Spatial variability of deep mixing in a Mid-Ocean Ridge fracture zone in the Brazil Basin

Mid-Ocean Ridge fracture zones channel bottom waters in the eastern Brazil Basin and are known to be regions of intensified deep mixing. Despite the essential role of bottom waters in the oceanic circulation, the mechanisms responsible for the deep turbulent mixing inside fracture zones are still subject to debate. To investigate the hydrography and the small scale turbulence in a particular Mid-Ocean Ridge fracture zone, a survey of cross-canyon tow-yos and individual CTD/LADCPs was carried out along 21°S, covering ~150 km of canyon axis. Based on the turbulent kinetic energy dissipation rate e inferred both from a Thorpe scale analysis and from vertical kinetic energy, a pattern is found of enhanced mixing near a major sill and over the canyon walls with e=10^-8 W kg^-1 compared with a background value of e=10^-9 W kg^-1. The locations and magnitudes of the density overturns along with the 3d bottom velocity field suggest hydraulic control at the sill. Based on a comparison between the topographic slopes of the lateral canyon walls and the slopes of semidiurnal internal wave energy propagation we hypothesize that enhanced turbulence over the sidewalls is due to a mechanism involving internal tides. The observations are interpreted in the context of a heat budget inside the canyon.