SCALE-DEPENDENT MERGING OF BAROCLINIC VORTICES

The influence of stratification on the merging of like-sign vortices o f equal intensity and shape is investigated by numerical simulations i n a quasi-geostrophic, two-layer stratified model. Two different types of vortices are considered: vortices defined as circular patches of u niform potential vorticity in the upper layer but no PV anomaly in the lower layer (referred to as PVI vortices), and vortices defined as ci rcular patches of uniform relative vorticity in the upper layer but no motion in the lower layer (referred to as RVI vortices). In particula r, it is found that, in the RVI case, the merging behaviour depends st rongly on the magnitude of the stratification (i.e. the ratio of inter nal Rossby radius and vortex radius). The critical point here appears to be whether or not the initial eddies have a deep flow signature in terms of PV. The specific phenomenon of scale-dependent merging observ ed is interpreted in terms of the competitive effects of hetonic inter action and vortex shape. In the case of weaker stratification, the bar oclinic structure of the eddies can be seen as dominated by a mechanis m of hetonic interaction in which bottom flow appears to counteract th e tendency of surface eddies to merge. In the case of larger stratific ation, the eddy interaction mechanism is shown to be barotropically do minated, although interface deformation still determines the actual ed dy vorticity profile during the initialization stage. Repulsion (heton ic) effect therefore oppose attraction (barotropic shape) effects in a competitive process dependent on the relationship between the origina l eddy lengthscale and the first internal Rossby radius. A concluding discussion considers the implications of such analysis for real situat ions, in the ocean or in the laboratory.