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−1, deepened by up to 3 m, and elongated alongshore at ≈4.5 cm s−1 km−1 (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−1 and dissipated onshore of 4Lsz. Surface HF IW 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.

Exchange across the surf-zone and inner-shelf affects coastal water quality and larval recruitment. Surf-zone generated transient rip-currents (TRC) exchange shoreline released tracers onto and across a stratified inner-shelf. Surface heat fluxes (SHF) modify inner-shelf stratification and surf-zone temperature, relative to the inner-shelf, inducing nearshore thermally driven exchange. The coupled effect of TRC and diurnal SHF forcing on cross-shore exchange is evaluated using idealized model surf-zone tracer releases with TRC-only, SHF-only, and combined SHF+TRC forcing. For conditions representing Fall in Southern California, the TRC mechanism dominates cross-shore exchange, relative to SHF, to 12L SZ offshore (L SZ = 100 m is the surf-zone width). Tracer and velocity derived estimates of exchange velocity indicate that the TRC cross-inner-shelf exchange mechanism is due to an alongshore mean baroclinic flow setup by TRC vertical mixing of inner-shelf stratification.Plain Language Summary Cross-shore transport (also called exchange) of material, for example, pollutants, larvae, nutrients, and plankton, is important in coastal oceanography. Natural 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.

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.