Driving mechanisms of the Port Alfred upwelling cell inshore of the Agulhas Current
The presence of a semi-permanent upwelling cell, with a surface expression more than 40 % of the time has previously been described inshore of the Agulhas Current in the vicinity of Port Alfred, South Africa. This study employs a combination of in-situ mooring data, hydrographic cruises and satellite remote sensing in order to investigate the nature and variability of this upwelling cell, as well as to investigate possible driving mechanisms. The study takes place over a period of 11 months. Special focus is given to the subsurface variability due to its possible implications for the greater Agulhas Bank environment. Upwelled water was found to be present on the shelf 85% of the time, highlighting the importance of subsurface variability in this area. The main timescales of variability were observed at 50-70 days, 8-12 days and 4-6 days. Upwelling was found to be maintained by continuous processes, driven by the interaction of the Agulhas Current with the changing bathymetry at Port Alfred. This upwelling is modulated by the effects of mesoscale features on the inshore edge of the current. While not a primary driver of upwelling, wind events were observed to have an effect on inshore bottom temperatures as well as the surface expression of the upwelling cell. A high degree of variability in was observed, with bottom temperatures at three mooring site fluctuating through a range of approximately 10° C. Future directions include further theoretical and idealised modeling studies to separate out the exact mechanisms of topographically driven, site-specific upwelling. The range of mesoscale interactions of the Agulhas Current with the shelf circulation also require further observational study.
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