume elongation region, suggesting a linkage between plume elongation and either the DIT or HF IW. This IS plume evolution differs from previous SZ tracer releases, highlighting the effects of release timing relative to the solar cycle or the internal tide.
Other exchange mechanisms were also explored. Surf-zone wave breaking leads to transient rip-currents (TRCs), episodic, offshore flows onto the inner-shelf, which vertically mix stratified waters creating a cross-shore exchange pathway. In many regions, such as Southern California, daily surface heating/cooling, or diurnal surface heat-fluxes (SHF), also drive cross-shore exchange, because thermal response varies with water depth. However, the dominant exchange mechanism is not known. Impacts of combined TRC and SHF forcing on exchange and their relative strength are analyzed using idealized numerical model simulations. Cross-shore transport is quantified using a tracer released within the surf-zone. Tracer transport is strongest for simulations including TRCs, relative to SHF forcing alone, and transport induced by TRCs extends well offshore of the surf-zone. Analyses indicate that enhanced TRC-driven inner-shelf exchange is associated with the vertical mixing mechanism.
Farther offshore, transport of shoreline-released tracer from the surfzone across the shelf can be affected by a variety of physical processes from wind-driven to submesoscale, with implications for shoreline contaminant dilution and larval dispersion. Here, a high-resolution wave–current coupled model that resolves the surfzone and receives realistic oceanic and atmospheric forcing is used to simulate dye representing shoreline-released untreated wastewater in the San Diego–Tijuana region. Surfzone and shelf alongshore dye transports are primarily driven by obliquely incident wave breaking and alongshore pressure gradients, respectively. At the midshelf to outer-shelf (MS–OS) boundary (25-m depth), defined as a mean streamline, along-boundary density gradients are persistent, dye is surface enhanced and time and alongshelf patchy. Using baroclinic and along-boundary perturbation dye transports, two cross-shore dye exchange velocities are estimated and related to physical processes. Barotropic and baroclinic tides cannot explain the modeled cross-shore transport. The baroclinic exchange velocity is consistent with the wind-driven Ekman transport. The perturbation exchange velocity is elevated for alongshore dye and cross-shore velocity length scales , 1 km (within the submesoscale) and stronger alongshore density gradient variability, indicating that alongfront geostrophic flows induce offshore transport.  Both surfzone and shelf processes influence offshore transport of shoreline-released tracers with key parameters of surfzone and shelf alongcoast currents and alongshelf winds.

Last Modified: 05/19/2021
Modified by: Falk Feddersen