Cross‐shore tracer exchange between the surfzone and inner‐shelf

Abstract

AbstractCross‐shore tracer exchange between the surfzone and inner‐shelf is examined using temperature and dye measurements at an approximately alongshore‐uniform beach. An alongshore‐oriented plume is created by releasing dye continuously for 4.5 h in a surfzone alongshore current. The plume is sampled for 13 h from the release point to 700 m downstream, between the shoreline and 250 m offshore (6 m water depth). Within the surfzone (≤2 m depth), water is relatively warm, and dye is vertically well mixed. On the inner‐shelf (3–6 m depth), alongshore currents are weak, and elevated temperature and dye co‐occur in 25–50 m wide alongshore patches. Within the patches, dye is approximately depth‐uniform in the warm upper 3 m where thermal stratification is weak, but decreases rapidly below 3 m with a strong thermocline. Dye and temperature vertical gradients are correlated, and dye is not observed below 18 °C. The observations and a model indicate that, just seaward of the surfzone, thermal stratification inhibits vertical mixing to magnitudes similar to those in the ocean interior. Similar surfzone and inner‐shelf mean alongshore dye dilution rates are consistent with inner‐shelf dye properties being determined by local cross‐shore advection. The alongshore‐patchy and warm inner‐shelf dye is ejected from the surfzone by transient rip currents. Estimated Stokes drift driven cross‐shore exchange is small. The transient rip current driven depth‐normalized heat flux out of the surfzone has magnitude similar to those of larger‐scale shelf processes. Dye recycling, from the inner‐shelf back to the surfzone, is suggested by relatively long surfzone dye residence times.