The Origin of an Anomalous Ring in the Southeast Atlantic

Abstract

A warm core ring in the southeast Atlantic, previously thought to have come from the Brazil?Falklands (Malvinas) confluence, is traced back to the Agulhas retroflection. The path of this ring, sampled at 36°S, 4°E on 23 January 1993 during the World Ocean Circulation Experiment one-time hydrographic section A11, is resolved using a combination of satellite altimeter data from ERS-1 and TOPEX/POSEIDON and infrared radiometer data from ERS-1?s Along Track Scanning Radiometer (ATSR). The ERS-1 35-day repeat and the TOPEX/POSEIDON 10-day repeat altimeter-derived sea surface height anomalies are combined and used to trace the ring back to 40°S, 10°E at the beginning of the ERS-1 35-day repeat mission in April 1992 when it had a sea surface height anomaly in excess of 50 cm. This anomaly in this part of the ocean is too large to be associated with a ring formed anywhere other than the Agulhas retroflection. The ring is identified in coincident 35-day altimetry fields and in a monthly average of ATSR sea surface temperature data in August 1992. That this anomaly was observed in the ATSR data indicates that the ring has not overwintered; the age and speed since formation inferred from this precludes formation outside of the Agulhas retroflection region. Using ATSR data prior to April 1992 and altimetry data from ERS-1?s 3-day repeat, this ring is traced back to the Agulhas retroflection where it formed in October 1991. The ring was unlike any previously sampled ring in this region, as it had a homogeneous core between 100 and 550 db, potential temperature 13°C, salinity 35.2 psu and potential density 26.55. This water lay on the same potential temperature?salinity (??S) curve as the water within a typical Agulhas ring sampled a few days later. The peak velocity measured within the ring by an acoustic Doppler current profiler at 200 m was 58 cm s?1. This velocity was similar to other Agulhas rings, but higher than the velocities measured in rings originating in the Brazil?Falklands (Malvinas) confluence region. The high oxygen and low nutrient concentrations (relative to typical Agulhas water) and ??S characteristics of the core of the anomalous ring were consistent with a mode water that forms in the subantarctic zone of the southwest Indian Ocean, subantarctic mode water (SAMW). The type of SAMW detected in the anomalous ring has been observed forming by deep convection in the subantarctic zone at 60°E. At this longitude water is readily subducted from the subantarctic zone into a pronounced and compact anticyclonic gyre in the western south Indian Ocean. A particle tracing experiment in the mean velocity field of the Fine Resolution Antarctic Model showed that it would take eight years (within a factor of 2) for water to recirculate from near 40°S, 60°E into the Agulhas retroflection in this compact gyre. Once in the retroflection, the SAMW is available to be pinched off into rings.