  Most collimated supersonic protostellar jets show a collimated wiggling, and
knotty structure (e.g., the Haro 6-5B jet) and frequently reveal a long gap
between this structure and the terminal bow shock. In a few cases, there is no
evidence of such a terminal feature. We present three-dimensional smoothed
particle hydrodynamical simulations which suggest that this morphology may be
due to the interaction of the propagating cooling jet with a non-homogeneous
ambient medium. In regions where the ambient gas has an increasing density (and
pressure) gradient, we find that it tends to compress the cold, low-pressure
cocoon of shocked material that surrounds the beam, destroy the bow shock-like
structure at the head, and enhance beam focusing, wiggling, and internal
traveling shocks. In ambient regions of decreasing density (and pressure), the
flow widens and relaxes, becoming very faint. This could explain ``invisible''
segments in systems like the Haro 6-5B jet. The bow shock in these cases could
be a relic of an earlier outflow episode, as previously suggested, or the place
where the jet reappears after striking a denser portion of the ambient medium.
