  Vortex formation through the Rossby wave instability (RWI) in protoplanetary
discs has been invoked to play a role in planet formation theory, and suggested
to explain the observation of large dust asymmetries in several transitional
discs. However, whether or not vortex formation operates in layered accretion
discs, i.e. models of protoplanetary discs including dead zones near the disc
midplane --- regions that are magnetically inactive and the effective viscosity
greatly reduced --- has not been verified. As a first step toward testing the
robustness of vortex formation in layered discs, we present non-linear
hydrodynamical simulations of global 3D protoplanetary discs with an imposed
kinematic viscosity that increases away from the disc midplane. Two sets of
numerical experiments are performed: (i) non-axisymmetric instability of
artificial radial density bumps in viscous discs; (ii) vortex-formation at
planetary gap edges in layered discs. Experiment (i) shows that the linear
instability is largely unaffected by viscosity and remains dynamical. The
disc-planet simulations also show the initial development of vortices at gap
edges, but in layered discs the vortices are transient structures which
disappear well into the non-linear regime. We suggest that the long term
survival of columnar vortices, such as those formed via the RWI, requires low
effective viscosity throughout the vertical extent of the disc, so such
vortices do not persist in layered discs.
