Connecting Antarctic Cross-Slope Exchange with Southern Ocean Overturning

Stewart, Andrew L.; Thompson, Andrew F.

doi:10.1175/jpo-d-12-0205.1

Cited by 69 publications

(82 citation statements)

References 71 publications

(93 reference statements)

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“…1a). Throughout this study we keep L s = 10 km and h = 600 m constant, while we vary both H and s. This profile has been often used as a generic bathymetry in previous works (Lozier et al, 2002;Lozier and Reed, 2005;Poulin and Flierl, 2005;Stewart and Thompson, 2013), and according to Poulin et al (2014), the hyperbolic tangent profile fits remarkably well the shelf bathymetry in the Bransfield Strait, from which the numerical setup of this work takes inspiration. We use a linear equation of state and set salinity to a constant; thus, the density stratification is a function of temperature only and is equal to ρ = −ρ 0 α T T .…”

Section: Numerical Model Setupmentioning

confidence: 96%

“…Hence, the impact of a sloping bathymetry on the development and the evolution of meanders and eddies has been the topic of several studies. The first studies were devoted to the linear stability of coastal flows, while the more recent numerical simulations focus on the non-linear formation of meso-and submesoscale eddies from shelf/slope density fronts or currents (Pennel et al, 2012;Stewart and Thompson, 2013;.…”

Section: Introductionmentioning

confidence: 99%

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Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

2017

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Abstract. This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the crossshore transport. We use the Regional Ocean Modeling System (ROMS) to run simulations in an idealized channel configuration, using a fixed coastal current structure and testing its unstable evolution for various depths and topographic slopes. The experiments are integrated beyond the linear stage of the instability, since our focus is on the non-linear end state, namely the formation of coastal eddies or meanders, to classify the dynamical regimes. We find three nonlinear end states, whose properties cannot be deduced solely from the linear instability analysis. They correspond to a quasi-stable coastal current, the propagation of coastal meanders, and the formation of coherent eddies. We show that the topographic parameter T p , defined as the ratio of the topographic Rossby wave speed over the current speed, plays a key role in controlling the amplitude of the unstable crossshore perturbations. This result emphasizes the limitations of linear stability analysis to predict the formation of coastal eddies, because it does not account for the non-linear saturation of the cross-shore perturbations, which is predominant for large negative T p values. We show that a second dimensionless parameter, the vertical aspect ratio γ , controls the transition from meanders to coherent eddies.We suggest the use of the parameter space (T p , γ ) to describe the emergence of coastal eddies or meanders from an unstable buoyant current. By knowing the values of T p and γ for an observed flow, which can be calculated from hydrological sections, we can identify which non-linear end state characterizes that flow -namely if it is quasi-stable, meanders, or forms eddies.

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Section: Numerical Model Setupmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

2017

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“…According to (4) we should have APE much larger than EKE in these regions. Further, this may also be contributed by the underestimate of EKE in our applied 18-km grid ECCO2 dataset, the suppression of baroclinic instability above continental slope and along the ocean front (Stewart and Thompson 2013;Su et al 2014), and the potential smallness of real-ocean parcel rearrangement scale relative to our applied eddy size. Global patterns of Lorenz APE density are again distinct from EKE (see Fig.…”

Section: Eddy Size-constrained Ape Densitymentioning

confidence: 99%

On the Minimum Potential Energy State and the Eddy Size–Constrained APE Density

Ingersoll

2016

Journal of Physical Oceanography

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Exactly solving the absolute minimum potential energy state (Lorenz reference state) is a difficult problem because of the nonlinear nature of the equation of state of seawater. This problem has been solved recently but the algorithm comes at a high computational cost. As the first part of this study, the authors develop an algorithm that is ;10 3 -10 5 times faster, making it useful for energy diagnosis in ocean models. The second part of this study shows that the global patterns of Lorenz available potential energy (APE) density are distinct from those of eddy kinetic energy (EKE). This is because the Lorenz APE density is based on the entire domainwide parcel rearrangement, while mesoscale eddies, if related to baroclinic instability, are typically generated through local parcel rearrangement approximately around the eddy size. Inspired by this contrast, this study develops a locally defined APE framework: the eddy size-constrained APE density based on the strong constraint that the parcel rearrangement/displacement to achieve the minimum potential energy state should not exceed the local eddy size horizontally. This concept typically identifies baroclinically unstable regions. It is shown to be helpful to detect individual eddies/vortices and local EKE patterns, for example, around the Southern Ocean fronts and subtropical western boundary currents. This is consistent with the physical picture that mesoscale eddies are associated with a strong signature in both the velocity field (i.e., EKE) and the stratification (i.e., local APE). The new APE concept may be useful in parameterizing mesoscale eddies in ocean models.

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“…There are three physical processes that are responsible for setting the interior stratification: (a) thermal forcing from the lateral boundaries; (b) modification of the isopycnals over the continental slope related to PV conservation (this tends to generate isopycnals that slope in a similar sense to the bottom topography) [17,24] and (c) Ekman convergence and divergence caused by frictional processes at both top and bottom boundaries [38]. Figure 7 shows that in all simulations, a broad region at the surface, which spans the latitudes that feel a surface wind forcing exhibits low PV reflecting a weak surface stratification.…”

Section: Interior Pv Gradientsmentioning

confidence: 99%

“…Stewart et al [18,24] find that submesoscale eddies are generated over the continental shelf and shelf break, but are suppressed over the continental slope due to the strong potential vorticity gradient [17]. To date, much of the work on submesoscale dynamics has been limited to idealized processes models or observational studies in strong western boundary currents [25][26][27] and the open ocean [28].…”

Section: Introductionmentioning

confidence: 99%

Submesoscale Turbulence over a Topographic Slope

Lazar

Zhang

Thompson

2018

Fluids

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Abstract:Regions of the ocean near continental slopes are linked to significant vertical velocities caused by advection over a sloping bottom, frictional processes and diffusion. Oceanic motions at submesoscales are also characterized by enhanced vertical velocities, as compared to mesoscale motions, due to greater contributions from ageostrophic flows. These enhanced vertical velocities can make an important contribution to turbulent fluxes. Sloping topography may also induce large-scale potential vorticity gradients by modifying the slope of interior isopycnal surfaces. Potential vorticity gradients, in turn, may feed back on mesoscale stirring and the generation of submesoscale features. In this study, we explore the impact of sloping topography on the characteristics of submesoscale motions. We conduct high-resolution (1 km × 1 km) simulations of a wind-driven frontal current over an idealized continental shelf and slope. We explore changes in the magnitude, skewness and spectra of surface vorticity and vertical velocity across different configurations of the topographic slope and wind-forcing orientations. All of these properties are strongly modulated by the background topography. Furthermore, submesoscale characteristics exhibit spatial variability across the continental shelf and slope. We find that changes in the statistical properties of submesoscale motions are linked to mesoscale stirring responding to differences in the interior potential vorticity distributions, which are set by frictional processes at the ocean surface and over the sloping bottom. Improved parameterizations of submesoscale motions over topography may be needed to simulate the spatial variability of these features in coarser-resolution models, and are likely to be important to represent vertical nutrient fluxes in coastal waters.

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Connecting Antarctic Cross-Slope Exchange with Southern Ocean Overturning

Cited by 69 publications

References 71 publications

Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

On the Minimum Potential Energy State and the Eddy Size–Constrained APE Density

Submesoscale Turbulence over a Topographic Slope

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