The Efficiency of Deep and Abyssal Ocean Turbulent Mixing

Turbulent mixing in the deep ocean, produced by various shear-driven mechanisms including internal wave breaking, plays a primary role in the climate through exerting a control upon the upwelling of deep dense waters formed at high latitudes, thereby driving the global ocean overturning circulation. A key parameter used to characterise turbulent mixing in observations, climate models, and global energy budgets is the turbulent flux coefficient $\Gamma$ (often referred to as `mixing efficiency'). $\Gamma$ is traditionally defined as the ratio of the portion of the energy input into the deep ocean that is invested in irreversible mixing, to the portion viscously dissipated into heat, and is conventionally assumed to be a constant of approximately twenty percent. We present evidence from both numerical simulations of idealised flows and observations that strongly suggests that $\Gamma$ varies significantly in the abyssal ocean, and that mixing is predicted to be most efficient, corresponding to values of $\Gamma \simeq 0.5$, near topographic features which host vigorous wave generation and breaking.