Mesoscale bio-physical interactions between the Agulhas Current and the Agulhas Bank, South Africa

The Agulhas Current on the east coast of South Africa is a major western boundary current that exchanges heat and salt between the Indian and South Atlantic Oceans. The current retroflects as it deflects away from the African continent at the southern tip of the Agulhas Bank, a biologically productive extension of the continental shelf south of the South African coast. The less energetic Benguela Current borders the Agulhas Bank to the west. Little is known about mesoscale interactions between the Agulhas Current and the shelf waters of the Agulhas Bank or how these processes influence the biology of the bank. In this study, physical and biological data collected during a dedicated cruise in September 2010 allowed the identification of several mesoscale features that indicate a strong effect of the current on the bank, including a Natal Pulse that forced the Agulhas Current onto the Agulhas Bank. While on the bank itself, the current entrained particles that were then transported offshore. We also found evidence of upwelling on the southeast edge of the Agulhas Bank, which is thought to be a source of water for a cold ridge that characterizes the eastern region of the bank. Large fluctuations of the thermocline, consistent with internal waves, were observed inshore of the Agulhas Current, with high phytoplankton concentrations at their crests. We suggest that this is a physical effect, with doming of the waves concentrating plankton at their crests, thereby creating episodic biological hotspots.