Boundary Mixing and Topographic Blocking on the Mid-Atlantic Ridge in the South Atlantic

Abstract

It is commonly observed in hydrographic sections crossing midocean ridges that the isopycnals on the ridge flanks slope downward toward the crests. Although the observed vertical scales of isopycnal dipping are not consistent with steady diffusive boundary layers on slopes, the cross-flank density gradients can nevertheless be caused by diapycnal mixing acting on timescales of several years. The corresponding pressure gradients are usually inferred to be associated with cyclonic along-flank flows. Recent observations of southward flow along the highly corrugated western flank of the Mid-Atlantic Ridge near 20°S are inconsistent with this conceptual picture, however. Data from seven zonal cross-ridge sections and from four meridional along-ridge sections were used to analyze the hydrography on the flanks of the Mid-Atlantic Ridge between 2°N and 30°S. The majority of the hydrographic stations were occupied over deep cross-flank canyons, which extend over a total length of ?90 000 km in the tropical and subtropical South Atlantic alone. The dipping of the isopycnal surfaces on the western ridge flank in the Brazil Basin is largely restricted to the canyons, where flow along the flank is topographically blocked. The magnitudes of the blocked apparent along-ridge transports are typically of order 1 Sv (1 Sv ≡ 106 m3 s?1) with values as high as 3 Sv, implying important consequences for circulation studies with both forward and inverse models. In the southern Brazil Basin the horizontal density gradients immediately above the blocking topography are reversed?that is, the densities increase toward the crest of the Mid-Atlantic Ridge, consistent with observations of southward flow along the flank. On the eastern flank in the Angola Basin there is a layer of crestward-increasing densities as well, but there it lies well above the ridge topography. Numerical solutions of the buoyancy equation indicate that the observed cross-flank density gradients of both signs are consistent with bottom-intensified diapycnal mixing, which causes a vertical buoyancy-flux dipole. Boundary mixing on slopes can therefore give rise to anticyclonic, as well as cyclonic, along-slope flows. The observed horizontal temperature and salinity gradients near the Mid-Atlantic Ridge in the South Atlantic cannot be accounted for by diapycnal mixing alone, on the other hand. The distributions of these properties are therefore largely determined by isopycnal processes.