An investigation of sea level and circulation response during a coastal trapped wave event on the Eastern Agulhas Bank, South Africa

Abstract

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 109 m3 which balanced (completion of a full wave cycle) over a period of 150h after the initial zero-crossing point of the transport flux.