Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

Abstract

The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically 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, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5°S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s\ . At the Brazil-Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.

Pathways and variability at intermediate depths in the tropical Atlantic

“…If it goes back down at a time when the current has meandered far enough south it may get caught in an opposing current. As mentioned above, earlier observations [Boebel et al, 1999a;Richardson and Fratantoni, 1999] and the small mean meridional velocities with large standard deviations (Figure 9) support the meandering of the tropical currents.…”

“…These characteristics of the meridional velocity field at intermediate depth can be interpreted as an indication for the presence of a meandering current. Similar (and clearer) signs were also found in trajectories from RAFOS floats [Boebel et al, 1999a] and SOFAR floats [ Richardson and Schmitz, 1993;Richardson and Fratantoni, 1999].…”

“…Most of these floats continued northward once they reached the western boundary. Only the float in Figure 10a experienced southward drift, probably in an eddy (Boebel et al, 1999a, observed the southward propagation of an eddy along the western boundary in this region in RAFOS trajectories). As the eddy-like drift terminated the float turned eastward, came back to the west and then turned southward once again.…”

“…Schott et al, 1998;Boebel et al, 1999a;Gouriou et al, 1999;Molinari et al, 1999;Richardson and Fratantoni, 1999;Schmid et al, 2001]. Essentially the equatorial flow is predominantly eastward (westward) in the first (second) half of the year.…”

“…For example, seasonal reversals of the zonal flow at intermediate depth have often been observed close to the equator [e.g. Schott et al, 1998;Boebel et al, 1999a;Gouriou et al, 1999;Molinari et al, 1999;Richardson and Fratantoni, 1999;Schmid et al, 2001]. It has been proposed that planetary waves cause the variability.…”