CHARACTERISTICS OF WARM RINGS IN THE AFRICAN SECTOR OF THE ANTARCTIC CIRCUMPOLAR CURRENT

Both models and observations of the Southern Ocean circulation suggest an important role for mesoscale eddies in the meridional transport of heat and salt and in the vertical transport of momentum. However, in contrast with western boundary currents, there are few observations of individual mesoscale features originating at the Antarctic Circumpolar Current (ACC). We used hydrographic data from a quasi-synoptic survey, Geosat altimetry data and historical hydrographic data to estimate some characteristics of warm rings in the sector 20-40 degrees E. A sequence of maps of sea surface height anomalies derived from the Geosat data shows anticyclonic rings with a typical diameter of 150-200 km. The rings are shed from the meanders of the ACC at the Antarctic Polar Front (APF), which evolve over the deep abyssal plain immediately behind the mid-ocean ridge. Their subsequent southward movement is confined to a rather narrow (300-400 km) corridor between 26 and 32 degrees E. The rings drift southward with an average speed of about 3 cm s(-1). After reaching 55-57 degrees S they tend to stagnate and decay. According to altimetry data, the lifetime of the rings is about one year. Formation of five warm rings was detected during the two-year period November 1986 to October 1988. Since the rings do not recoalesce again with the ACC, their translation results in net southward heat and salt transport. We also describe the hydrographic structure and southward translation of an individual ring found in February 1988 near 28 degrees E, 56 degrees S. Estimates of heat and salt anomalies for this ring are 6 x 10(19) J and 13 x 10(11) kg relative to the Warm and 13 x 10(19) J and 26 x 10(11) kg relative to the Cold regime of the Weddell Gyre, i.e. an order of magnitude larger than corresponding estimates for the previously described Drake Passage cold rings. Mesoscale variability patterns suggest a similar hydrographic regime in a few other sectors of the Southern Ocean. However, available hydrographic data do not allow definite assessment about the overall importance of warm rings for the meridional transports of heat and salt south of the APF.