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

Abstract

a b s t r a c tSix 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 30-80 m bottom depths and two current metres (30 m 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 1.8 m day À 1 as the cold bottom layer increased in thickness to 40-60 m, 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 80 cm s À 1 . During all Natal Pulses cold bottom water (10-12°C) flooded over the 80 m 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.

Observations of the vertical and temporal evolution of a Natal Pulse along the Eastern Agulhas Bank

“…Goschen et al . [] also observed that the propagation of the Natal Pulse downstream of the Eastern Agulhas Bank could induce upwelling of cold water.…”

“…Such warming of coastal waters has already been observed by Goschen et al . [] when a warm plume originating from the crest of an Agulhas Current meander penetrated over the shelf and moved toward the shore. The cooling episodes were most likely induced by coastal upwelling.…”

“…This cooling could be attributed to (i) the entrainment of cold coastal water from Algoa Bay into the cyclonic core of the Natal Pulse, in a manner similar to that described by Tsugawa and Hasumi [] during the initial formation of Natal Pulses, and (ii) the upwelling of very shallow water within the Pulse. Previous observations [ Porri et al ., ; Goschen et al ., ] have shown that coastal water from Algoa Bay is advected offshore during the passage of Natal Pulses. Our observations seem to support the first assumption.…”

Environmental controls on marine productivity near Cape St. Francis, South Africa

“…The southern coast of South Africa is swept by the prevailing warm Agulhas Current (Lutjeharms et al, 2000). Shelfedge upwelling is induced by cyclonic shear of the current and downstream widening of the shelf (Schumann, 1986, 1987, Lutjeharms, 2006Goschen et al, 2015;Malan et al, 2018). Coastal winds average 7 m s −1 (Schumann and Martin, 1991) and tend to blow from east in summer and from the west in winter.…”

Fine scale depth regulation of invertebrate larvae around coastal fronts

“…A major mesoscale frontal feature separates the shelf Agulhas Bank waters from the warm core of the Agulhas Current (Lutjeharms 2006). In addition, Ekman forcing is not the only source of upwelled waters in the region: fast flow velocities typical of the Agulhas Current and its meanders onto the shelf cause shelf edge upwelling, semi-permanent upwelling cells and their associated fronts (Schumann et al 1982;Goschen and Schumann 1990;Lutjeharms et al 2000;Goschen et al 2015). Internal waves have also been observed across the Agulhas Bank (Largier and Swart 1987;Jackson et al 2012).…”