  We study the long term evolution of a set of 22 bipolar active regions (ARs)
in which the main photospheric polarities are seen to rotate one around the
other during several solar rotations. We first show that differential rotation
is not at the origin of this large change in the tilt angle. A possible origin
of this distortion is the nonlinear development of a kink-instability at the
base of the convective zone; this would imply the formation of a non-planar
flux tube which, while emerging across the photosphere, would show a rotation
of its photospheric polarities as observed. A characteristic of the flux tubes
deformed by this mechanism is that their magnetic twist and writhe should have
the same sign. From the observed evolution of the tilt of the bipoles, we
derive the sign of the writhe of the flux tube forming each AR; while we
compute the sign of the twist from transverse field measurements. Comparing the
handedness of the magnetic twist and writhe, we find that the presence of
kink-unstable flux tubes is coherent with no more than 35\% of the 20 cases for
which the sign of the twist can be unambiguously determined. Since at most only
a fraction of the tilt evolution can be explained by this process, we discuss
the role that other mechanisms may play in the inferred deformation. We find
that 36\% of the 22 cases may result from the action of the Coriolis force as
the flux tube travels through the convection zone. Furthermore, because several
bipoles overpass in their rotation the mean toroidal (East-West) direction or
rotate away from it, we propose that a possible explanation for the deformation
of all these flux tubes may lie in the interaction with large-scale vortical
motions of the plasma in the convection zone, including also photospheric or
shallow sub-photospheric large scale flows.
