Early evolution of an Agulhas Ring

Rings shed at the Agulhas retroflection are an integral part of interoceanic exchange south of Africa. There is clear evidence of westward ring translation from the northern Cape Basin across the South Atlantic Ocean. Early ring development and translation from the southern to the northern Cape Basin, however, are obscured by an intensely variable kinematic field close to the spawning site. In this study unique in situ observations, obtained in March to September 1997, are analyzed to improve the understanding of the early development of a juvenile Agulhas Ring. In March the ring was surveyed near 37°S, 16°E, approximately 4 months after its generation. Its strength and size were in the upper range typical for Agulhas Rings, and its trapping depth extended down to at least 1600 dbar according to geostrophic velocities and RAFOS trajectories in the ring. Between March and September the ring propagated in a general northwestward direction; however, RAFOS trajectories and MODAS sea-surface steric height fields revealed a large variability of the translation speed (3 cm s−1 to more than 20 cm s−1) and direction. In September 1997, the mature ring was examined near 31°S, 9°E. By this time, its available heat and salt anomaly were reduced by about 30% and its available potential energy was reduced by about 70%. This indicates that a significant loss of the ring characteristics occurred on the way from the southern to the northern Cape Basin. One-third of this loss is due to changes at intermediate depth (between 800 and 1600 m).