Evolution of initially axisymmetric buoyancy jets: a numerical study

Abstract

Baroclinic evolution of coastal currents associated with a surface front over variable topography is investigated numerically, employing a two-layer frontal-geostrophic model. In the frontal-geostrophic dynamical limit the frontal layer velocity is geostrophic to leading order; however the advective terms from the momentum equation are accounted for in the leading-order balance, and the mass conservation equation includes both the temporal and advective contributions. In this study, special focus is placed on the role of topography and the development of coherent vortex features. Perturbed axisymmetric currents in an annular domain are seen to develop typical breaking-wave instabilities at the outside edge. In general, the topography is found to be a destabilizing influence; however very steep topography is shown to inhibit cross-front motions, and thus delay subsequent vortex shedding. In simulations involving a continuous source of buoyant fluid, the ambient layer exhibits a marked increase in anticyclonic vorticity, which prevents eddy pinch-off. The results are compared with previous laboratory experiments involving surface currents, and implications for the stability of fronts at a shelf break are discussed.