Editorial Type:
Article
Article Type:
Research Article

Weak Mixing in the Eastern North Atlantic: An Application of the Tracer-Contour Inverse Method

Jan D. Zika Climate Change Research Centre, University of New South Wales, Sydney, and CSIRO Wealth from Oceans National Research Flagship, Hobart, Australia

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Trevor J. McDougall Centre for Australian Weather and Climate Research, CSIRO and the Bureau of Meteorology, and CSIRO Wealth from Oceans National Research Flagship, Hobart, Australia

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Bernadette M. Sloyan Centre for Australian Weather and Climate Research, CSIRO and the Bureau of Meteorology, and CSIRO Wealth from Oceans National Research Flagship, Hobart, Australia

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Print Publication:
01 Aug 2010
DOI:
https://doi.org/10.1175/2010JPO4360.1
Page(s):
1881–1893
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Abstract

The tracer-contour inverse method is used to infer mixing and circulation in the eastern North Atlantic. Solutions for the vertical mixing coefficient D, the along-isopycnal mixing coefficient K, and a geostrophic streamfunction Ψ are all direct outputs of the method. The method predicts a vertical mixing coefficient O(10−5 m2 s−1) in the upper 1000 m of the water column, consistent with in situ observations. The method predicts a depth-dependent along-isopycnal mixing coefficient that decreases from O(1000 m2 s−1) close to the mixed layer to O(100 m2 s−1) in the interior, which is also consistent with observations and previous hypotheses. The robustness of the result is tested with a rigorous sensitivity analysis including the use of two independently constructed datasets.

This study confirms the utility of the tracer-contour inverse method. The results presented support the hypothesis that vertical mixing is small in the thermocline of the subtropical Atlantic Ocean. A strong depth dependence of the along-isopycnal mixing coefficient is also demonstrated, supporting recent parameterizations for coarse-resolution ocean models.

Corresponding author address: Jan Zika, Laboratoire des écoulements géophysiques et industriels (LEGI), BP 53-38041 Grenoble, CEDEX 9, France. Email: jan.zika@hmg.inpg.fr

Abstract

The tracer-contour inverse method is used to infer mixing and circulation in the eastern North Atlantic. Solutions for the vertical mixing coefficient D, the along-isopycnal mixing coefficient K, and a geostrophic streamfunction Ψ are all direct outputs of the method. The method predicts a vertical mixing coefficient O(10−5 m2 s−1) in the upper 1000 m of the water column, consistent with in situ observations. The method predicts a depth-dependent along-isopycnal mixing coefficient that decreases from O(1000 m2 s−1) close to the mixed layer to O(100 m2 s−1) in the interior, which is also consistent with observations and previous hypotheses. The robustness of the result is tested with a rigorous sensitivity analysis including the use of two independently constructed datasets.

This study confirms the utility of the tracer-contour inverse method. The results presented support the hypothesis that vertical mixing is small in the thermocline of the subtropical Atlantic Ocean. A strong depth dependence of the along-isopycnal mixing coefficient is also demonstrated, supporting recent parameterizations for coarse-resolution ocean models.

Corresponding author address: Jan Zika, Laboratoire des écoulements géophysiques et industriels (LEGI), BP 53-38041 Grenoble, CEDEX 9, France. Email: jan.zika@hmg.inpg.fr

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