Cross-shore deformation of a surfzone released dye plume by an internal tide on the inner-shelf

Abstract

An inner-shelf (IS) dye plume that formed following a 3.84 h early morning surfzone (SZ) dye release off of Imperial Beach, CA is analyzed with in situ and aerial remotely-sensed observations. Mid morning, 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 re-enter the warm surfzone, which potentially acted as a thermal barrier. High-frequency internal waves (HF IW) 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.