Monitoring the dynamics of the Agulhas Current System off Port Edward, Kwazulu-Natal.

Abstract

In order to validate remote sensing products and to provide data for model assimilation, a real-time monitoring line consisting of three moorings was deployed across the Agulhas Current off Port Edward, South Africa. This deployment formed part of a Technology and Human Resource for Industry Programme (THRIP) funded initiative to develop a real-time mooring system capable of measuring ocean parameters in the Agulhas Current during 2011. The slope and offshore moorings displayed a distinct stratified regime within the Agulhas Current, a northeastward flowing Agulhas Undercurrent and the southwestward flowing Agulhas Current. Three major reversal events, with northeastward currents occurred on 23 July, 02 September and on 11 October 2011. All current reversals caused a decrease in current velocity. The Agulhas Undercurrent was a persistent feature and average velocities between the line of moorings ranged between 13.38 cm/s and 15.52 cm/s. The results obtained from the mooring systems were consistent in terms of velocity, direction and hydrographic properties of the Agulhas Current as described in previous literature. The low directional variability in the surface layers at the offshore mooring and dominant southwestward flow, except during reversal events indicate the strong influence of the Agulhas Current in this region. The inshore mooring showed less occurrences of the Agulhas Undercurrent if northward flow in the bottom layers was to be considered as signs of the Agulhas Undercurrent. General current characteristics as well as the characterisation of the mesoscale features affecting the coast off Port Edward was accomplished through the use of the in situ moorings. All current reversals encountered were associated with the process of vortex shedding from the Natal Bight. These events may be related to the shedding of the Durban Cyclonic Eddy from its origin in the Natal Bight. Data from the offshore mooring suggested that for monitoring Agulhas Current core dynamics, it was ideally placed as highest surface velocities were measured by this mooring system. The slope mooring recorded highest velocities within the Agulhas Undercurrent and was thus ideally placed to measure the Agulhas Undercurrent’s core. Shelf dynamics were under the influence of the Agulhas Current and northerly current reversals and were aptly recorded by the inshore mooring which was placed on the continental shelf, close to the shelf break.