Crossing of the Equator by the Deep Western Boundary Current in the Western Atlantic Ocean

Abstract

Property distributions and geostrophic shear from a hydrographic section near 37°W in the Atlantic Ocean show the deep western boundary current (DWBC) in the North Atlantic Deep Water (NADW) established against the western boundary of the Brazil Basin immediately south of the equator (between 2° and 5°S). The DWBC thus has directly crossed the equator to the South Atlantic following the east-southeast trend of the continental slope isobaths. The estimated DWBC transport of NADW is 35 ? 106 m3 s?1, similar to other estimates from the tropics discussed here. These large DWBC transports are opposed by flow of deep water to the North Atlantic immediately offshore of the DWBC, with as much as two-thirds of the DWBC transport being returned as these recirculations. One recirculation center is the Guiana Basin north of the equator but extends at least a few hundred kilometers south of the equator; another is visible at 11°S in the Brazil Basin. The degree of connection of these two observed recirculations is not established. These recirculations spread the northern source influences over the width of the recirculation (rather than the DWBC width) and efficiently dilute the northern source concentration with South Atlantic influences, with the self-mixing of the recirculation complicating the interpretation of tracer distributions. A further complication occurs for the uppermost levels of the NADW, for the DWBC flows to the Southern Hemisphere beneath an opposing western boundary current of Antarctic Intermediate Water (AAIW), and downgradient property fluxes mutually erode the upper NADW and the AAIW core characteristics. This causes a displacement of the axis of maximum northern source concentration offshore front the axis of maximum transport of upper NADW in the DWBC, a demonstration that the relationship between a tracer tongue and the flow field can be obscure.