Atmospheric and advective forcing of upwelling on South Africa's central Agulhas Bank

Abstract

Current and temperature structures on South Africa's central Agulhas Bank are described from six months of moored observations and ancillary data collected during the seasonal transition from austral spring to summer (October 2018–March 2019). The occurrence of an intermittend mid-shelf upwelling ridge, associated with increased productivity, is an important part of the shelf's thermal structure. However, the subsurface evolution of this cold ridge has not been documented to date. A mooring array that transected the wide central shelf captured the seasonal increase in stratification that culminated in the formation of a cold ridge in late summer. We show that the cold ridge originates in the wind-driven upwelling zone on the eastern Agulhas Bank and emphasise the importance of oceanic forcing in maintaining the subsurface thermal structure through advective steering of wind-driven coastal upwelling plumes and through dynamic shelf edge upwelling. The main source of the cold basal layer at the mooring transect originated from upwelling further east, but persistent near-seabed temperatures <9 °C on the outer shelf during the encroachment of the Agulhas Current confirm the contribution of shelf edge upwelling, offshore of the mooring transect, to the cold bottom layer on the Agulhas Bank. Of particular interest is the strengthening of the south-westward shelf current, inshore of cyclonic flow in the Agulhas Bight that accelerated the offshore advection of productive coastal water in the shape of the cold ridge. We show the shelf circulation to be highly coherent across and along the shelf. Shelf-wide barotropic pulses were driven by increased zonal wind stress that was in turn associated with variation in coastal sea level. In the light of global climate change, this work highlights the importance of long term in-situ monitoring in understanding ecosystem functioning on the highly dynamic Agulhas Bank.