  We investigate the effects of interatomic interactions and expansion on the
distortion of interference fringes of a pair of initially well-separated, but
coherent, condensate clouds trapped in a harmonic trap. The distortion of
interference fringes, which can lead to the spontaneous formation of vortices
in the atom clouds, depends crucially on two relevant parameters: the
center-of-mass velocity and peak density of the initial state. We identify
three qualitatively distinct regimes for the interfering condensates:
collision, expansion, and merging, by the spatial and temporal features of the
fringe spacings. Using a comprehensive set of numerical simulations based on
the Gross-Pitaevskii equation, we specify the cross-overs between these regimes
and propose the optimal the system parameters required for dynamical
instabilities and vortex creation.
