  In this Letter, we report on the effects of a vorticity filament on the
coherent backscattering cone. Using ultrasonic waves in a strongly
reverberating cavity, we experimentally show that the discrete number of loops
of acoustic paths around a pointlike vortex located at the center of the cavity
drives the cancellation and the potential rebirth of the coherent
backscattering enhancement. The vorticity filament behaves, then, as a
topological anomaly for wave propagation that provides some new insight between
reciprocity and weak localization.
