Eastward Flow through the Mid-Atlantic Ridge at 11°N and Its Influence on the Abyss of the Eastern Basin

Abstract

The dilute Antarctic Bottom Water of the North Atlantic eastern trough is supplied from the western trough through fractures in the Mid-Atlantic Ridge. In particular, the influence on eastern trough property distributions of flow through the Romanche and Vema fracture zones, near the equator and 11°N, respectively, has been noted previously. Here, new observations are reported that document the abyssal circulation of the northeastern Atlantic basins (Gambia Abyssal Plain, South Canary Basin, and North Canary Basin) in particular, the dominance of Vema influence, the absence of Romanche influence, and the existence of a system of deep western boundary currents and estimated transport. Deep isopycnals slope steeply across the Vema's eastern end near 39°W, corresponding to a geostrophic transport through the Vema of 2.1 to 2.3 (?106 m3s?1) colder than 2.0°C. This is half or more of the estimated Bottom Water that flows north across the equator into the subtropical western North Atlantic. This transport in the Vema debouches into the Gambia Abyssal Plain. A deep western boundary current with 1.3 to 3.0 (?106m3s?1) transport colder than 2.0°C flows eastward. This current subsequently bifurcates into 1) a nearly zonal eastward current (≤1.0 ? 106m3s?1 transport) along the plain's southern boundary, the Sierra Leone Rise, and 2) a northward western boundary current along the flank of the Mid-Atlantic Ridge 1.8 to 3.9 ? 106m3s?1). Property and shear fields indicate that, for water colder than 2.0°C, basically none of the eastward flow along the rise passes southward through its deepest passage, the Kane Gap, nor does Romanche-derived water flow north there. The poleward Mid-Atlantic Ridge flank flow in the plain continues northward across the Cape Verde Ridge into the South Canary Basin and from there poleward into the North Canary Basin.