We study symmetric vortex merger in quasi-geostrophic flows using numerical simulations
with high vertical resolution. We analyse the effect of varying the vertical aspect
ratio of the vortices and compare with the barotropic case. During the merging of
potential vorticity cores with small aspect ratio, we observe the birth of secondary
instabilities on the filaments. This is a new phenomenon not seen in baroclinic
simulations having low vertical resolution. Passive Lagrangian tracers are used to
explore the transport of fluid parcels during vortex merger, to provide a detailed view
of the flow evolution, and to determine the value of the critical merging distance for
baroclinic vortices.