The formation of compound vortices from a ‘shielded’ vortex monopole embedded in a two-dimensional background straining flow is studied numerically. The computations are performed in plane polar coordinates using an infinite radial domain Navier–Stokes solver. The straining flow excites an azimuthal instability, leading to vortex tripole formation in the case of small strain rates. For larger strain rates, tripole formation proceeds briefly before the structure is destroyed by the dominant background straining flow.