Upwelling in the coastal zone off Algoa Bay driven by wind and large episodic meanders in the Agulhas Current

Goschen, W. S., T. G. Bornman, S. H. P. Deyzel and S. E. Bailey (2012). In: The Agulhas System and its Role in Changing Ocean Circulation, Climate and Marine Ecosystems, American Geophysical Union Chapman Conference, Stellenbosch, Western Cape, Africa, 8-12 October 2012.

The emergence of cold water is frequently observed in Algoa Bay and along the coast north to Port Alfred, on the Eastern Cape coast, South Africa. It was found that upwelling in this area was driven by both wind and the Agulhas Current. Wind-driven upwelling appears to be initiated at Woody Cape/Cape Padrone, the northeastern promontory of Algoa Bay, and then spreads into Algoa Bay and north towards Port Alfred during northeasterly component winds. Cold water is upwelled along the shoreline at inertial periods and greater after the wind changes to northeasterly, although uplifting of the isotherms in the bottom layer begins with the wind change. During wind-driven upwelling the winds, currents, sea level, and sea temperatures are highly correlated, with the local response of currents and sea level signifying the activity of coastal trapped waves. When four large solitary meanders in the Agulhas Current (so-called “Natal Pulses”) propagated past Algoa Bay in 2009 and 2010, enhanced upwelling was observed along the shoreline and in the coastal zone. Two such events occurred in autumn and winter during mainly southwesterly, downwelling favourable winds, but the currents had strong southwestward (downstream) components, as with wind-driven upwelling although there was little correlation between winds and currents. The role of large solitary meanders in the Agulhas Current in causing these strong currents, and their contribution to coastal upwelling off Algoa Bay, are investigated.