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 separatin g the various abyssal basins. At the equator, the western trough is de formed by major east-west offsets of the Mid-Atlantic Ridge and the co ntinental margin of Brazil, forming a nearly zonal abyssal channel abo ut 250 km wide, centered at the equator, and extending approximately 1 000 km along its axis, in which the AABW is confined. Thus, the genera l northward flow of AABW is topographically constrained to be westward as it crosses the equator. A hydrographic section across this channel at 37-degrees-W shows the AABW isopycnals to be ''bowl'' shaped withi n and beneath the level of the channel walls. The equatorial geostroph ic relation permits us to compute a zonal velocity from the well-defin ed parabolic distribution of dynamic height, relative to a reference l evel at the transition between AABW and the overlying deep water. Here 4.3 x 10(6) m3 s-1 is estimated for the westward-and ultimately north ward-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 in to an overall scenario constructed from transport estimates made from section data collected during the 1980s. This scenario includes a flow of approximately 7 x 10(6) 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 x 10(6) m3 s-1 at 23- degrees-S and 5.5 x 10(6) m3 s-1 at 11-degrees-S. At the equator, 4.3 x 10(6) m3 s-1 exits the Brazil Basin to continue northward across the Guiana Basin, and an unquantified amount flows through the Romanche F racture Zone into the eastern basin. The northward decrease in AABW su ggests an upwelling across isotherms. The difference in transports bet ween 11-degrees-S and the equator, 1.2 x 10(6) m3 s-1, is an estimate of the combined amounts of AABW being upwelled and exiting the basin t hrough the Romanche Fracture Zone. In the Guiana Basin at 4-degrees-N, AABW transport is estimated at 4.0 x 10(6) m3 s-1. This flow subseque ntly splits into two approximately equal flows: continued northward fl ow through the Guiana Basin, and eastward flow through the Vema Fractu re Zone at 11-degrees-N to the eastern basin.