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Investigating the Agulhas Current using Argo
By Jarred Voorneveld1 and Katherine Moffett2
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The inter-ocean exchange of salt and heat south of Africa is thought to have great importance on the global meridional overturning circulation, and is regulated by one of the most energetic western boundary currents in the world - the Agulhas Current.
This current flows polewards along the east coast of southern Africa and extends to depths of 2 300 metres. In its southern reaches it follows the bathymetry of the Agulhas Bank and then retroflects eastward to form the Agulhas Return Current, which makes up part of the South-West Indian Ocean sub-gyre.
Approximately four to six times a year, the Agulhas Current sheds anticyclonic warm-core eddies that propagate westward into the South Atlantic Ocean. This process is known as Agulhas Leakage and is climatologically important in terms of heat and salt flux.
Argo float technology
Argo float technology has revolutionised the coverage of the ocean and can be used to study the salt and heat exchange described above. Profiling the ocean with Argo float technology will allow a more accurate representation of ocean dynamics down to a depth of 2 000 metres, compared to satellite imagery and stationary moored instruments.
Two Cape Peninsula University of Technology (CPUT) Oceanography B.Tech students, Jarred Voorneveld and Katherine Moffett, are investigating profiling Argo floats that were released into the Agulhas Current.
The first study, by Jarred Voorneveld, looks at the dynamic variability of the Agulhas Retroflection by comparing two sets of Argo floats. Three MRV SOLO II Argo floats, donated by Woods Hole Oceanographic Institute for a study on mesoscale eddy dynamics, were deployed off the northern KwaZulu-Natal coast in April 2013. These floats were programmed to do daily profiling from 1 000 m and at varying park depths and all three are still operational.
Two ARVOR floats, procured by the SAEON Egagasini Node, were deployed on the Good Hope transect from Cape Town en route to SANAE in 2009. The SAEON floats were set to the standard Argo profiling mission of 10 days, parking at 1 000 m and profiling from 2 000 m.
Of the SAEON floats, one became grounded at Marion Island, while the other became trapped within the Agulhas Retroflection and has been trapped in this region ever since. All three MRV SOLO-II floats were entrained within the Agulhas Retroflection, however their trajectories became involved in different features - the Agulhas Return Current moving eastwards away from Africa, the recirculation around the South-West Indian Ocean sub-gyre and southwards towards the Southern Ocean.
The three SOLO-II floats were more energetic than the SAEON floats, which were sedate, and the aim is to find out why this was the case.
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Investigating Argo float trajectories
The second study, by Katherine Moffett, investigates the trajectories of two sets of Argo floats released in the northern reaches of the Agulhas Current in April and July 2013. The two sets of floats had different profiling schedules. The April floats had varying park depths and profiling frequencies, while the July floats had a park depth of 500 metres and profiled every five days. The April floats remained in the South-West Indian Ocean and the July floats drifted into the South Atlantic Ocean.
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The study looks at whether these differences in trajectories are due to anomalies present in the Agulhas Current at the time of deployment, or whether the differing profiling regimes affected the performance of the Argo floats.
Both these studies are part of ongoing investigations into the Agulhas Current and will form part of Katherine’s and Jarred’s B.Tech. degree at CPUT. The students will be presenting their preliminary findings in a SANCOR talk at the end of August 2015.
1 Cape Peninsula University of Technology
2 SAEON Egagasini Node
