Intense mixing of Antarctic Bottom Water in the equatorial Atlantic Ocean

Abstract

THE spreading of Antarctic Bottom Water—the densest global-scale water mass—is highly constrained by ocean-floor topography. In the Atlantic Ocean, the Mid-Atlantic Ridge confines this water mass mainly to the western basins, the bottom waters in the eastern basins being renewed by flows through gaps in the ridge¹. One such gap is the Romanche fracture zone, a large offset of the ridge which straddles the Equator. It has been observed² that sills within this fracture zone block the passage of waters colder than ∼0.9 °C; warmer, less dense waters passing over the sills appear to cascade downslope where they are modified by mixing. Here we present direct measurements which quantify these processes. The flow is vertically sheared and exhibits remarkably intense turbulence, comparable to that seen at the ocean surface in the presence of winds of ∼l0ms ⁻¹. This turbulence mixes the densest waters passing through the fracture zone with the warmer, overlying waters, so that the coldest waters exiting this region have been warmed by ∼0.6 °C during transit. Topographic obstructions and turbulent mixing together thus determine the properties of the flows renewing the deepest waters of the Atlantic Ocean's eastern basins.