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Properties of the lateral mesoscale eddy-induced transport in a high-resolution ocean model: Beyond the flux-gradient relation

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  • 1 aRosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida
  • | 2 bDepartment of Mathematics, Imperial College London, London, United Kingdom
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Abstract

Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux-gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatio-temporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of non-divergent (“rotational”) component of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, re-evaluate the utility of the flux-gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux-gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux-gradient relation for representing the eddy-induced tracer transport.

Corresponding author: Yueyang Lu, yueyang.lu@rsmas.miami.edu

Abstract

Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux-gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatio-temporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of non-divergent (“rotational”) component of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, re-evaluate the utility of the flux-gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux-gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux-gradient relation for representing the eddy-induced tracer transport.

Corresponding author: Yueyang Lu, yueyang.lu@rsmas.miami.edu
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