On the Vertical Structure of Oceanic Mesoscale Tracer Diffusivities

Zhang, Wenda; Wolfe, Christopher L.

doi:10.1029/2021ms002891

Cited by 10 publications

(35 citation statements)

References 110 publications

(291 reference statements)

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“…Bringing in the eddy efficiency E eff (see Figure S6 and Text S6 in Supporting Information S1 for an estimate of the E eff pattern) leads to additional and more severe diffusivity reduction globally, also away from topographic features (bottom row). This is also in agreement with recent studies (admittedly focusing on Redi mixing) that found that, at large scales, the scaling by mean-flow dependent suppression has the largest impact on diffusivity (Holmes et al, 2022;Stanley et al, 2020;W. Zhang & Wolfe, 2022).…”

Section: Model Response In the Global Domainsupporting

confidence: 93%

Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

Nummelin,

Isachsen

2024

J Adv Model Earth Syst

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Most of the ocean's kinetic energy is contained within the mesoscale eddy field. Models that do not resolve these eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large‐scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models rely on an assumption of a flat ocean floor even though observations and high‐resolution modeling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. We show that buoyancy transport coefficient diagnosed from idealized eddy‐resolving simulations is indeed reduced over both prograde and retrograde bottom slopes (topographic wave propagation along or against the mean flow, respectively) and that the reduction can be skilfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale. This modified “GM” parameterization enhances the strength of thermal wind currents over the slopes in coarse‐resolution, non‐eddying, simulations. We find that in realistic global coarse‐resolution simulations the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean‐flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean biases.

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Section: Model Response In the Global Domainsupporting

confidence: 93%

Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

Nummelin,

Isachsen

2024

J Adv Model Earth Syst

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“…The coherent eddy diffusivity obtains a stable value much faster than the particle diffusivity, suggesting that using coherent eddy tracks may be more efficient than using surface drifters and subsurface floats to estimate diffusivity, though the coherent eddy diffusivity does not estimate the tracer diffusivity at the same depth as the drifters and floats do. Connecting these diffusivity estimates require knowledge of the vertical structure of eddy velocity (W. Zhang & Wolfe, 2022). Ni et al (2023) provides an estimate of full-depth eddy kinetic energy based on composite analysis of altimeter and Argo observations.…”

Section: Discussionmentioning

confidence: 99%

“…The direction of the associated eigenvector is generally in the meridional direction and is almost perpendicular to the direction of the mean flow in the upper ocean. The tracer diffusivity has a complicated vertical structure and tends to have a subsurface maximum due to the variation of eddy velocity with depth and the mean flow suppression effect, discussed in detail in W. Zhang and Wolfe (2022).…”

Section: Comparison Of the Coherent Eddy Diffusivity To The Tracer Di...mentioning

confidence: 99%

“…The reason for this difference is that the coherent eddies themselves are 3D structures and move as deep water columns. The eddy swirling velocity and the mean flow tends to decrease with depth, which causes the vertical variation of tracer diffusivity (W. Zhang & Wolfe, 2022). The coherent and tracer diffusivities are most likely to be comparable at the vertical level where the translation speed of coherent eddies is close to the eddy swirling velocity.…”

Section: Comparison Of the Coherent Eddy Diffusivity To The Tracer Di...mentioning

confidence: 99%

“…The PE model is an idealized configuration of the Massachusetts Institute of Technology general circulation model (MITgcm checkpoint67r; Campin et al, 2020;, which has been used for several previous studies Cessi et al, 2010;Wolfe, 2014;, 2011Wolfe et al, 2008;W. Zhang & Wolfe, 2022).…”

Section: Primitive Equation Modelmentioning

confidence: 99%

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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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Vertical Structure of Buoyancy Transport by Ocean Baroclinic Turbulence

Meunier,

Miquel,

Gallet

2023

Geophysical Research Letters

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Ocean mesoscale eddies enhance meridional buoyancy transport, notably in the Antarctic Circumpolar Current where they contribute to setting the deep stratification of the neighboring ocean basins. The much‐needed parameterization of this buoyancy transport in global climate models requires a theory for the overall flux, but also for its vertical structure inside the fluid column. Based on the quasi‐geostrophic dynamics of an idealized patch of ocean hosting an arbitrary vertically sheared zonal flow, we provide a quantitative prediction for the vertical structure of the buoyancy flux without adjustable parameters. The prediction agrees quantitatively with meridional flux profiles obtained through numerical simulations of an idealized patch of ocean with realistic parameter values. This work empowers modelers with an explicit and physically based expression for the vertical profile of buoyancy transport by ocean baroclinic turbulence, as opposed to the common practice of using arbitrary prescriptions for the depth‐dependence of the transport coefficients.

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On the Vertical Structure of Oceanic Mesoscale Tracer Diffusivities

Abstract: Ocean mesoscale eddies, with scales of 10-100s of kilometers, represent the majority of the kinetic energy of the ocean circulation (Ferrari & Wunsch, 2009). Stirring by mesoscale eddies plays an important role in the transport and mixing of oceanic tracers, which impacts ocean dynamics

Cited by 10 publications

References 110 publications

Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography

Inferring Tracer Diffusivity From Coherent Mesoscale Eddies

Vertical Structure of Buoyancy Transport by Ocean Baroclinic Turbulence

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