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Ocean Science An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/os-2017-62
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
28 Aug 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).
Isoneutral control of effective diapycnal mixing in numerical ocean models with neutral rotated diffusion tensors
Antoine Hochet, Rémi Tailleux, David Ferreira, and Till Kuhlbrodt University of Reading
Abstract. The current view about the mixing of heat and salt in the ocean is that it should be parameterised by means of a rotated diffusion tensor based on mixing directions parallel and perpendicular to the local neutral vector. However, the impossibility to construct a density variable in the ocean that is exactly neutral because of the coupling between thermobaricity and density-compensated temperature/salinity anomalies implies that the effective diapycnal diffusivity experienced by any possible density variable is partly controlled by isoneutral diffusion when using neutral rotated diffusion. Here, this effect is quantified by evaluating the effective diapycnal diffusion coefficient for five widely used density variables: Jackett and McDougall (1997) γn, Lorenz reference state density ρref of Winters and D’Asaro (1996); Saenz et al. (2015), and three potential density variables σ0, σ2 and σ4. Computations use the World Ocean Circulation Experiment climatology, assuming either a uniform value for isoneutral mixing or spatially varying values inferred from an inverse calculation. Isopycnal mixing contributions to the effective diapycnal mixing yields values systematically larger than 10−3 m2/s in the deep ocean for all density variables, with γn suffering the least from the isoneutral control of effective diapycnal mixing, and σ0 the most. These high values are due to spatially localised large values of non-neutrality, mostly in the deep Southern Ocean. Removing only 5 % of these high values on each density surface reduces the effective diapycnal diffusivities to less than 10−4 m2/s. This work highlights the potential pitfalls of estimating diapycnal diffusivities by means of Walin-like water masses analysis or in using Lorenz reference state for diagnosing spurious numerical diapycnal mixing.

Citation: Hochet, A., Tailleux, R., Ferreira, D., and Kuhlbrodt, T.: Isoneutral control of effective diapycnal mixing in numerical ocean models with neutral rotated diffusion tensors, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-62, in review, 2017.
Antoine Hochet et al.
Antoine Hochet et al.
Antoine Hochet et al.

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