INTENSE MIXING OF ANTARCTIC BOTTOM WATER IN THE EQUATORIAL ATLANTIC-OCEAN

The spreading of Antarctic Bottom Water--the densest global-scale wate r mass--is highly constrained by ocean-floor topography. In the Atlant ic Ocean, the Mid-Atlantic Ridge confines this water mass mainly to th e western basins, the bottom waters in the eastern basins being renewe d by flows through gaps in the ridge(1). One such gap is the Romanche fracture zone, a large offset of the ridge which straddles the Equator . It has been observed(2) that sills within this fracture zone block t he passage of waters colder than similar to 0.9 degrees C; warmer, les s dense waters passing over the sills appear to cascade downslope wher e they are modified by mixing. Here we present direct measurements whi ch quantify these processes. The flow is vertically sheared and exhibi ts remarkably intense turbulence, comparable to that seen at the ocean surface in the presence of winds of similar to 10 m s(-1). This turbu lence 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 similar to 0.6 degrees C during transit, To pographic obstructions and turbulent mixing together thus determine th e properties of the flows renewing the deepest waters of the Atlantic Ocean's eastern basins.