The instability of a forced, circular shear layer in a rotating fluid has been studied experimentally and numerically. The experiments were performed with a shallow layer of water in a parabolic tank, in which it is possible to apply radial pumping and to model a geophysical beta-effect. A shear layer was produced by a secondary rotation of the central part of the parabolic vessel. In most experiments, the shear layer takes on the appearance of a sequence of vortices, the number of which decreases with increasing strength of the shear. A beta-effect may prevent the formation of a steady vortex chain. Continuous pumping of fluid from the periphery to the centre or vice versa leads to an azimuthal velocity field corresponding to a point vortex. This azimuthal flow appears to stabilize the shear flow if it is opposite to the inner rotation, and to be destabilizing otherwise.

The numerical investigations consist of the solution of the quasi-geostrophic equation in a geometry similar to the experimental situation and with a term modelling the experimental forcing. Though the numerical computations are based on a two-dimensional model, they capture the essential features of the instability and the resulting vortex structures.