Turbulent mixing plays a fundamental role in the ventilation of the lower branch of the overturning circulation. In the topographically closed Brazil Basin of the South Atlantic, which is connected to neighboring basins via a small number of deep passages, heat budgets previously estimated that ~4 Sv of Antarctic Bottom Water must upwell across isopycnals. Despite the presence of a tidal modulation of the dissipation rate of kinetic energy over the flanks of the Mid-Atlantic Ridge, the mechanisms responsible for the diapycnal upwelling of bottom waters is still surrounded by many uncertainties. On the face of it, vertical profiles of dissipation appear to imply a counterintuitive deep-water downwelling.
In this study, we investigated two mechanisms that affect the turbulent mixing of mid-ocean ridge fracture zones. Within a fracture zone, a two-layer exchange flow is revealed by 1-year measurements of two McLane Moored Profilers. Firstly, at the location of a large sill, the fortnightly modulation of temperature overturns indicates the presence of an overflow, which is potentially tidally modulated. This observation is consistent with the fortnightly modulation of cross-sill transects of density and microstructure dissipation rate of kinetic energy.
Secondly away from the sill, downward-propagating energy of near-inertial waves dominates the internal wave field above 4000 m. Given the background flow and stratification of the 2-layer mean flow, ray-tracing simulations are used to predict the evolution of near-inertial waves. Those simulations indicate a reduced near-inertial energy at 4100 m below a peak in energy. Consistent with near-inertial waves approaching a critical layer, 45% of the microstructure profiles taken within fracture zones have their peak in dissipation several hundreds of meters above the seafloor. These mid-depth dissipation maxima are associated with enhanced downward-propagating kinetic energy. Within the ~40 fracture zones of the Brazil Basin, we estimate that this wave-mean flow interaction mechanism can ventilate up to 0.59—0.89 Sv of bottom water.