Lagrangian circulation and transports of the Antarctic Intermediate Water in the south and tropical Atlantic

Abstract

This study uses a data set of 611 float years in the south and tropical Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). The data set includes pop-up and acoustically tracked floats that drifted within more than one decade in the area 60 degrees W to 30 degrees E and 70 degrees S to 10 degrees N. Float data is constrained in the vertical according to two isoneutral surfaces (gn = 27.25 and gn = 27.55), according to the characteristics of 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). Judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a cell size of 3 degrees (latitude) by 4 degrees (longitude) with cells 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. Since these space-time averages and the corresponding objective maps were unable to reveal the Intermediate Western Boundary Current (iWBC), an alternative approach based on an objective mapping with primal data in overlapping subsets was also applied. With this goal, an isotropic longitudinal covariance function was estimated considering cylin