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Agulhas eddies invade the south Atlantic: Evidence From Geosat altimeter and shipboard conductivity-temperature-depth survey

Authors

  • Arnold L. Gordon,

  • William F. Haxby


Abstract

There is mounting evidence that Indian Ocean water penetrates into the South Atlantic. A sequence of Geosat altimeter data from November 1986 to November 1987 discloses numerous transient anticyclonic mesoscale features within the South Atlantic subtropical gyre. They migrate from the Agulhas Retroflection towards the northwest, then after stalling, cross the Walvis Ridge, to continue drifting to the west, at 5 to 8 cm s−1. The “family” of these features can be traced to the western South Atlantic in the latitude range of 20°–30°S. The 1987 population of eddies in the South Atlantic and their mean drift velocity suggests a production rate of about five eddies per year. As much as 15×106 m3 s−1 of Indian Ocean water may pass into the subtropical South Atlantic by this process. One of the larger eddies disclosed by the Geosat altimeter was crossed by the Discovery during a conductivity-temperature-depth survey of the Benguela Current in May 1987. The dynamic height of the sea surface relative to 3000 dbar reveals a 30 dyn cm crest, in excellent agreement with the Geosat data. The eddy displays a distinct sign of Indian Ocean stratification: an isohaline layer in the 15°–19°C interval. The colder South Atlantic water cannot achieve these characteristics by local interaction with the atmosphere. However, there is evidence for some surface modification: surface salinity values are higher than those found within the Agulhas Current. This is explained by enhanced evaporation driven by the warm temperatures of the eddy below the colder atmosphere. This process would eventually remove the Indian Ocean signature, converting the water within the eddy to typical South Atlantic thermocline water.

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