  We study the creation and breakdown of a quantized vortex shear layer forming
between a stationary Bose-Einstein condensate and a stirred-in persistent
current. Once turbulence is established, we characterize the progressive
clustering of the vortices, showing that the cluster number follows a power law
decay with time, similar to decaying turbulence in other two-dimensional
systems. Numerical study of the system demonstrates good agreement of the
experimental data with a point vortex model that includes damping and noise.
With increasing vortex number in the computational model, we observe a
convergence of the power-law exponent to a fixed value.
