Surfzone Vorticity

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Breaking waves dissipate energy while transferring momentum that forces water in the direction of wave propagation. In the shallow water of the surf zone the time-averaged wave-driven forcing raises water levels near the shoreline and, in the case of obliquely incident waves, drives alongshore currents. The resulting circulation patterns include a rich field of eddies and vortices. Surfzone eddies have been hypothesized to be generated by instabilities of sheared alongshore currents, by wave-group modulations of the sea surface, by the ends of breaking waves, and by currents interacting with complex bathymetry. This project will address these hypotheses with a unique suite of high spatial resolution estimates of currents from remote sensing and in situ sensors obtained during prior field studies that include a wide range of wave conditions and bathymetries. Existing extensive field observations of surfzone waves, circulation patterns, and underlying bathymetry will be used to test hypotheses about surfzone vorticity that were derived from numerical studies. These hypotheses include: (1) on an alongshore variable seafloor, the dominant spatial scales of surfzone vorticity depend on spatial scales of the underlying bathymetry, and these large-scale eddies are tied to the seafloor inhomogeneities; (2) small spatial-scale vorticity also depends on the vorticity injected into the water column at the ends of breaking waves, which increases with the directional spread of the wave field; (3) during energetic conditions, the momentum of alongshore currents can overcome forces (such as bathymetry-induced pressure gradients) that cause complex circulation patterns, resulting in alongshore-uniform flow with significantly reduced large-scale vorticity. Although the data used here are from a specific field site, the wide range of wave and bathymetric conditions observed over a decade allows development of a surfzone vorticity climatology that will be applicable at many locations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.