Coastal upwelling on the far eastern Agulhas Bank associated with large meanders in the Agulhas Current

Abstract

Six large solitary meanders in the Agulhas Current, so-called Natal Pulses, propagated down the eastern coast of South Africa between 2009 and 2011. Their influence on the coastal waters between Port Alfred and Algoa Bay, on the far eastern Agulhas Bank, was measured by thermistor strings moored at 30–80m bottom depths and two current metres (30m bottom depth) located at both sides of Algoa Bay. During all events active upwelling lasting 1–3 weeks was observed over the inner shelf and in Algoa Bay. During upwelling the isotherms ascended at an average rate of 1.8mday−1 as the cold bottom layer increased in thickness to 40–60m, although upwelled water did not break the surface in all cases. Cold water remained in the area for a further 2–3 weeks. During three Natal Pulses the water temperatures at the outer moorings initially increased as the plume of the leading edge (crest) of the meander moved onshore. During one Natal Pulse upwelling was recorded before the warm water plume impacted the moorings. At the onset of upwelling currents switched to the southwest in the case of Bird Island and southward at the Cape Recife inner-bay site and reached a maximum speed of 80cms−1. During all Natal Pulses cold bottom water (10–12°C) flooded over the 80m bottom depth moorings as the crest of the meander moved onshore, but also around the same time the core of the Agulhas Current began to move offshore. In all cases upwelling was wide-spread.