Three Agulhas rings observed during the Benguela Current Experiment

Abstract

Abstract. A field program to study the circulation of the Benguela Current and its extension into the southeastern Atlantic Ocean has completed the survey and instrument deployment phase. We report here new observations of three Agulhas rings north and west of Cape Town, South Africa. Three mesoscale anticyclonic rings initially identified by means of TOPEX/POSEIDON altimetry were surveyed with expendable bathythermographs, conductivity-temperature-depth-oxygen profiles, direct current measurements from a lowered acoustic Doppler current profiler, a hull-mounted acoustic Doppler current profiler, and satellite-tracked surface drifters. Characteristics of the rings are presented, and their origins are discussed. Two are typical Agulhas rings surveyed at different times after their generation; the third Agulhas ring has an anomalous water mass structure whose most likely origin is the Subtropical Front.

Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

“…An example of a well-measured blob-like ring was described by Schmid et al, (2003), Garzoli et al, (1999), and Richardson and Garzoli, (2003). This ring was observed by ship near 38°S 15°E in March 1997 and several RAFOS floats were launched in it .…”

“…Typical average ring diameters are around 200 km and maximum diameters are 250 km for most of the rings with little or no trend with distance. Note that the overall diameter of rings (outer limits) is larger than the largest loops (Olson and Evans, 1986;Arhan et al, 1999;Garzoli et al, 1999). Virtually all the KAPEX RAFOS floats that crossed over the Walvis Ridge did so associated with five rings and one cyclone.…”

“…During KAPEX, 6 drifters and 8 RAFOS floats were launched on purpose in three rings that were surveyed by ship near 30°S at 31.1°S 9.0°E, 31.2°S 3.8°W, and 29.8°S 6.1°W (Garzoli et al, 1999;Richardson and Garzoli, 2003). Thus, trajectories of 3 rings in Fig.…”

“…All the KAPEX rings tracked by drifters and floats were also tracked by altimetry (Schouten et al, 2005; see also Garzoli et al, 1999;Schmid et al, 2003), which provided some longer ring trajectories including back in time to ring formation. The two techniques give similar ring trajectories.…”

“…Many rings have been measured at sea as described by Dumcombe Rae (1991), Lutjeharms (1996), Arhan et al (1999), de Ruijter et al (1999, Garzoli et al (1999), andMcDonagh et al (1999). Agulhas rings are among the most energetic eddies in the world (Olson and Evans, 1986).…”