Merging of Frontal Eddies

Abstract

Merging of two anticyclonic vortices is studied in the context of a reduced-gravity model and with emphasis on frontal dynamics. Using a particle-in-cell method, numerical experiments illustrate the merging process and the accompanying Lagrangian motions. In any merger event, three stages can be distinguished. In the first stage, intrusions from each eddy wrap around the other eventually substituting particles of one eddy almost completely with those of the other. The second and rapid stage is the merging per se, leaving a new, elongated eddy. In a prolonged third stage, stabilization of this eddy proceeds with axisymmetrization and rejection of fluid to the surroundings. Such rejection of fluid, which is demonstrated to be essential for the conservation of potential vorticity, energy and angular momentum, proceeds with the formation not of filaments, as in Euler's dynamics, but of satellite vortices. The center eddy may or may not axisymmetric completely, but, in all cases, consists of a thorough mixture of particles from both original eddies. Other numerical experiments with cyclones and zero?potential-vorticity anticyclones indicate that these vortices are resistant to merging. The impacts of the planetary beta effect and of eddy pulsation on merging are also considered.