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

TL;DR

This paper demonstrates that boundary-intensified mixing, rather than interior mixing alone, explains the observed exponential deep ocean temperature profile, reconciling weak interior diffusivity with Southern Ocean dynamics.

Contribution

It introduces a boundary-focused mixing model that accounts for the exponential stratification, challenging the notion that interior mixing solely determines deep ocean profiles.

Findings

Boundary mixing leads to exponential temperature profiles.

Southern Ocean eddies link surface fluxes to deep stratification.

Interior mixing is insufficient alone to explain observed profiles.

Abstract

Munk (1966) showed that the deep (1000-3000 m) vertical temperature profile is consistent with a one-dimensional vertical advection-diffusion balance, with a constant upwelling and an interior diapycnal diffusivity of $\mathcal{O}(10^{-4})$ m$^{2}$ s$^{-1}$. However, typical observed diffusivities in the interior are $\mathcal{O}(10^{-5})$ m$^{2}$ s$^{-1}$. Recent work suggested that the deep stratification is set by Southern Ocean (SO) isopycnal slopes, fixed by SO eddies, that communicate the surface outcrop positions to the deep ocean. It is shown here, using an idealized ocean general circulation model, that SO eddies alone cannot lead to the observed exponential temperature profile, 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 deep temperature stratification, which propagates via isopycnal 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. Southern Ocean eddies link the surface water mass transformation by air-sea fluxes with the deep stratification, but the eddies do not determine the stratification itself. These results reconcile the observed exponential interior deep temperature stratification, the weak diapycnal diffusivity observed in tracer release experiments, and the role of Southern Ocean dynamics.