Instability and Splitting of Mesoscale Rings using a Two-Layer Quasi-Geostrophic Model on anf-Plane

Abstract

Abstract Asymmetric instability of a circular Gaussian ring and evolution of a ring shape in a nonlinear case are studied using a two-layer quasi-geostrophic model on an f plane. Ring behavior is classified on an F-λ plane with other parameters fixed, where F is an internal rotational Froude number associated with a two-layer model, and λ is a ratio of the rotation speed in the lower layer to that in the upper layer. In the range concerned where F 0, the mode having an azimuthal wavenumber of 2 is the fastest growing, followed by the mode of the wavenumber 3. The ring is more unstable as F increases and as λ decreases, where larger F corresponds to smaller density difference or to a larger ring size. By numerical calculations, for which an initial perturbation of the wavenumber 2 is superimposed on a ring, ring behavior is classified into four groups from the most unstable to the most stable case: (i) a ring that splits into two small rings, (ii) a ring that rapidly becomes a slender ellipse a...