Abstract
The Deep Western Boundary Current (DWBC) is the main component of the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). Off northeast Brazil, the DWBC breaks up into southwestward‐propagating anticyclones. In this study, we investigate the breakup mechanism with hydrographic observations, eddy‐resolving numerical model outputs, and theory. Here, we present a quasi‐synoptic map of geostrophic velocities and stream function at the DWBC core level between 2.5°S and 11°S. We observe, in horizontal distributions of velocities, that the DWBC breakup site is linked to a topographic feature of the Brazilian continental margin centered at 8°S: the Pernambuco Plateau (PP). Moreover, both observations and model outputs hint at a possible DWBC separation near the PP preceding anticyclone genesis. We test, with three different theories from the literature, whether or not the DWBC separates at 8°S. The results of the tests converge to indicate that the DWBC undergoes a local and intermittent inertial separation while contouring the PP. Downstream of its separation at the plateau, the DWBC sheds eddies similarly to previously reported laboratory experiments. In addition, a regional analysis of energy transfer shows that barotropic instability significantly contributes to the anticyclones growth between 8°S and 13°S. Analysis of the energy budget and separation of waters related to the AMOC pathways into the basin interior provide a better understanding for later studies about heat fluxes and ventilation in the deep tropical South Atlantic.
Plain Language Summary
The Deep Western Boundary Current transports southward the cold and dense waters linked to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). Among many properties, these waters transport energy from the Northern to the Southern Hemisphere, thousands of meters below the surface. Along the deep ocean off northeast Brazil, this current breaks up into large whirlpools of water that rotate counterclockwise while propagating southwestward. These whirlpools in the ocean are know as eddies. We observe that the eddy formation is linked to a feature of the Brazilian continental margin: the Pernambuco Plateau (PP). Moreover, both shipboard data and numerical model outputs hint at a possible separation of the deep current near the PP prior to the eddy formation. We test the separation with theories and the results converge to indicate that the deep current undergoes a local separation off the continental slope while contouring the PP. In addition, we present that the eddies grow by feeding off the energy from the deep current. The local separation and regional energy exchange along this deep current contribute to further discussions about the pathways of the AMOC and heat fluxes in the South Atlantic.
Key Points
Part of the Deep Western Boundary Current (DWBC) separates inertially off the continental slope while crossing the Pernambuco Plateau at 8°S
The DWBC separation plays a crucial role in the formation of the DWBC deep anticyclonic eddies
Barotropic instability significantly contributes to the growth of the deep anticyclonic eddies