Various Regimes of Instability and Formation of Coastal Eddies Along the Shelf Bathymetry

Abstract:

This numerical study aims at understanding the impact of the bottom topography on the stability of a geostrophic surface coastal current and the formation of coastal meanders and eddies.

A continuously stratified model is used, extending the analysis presented in Poulin et al., 2014 for a two-layer stratification. Simulations are performed using the Regional Ocean Modeling System (ROMS). We use an idealized periodic channel configuration with a geostrophic coastal current jet (modeled with a linear-gaussian profile) flowing along an hyperbolic tangent shelf bathymetry. The impact of the shelf bathymetry on the current stability is assessed by varying the shelf slope, the depth under the current jet and the vertical stratification (i.e. the first baroclinic deformation radius), while the geostrophic current remains unchanged.

Three distinct dynamical regimes are identified: a standard baroclinic instability, characterized by large mesoscale anticyclones that detach from the current; a trapped coastal instability, with small eddies propagating along the shelf and showing a marked baroclinic nature, i.e. anticyclonic (cyclonic) structures in the upper layer and cyclonic (anticyclonic) structures in the lower layer; finally, a stronger topographic stabilization leads to a weakly unstable current with no eddies formation. Three governing parameters are used to distinguish the three regimes, namely: the topographic parameter (Tp), a depths ratio (γ) and the Burger number (Bu). Besides, we show that the non-linear amplitude of the unstable perturbations is controlled by the shelf bathymetry. In the standard baroclinic regime, the unstable perturbations reach a finite amplitude leading to the formation of large meanders and mesoscale eddies. In the trapped coastal instability regime, both the meanders amplitude and the unstable wavelength are reduced. Once the first coastal eddies are formed along the shelf, a secondary process breaks them out and lead to sub-mesoscale perturbations. This direct cascade towards small scale induces a re-stabilization of the coastal current.