Title Port St Johns Conductivity Temperature Depth (CTD) and Bathymetry Survey on Algoa Voyage 198, May 2013

Oceans and Coastal Research
Department of Forestry, Fisheries and the Environment (DFFE)

Publisher Department of Forestry, Fisheries and the Environment (2017)

Contact Person: Marcel van den Berg
Oceans and Coastal Research, Department of Forestry, Fisheries and the Environment (DFFE), email: mvandenberg.dea@gmail.com

Abstract The Port St Johns Conductivity, Temperature, Depth (CTD) and Bathymetry Survey was conducted on the Algoa Voyage 198, 8 May - 13 May, 2013. This cruise has two scientific objectives: (1) to investigate the dynamics of the Port St. Johns eddy and biological implications and (2) to investigate the existence of mesoscale eddies in the Agulhas Current. In the case of the former, Oceans and Coasts was asked to provide input to the recent spate of shark attacks off Port St. Johns. Very little is known about the oceanography except that Roberts et al (2010) discovered during a cruise a lee-trapped cyclonic eddy off Port St. Johns that caused substantial slope upwelling. The cold upwelled water is nutrient-rich and appears to provide a biological pump to the local ecosystem. This cruise will deploy an ADCP in a depth of approximately 80 m to determine the frequency of appearance of this eddy coupled with Conductivity, Temperature, Depth (CTD) lines to measure the biological implications. In the case of the second objective - offshore mesoscale eddies interact strongly with the Agulhas Current, and have been shown to initiate the Natal Pulse, cause early retroreflections, and influence slope upwelling at Port Alfred. They also act as vectors carrying biological material to areas along their trajectory and therefore play a role in shaping biodiversity patterns. A recent study however using altimetry and VSP drifters indicates that mesoscale eddies from both Madagascar and the Mozambique Channel tend to move down the Agulhas Current on its offshore boundary, but that these eddies dissapate southwest of East London. On this cruise we investigated whether mesoscale eddies exist in the lower region of the Agulhas Current using in situ measurements. The results may have a profound impact on local altimetry processing and our understanding of the regional oceanography.
Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
Temporal extent 08 May 2013 – 13 May 2013
Geographic extent

North: -31.43
South: -31.89
West: 29.86
East: 29.97

Keywords Agulhas Current, Algoa, Algoa 198, cruise report, East London, Madagascar, mesoscale eddies, Mozambique Channel, Natal Pulse, Port St. Johns eddy, sailing orders, Shark attacks