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Fluids 2016, 1(3), 25; doi:10.3390/fluids1030025

Diapycnal Velocity in the Double-Diffusive Thermocline

Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA
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Author to whom correspondence should be addressed.
Academic Editor: Pavel S. Berloff
Received: 22 May 2016 / Revised: 30 July 2016 / Accepted: 12 August 2016 / Published: 25 August 2016
(This article belongs to the Collection Geophysical Fluid Dynamics)
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Abstract

A series of large-scale numerical simulations is presented, which incorporate parameterizations of vertical mixing of temperature and salinity by double-diffusion and by small-scale turbulence. These simulations reveal the tendency of double-diffusion to constrain diapycnal volume transport, both upward and downward. For comparable values of mixing coefficients, the average diapycnal velocity in the double-diffusive thermocline is much less than in the corresponding turbulent regime. The insulating effect of double-diffusion is rationalized using two theoretical models. The first argument is based on the assumed vertical advective-diffusive balance. The second theory uses the Rhines and Young technique to evaluate the net diapycnal transport across regions bounded by closed streamlines at a given density surface. The numerical simulations and associated analytical arguments in this study underscore fundamental differences between double-diffusive mixing and mechanically generated small-scale turbulence. When both double-diffusion and turbulence are taken into account, we find that the constraints on diapycnal velocity loosen (tighten) with the increase (decrease) of the fraction of the overall mixing attributed to turbulence. The range of diapycnal velocities that could be realized in doubly-diffusive fluids is determined by the variation in the heat/salt flux ratio. We hypothesize that the unique ability of double-diffusive mixing to actively control diapycnal volume transport may have significant ramifications for the structure and dynamics of thermohaline circulation in the ocean. View Full-Text
Keywords: double-diffusion; diapycnal transport; ocean dynamics double-diffusion; diapycnal transport; ocean dynamics
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Radko, T.; Edwards, E. Diapycnal Velocity in the Double-Diffusive Thermocline. Fluids 2016, 1, 25.

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