Скачать или смотреть Talk: Insights into the mixing efficiency of submesoscale Centrifugal-Symmetric instabilities

This presentation was recorded for the 2022 AGU Ocean Sciences Meeting, session PS02: Multi-scale turbulent mixing of the ocean surface boundary layer 04.

Abstract:

Submesoscale processes provide a pathway for energy to transfer from the balanced circulation to turbulent dissipation. One class of submesoscale phenomena that has been shown to be quite effective at mixing and removing energy from the balanced flow are centrifugal-symmetric instabilities (CSIs), which have been observed to generate significant mixing in both the surface boundary layer and bottom boundary layer flows along bathymetry, where they have been implicated in the mixing and watermass transformation of Antarctic Bottom Water. However, the mixing efficiencies (i.e. the fraction of the energy extracted from the flow used to irreversibly mix the fluid) of these instabilities remain uncertain, making estimates of global mixing and energy dissipation due to CSI difficult. In this work we use large-eddy simulations to investigate the mixing efficiency of CSIs in the submesoscale range. We find that centrifugally-dominated CSIs (i.e. CSI mostly driven by horizontal shear production) tend to have a higher mixing efficiency than symmetrically-dominated CSIs (i.e. CSI driven by vertical shear production). The mixing efficiency associated with CSIs can therefore alternately be significantly higher or significantly lower than the canonical value used by most studies (~0.2) depending on the dominant source of shear production. These results can be understood in light of recent work on stratified turbulence, whereby CSIs control the background state of the flow in which smaller-scale secondary overturning instabilities develop, thus actively modifying the characteristics of mixing by Kelvin-Helmholtz instabilities. Our results also suggest that it may be possible to predict the mixing efficiency with more readily measureable parameters (namely the Richardson and Rossby numbers), possibly allowing for parameterization of this effect.