AGULHAS RING DYNAMICS FROM TOPEX POSEIDON SATELLITE ALTIMETER DATA/

The transfer of warm water from the Indian Ocean into the South Atlant ic subtropical gyre takes place in the form of rings and filaments for med when the Agulhas Current retroflects south of Africa between 15 an d 25E. A survey of the rings formed from September 1992 until December 1995 in the Retroflection region was carried out using TOPEX/POSEIDON altimeter data. A two-layer model was used to estimate the upper laye r thickness from the altimeter-derived sea-surface height anomaly data . An objective analysis scheme was used to construct a map of upper la yer thickness every ten days. Seventeen rings and their trajectories w ere identified using these maps. The shedding of rings from the Agulha s Current was neither continuous nor periodic, and for long periods th ere is no formation of rings. Several rings remained in the region for more than a year and, at any given time, 2 to 6 rings coexisted in th e region east of the Walvis Ridge. The results showed that the number of rings translating simultaneously in this region is larger during th e first half of each year. The upper layer transport of the Agulhas Cu rrent in the Retroflection region was computed and a close association between high variations in transport and ring shedding was found. Rin gs translated WNW at translation speeds ranging from 5 to 16 km day(-1 ) following formation. The values of available potential energy comput ed for the rings place them among the most energetic rings observed in the world oceans, with values of up to 70 x 10(15) J. Transport compu tations indicate that each ring contributes in the average approximate ly 1 Sv of Agulhas Current waters to the Benguela Current.