How Variable Is Mixing Efficiency in the Abyss?

Abstract

Mixing efficiency is an important turbulent flow property in fluid dynamics, whose variability potentially affects the large-scale ocean circulation. However, there are several confusing definitions. Here we compare and contrast patch-wise versus bulk estimates of mixing efficiency in the abyss by revisiting data from previous extensive field surveys in the Brazil Basin. Observed patch-wise efficiency is highly variable over a wide range of turbulence intensity. Bulk efficiency is dominated by rare extreme turbulence events. In the case where enhanced near-bottom turbulence is thought to be driven by breaking of small-scale internal tides, the estimated bulk efficiency is 20%, close to the conventional value of 17%. On the other hand, where enhanced near-bottom turbulence appears to be convectively driven by hydraulic overflows, bulk efficiency is suggested to be as large as 45%, which has implications for a further significant role of overflow mixing on deep-water mass transformation.Plain Language Summary Ocean turbulence can vertically mix the stratified water column to raise the background gravitational potential energy, maintaining the ocean stratification. "Mixing efficiency" is a parameter representing the fraction of the available turbulent mechanical energy that is irreversibly converted to the background gravitational potential energy during a mixing event. Efficiency has traditionally been treated as a global constant of 17%, but there is growing evidence for its highly variable nature during individual turbulence events. It remains unknown how variable bulk mixing efficiency is (defined as the average over multiple turbulence events, including rare extreme cases). Here we show that bulk efficiency can be significantly different from the corresponding space-time average of local-instantaneous patch-wise efficiency estimates at deep depths with weak stratification. This warns against the application of variable mixing efficiency revealed in individual turbulence events to large-scale ocean circulation models. The use of the conventional efficiency in such models is supported in a tidal mixing hotspot over rough bathymetry. On the other hand, significantly higher mixing efficiency, up to 45%, is suggested in a localized mixing hotspot over a fracture zone sill, implying a more significant role of overflow mixing on deep-water mass transformation than previously thought.

Simulating turbulent mixing caused by local instability of internal gravity waves

“…This thought agrees with Ijichi et al. (2020), who found values as large as near the sea floor in the Brazil Basin where hydraulic overflows are thought to cause convection, and also with the latest DNS study of Howland, Taylor & Caulfield (2020). In conclusion, we should not consider that the mixing efficiency is parameterised solely from a single parameter , but always pay attention to the mechanisms of turbulence generation.…”

A Marginal Stability Paradigm for Shear‐Induced Diapycnal Turbulent Mixing in the Ocean

“…The simulation setup details were discussed in . The data in Figure 3 is from Ijichi et al (2020) and is openly available as their supplementary material (https://doi.org/10.1029/2019GL086813).…”

Spatial and Temporal Variability of Diapycnal Mixing in the Indian Ocean

“…Both the first (Gargett, 2003) and second (Ijichi et al., 2020) assumptions are debatable. The fluxes of heat and salt are converted to a flux of buoyancy using the thermal expansion coefficient α and haline contraction coefficient β , both of which are calculated for the bin‐averaged temperature and salinity.…”