Steep Shelf Stabilization of the Coastal Bransfield Current: Linear Stability Analysis

Poulin, Francis; Stegner, Alexandre; Hernández-Arencibia, Mónica; Marrero-Díaz, Ángeles; Sangrà, Pablo

doi:10.1175/jpo-d-13-0158.1

Cited by 25 publications

(44 citation statements)

References 40 publications

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“…On the one hand, many coastal currents such as the Algerian Current (Millot, 1999;Obaton et al, 2000;Puillat et al, 2002), the West Greenland Current (Eden and Boning, 2002;Pickart et al, 2005;Hátún et al, 2007), and the Norwegian Coastal Current (Björk et al, 2001) shed coherent and long-lived eddies off the coast. On the other hand, some coastal flows, such as the Bransfield Current (Savidge and Amft, 2009;Sangrá et al, 2011;Poulin et al, 2014) or the Northern Current in the western Mediterranean Sea (Sammari et al, 1995;Millot, 1999;Birol et al, 2010), appear to be quasi-stationary along the shelf slope.…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, some coastal flows, such as the Bransfield Current (Savidge and Amft, 2009;Sangrá et al, 2011;Poulin et al, 2014) or the Northern Current in the western Mediterranean Sea (Sammari et al, 1995;Millot, 1999;Birol et al, 2010), appear to be quasi-stationary along the shelf slope. Hence, the impact of a sloping bathymetry on the development and the evolution of meanders and eddies has been the topic of several studies.…”

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: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2017

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“…The homogenization of PV together with wind forcing determines the equilibrated stratification and jet structure. Stability analyses have been carried out for barotropic jets (e.g., Poulin and Flierl [15]), and for some simple cases for baroclinic jets [17,44]. However, these studies do not cover all aspects of our numerical simulations, for instance, the impact of a surface wind stress and the fact that density surfaces may incrop on the bottom topography.…”

Section: Resultsmentioning

confidence: 99%

“…However, these studies do not cover all aspects of our numerical simulations, for instance, the impact of a surface wind stress and the fact that density surfaces may incrop on the bottom topography. Isachsen [17] shows that a quasi-geostrophic (QG) stability analysis does not predict diffusivities diagnosed from primitive equation simulations well, while Poulin et al [44] focus on prograde jets. In any case, in this paper we investigate the non-linear saturation, which is very different in these cases, especially with steep slopes [17,45].…”

Section: Resultsmentioning

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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“…The QG approximation also requires the bathymetry and isopycnals to exhibit small variations relative to their respective domain-wide averages. While these conditions are not necessarily satisfied over continental slopes, previous studies suggest that QG captures the essential features of large-scale flows over topographic steepnesses typical of the ocean's continental slopes (Williams et al 2010;Stewart et al 2011Stewart et al , 2014Poulin et al 2014;Stern et al 2015).…”

Section: Quasi-geostrophic Model Equations In Cylindrical Coordinatesmentioning

confidence: 97%

Baroclinic instability of axially symmetric flow over sloping bathymetry

2016

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Observations and models of deep ocean boundary currents show that they exhibit complex variability, instabilities and eddy shedding, particularly over continental slopes that curve horizontally, for example around coastal peninsulas. In this article the authors investigate the source of this variability by characterizing the properties of baroclinic instability in mean flows over horizontally curved bottom slopes. The classical 2-layer quasi-geostrophic solution for linear baroclinic instability over sloping bottom topography is extended to the case of azimuthal mean flow in an annular channel. To facilitate comparison with the classical straight channel instability problem of uniform mean flow, the authors focus on comparatively simple flows in an annulus, namely uniform azimuthal velocity and solid-body rotation. Baroclinic instability in solid-body rotation flow is analytically analogous to the instability in uniform straight channel flow due to several identical properties of the mean flow, including vanishing strain rate and vorticity gradient. The instability of uniform azimuthal flow is numerically similar to straight channel flow instability as long as the mean barotropic azimuthal velocity is zero. Nonzero barotropic flow generally suppresses the instability via horizontal curvatureinduced strain and Reynolds stresses work. An exception occurs when the ratio of the bathymetric to isopycnal slopes is close to (positive) one, as is often observed in the ocean, in which case the instability is enhanced. A non-vanishing mean barotropic flow component also results in a larger number of growing eigenmodes and in increased non-normal growth. The implications of these findings for variability in deep western boundary currents are discussed.

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Steep Shelf Stabilization of the Coastal Bransfield Current: Linear Stability Analysis

Cited by 25 publications

References 40 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

Submesoscale Turbulence over a Topographic Slope

Baroclinic instability of axially symmetric flow over sloping bathymetry

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