Mixing Associated with Sills in a Canyon on the Midocean Ridge Flank

Abstract

To close the global overturning circulation, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion. Hydrographic and microstructure observations from the Brazil Basin in the South Atlantic Ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge. In previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents. Here, the results from a detailed analysis of velocity, hydrographic, and microstructure data from a ridge-flank canyon are presented. Two-year-long current-meter records indicate that within the canyon there is a significant along-axial mean flow down the density gradient toward the ridge crest. Five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2. The mean dissipation of kinetic energy inside the canyon exceeds that above the ridge-flank topography by approximately a factor of 5. The largest dissipation values were observed downstream of a narrow, 1000-m-high sill that extends across the full width of the canyon. Along the entire canyon, there is a strong association between the presence of sills and along-axial density gradients, while there is no similar association between the presence of depressions and density gradients. Together, these observations suggest that sill-related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal-wave breaking, which is not expected to be associated preferentially with sills. While only ≈15% of the interfacial area between Antarctic Bottom Water and North Atlantic Deep Water in the Brazil Basin lie inside canyons, the available data suggest that approximately one-half of the diapycnal buoyancy fluxes take place there. In comparison, the region above the ridge-flank topography accounts for about one-third of the buoyancy fluxes. The apparent importance of sill-related processes for mixing in ridge-flank canyons is therefore of global significance, especially considering that such canyons occur on average every 50 km along 2/3 of the global midocean ridge system, and that sills partially block the canyon axes every few tens of kilometers.