Measurements of Turbulence Generated by Wake Eddies Near a Steep Headland

Tidal and low-frequency flows interact with abrupt topographic features, giving rise to a variety of small-scale phenomena including lee waves, vortical motions and turbulence. With the goal of identifying the processes that transfer momentum and energy between the larger-scale and smaller-scale flows, detailed tidally-resolving shipboard surveys were conducted near a sharp and steep headland north of Velasco Reef, Palau, in the Western Pacific. We present velocity, microstructure and rapid-cycling conductivity and temperature measurements, contextualized with a nearby 10-month moored record, to examine wake and vortex structures. Observed flow near Velasco Reef was layered and multi-directional in depth such that the "downstream" side of the headland depended on depth. Downstream of the headland, during a period where mean flow dominated over the tide in the top 100 m of the water column, turbulent kinetic energy dissipation rates were enhanced by up to three orders of magnitude relative to those upstream. During a period dominated by tides, the flow reversed periodically and wake eddies were associated with turbulent dissipation rates up to O(10(-5)) W kg(-1). Turbulent dissipation rates in the wake increased with inverse Richardson number for both tidal and mean flows past the headland, and were larger for stronger vorticity flows. Our analysis suggests that the eddies themselves lead to shear instabilities and subsequent turbulence, either through tilting or their limited vertical extent. We associate the enhanced downstream dissipation with a combination of breaking internal tides and either these eddy processes or a multi-scaled turbulent wake. Plain Language Summary Tall, steep underwater features such as ridges and headlands interact with tides and large currents to alter the flow downstream. Here, we present observations near Velasco Reef, Palau, an example of a sharp headland in the western Pacific. Turbulence in the wake of the headland is higher than upstream. During a time when the tides were strong the flow changed direction such that eddies were shed from both sides of the headland. These patches of rotating flow were associated with stronger turbulence than elsewhere around the headland. Shearing of the flow in the vertical direction by the eddies and the internal tides are possible mechanisms by which the turbulence is generated.