Cyclonic gyre in the tropical South Atlantic

Abstract

A cyclonic gyre within the eastern tropical South Atlantic is resolved by an extensive oceanographic station array obtained in 1983 and 1984. The gyre is centered near 13°S and 5°E with a sea surface relief relative to 1500 decibars (db) of 8 dyn cm. The 500 db surface relative to 1500 db reveals a much diminished cyclonic circulation, shifted slightly to the south. The weak baroclinic expression of the cyclonic gyre is confined for the most part to the upper 300 db, with a surface characteristic speed of only 3 cm s-1 . A transport of 5 × 106 m3 s-1 across a line from the gyre center to the African continental margin, including the Angola Current, may best depict the gyre 0-300 db transport (relative to 1500 db). Ship drift data fvr the region do not show the presence of the cyclonic gyre because the wind-induced Ekman layer masks the gyre. The thermocline of the cyclonic gyre is significantly saltier and lower in oxygen than the suspected source water: the main thermocline of the South Atlantic subtropical gyre. A strong front near 18°S, the Angola-Benguela Front, separates the cyclonic gyre regime from that of the subtropical gyre. Using the regional freshwater balance, gyre thermocline residence time is determined to be between 4.4 and 8.5 years, implying an oxygen utilization of 0.3-0.5 mll-1 y-1 . Below the thermocline there is some evidence for southward Plow of North Atlantic Deep Water along the eastern boundary. It is inferred that this flow is fed by eastward spreading of North Atlantic Deep Water along the equatorial belt.