DYNAMICAL PROPERTIES OF VORTICAL STRUCTURES ON THE BETA-PLANE

The long-time evolution of monopolar and dipolar vortices influenced b y the large-scale gradient of the ambient potential vorticity (the bet a-effect) is studied by direct numerical solutions of the equivalent b arotropic quasi-geostrophic equation. Translation and reorganization o f vortical structures are shown to depend strongly on their intensity. Transport of trapped fluid by vortical structures is illustrated by c alculating particle trajectories and by considering closed isolines of potential vorticity and the streamfunction in a co-moving reference f rame. The initial behaviour of strong monopoles is found to be well de scribed by a recent approximate theory for the evolution of azimuthal mode one, even for times longer than the linear Rossby wave period. In the long-time limit, strong monopoles transport particles mainly west ward, although the meridional displacement is several times larger tha n the initial vortex size. The appearance of an annulus with opposite radial gradient of the potential vorticity around the vortex core is d emonstrated. This annulus forms owing to the meridional vortex drift o n the beta-plane and results in reorganization of a strong monopolar v ortex into a rotating tripole. A critical value of the vortex intensit y is found, below which the tripolar structure does not appear even in the case of an initially shielded vortex. Weak monopolar vortices are able to trap particles and provide some west-meridional fluid transpo rt, even in the case when they decay like a linear Rossby wave packet. The evolution of initial f-plane dipoles on the beta-plane is strongl y dependent on the initial direction of propagation. Strong dipoles ad just to steadily propagating modon solutions either accelerating (west ward case), decelerating (eastward case) or oscillating with a decayin g amplitude (meridional case), thereby carrying trapped particles pred ominantly eastward. A steady state is not reached if the dipole intens ity is below a critical value which depends on the initial direction o f propagation. Weak dipoles either decay and shrink owing to Rossby wa ve radiation (westward case), gradually separate and split (eastward c ase), or disintegrate (meridional case) without long-distance fluid tr ansport. Thus, on the beta-plane monopoles provide mainly westward tra nsport of trapped fluid, whereas dipoles provide mainly eastward trans port. Only strong monopoles are found to provide significant meridiona l fluid transport.