Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

The northward flowing Antarctic Intermediate Water (AAIW) is a major contri butor to the large-scale meridional circulation of water masses in the Atla ntic. Together with bottom and thermocline water, AAIW replaces North Atlan tic Deep Water that penetrates into the South Atlantic from the North. On t he northbound propagation of AAIW from its formation area in the south-west ern region of the Argentine Basin, the AAIW progresses through a complex sp reading pattern at the base of the main thermocline. This paper presents tr ajectories of 75 subsurface floats, seeded at AAIW depth. The floats were a coustically tracked, covering a period from December 1992 to October 1996, Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermi ttent westward and eastward currents were observed, suggesting a seasonal c ycle of the AAIW flow direction. At tropical latitudes, just offshore the i ntermediate western boundary current, the southward advection of an anticyc lonic eddy was observed between 5 degrees S and 11 degrees S. Farther offsh ore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28 degrees S shows c onsiderable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic C urrent is dominated by large-scale meanders with particle velocities in exc ess of 60 cm s(-1). At the Brazil-Falkland Current Confluence Zone, a cyclo nic eddy near 40 degrees S 50 degrees W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of th e various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions. (C) 1999 Elsevier Science Ltd. All r ights reserved.