Evolution of Vortical Flow Structure in an Ocean Boundary Process

Flow across a rough boundary generates vortical structure over a broad range of spatial scales. The change in the pressure field associated with these structures is manifested as topographic drag. Relevant roughness scales can extend from less than a meter, for surface wave-driven flows, up to kilometers, for tidal currents. Here, we explore the relationship between the energy invested into and subsequently dissipated by the vorticity field and the work done by drag forces along the boundary. The case of oscillatory flow, as induced by tides or waves, further enables interactions between long-lived vortical structure of opposite signs. These interactions can introduce secondary circulations that provide a mechanism for the transport of interior fluid to the boundary as well as for redistribution of turbulent energy originating at the boundary and its eventual dissipation. We present analysis of observations and numerical simulations of vortex evolution in oscillatory flow and examine the dissipation mechanisms that lead to decay of the vorticity field. The flow structure and its longevity are found to be strongly affected by baroclinic mechanisms including lee wave generation and internal tides. Vortex longevity and interactions on successive oscillation periods, in turn, determine the fate of boundary turbulent energy.