UC San Diego

Tracer Exchange across the inner-shelf is critical in nutrient cycling and larval recruitment, and coastal water quality. Many processes contribute to inner-shelf exchange e.g., rip-currents, internal bores, tides, and winds, with varying time scales from minutes to weeks and alongshore length scales from 100 m to 10 km, but in many conditions a single mechanism dominates and all are affected by stratification. Here, a sub-set of exchange mechanisms are studied using field observations of a surf-zone released tracer on the stratified inner-shelf and a series of idealized coupled surf-zone and stratified inner-shelf model simulations.

The cross-shore deformation of 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. After 5 h, the inner-shelf plume was surface intensified, extending to 800 m from shore. Over the next ≈2 h, the IS-plume deformed (narrowed) cross- shore, deepened, and elongated alongshore. Offshore tracer transport and subsequent IS-plume deformation and deepening is related to advection by two phases of the diurnal internal tide (DIT).

Exchange between SZ and IS is predominately due to rip-currents, strong concentrated offshore flows. On alongshore uniform coasts, directionally spread swell generate transient rip-currents (TRC), that are episodic without preferred formation site. Diurnal heating and cooling can also induce thermally driven exchange. A series of idealized simulations with TRC and diurnal surface heat flux (SHF) forcing (separately and combined) are used to evaluate the relative strength of these two mechanism. Based on multiple exchange definitions, the TRC mechanism is similar with and without SHF, and is dominant out to 1 km offshore, relative to SHF only or thermally driven exchange. Additional idealized simulations with only TRC-forcing and uniform initial thermal stratification dT0/dz [◦Cm−1] (or N02) varying from 0 (unstratified) to 0.75 ◦Cm−1 (highly stratified) indicate the inner-shelf response is self-similar and related to mixing modification of background potential energy.