On non-uniqueness of the mesoscale eddy diffusivity

Luolin Sun, Michael Haigh, Igor Shevchenko, Pavel Berloff, Igor Kamenkovich

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

Oceanic mesoscale currents ('eddies') can have significant effects on the distributions of passive tracers. The associated inhomogeneous and anisotropic eddy fluxes are traditionally parametrised using a transport tensor (-tensor), which contains both diffusive and advective components. In this study, we analyse the eddy transport tensor in a quasigeostrophic double-gyre flow. First, the flow and passive tracer fields are decomposed into large- and small-scale (eddy) components by spatial filtering, and the resulting eddy forcing includes an eddy tracer flux representing advection by eddies and non-advective terms. Second, we use the flux-gradient relation between the eddy fluxes and the large-scale tracer gradient to estimate the associated -tensors in their entire structural, spatial and temporal complexity, without making any additional assumptions or simplifications. The divergent components of the eddy tracer fluxes are extracted via the Helmholtz decomposition, which yields a divergent tensor. The remaining rotational flux does not affect the tracer evolution, but dominates the total tracer flux, affecting both its magnitude and spatial structure. However, in terms of estimating the eddy forcing, the transport tensor prevails over its divergent counterpart because of the significant numerical errors induced by the Helmholtz decomposition. Our analyses demonstrate that, in general, the -tensor for the eddy forcing is not unique, that is, it is tracer-dependent. Our study raises serious questions on how to interpret and use various estimates of -tensors obtained from either observations or eddy-resolving model solutions.

Original languageEnglish (US)
Article numberA32
JournalJournal of Fluid Mechanics
Volume920
DOIs
StatePublished - 2021

Keywords

  • mixing and dispersion
  • quasi-geostrophic flows
  • turbulent mixing

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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