Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic

Abstract

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 spacetime 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 3º (latitude) by 4º (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 36ºS and spanning from 23º±1°S to 46° ± 1ºS. The South Atlantic Current meanders from 33ºS to 46ºS and shows a mean speed of 9.6 ± 7.8 cm s-1 (8.5 Sv ± 3.5 Sv; 1 Sv = 1×106 m3 s-1). The northern branch of the Subtropical Gyre is located between 22ºS and 32ºS and flows westward with a mean speed of 4.7 ± 3.3 cm s-1 (9.3 Sv ± 3.4 Sv). Evidence of a cyclonic Tropical Gyre divided in two sub-cells is visible on the stream function.