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On the Vertical Structure of Oceanic Mesoscale Tracer Diffusivities
  • Wenda Zhang,
  • Christopher Lee Pitt Wolfe
Wenda Zhang
Stony Brook University, Stony Brook University

Corresponding Author:wenda.zhang@stonybrook.edu

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Christopher Lee Pitt Wolfe
School of Marine and Atmospheric Sciences, School of Marine and Atmospheric Sciences
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Isopycnal mixing of tracers is important for ocean dynamics and biogeochemistry. Previous studies have primarily focused on the horizontal structure of mixing, but what controls the vertical structure is still unclear. This study investigates the vertical structure of the isopycnal tracer diffusivity diagnosed by a multiple-tracer inversion method in an idealized basin circulation model. The first two eigenvalues of the symmetric part of the 3D diffusivity tensor are approximately tangent to isopycnal surfaces. The isopycnal mixing is anisotropic, with principle directions of the large and small diffusivities generally oriented along and across the mean flow direction. The cross-stream diffusivity can be reconstructed from the along-stream diffusivity after accounting for suppression of mixing by the mean flow. In the circumpolar channel and above the thermocline in the gyres, the vertical structure of the along-stream diffusivity follows that of the rms eddy velocity, with the depth-independent constant of proportionality a local energy-containing scale defined by the peak of the surface eddy kinetic energy (EKE) spectrum. The diffusivity below the thermocline in the gyres instead follows the profile of the EKE times a depth-independent mixing time scale. The transition between the two mixing regimes is attributed to the dominance of nonlinear interactions and linear waves in the upper and deep ocean, respectively. A scaling is proposed that accounts for both regimes and captures the vertical variation of diffusivities better than extant theories. These results inform efforts to parameterize the vertical structure of isopycnal mixing in coarse-resolution ocean models.
Jun 2022Published in Journal of Advances in Modeling Earth Systems volume 14 issue 6. 10.1029/2021MS002891