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Shelf Impact on Buoyant Coastal Current Instabilities

Romain PennelUnité de Mécanique, ENSTA-ParisTech, Palaiseau, France

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Alexandre StegnerLaboratoire de Météorologie Dynamique, École Polytechnique, Palaiseau, France

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Karine BérangerUnité de Mécanique, ENSTA-ParisTech, Palaiseau, France

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Abstract

The impact of shelf slope on the linear stability of buoyant coastal currents and on the nonlinear formation of coastal meanders and eddies is investigated. The authors consider a simplified two-layer stratification in cylindrical geometry where a buoyant surface current flows along the coast above a denser water, with a flat bottom or steep shelves. Simulations were performed using the Nucleus for European Modelling of the Ocean (NEMO) ocean global circulation model. The initial state of these simulations was defined according to laboratory experiments performed in the same configuration. Comparisons between laboratory and numerical results highlight the role of momentum diffusion and of the initial perturbations amplitude. The authors’ results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the buoyant coastal current is controlled by the baroclinic instability, the increase of To yields a selection of smaller unstable wavelengths and a decrease of the unstable growth rates. For finite values of To, a complete stabilization of the surface current can be reached. The typical radius of the first eddies generated by the coastal current is set by the linear stage of the baroclinic instability. However, secondary nonlinear processes may lead to larger or smaller structures. The authors exhibit a new dynamical sequence, leading to the formation of submesoscale cyclonic eddies over a steep shelf by splitting of mesoscale eddies. These cyclonic eddies trap and transport water masses and may play an important role in the cross-shelf exchanges.

Corresponding author address: Romain Pennel, UME/ENSTA-ParisTech, Chemin de la Hunière, 91761 Palaiseau CEDEX, France. E-mail: romain.pennel@ensta.org

Abstract

The impact of shelf slope on the linear stability of buoyant coastal currents and on the nonlinear formation of coastal meanders and eddies is investigated. The authors consider a simplified two-layer stratification in cylindrical geometry where a buoyant surface current flows along the coast above a denser water, with a flat bottom or steep shelves. Simulations were performed using the Nucleus for European Modelling of the Ocean (NEMO) ocean global circulation model. The initial state of these simulations was defined according to laboratory experiments performed in the same configuration. Comparisons between laboratory and numerical results highlight the role of momentum diffusion and of the initial perturbations amplitude. The authors’ results confirm that the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the current) is the relevant parameter to quantify the shelf impact on the linear and nonlinear dynamics of the surface current. When the evolution of the buoyant coastal current is controlled by the baroclinic instability, the increase of To yields a selection of smaller unstable wavelengths and a decrease of the unstable growth rates. For finite values of To, a complete stabilization of the surface current can be reached. The typical radius of the first eddies generated by the coastal current is set by the linear stage of the baroclinic instability. However, secondary nonlinear processes may lead to larger or smaller structures. The authors exhibit a new dynamical sequence, leading to the formation of submesoscale cyclonic eddies over a steep shelf by splitting of mesoscale eddies. These cyclonic eddies trap and transport water masses and may play an important role in the cross-shelf exchanges.

Corresponding author address: Romain Pennel, UME/ENSTA-ParisTech, Chemin de la Hunière, 91761 Palaiseau CEDEX, France. E-mail: romain.pennel@ensta.org
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