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Dependence of Differential Mixing on N and Rρ

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  • 1 Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, Canada
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Abstract

Mechanisms and parameter dependence of differential mixing of heat and salt by ocean turbulence are investigated numerically by extending a previous study to examine dependence upon buoyancy frequency N and density gradient ratio Rρ. In these experiments a burst of turbulence mixes temperature T and pseudosalinity S having molecular diffusivity 0.1 times that of T across background vertical gradients of both quantities. In contrast to previous results, which found turbulent diffusivity ratios d = KS/KT < 1 at a fixed N, the present study finds that d > 1 when N = 0 and that d tends to approach this value as N → 0. In all cases considered, d is larger at high Rρ (buoyancy dominated by T) than at low Rρ (buoyancy dominated by S). It is shown that this tendency is consistent with differential mixing being largely due to preferential restratification of the slower-diffusing component S. This conclusion is reinforced by the finding that d scales linearly with a fractional restratification measure over a wide range of conditions.

Corresponding author address: Dr. William Merryfield, Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 1700, Victoria, BC V8W 2Y2, Canada. Email: bill.merryfield@ec.gc.ca

Abstract

Mechanisms and parameter dependence of differential mixing of heat and salt by ocean turbulence are investigated numerically by extending a previous study to examine dependence upon buoyancy frequency N and density gradient ratio Rρ. In these experiments a burst of turbulence mixes temperature T and pseudosalinity S having molecular diffusivity 0.1 times that of T across background vertical gradients of both quantities. In contrast to previous results, which found turbulent diffusivity ratios d = KS/KT < 1 at a fixed N, the present study finds that d > 1 when N = 0 and that d tends to approach this value as N → 0. In all cases considered, d is larger at high Rρ (buoyancy dominated by T) than at low Rρ (buoyancy dominated by S). It is shown that this tendency is consistent with differential mixing being largely due to preferential restratification of the slower-diffusing component S. This conclusion is reinforced by the finding that d scales linearly with a fractional restratification measure over a wide range of conditions.

Corresponding author address: Dr. William Merryfield, Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 1700, Victoria, BC V8W 2Y2, Canada. Email: bill.merryfield@ec.gc.ca

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