A wave-driven surface circulation feature in Table Bay

Abstract

Table Bay, located in the Cape Peninsula region of South Africa, supports a variety of human and ecological interests. Notably it hosts a major port, with significant shipping and smaller maritime activity in and near the bay. Despite this, knowledge of its circulation dynamics remains cursory. In this study, surface gravity waves, particularly those with longer periods and higher wave heights such as swells, are shown to be important in driving near surface currents and establishing circulation patterns within Table Bay. A surface circulation feature, linked to large wave conditions and established by strong wave-driven flows near Robben Island, is identified and described by means of two coastal ocean model simulations. One simulation is dynamically coupled to a wave model and includes current forcing due to waves, whereas the other neglects waves. The influence of these wave-driven currents is relevant at the event scale, but also affects the monthly means of the simulation periods. Finally, the importance of including accurate surface gravity wave forcing in simulations of coastal currents, for applications of coastal models, is elucidated. This is achieved by analysing differences in the drift of a series of drogues deployed in the coupled and uncoupled simulations. Trajectories, drift speeds and drogue fates differed materially between the two configurations, underscoring the implications of wave-driven currents for common use cases.