Direct measurement of the deep circulation within the Brazil Basin

Abstract

In support of the Deep Basin Experiment, part of the World Ocean Circulation Experiment, a large number of neutrally buoyant floats were released within the Brazil Basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior. Three levels corresponding to the three major subthermocline water masses were selected, and results from the deeper two (North Atlantic Deep Water, NADW, and Antarctic Bottom Water, AABW) are described. At this writing processing of acquired tracking data is incomplete. Hence, this paper reports on the progress of the observational program and gives our initial conclusions. It appears that the flow in the deep Brazil Basin is unlike previous conjectures in which the circulation patterns can be characterized as being primarily meridional, both along the western boundary and in the interior. The existence of a deep western boundary current (DWBC) is quite clear in the float data at the NADW level, but less prominent in the AABW, and the interior flow is dominantly zonal with unexpectedly small meridional space scales. Integral time scales are long, of order 20–30 days, and eddy kinetic energy levels are low, of order 1 cm 2/s 2. In spite of the low energy levels a surprising number of our floats became caught up in vortices. A line of seamounts extending offshore near 20°S, known as the Vitória–Trindade Seamounts, interrupts the DWBCs and is the location for eddy formation and apparent flow away from the boundary into the interior. Although it has been speculated that this could feed a narrow zonal current of NADW (the “Namib Col Current”) our float trajectories suggest a return to the western boundary, rather than a continuation to the east.