Mixing in the Romanche Fracture Zone

Abstract

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data. The finestructure of density profiles is described using the Thorpe scales LT. It is shown from microstructure data taken in the bottom water that the Ozmidov scale LO is related to LT by the linear relationship LO = 0.95LT, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 ? 10?4 m2 s?1 is found for the bottom water. Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4°C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.8°C isotherm, emphasizing the role of these passages for bottom-water property modifications.