The disparate evolution of three Agulhas rings in the South Atlantic Ocean


  • Michel Arhan,

  • Herlé Mercier,

  • Johann R. E. Lutjeharms


Hydrographic sections carried out in January-March 1995 across the pathway of Agulhas rings in the Cape Basin are used for a brief description of the mesoscale thermohaline variability in this region and a detailed study of three rings that were identified in the data. The three eddies exhibited remarkably diverse dimensions, vertical structures, and water mass characteristics. One of them, R1, was located near the Agulhas retroflection, had a diameter of 200 km, a maximum azimuthal speed of 0.40 m s−1, core oxygen values in excess of 260 μmol kg−1 and was characterized by a well-developed thermostad of 11.6°C. A second ring, R2, at 31°30′S, 9°W, by contrast, had a diameter of about 500 km, a core temperature of 17.1°C, and azimuthal speeds of 0.50 m s−1, suggesting a very different history. A third ring, R3, at 26°S, 9°W, although farthest north of the three, had characteristics similar to ring R1, but with a deeper thermostad. Satellite altimetric data allow one to infer the natural histories of these vortices. Ring Rl detached from the retroflection at the beginning of March 1994 and spent the whole of the subsequent winter south of 42°S. This could explain the estimated heat loss of 620 W m−2. The two rings observed at 9°W were spawned as one feature in April 1993, but interaction with the Erica seamount split it into two eddies; R3 being stalled in the retroflection region for the winter, while R2 moved off rapidly into the South Atlantic, by contrast, retaining most of its heat. These histories account for the observed differences between the rings. They also demonstrate that the interaction of Agulhas rings with their environment, including ambient water masses, the overlying atmosphere, and the bottom topography, is critical to their eventual hydrographic characteristics and the manner in which they contribute to the transport of heat and salt from the Indian to the South Atlantic Ocean.