Sea level and circulation response during a protracted wind and coastal trapped wave event observed in in situ instrument data and a ROMS ocean model on the eastern Agulhas Bank, South Africa

Coastal trapped waves (CTW) have the potential to capture and transport a significant amount energy from atmospheric wind systems, in turn inducing sudden changes to the coastal marine environment over large areas, even those remote to the propagating wind system. In this study, the effects of a large, eastward travelling CTW that was captured on in situ moored acoustic doppler current profiler (ADCP) (∼30 m bottom depth) and sea level recording instrumentation in Algoa Bay on the eastern Agulhas Bank off South Africa is examined using a high-resolution (dx∼1 km) Regional Oceanographic Modelling System (ROMS) ocean model. Alongshore current reversals to the full depth were seen in both the model and two ADCP moorings as the CTW traversed the study domain, with strong currents exceeding 1 m/s developing directly to the south of Cape St Francis and Cape Recife at the peak of the event within the model. In a special model run with surface wind stresses removed, it was shown that the CTW carries much of the energy into the domain from remote wind forcing to the west, contributing an estimated 82% of the eastward flux of shelf water. The total volume of water displacement during the CTW was estimated at 335.2 x 10 9 m 3 which balanced (completion of a full wave cycle) over a period of 150h after the initial zero-crossing point of the transport flux. • The model has predictive skill in reproducing coastal trapped wave events along the south coast of South Africa. • Sudden alongshore current reversals are seen to full depth in the model and ADCP mooring data. • The model shows very strong eastward currents develop off Cape St Francis and Cape Recife. • The model shows a large volume displacement caused by CTW, carrying most energy from remote wind forcing west of Oyster Bay.