Reconciling the Observed Mid-Depth Exponential Ocean Stratification with\n Weak Interior Mixing and Southern Ocean Dynamics via Boundary-Intensified\n Mixing

Munk (1966) showed that the deep (1000-3000 m) vertical temperature profile\nis consistent with a one-dimensional vertical advection-diffusion balance, with\na constant upwelling and an interior diapycnal diffusivity of\n$\\mathcal{O}(10^{-4})$ m$^{2}$ s$^{-1}$. However, typical observed\ndiffusivities in the interior are $\\mathcal{O}(10^{-5})$ m$^{2}$ s$^{-1}$.\nRecent work suggested that the deep stratification is set by Southern Ocean\n(SO) isopycnal slopes, fixed by SO eddies, that communicate the surface outcrop\npositions to the deep ocean. It is shown here, using an idealized ocean general\ncirculation model, that SO eddies alone cannot lead to the observed exponential\ntemperature profile, and that interior mixing must contribute. Strong diapycnal\nmixing concentrated near the ocean boundaries is shown to be balanced locally\nby upwelling. A one-dimensional Munk-like balance in these boundary mixing\nareas, although with much larger mixing and upwelling, leads to an exponential\ndeep temperature stratification, which propagates via isopycnal mixing to the\nocean interior. The exponential profile is robust to vertical variations in the\nvertical velocity, and persists despite the observed weak interior diapycnal\nmixing. Southern Ocean eddies link the surface water mass transformation by\nair-sea fluxes with the deep stratification, but the eddies do not determine\nthe stratification itself. These results reconcile the observed exponential\ninterior deep temperature stratification, the weak diapycnal diffusivity\nobserved in tracer release experiments, and the role of Southern Ocean\ndynamics.\n