Reconciling the observed mid-depth exponential ocean stratification with weak interior mixing and Southern Ocean dynamics via boundary-intensified mixing

Abstract

Munk (Deep Sea Res Oceanogr Abstr 13(4):707–730, 1966) showed that the mid-depth (1–3 km) vertical temperature profile is consistent with a one-dimensional vertical advection–diffusion balance, with a constant upwelling and an interior diapycnal diffusivity of ${\mathcal {O}}(10^{-4})\,\hbox {m}^{2}\,\hbox {s}^{-1}$. However, typical observed diffusivities in the interior are ${\mathcal {O}}(10^{-5})\,\hbox {m}^{2}\,\hbox {s}^{-1}$. Recent work suggested that the mid-depth stratification is set by Southern Ocean (SO) isopycnal slopes, governed by SO wind and eddies, that communicate the surface outcrop positions to the mid-depth ocean. It is shown here, using an idealized ocean general circulation model, that while SO dynamics play an important role by linking the surface water mass transformation by air-sea fluxes with the mid-depth interior stratification, they do not set the observed exponential stratification and that interior mixing must contribute. Strong diapycnal mixing concentrated near the ocean boundaries is shown to be balanced locally by upwelling. A one-dimensional Munk-like balance in these boundary-mixing areas, although with much larger mixing and upwelling, leads to an exponential mid-depth temperature stratification, which spreads via isopycnal advection and mixing to the ocean interior. The exponential profile is robust to vertical variations in the vertical velocity and persists despite the observed weak interior diapycnal mixing. These results may suggest a way to reconcile the observed exponential interior mid-depth temperature stratification, the weak diapycnal diffusivity observed in tracer release experiments, and the role of Southern Ocean dynamics.