The water masses along the western boundary of the south and equatorial Atlantic

A quasi-meridional hydrographic section located offshore from South America from 50 degrees S to 10 degrees N, and three shorter transverse lines to t he continental slope, are used for a descriptive study of the water masses along the western boundary of the South and Equatorial Atlantic. At the upp er and intermediate levels, the tracer analysis provides geographical limit s of the wind-driven circulation regimes, and a comparison of the tracer va lues at the continental slope and along the meridional section shows where the boundary currents originate. At depths shallower than about 200 m, the subdivision of the subtropical gyre into two cells separated by the Subtrop ical Countercurrent near 28 degrees S, that was pointed out in a previous s tudy, is corroborated. South of this front, a warm variety (similar to 18 d egrees C) of Subtropical Mode Water in the inner recirculation of the Brazi l Current appears, despite its limited extent, as a southern counterpart of the North Atlantic 18 degrees C water. At the deep levels, the Upper Circu mpolar Water and Upper North Atlantic Deep Water enter the South Atlantic i n a significantly overlapping density range, The ensuing lateral encounter of both water masses occurs at 26 degrees S near the western boundary, wher e most of the boundary flow of the latter water is stopped and deflected se award by the base of the subtropical gyre. Other tracer anomalies signal si gnificant eastward escapes of North Atlantic Deep Water: within two jets at about two degrees of latitude on either side of the equator, in another na rrow current at 10 degrees S, and at 34 degrees S. The latter latitude mark s the confluence, and eastward deflection, of the opposite boundary cut-ren ts of Lower North Atlantic Deep Water and Lower Circumpolar Water. Near the bottom of the Argentine Basin, the Weddell Sea Deep Water that flows westw ard nor th of the Zapiola Ridge is more recently ventilated than the water carried by the boundary current near the Falkland Escarpment. While a part of it Rows anticyclonically around the ridge, another part turns equatorwar d and enhances the southern property signatures of the water farther north. (C) 2000 Elsevier Science Ltd. All rights reserved.