Horizontal and vertical Lagrangian dispersion in decaying cyclonic and anticyclonic vortices with lateral and bottom friction

Abstract

The horizontal and vertical dispersion of passive particles in a decaying circular vortex in a rotating system is investigated analytically and numerically. The vortex decay is due to lateral viscosity and bottom friction effects (associated with the Ekman boundary layer). The vortex model comprises the three-dimensional velocity field, where the azimuthal component is much larger than the radial and vertical components, so the structure remains circular. The particles are dispersed by the deterministic velocity field plus stochastic perturbations. The analytical model allows the examination of frictional effects separately. The experiments show that an initial point charge of particles is dispersed around the vortex. The role of lateral viscosity is to delay the angular distribution of the particles. Bottom friction, on the other hand, generates radial motions, thus inducing outward advection in cyclones and inward advection in anticyclones. The intensity of lateral and bottom friction slows down the expulsion or retention of particles. Regarding vertical dispersion, cyclonic vortices can lift particles by a substantial fraction of the fluid column, while anticyclones sink particles as in a bathtub vortex. It is shown that the vertical distributions of the particles are significantly affected by the strength of the decaying mechanisms. Some consequences for the dispersion of tracers in oceanic vortices are discussed.