Infrared Remote Sensing of Surf-Zone Eddies

Airborne infrared imagery is used for the first time to investigate characteristics of surf-zone eddies, occurring along an along-shore uniform beach. Eddies are found to have diameters of 100 to 150 m, an alongshore spacing of about 375 m (2.5 times the surf-zone width), and lifetimes of 40 min and longer. Eddies travel alongshore at speeds of ~ 0.3 to 0.5 m/s, at a distance from the shoreline of about 1.3 surf-zone widths. Eddies are also observed to move directly offshore, two surf-zone widths from the shoreline. Fine-scale thermal structure within an eddy includes ~ 6-m-wide cold patches, possibly the surface imprints of bottom-induced turbulence, and a small (<; 10-m wide) `cold core´, likely from accumulation of surfactant by radially inward surface flow. Visualization of an eddy is enhanced through flow deformation of surfactant slicks. In particular, narrow slicks appear to be wound-up by an eddy into spirals similar in form to those of a free vortex. In this sense, these nearshore eddies resemble ocean sub-mesoscale `spiral eddies´ as seen in synthetic-aperture radar and sunglint imagery.