Abyssal Circulation Driven By Near-Boundary Mixing: Water Mass Transformations and Interior Stratification

Classical theories describe a steady-state abyssal ocean stratification and overturning forced by uniform vertical mixing over a flat ocean seafloor. Ship-based measurements, however, reveal heterogeneities in abyssal mixing: weak, uniform mixing over smooth continental slopes and abyssal plains, and strong, bottom-enhanced mixing over rough mid-ocean ridges. In the interior ocean, bottom-enhanced mixing results in a buoyancy-flux divergence which forces a transformation of waters from light to dense and thus downwelling. Near the bottom, however, buoyancy flux must decrease downwards to meet an insulating condition at the boundary, converging buoyancy, transforming waters to lighter density classes and forcing upwelling. In the limit of slowly-varying stratification and topographic slope, theory predicts that the up-slope and down-slope flows largely compensate, such that net watermass transformations are small. However, simulations with a Planetary-Geostrophic Circulation Model that resolves both the boundary-layer dynamics and the large-scale overturning show that at equilibrium variations in background stratification become sufficiently large to reduce the degree of compensation, resulting in watermass transformations similar to diagnostic estimates for the abyssal ocean. This suggests that boundary mixing plays a crucial role in setting the global ocean stratification and requires a revision of our theories of the ocean stratification and overturning circulation.