Transequatorial Flow of Antarctic Bottom Water in the Western Atlantic Ocean: Abyssal Geostrophy at the Equator

Abstract

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 newly 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°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 AARW and the overlying deep water. Here 4.3 ? 106 m3 s?1 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. This scenario includes a flow of approximately 7 ? 106 m3 s?1 of AABW into the Brazil Basin from the south. The magnitude of the northward flow diminishes as it moves toward the equator indicated by estimates of 6.7 ? 106 m3 s?1 at 23°S and 5.5 ? 106 m3 s?1 at 11°S. At the equator, 4.3 ? 106 m3 s?1 exits the Brazil Basin to continue northward across the Guiana Basin, and an unquantified amount flows through the Romanche Fracture Zone into the eastern basin. The northward decrease in AABW suggests an upwelling across isotherms. The difference in transports between 11°S and the equator, 1.2 ? 106 m3 s?1, is an estimate of the combined amounts of AABW being upwelled and exiting the basin through the Romanche Fracture Zone. In the Guiana Basin at 4°N, AABW transport is estimated at 4.0 ? 106 m3 s?1. This flow subsequently splits into two approximately equal flows: continued northward flow through the Guiana Basin, and eastward flow through the Vema Fracture Zone at 11°N to the eastern basin.