Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic

This study combines float data from different projects collected between 1991 and 2003 in the South Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). Velocity space–time averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW layer). When judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a nominal cell size of 3° (latitude) by 4° (longitude) with cell shapes deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Results show an anticyclonic Subtropical Gyre centred near 36°S and spanning from 23±1°S to 46±1°S. The South Atlantic Current meanders from 33°S to 46°S and shows a mean speed of 9.6±7.8 cm s−1 (8.5±3.5 Sv; 1 Sv=1×106 m3 s−1). The northern branch of the Subtropical Gyre is located between 22°S and 32°S and flows westward with a mean speed of 4.7±3.3 cm s−1 (9.3±3.4 Sv). Evidence of a cyclonic Tropical Gyre divided in two sub-cells is visible on the stream function.