Anticyclonic selection by instability of parallel flows in a frontal regime

Abstract

Large-scale flows are known to present a predominance of anticyclonic vortices. A previous study (Perret et. al. 2006) showed a strong cyclone-anticyclone asymmetry in large-scale wakes, anticyclones are circular whereas cyclones are deformed. To determine the mechanisms responsible for the asymmetry, we perform a stability analysis of parallel wake flows associated with experimental velocity profiles. It is shown that the most unstable mode, in a frontal regime, is localized in the anticyclonic shear leading to a strong cyclone-anticyclone asymmetry in the nonlinear evolution of the perturbation. Moreover, the wake instability changes from the absolute instability in the quasigeostrophic regime to the strongly convective instability of the frontal regime. To determine whether the stability property of a parallel flow in a frontal regime could be a mechanism of anticyclones selection, we extend this stability analysis to parallel jets and shears. The anticyclonic shear, in a frontal regime, is much more unstable than the cyclonic one, but the nonlinear evolution of the flow leads, in both cases, to circular vortices.