  Large-scale collective oscillation is discovered in the two-dimensional Euler
equations. For initial conditions far from a base stationary flow, the system
does not relax to the base stationary flow, but instead shows pairs of coherent
vortices moving along the base stream line, which leads to large-scale
oscillatory fields. The investigation of the vicinity of a bifurcation point
suggests that this oscillation appears through Hopf bifurcation. Furthermore, a
dynamic self-consistent theory explains that this oscillation results from the
collective organization of a state of self-oscillation.
