Cautionary tales from the mesoscale eddy transport tensor

Uchida, Takaya; Balwada, Dhruv; Jamet, Quentin; Dewar, William K.; Deremble, Bruno; Penduff, Thierry; Sommer, Julien Le

doi:10.1016/j.ocemod.2023.102172

Cited by 4 publications

(4 citation statements)

References 69 publications

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“…This is not meant to be an exact number but rather an order of magnitude of the eddy diffusivity. This value is well in the range of values used for eddy diffusivity diagnosed in high resolution models (Abernathey et al., 2013; Uchida et al., 2023) and used in low resolution ocean models (Nakamura & Chao, 2000). There are zones with upgradient PV flux near the western boundary (a region of intense eddy activity) and in banded structures in the middle of the domain.…”

Section: Rectification Termsupporting

confidence: 59%

Eddy‐Mean Flow Interaction With a Multiple Scale Quasi Geostrophic Model

Deremble,

Uchida,

Dewar

et al. 2023

J Adv Model Earth Syst

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Parameterization of mesoscale eddies in coarse resolution ocean models is necessary to include the effect of eddies on the large‐scale oceanic circulation. We propose to use a multiple‐scale Quasi‐Geostrophic (MSQG) model to capture the eddy dynamics that develop in response to a prescribed large‐scale flow. The MSQG model consists in extending the traditional quasi geostrophic (QG) dynamics to include the effects of a variable Coriolis parameter and variable background stratification. Solutions to this MSQG equation are computed numerically and compared to a full primitive equation model. The large‐scale flow field permits baroclinically unstable QG waves to grow. These instabilities saturate due to non‐linearities and a filtering method is applied to remove large‐scale structures that develop due to the upscale cascade. The resulting eddy field represents a dynamically consistent response to the prescribed background flow, and can be used to rectify the large‐scale dynamics. Comparisons between Gent‐McWilliams eddy parameterization and the present solutions show large regions of agreement, while also indicating areas where the eddies feed back onto the large scale in a manner that the Gent‐McWilliams parameterization cannot capture. Also of interest is the time variability of the eddy feedback which can be used to build stochastic eddy parameterizations.

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Section: Rectification Termsupporting

confidence: 59%

Eddy‐Mean Flow Interaction With a Multiple Scale Quasi Geostrophic Model

Deremble,

Uchida,

Dewar

et al. 2023

J Adv Model Earth Syst

Self Cite

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“…(2020), where coherent eddies are identified and tracked as Lagrangian coherent structures. While the PV diffusivity can become ill‐defined in regions with weak PV gradients (Uchida et al., 2023), the background PV gradient is held fixed in this model and Lagrangian and Eulerian estimates of the PV diffusivity are expected to agree.…”

Section: Coherent Eddies and Tracer Diffusivity In Qg Turbulencementioning

confidence: 99%

Inferring Tracer Diffusivity From Coherent Mesoscale Eddies

Zhang,

Wolfe

2024

J Adv Model Earth Syst

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Mixing along isopycnals plays an important role in the transport and uptake of oceanic tracers. Isopycnal mixing is commonly quantified by a tracer diffusivity. Previous studies have estimated the tracer diffusivity using the rate of dispersion of surface drifters, subsurface floats, or numerical particles advected by satellite‐derived velocity fields. This study shows that the diffusivity can be more efficiently estimated from the dispersion of coherent mesoscale eddies. Coherent eddies are identified and tracked as the persistent sea surface height extrema in both a two‐layer quasigeostrophic (QG) model and an idealized primitive equation (PE) model. The Lagrangian diffusivity is estimated using the tracks of these coherent eddies and compared to the diagnosed Eulerian diffusivity. It is found that the meridional coherent eddy diffusivity approaches a stable value within about 20–40 days in both models. In the QG model, the coherent eddy diffusivity is a good approximation to the upper‐layer tracer diffusivity in a broad range of flow regimes, except for small values of bottom friction or planetary vorticity gradient, where the motions of same‐sign eddies are correlated over long distances. In the PE model, the tracer diffusivity has a complicated vertical structure and the coherent eddy diffusivity is correlated with the tracer diffusivity at the e‐folding depth of the energy‐containing eddies where the intrinsic speed of the coherent eddies matches the rms eddy velocity. These results suggest that the oceanic tracer diffusivity at depth can be estimated from the movements of coherent mesoscale eddies, which are routinely tracked from satellite observations.

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“…The dataset has been used to quantify the effect of oceanic chaos on the Atlantic Meridional Overturning Circulation (Jamet et al 2019(Jamet et al , 2020cDewar et al 2022), and spatial variability of eddies, here defined about the ensemble mean and interpreted as the expression of oceanic chaos (Uchida et al 2021c(Uchida et al , 2022b(Uchida et al , 2023a. In this study, we shift our attention to the temporal variability by examining the energy cycle.…”

Section: A Model Descriptionmentioning

confidence: 99%

“…We argue that this deviation from the Lorenz cycle of a non-explicit eddy APE reservoir results from the fact that quasi geostrophy corresponds to the thickness-weighted primitive equations of motion in isopycnal coordinates, and not the unweighted equations in geopotential coordinates. Under quasi geostrophy, the isopycnal layer thickness is constant, leading to quasi-geostrophic (QG) variables being implicitly thickness-weighted averaged (Marshall et al 2012;Maddison and Marshall 2013;Uchida et al 2023a;Meunier et al 2023). Nonetheless, an energy budget can be formulated for non-thickness weighted primitive equations under geopotential coordinates (cf.…”

Section: B Ocean Energy Cyclementioning

confidence: 99%

Imprint of chaos on the ocean energy cycle from an eddying North Atlantic ensemble

Uchida,

Jamet,

Dewar

et al. 2023

Preprint

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We examine the ocean energy cycle where the eddies are defined about the ensemble mean of a partially air-sea coupled, eddy-rich ensemble simulation of the North Atlantic. The decomposition about the ensemble mean leads to a parameter-free definition of eddies, which is interpreted as the expression of oceanic chaos. Using the ensemble framework, we define the reservoirs of mean and eddy kinetic energy (MKE and EKE respectively) and mean total dynamic enthalpy (MTDE). We opt for the usage of dynamic enthalpy (DE) as a proxy for potential energy due to its dynamically consistent relation to hydrostatic pressure in Boussinesq fluids and non-reliance on any reference stratification. The curious result that emerges is that the potential energy reservoir cannot be decomposed into its mean and eddy components, and the eddy flux of DE can be absorbed into the EKE budget as pressure work.We find from the energy cycle that while baroclinic instability, associated with a positive vertical eddy buoyancy flux, tends to peak around February, EKE takes its maximum around September in the wind-driven gyre. Interestingly, the energy input from MKE to EKE, a process sometimes associated with barotropic processes, becomes larger than the vertical eddy buoyancy flux towards the summer and autumn. Our results question the common notion that the inverse energy cascade of winter-time EKE energized by baroclinic instability within the mixed layer is solely responsible for the summer-to-autumn peak in EKE, and suggest that the non-local eddy transport of DE and local transfer of energy from MKE to EKE could also contribute to the seasonal EKE maxima.

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Cautionary tales from the mesoscale eddy transport tensor

Cited by 4 publications

References 69 publications

Eddy‐Mean Flow Interaction With a Multiple Scale Quasi Geostrophic Model

Eddy‐Mean Flow Interaction With a Multiple Scale Quasi Geostrophic Model

Inferring Tracer Diffusivity From Coherent Mesoscale Eddies

Imprint of chaos on the ocean energy cycle from an eddying North Atlantic ensemble

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