The Structure of the Agulhas Current System during the Agulhas Undercurrent Experiment

The Agulhas Undercurrent Experiment took place in February-March 2003 off the east coast of South Africa and included 112 CTD and LADCP casts along four cross-slope sections and three offshore sections. Direct absolute velocities in the Agulhas Current show a narrow and swift current, 180 km wide and up to 2 m s-1 in speed, that deepens as it flows south, eventually detaching from the continental slope at 36ºS. Results also show the northward Agulhas Undercurrent against the continental slope, beneath the Agulhas Current with peak velocities of 10 cm s-1. Several mesoscale cyclonic eddies extending down to the intermediate layer were sampled during the survey, in particular a shear-edge eddy inshore of the Agulhas Current at 36ºS. A deep water anticyclonic eddy was found for the first time in this region centered at 2800 m in the northward flowing North Atlantic Deep Water (NADW) layer. Anomalous water properties reveal that it was formed in the Agulhas Retroflection region and may have been generated by the coupling of a deep Agulhas Ring with the NADW slope current in the SE Atlantic and later entrained into the deep flow of the Agulhas Return Current, until ejected in the Agulhas Current region by localized recirculations in the deep layers of the Agulhas Current system. An inverse model was applied to the hydrographic and LADCP data; results show that the Agulhas Current had a considerably higher transport of 103 Sv at the historical 32ºS section than earlier estimates, consistent with altimetry time series for the region. The growth of the Agulhas Current transport is given primarily by the Sverdrup transport from the supergyre connecting the southern Pacific and Indian Oceans, and the Indonesian Throughflow and Indian Ocean overturning need to be included to account for the total transport. The bulk of the Agulhas Current transport is concentrated in the thermocline layer in the cross-sections and in the intermediate layer in the offshore sections. Inshore of the Agulhas Current core, mixing is inhibited from the surface to the thermocline layers, with no transport growth downstream. Cross-stream mixing does appear to occur in the intermediate layer. The Mozambique Channel and East Madagascar Current appear to have similar contributions as sources to the Agulhas Current at the northern most section of 16 Sv each, with the Indian Ocean wind-driven sub-gyre contribution increasing as the current flows southward. In the intermediate layer, Red Sea Water is actively mixing with Antarctic Intermediate Water when eddies are present. Red Sea Water appears to advect in the form of parcels and not as a continuous flow. Results also suggest the occurrence of small localized recirculations in the deep layers. In the deepest layer of lower NADW the flow is upwelling into the overlaying layer due to the shallowing topography at the northern most section