The spreading of Antarctic bottom water in the tropical Atlantic

Abstract

Hydrographic and tracer [chlorofluorocarbon (CFC), component F11] data in the tropical Atlantic off Brazil taken in spring 1994 are used to describe the development of the water mass characteristics of Antarctic Bottom Water (AABW) between 10°S and 11°N. To compute the AABW transports, geostrophic computations and directly measured velocity fields are combined. Velocity profiles were measured with the Pegasus profiling system and an ADCP attached to the CTD. The F11 increase from 10°S to 11°N, mainly in the upper part of the tracer-poor AABW, reveals the mixing of AABW along its path with the overlying North Atlantic Deep Water, which carries a significant F11 signal in the equatorial Atlantic. While propagating north of 5°S, the AABW shifts to higher salinities at a given temperature. About one-third of the northward flowing AABW at 10°S (4.8 Sv) and at 5°S (4.7 Sv) west of about 31°30′W enters the Guiana Basin, mainly through the southern half of the Equatorial Channel at 35°W (1.5–1.8 Sv). The other part recirculates and some of it flows through the Romanche Fracture Zone into the eastern Atlantic. In the Guiana Basin, west of 40°W, the sloping topography and the strong, eastward flowing deep western boundary current might prevent the AABW from flowing west: thus it has to turn north at the eastern slope of the Ceara Rise (2.2 Sv). At 44°W, north of the Ceara Rise, AABW flows west in the interior of the basin in a main core near 7°15′N (1.9 Sv). A net return flow of about 0.5 Sv was found north of 8°43′N. A large fraction of the AABW (1.1 Sv) enters the eastern Atlantic through the Vema Fracture Zone, leaving only 0.3 Sv of AABW for the western Atlantic basins.