Transequatorial flow of Antarctic bottom water in the Western Atlantic Ocean: abyssal geostrophy at the Equator

In its general northward flow along the western trough of the Atlantic, Antarctic Bottom Water (AABW) must pass over several sills separating the various abyssal basins. At the equator, the western trough is deformed by major east-west offsets of the Mid-Atlantic Ridge and the continental margin of Brazil, forming a nearly zonal abyssal channel about 250 km wide, centered at the equator, and extending approximately 1000 km along its axis, in which the AABW is confined. Thus, the general northward flow of AABW is topographically constrained to be westward as it crosses the equator. A hydrographic section across this channel at 37 degree W shows the AABW isopycnals to be "bowl" shaped within and beneath the level of the channel walls. The equatorial geostrophic relation permits us to compute a zonal velocity from the well-defined parabolic distribution of dynamic height, relative to a reference level at the transition between AABW and the overlying deep water. Here 4.3 x 10 super(6) m super(3)/s is estimated for the westward-and ultimately northward-transport of AABW. Although this value exceeds previous estimates of net northward transport in the Brazil and Guiana basins made from International Geophysical Year data of the late 1950s, it fits well into an overall scenario constructed from transport estimates made from section data collected during the 1980s.