Where did it go, and why? Surf-zone tracer release 3-D hindcast with obliquely incident swell.

The fate of surf-zone released tracers is an important aspect of coastal oceanography and relevant to policy making and mitigation efforts of adjacent communities. For example, the dispersion (connectivity) and recruitment of intertidal larval populations depend on surf-zone and inner-shelf hydrodynamics and alongshore transport (e.g., Morgan et al., 2017 Ann Rev) . The evolution of passive tracers in the surfzone also directly reflects the spreading of pollution and impacts of poor coastal water quality on adjacent coastal communities (e.g., Boehm et al., 2019). Development of operational products, with the goal of real-time forecasting of coastal tracer evolution, requires evaluation of current modeling techniques and comparison with field observations. The CSIDE (Cross Surfzone / Inner-shelf Dye Exchange) experiment (Sept & Oct 2015; Imperial Beach, CA) combined both in-situ physical oceanographic (temperature, velocity) observations with coordinated in-situ and remote passive fluorescent tracer (dye) measurements from a series of nearshore tracer releases (Grimes et al. 2020). Here, observations from a surf-zone tracer release are compared to quasi-realistic 3-D COAWST (ROMS/SWAN) hindcast simulations. Simulations are down-scaled from synoptic to regional scales, with the study region resolution ranging from 8-100 m (Wu et al., 2020). In situ surf-zone and remote near-surface inner-shelf tracer observations are compared with simulated dye tracer fields. The sensitivity of surf-zone alongshore transport to wave boundary conditions are evaluated. We also explore the relationship between modeled/observed inner-shelf tracer evolution and transport and the timing of inner-shelf events, e.g. arrival of internal tides. Preliminary results are promising and reproduce several aspects of tracer evolution, including alongshore arrival times, nominal duration and peak surf-zone tracer concentrations.