How Variable Is Mixing Efficiency in the Abyss?

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.