We study symmetric vortex merger in quasi-geostrophic hows 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 rat
io, 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.