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Volume 13, issue 6 | Copyright
Ocean Sci., 13, 905-923, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 14 Nov 2017

Research article | 14 Nov 2017

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

Laura Cimoli1,3, Alexandre Stegner2, and Guillaume Roullet3 Laura Cimoli et al.
  • 1University of Brest, CNRS, IRD, Ifremer, Laboratoire d'Océanographie Physique et Spatiale, IUEM, Brest, France
  • 2Laboratoire de Météorologie Dynamique, CNRS/École Polytechnique/Université Paris-Saclay, Palaiseau, France
  • 3Department of Physics, University of Oxford, Oxford, UK

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 cross-shore 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 non-linear 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 Tp, 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 cross-shore 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 Tp 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 (Tp, γ) to describe the emergence of coastal eddies or meanders from an unstable buoyant current. By knowing the values of Tp 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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Short summary
The dispersion of coastal waters offshore strongly depends on the dynamical regime of the current that characterizes the local coastal circulation. By using an idealized model configuration, we identify some key parameters – which can be calculated from observations – that describe when a coastal current flowing over a sloping topography acts as a source of meanders or eddies or as a dynamical barrier to the cross-shore transport.
The dispersion of coastal waters offshore strongly depends on the dynamical regime of the...