  Relativistic jets associated with active galactic nuclei and gamma-ray bursts
propagate over huge distances without significant loss of momentum. At the same
time they are bright emitters, which is indicative of strong energy
dissipation. This points towards a mechanism of internal dissipation which does
not result in a global disruption of the flow. One possibility is internal
shocks and another one is turbulence driven by local instabilities. Such
instabilities can be triggered when a freely expanding jet is reconfined by
either the cocoon or external gas pressure. In this paper we study the dynamics
of two-component spine-sheath hydrodynamic jets coming into pressure
equilibrium with external gas using 2D computer simulations. We find that the
jet oscillations lead to a rapid onset of Rayleigh-Taylor-type instabilities,
which results in additional internal dissipation and mixing of the jet
components. Although slightly different in details, this outcome holds both for
the heavy-spine-light-sheath and light-spine-heavy-sheath configurations. The
results may provide an explanation to the spatial flaring observed in some AGN
jets on kpc-scales.
