Evolution of Tidal Vorticity in Stratified Coastal Flow

The lifespan of a tidal eddy generated by flow around a coastal headland is examined. Field observations of a tidal headland eddy at Three Tree Point, WA (USA) are presented that examine the temporal evolution of the flood tide separation eddy from its generation, through the eddy release at the turn of the tide, until its dissipation during subsequent tidal cycles. Ship-based acoustic profiling examines the vertical structure of the velocity field and subsurface drogued drifters are used to track the horizontal motion of the flow structure. Drifter tracks from successive days at similar phases of the tide indicate that flow structure is repeatable. The combined set of drifter tracks is used to obtain an estimate of eddy lifetime. Time scales for vorticity decay of less than a tidal period are significantly shorter than simple estimates using boundary friction would imply. This finding suggests that the internal wave response of the stratified flow over the sloping headland plays a significant role in the dissipation of vorticity. Field observations are compared with results from numerical modeling that also suggest that baroclinic effects are significant.