Transient Rip-Current Exchange Across the Inner-Shelf Modulated by Diurnal Heating/Cooling.

Exchange of tracers (e.g., nutrients, pollutants, larvae) across the inner-shelf affects littoral ecosystem health and coastal water quality. Transient rip-currents are episodic offshore directed flows that are the dominant surf-zone to inner-shelf tracer exchange mechanism for alongshore uniform and wave exposed coastlines. On the stratified inner-shelf over time-scales of 1-24 hours transient rip-current enhanced vertical mixing in idealized 3D numerical simulations generates a subsurface baroclinic exchange pathway across the inner-shelf. The interaction of transient rip-currents with inner-shelf stratification is critical for this mechanism. Processes like surface heat fluxes, wind, waves, and tides set and modify stratification on the inner-shelf. The solar heating and cooling cycle modulates both vertical and cross-shore stratification, and generates thermally driven exchange in many contexts (e.g., lakes and reef-flats). Coupled COAWST--funwave-C simulations are used to examine how the combined influence of transient rip-current exchange and mixing with diurnal surface heat flux forcing affects cross-shore tracer exchange. Results indicate that, averaged over the solar cycle, the strength of the transient rip-current exchange pathway is not significantly altered. However, the vertical structure of the inner-shelf baroclinic exchange pathway is modulated over the solar cycle. At time-scales of 1-6 hours, the time of day of a shoreline tracer release can impact bulk cross-shore plume growth. These results have implications for the spreading rate of tracer plumes and their vertical distribution on the inner-shelf.