Remote Sensing and Modeling of Warm and Cool Plumes Connecting the Surf Zone and Inner Shelf

Rip currents are important for the cross-shore exchange of heat, larvae, and other material, but the along-coast variability in rip-current driven exchange is poorly understood, and the importance of buoyancy is not known. Nearshore 'plumes' of water on the inner shelf resulting from rip currents often have strong turbidity and sea-surface-temperature signatures in remote sensing imagery, but these signatures have not yet been used to characterize exchange variability and dynamics. Here, airborne remote sensing surveys and in situ measurements from the Office of Naval Research Inner Shelf DRI (Sept–Oct 2017) and coupled wave-current numerical model simulations (COAWST) are used to improve understanding of exchange resulting from bathymetric rip currents. Rip currents observed in the airborne visible and infrared imagery transport fluid and material out to several surfzone widths. The plumes were often cold relative to water on the shelf, while warm plumes were observed less often. Warm plumes exhibit more lateral spreading and on average transport material further offshore (~3 surfzone widths) than cool plumes (~2 surfzone widths). Simulated bathymetric rip currents in the presence of initial lateral temperature gradients suggest that the relative surf-shelf temperature is a strong control on plume structure and behavior. The observations and simulations are used to assess the variability of plumes as a function of wave conditions, tidal elevation, coastal bathymetry, shelf stratification, and meteorological conditions.