How variable is mixing efficiency in the abyss?

Takashi Ijichi1, Louis St Laurent2, Kurt L Polzin3 and John Merrill Toole3, (1)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (2)Applied Physics Laboratory, University of Washington, Seattle, United States, (3)Woods Hole Oceanographic Institution, Woods Hole, United States
Abstract:
Mixing efficiency is an important turbulent flow property in fluid dynamics and its representation can be a potentially significant source of uncertainty in large-scale ocean circulation models. However, there are several confusing definitions of mixing efficiency. Here we compare and contrast patch-wise mixing efficiency estimates and bulk mixing efficiency estimates in the abyssal ocean by revisiting data from previous field surveys in the Brazil Basin, one of the most prominent mixing hotspots in the abyssal interior of the global ocean. Although observed patch-wise mixing efficiency is highly variable over a wide range of turbulence intensity, bulk mixing efficiency is dominated by a small number of very energetic turbulent events. In the case where enhanced near-bottom turbulence is thought to be driven by breaking of small-scale internal tides, the estimated bulk mixing efficiency is 16.7%, as conventionally used. On the other hand, where enhanced near-bottom turbulence is thought to be driven by hydraulic overflows, bulk mixing efficiency is suggested to be as large as 50%, which has implications for a more significant role of overflow mixing on deep-water mass transformation than previously thought.
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