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

Abstract

Munk (1966) showed that the deep (1000-3000 m) vertical temperature profile is consistent with a one-dimensional vertical advectiondiffusion balance, with a constant upwelling and an interior diapycnal diffusivity of O(10 −4 ) m 2 s −1 . However, typical observed diffusivities in the interior are 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.

Mid-depth Ocean Stratification: Southern Ocean eddies vs interior vertical diffusivity

“…We find that both isopycnal slopes and eddy activity in the SO, as reflected in measures of both the marginal criticality and residual circulation, are responsive to changes in the interior stratification. We also show, similar to the coarser resolution model of Miller et al (2019), that when the interior vertical diffusivity is small, the resulting adiabatic MOC dynamics and SO eddies are unable to lead to an observed-like interior mid-depth exponential vertical stratification. We conclude that the mid-depth ocean stratification must be determined by an interplay of interior vertical diffusion and SO eddy dynamics.…”

“…Observations show that both the mean density profile and the mean stratification (N 2 ) profiles are very nearly exponential over large regions of the world ocean (e.g., Miller et al, 2019). A vertical advective-diffusive balance was proposed to explain the exponential profile of temperature in the Pacific Ocean by Munk (1966).…”

“…The consequences of enhanced ocean margin mixing for the meridional overturning circulation were studied by Samelson (1998) and Scott and Marotzke (2002). More relevant to this work, the consequences for what might be setting the mid-depth stratification were examined in Miller et al (2019). They showed that a Munk-like balance can hold in narrow boundary regions near ocean margins, with much larger vertical diffusion and vertical velocity there, resulting in a Munk-like stratification that is then advected toward the ocean interior where the diffusivity may be small, consistent with tracer release experiments and with a similar suggestion made by Munk and Wunsch (1998).…”

“…In contrast, when the Southern ocean dynamics are included, interior stratification does not vanish with vertical mixing (Vallis, 2000;Wolfe and Cessi, 2010;Nikurashin and Vallis, 2011). However, it has been pointed out by Miller et al (2019) that the stratification in this case is not exponential. Nikurashin and Vallis (2011) pointed out that in the presence of large vertical diffusion, the stratification is determined by the Munk balance rather than by the SO wind and eddies.…”

“…These studies show that upwelling is concentrated where vertical mixing is enhanced, instead of being uniform over the global ocean. It was also suggested that a Munk-like balance in strong-mixing boundary regions, with a large vertical velocity and large diapycnal mixing there, can lead to the observed 1 km scale that is then communicated to the interior vial horizontal/eddy mixing (Munk and Wunsch, 1998;Miller et al, 2019).…”