Parity-breaking transitions of modulated patterns in hydrodynamic systems

A model for subcritical parity-breaking bifurcation of a periodic pattern is introduced. It is found that nucleated regions of the new asymmetric state propagate with a well-defined velocity in a direction determined by their parity, and leave in their wake a pattern with an altered wavelength. Successive passage of these ``parity bubbles'' enables the system to relax to a selected wavelength. The possible relevance of these findings to recent observations of ``solitary modes'' in directional solidification and hydrodynamic experiments is discussed.