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Agulhas Current Influence on the Shelf Dynamics of the KwaZulu-Natal Bight

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  • 1 Oceans and Coastal Research, Department of Environmental Affairs, Cape Town, and Marine Research Institute and Department of Oceanography, University of Cape Town, Rondebosch, South Africa
  • | 2 Oceans and Coastal Research, Department of Environmental Affairs, Cape Town, South Africa
  • | 3 Bayworld Centre for Research and Education, Constantia, Cape Town, and Marine Research Institute and Department of Oceanography, University of Cape Town, Rondebosch, South Africa
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Abstract

The Agulhas Current is a fast western boundary current flowing along the South African east coast, strongly influencing oceanographic variability on the adjacent KwaZulu-Natal shelf. Hydrographic observations in January 2010, July 2010, and July 1989 have been used to describe variations in geostrophic currents and water masses and to elucidate mechanisms of change in circulation and thermohaline structure. Differences in wind forcing, precipitation, and solar insolation caused modifications in stratification and mixing that resulted in seasonal changes in surface temperature, salinity, and mixed layer depth ZMLD between the austral summer and winter surveys. During January 2010, surface temperatures were elevated and there was a large range in salinity as the upper 10 m was influenced by river outflow and rainfall. In July 2010 and 1989, surface temperatures were lower and the salinity range was significantly less. The ZMLD in January 2010 was much shallower, varying between 5 and 40 m, while deeper ZMLD, from 9 to 107 m, was found in July 2010 and 1989. Cross-shelf, alongshore, and vertical variations in water masses during July 2010 and July 1989 were driven by deviations in the position of the inshore edge of the Agulhas Current and the influence of offshore cyclonic eddies. In January 2010, the close proximity of the current to the coast and shelf break caused increased Ekman veering in the bottom layers along the shelf edge and slope in the north and recirculation in the south.

Corresponding author address: T. Lamont, Oceans and Coastal Research, Department of Environmental Affairs, P.O. Box 52126, Victoria and Alfred Waterfront, Cape Town, South Africa. E-mail: tarron.lamont@gmail.com

Abstract

The Agulhas Current is a fast western boundary current flowing along the South African east coast, strongly influencing oceanographic variability on the adjacent KwaZulu-Natal shelf. Hydrographic observations in January 2010, July 2010, and July 1989 have been used to describe variations in geostrophic currents and water masses and to elucidate mechanisms of change in circulation and thermohaline structure. Differences in wind forcing, precipitation, and solar insolation caused modifications in stratification and mixing that resulted in seasonal changes in surface temperature, salinity, and mixed layer depth ZMLD between the austral summer and winter surveys. During January 2010, surface temperatures were elevated and there was a large range in salinity as the upper 10 m was influenced by river outflow and rainfall. In July 2010 and 1989, surface temperatures were lower and the salinity range was significantly less. The ZMLD in January 2010 was much shallower, varying between 5 and 40 m, while deeper ZMLD, from 9 to 107 m, was found in July 2010 and 1989. Cross-shelf, alongshore, and vertical variations in water masses during July 2010 and July 1989 were driven by deviations in the position of the inshore edge of the Agulhas Current and the influence of offshore cyclonic eddies. In January 2010, the close proximity of the current to the coast and shelf break caused increased Ekman veering in the bottom layers along the shelf edge and slope in the north and recirculation in the south.

Corresponding author address: T. Lamont, Oceans and Coastal Research, Department of Environmental Affairs, P.O. Box 52126, Victoria and Alfred Waterfront, Cape Town, South Africa. E-mail: tarron.lamont@gmail.com
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