Weak vorticity can behave in many circumstances like a passive scalar, being
carried in the fluid flow and subject to enhanced dissipation through the
interaction of molecular diffusion and stretching of fluid elements. However
vorticity is coupled into the flow field, and this can cause interesting
effects. We discuss how the spiral wind-up of weak background vorticity by a
coherent vortex can lead to vortex motion, and compare analysis and numerical
simulation.