  The deceleration mechanisms for relativistic jets in active galactic nuclei
remain an open question, and in this paper we propose a model which could
explain sudden jet deceleration, invoking density discontinuities. This is
particularly motivated by recent indications from HYMORS. Exploiting high
resolution, numerical simulations, we demonstrate that for both high and low
energy jets, always at high Lorentz factor, a transition to a higher density
environment can cause a significant fraction of the directed jet energy to be
lost on reflection. This can explain how one-sided jet deceleration and a
transition to FR I type can occur in HYMORS, which start as FR II (and remain
so on the other side). For that purpose, we implemented in the relativistic
hydrodynamic grid-adaptive AMRVAC code, the Synge-type equation of state
introduced in the general polytropic case by Meliani et al. (2004). We present
results for 10 model computations, varying the inlet Lorentz factor from 10 to
20, including uniform or decreasing density profiles, and allowing for
cylindrical versus conical jet models. As long as the jet propagates through
uniform media, we find that the density contrast sets most of the propagation
characteristics, fully consistent with previous modeling efforts. When the jet
runs into a denser medium, we find a clear distinction in the decelaration of
high energy jets depending on the encountered density jump. For fairly high
density contrast, the jet becomes destabilised and compressed, decelerates
strongly (up to subrelativistic speeds) and can form knots. We point out
differences that are found between cylindrical and conical jet models, together
with dynamical details like the Richtmyer-Meshkov instabilities developing at
the original contact interface.
