The dynamics of pressure and form drag on a sloping headland: Internal waves versus eddies



Topographically generated eddies and internal waves have traditionally been studied separately even though bathymetry that creates both phenomena is abundant in coastal regions. Here a numerical model is used to understand the dynamics of eddy and wave generation as tidal currents flow past Three Tree Point, a 1 km long, 200 m deep, sloping headland in Puget Sound, WA. Bottom pressure anomalies due to vertical perturbations of the sea surface and isopycnals are used to calculate form drag in different regions of the topography to assess the relative importance of eddies versus internal waves. In regions where internal waves dominate, sea surface and isopycnal perturbations tend to work together to create drag, whereas in regions dominated by eddies, sea surface, and isopycnal perturbations tend to counteract each other. Both phenomena are found to produce similar amounts of form drag even though the bottom pressure anomalies from the eddy have much larger magnitudes than those created by the internal waves. Topography like Three Tree Point is common in high latitude, coastal regions, and therefore the findings here have implications for understanding how coastal topography removes energy from tidal currents.