Goldilocks mixing in oceanic shear-induced turbulent overturns

Abstract: We present a new physically-motivated parameterization, based on the ratio of Thorpe and Ozmidov scales, for the irreversible turbulent flux coefficient $\Gamma_{\mathcal M}= {\mathcal M}/\epsilon$, i.e. the ratio of the irreversible rate ${\mathcal M}$ at which the background potential energy increases in a stratified flow due to macroscopic motions to the dissipation rate of turbulent kinetic energy. Our parameterization covers all three key phases (crucially, in time) of a shear-induced stratified turbulence life cycle: the initial, hot' growing phase, the intermediate energetically forced phase, and the finalcold' fossilization decaying phase. Covering all three phases allows us to highlight the importance of the intermediate one, to which we refer as the `Goldilocks' phase due to its apparently optimal (and so neither too hot nor too cold, but just right) balance, in which energy transfer from background shear to the turbulent mixing is most efficient. $\Gamma_{\mathcal M}$ is close to 1/3 during this phase, which we demonstrate appears to be related to an adjustment towards a critical or marginal Richardson number for sustained turbulence $\sim 0.2-0.25$.