Observations of northeastward flow on a narrow shelf dominated by the Agulhas Current

Abstract

Water column currents and bottom temperature from September 2005 to September 2006 across the narrow shelf region off Port Edward on the east coast of South Africa, combined with satellite observations, highlighted the dominant influence of the Agulhas Current (AC) on shelf circulation. Stronger southwestward flow occurred across the shelf during austral summer and autumn suggesting an intensification and shoreward movement of the AC during these seasons. In situ moored observations also showed that the AC extended as far inshore as 3.2 km from the coast. Weak and non-significant correlations between wind and current, and between wind and bottom temperature, indicated minimal influence of local wind forcing on current variations and temperature responses. A total of 11 northeasterly flow events, ranging from 1 to 15 days, were observed, with only 4 events associated with the passage of mesoscale cyclonic eddies along the inshore edge of the AC. The remaining events all suggested small-scale movements of the AC resulting in enhanced Ekman veering in the bottom layers further offshore, and full water column current reversals close inshore. Contrary to expectation, no clear seasonal variation in bottom temperatures was evident, and temperatures and currents were poorly correlated, since both warming and cooling occurred during southwesterly and northeasterly flows. Substantial short-term fluctuations of up to 4 °C were associated with shoreward/offshore movements of the AC, and southward propagation of eddies. Advection of warmer AC waters around eddies and southward advection of waters upwelled further north served to complicate the observed temperature responses.