The vertical structure of ocean eddies

Lama, M. S. de la; LaCasce, J. H.; Fuhr, Helle Kristine

doi:10.1093/climsys/dzw001

Cited by 49 publications

(70 citation statements)

References 40 publications

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“…The present results suggest that the preferred set of baroclinic modes in the ocean are the surface modes, with topographic waves accounting for velocity fluctuations at the bottom. This would explain the gravest EOFs of Wunsch (1997) and de La Lama et al (2016), which resembled the gravest surface mode in many extratropical regions. As surface modes obtain with only modest bottom topography and friction, they should apply essentially everywhere in the ocean.…”

Section: Discussionmentioning

confidence: 89%

“…The present arguments pertain to large‐scale waves, and variability on smaller scales and at higher frequencies could differ. The current meter results of de La Lama et al () suggest that motions with periods as low as a day can also be described by the gravest surface mode, but the wider applicability of this result deserves further examination. To what extent linear baroclinic modes apply to nonlinear eddies must also be studied (despite that the modes are often used in describing such features).…”

Section: Discussionmentioning

confidence: 92%

“…As surface modes obtain with only modest bottom topography and friction, they should apply essentially everywhere in the ocean. Higher baroclinic modes are found with current meter data nearer the equator, but these modes also have nearly zero flow at the bottom (de La Lama et al, ).…”

Section: Discussionmentioning

confidence: 94%

“…The flat bottom baroclinic modes, on the other hand, ought to apply in the ocean interior, over the abyssal plains. But EOF1 resembles the gravest surface mode in most extratropical regions, regardless of bathymetry (de La Lama et al, ).…”

Section: Introductionmentioning

confidence: 99%

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The Prevalence of Oceanic Surface Modes

LaCasce

2017

Geophysical Research Letters

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Satellite observations have revolutionized oceanography, capturing diverse phenomena over much of the globe. However, it remains to understand how surface fields, like sea surface height, reflect the motion occurring at depth. The vertical structure of ocean eddies is often expressed in terms of "baroclinic modes," which are basis functions derived assuming a flat ocean bottom. Bathymetry alters the modes though, weakening the bottom velocities. Using analytical solutions, we demonstrate that with realistic bathymetry and/or bottom friction, the bottom velocities are nearly zero. The resulting "surface modes" should be ubiquitous in the ocean. This would explain the dominant mode of variability obtained from globally distributed current meter data and is consistent with energy spectra derived from sea surface height data. The results yield a simple way to infer subsurface velocities from satellite data and suggest that ocean analyses should be made in terms of surface modes and topographic waves. Plain Language Summary Observations of the sea surface from satellites have revolutionized oceanography. But it is important to understand how the surface fields, such as sea height and surface temperature, reflect the motion occurring at depth. Traditionally, oceanographers have represented the vertical structure of ocean currents using "modes." One of these, for example, does not vary with depth. The present work demonstrates how these modes change when realistic bottom topography is taken into account. The results suggest a simple way to predict subsurface velocities and have implications for our understanding of ocean dynamics. Interestingly, Wunsch also found that the primary empirical orthogonal function (EOF1), calculated with the same current meter data, often did not resemble either the BT or BC1 modes. Rather, it decayed monotonically RESEARCH LETTER

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Section: Discussionmentioning

confidence: 89%

Section: Discussionmentioning

confidence: 92%

Section: Discussionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

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The Prevalence of Oceanic Surface Modes

LaCasce

2017

Geophysical Research Letters

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“…Interestingly, Wunsch () found temporal and geographical variations of their results and a certain degree of modal coupling. Such coupling led Sanchez de La Lama et al () to compute empirical orthogonal modes (EOF) of current meter records. The first EOF at most locations appeared to explain more than half of the current meter variance (their Figure ).…”

Section: Introductionmentioning

confidence: 99%

Reconstruction of the 3‐D Dynamics From Surface Variables in a High‐Resolution Simulation of North Atlantic

et al. 2018

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Several methods that reconstruct the three‐dimensional ocean dynamics from sea level are presented and evaluated in the Gulf Stream region with a 1/60° realistic numerical simulation. The use of sea level is motivated by its better correlation with interior pressure or quasi‐geostrophic potential vorticity (PV) compared to sea surface temperature and sea surface salinity, and, by its observability via satellite altimetry. The simplest method of reconstruction relies on a linear estimation of pressure at depth from sea level. Another method consists in linearly estimating PV from sea level first and then performing a PV inversion. The last method considered, labeled SQG for surface quasi‐geostrophy, relies on a PV inversion but assumes no PV anomalies. The first two methods show comparable skill at levels above −800 m. They moderately outperform SQG which emphasizes the difficulty of estimating interior PV from surface variables. Over the 250–1,000 m depth range, the three methods skillfully reconstruct pressure at wavelengths between 500 and 200 km whereas they exhibit a rapid loss of skill between 200 and 100 km wavelengths. Applicability to a real case scenario and leads for improvements are discussed.

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Parameterizing eddy form stress in a thickness-weighted average isopycnal ocean model

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Marques²,

Adcroft

et al. 2022

Preprint

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Mesoscale eddies are unresolved in most of today's global ocean models, and their effect on the large-scale ocean circulation needs to be parameterized. Greatbatch and Lamb (1990, GL90) suggested an eddy form stress parameterization that mixes geostrophic momentum in the vertical. The GL90 vertical viscosity scheme, which is equivalent to the Gent and McWilliams (1990, GM90) parameterization under the geostrophic assumption, has seen only very limited use, and exclusively in models that use z-coordinates and are of very coarse resolution. In this paper, we explore the GL90 parameterization in an idealized isopycnal coordinate model, both from a theoretical and practical perspective. We further compare the effects of the GM90 and GL90 parameterizations across a range of non-eddying to eddy-permitting resolutions. From a theoretical perspective, the GL90 parameterization is more attractive than the GM90 scheme for isopycnal coordinate models because GL90 provides an interpretation that is fully consistent with thickness-weighted isopycnal averaging, while GM90 cannot be entirely reconciled with any fully isopycnal averaging framework. From a practical perspective, the GL90 and GM90 parameterizations lead to extremely similar energy levels, flow and vertical structure, even though their energetic pathways are very different. The equivalence holds true from non-eddying through eddy-permitting resolution. We conclude that in isopycnal coordinate models, the GL90 parameterization provides a good-if not equivalent-alternative to the GM90 parameterization. The GL90 parameterization is more advantageous in terms of computational efficiency, ease of implementation, and numerical stability.

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The vertical structure of ocean eddies

Cited by 49 publications

References 40 publications

The Prevalence of Oceanic Surface Modes

The Prevalence of Oceanic Surface Modes

Reconstruction of the 3‐D Dynamics From Surface Variables in a High‐Resolution Simulation of North Atlantic

Parameterizing eddy form stress in a thickness-weighted average isopycnal ocean model

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