Ocean jet instability: a model comparison

Abstract
We investigate numerically the instability of three jets of the North Atlantic ocean (the Gulf-Stream, the North Atlantic current and the Azores current), and the vortices resulting from this instability. For this problem, we compare two hydrodynamical models (the shallow-water and the quasi-geostrophic equations) which we implement in identical conditions (two-layer stratification). First, both models are linearized and growth rates are calculated for zonal, normal-mode perturbations on such jets. When we vary the Burger and the Rossby numbers of the jets and the wavenumber of the perturbations, the two linear models yield fairly similar results. Secondly, both nonlinear models (discretized on a fixed regular grid) are used to compute the finite-amplitude evolutions of the perturbed jets. These evolutions are qualitatively similar in both models, but the dynamical features (jet and vortices) are longer-lived in the shallow-water than in the quasi-geostrophic model. Also, only the former model exhibits an asymmetry between cyclones and anticyclones, which is physically explained.