The Breakup of Frontal Filaments

Abstract

Unstable, infinitely long filaments, with a front at each side delimiting their width, are studied with the aid of a particle-in-cell numerical model. Two dynamical systems are considered: the shallow-water primitive equations and the frontal-geostrophic approximation. Invariably, the filaments break into a series of disconnected vortices. A case with zero potential vorticity (integrated with the primitive equations) yields a set of elongated eddies, which slightly depart from the main axis. Shape and dealignment from the axis are both due to a surplus in angular momentum, which results from the transition from filament to eddies. Integration with the frontal-geostrophic equations (nonzero potential vorticity) gives qualitatively similar results, but the eddies are less eccentric, and the departure from the axis is more notorious. This latter case is also investigated analytically, verifying the numerical results.