OCEANIC CIRCULATION AND TURBULENCE IN THE COASTAL ZONE

Measurements of turbulent eddy viscosities within 1 km of Bass Point (a 4 km wide headland near Sydney, Australia) indicate the presence of a relatively high level of turbulent energy, apparently caused by the complex subsurface topography near the point. This energy has typical length scale approximately 300 m, typical time scale approximately 100 min and an eddy viscosity of approximately 15 m2 s-1, one to two orders of magnitude greater than that due to mixed layer turbulence alone. Despite the fact that the headland protrudes well out onto the continental shelf where strong longshore (approximately 0.2-0.5 m s-1) currents often exist, large-scale recirculation in the lee of the headland is rarely observed. The turbulence near the tip of the point prevents the formation of a single narrow shear layer (caused by separation at the point), and instead provides a much wider turbulent shear layer where the non-linear terms are apparently ineffective at driving a large-scale recirculation. The overall effect appears to be analogous to a ''trip wire'' or ''vortex generator'' whereby the enhanced generation of turbulence acts to alter the overall pressure field and pushes the separation point far downstream (or eliminates it entirely). The result is that the large scale flow appears often to be almost laminar, and somewhat analogous to that which is expected from a truly low Reynolds number flow.