An experimental and analytical study into the vortex dynamics of a stratified shear layer subjected to a spatial acceleration is presented. The outer flow is dictated by a hydraulically controlled wedge flow which provides a spatially accelerating shear layer and baroclinic generation of vorticity along the inclined interface. A new, finite-amplitude mechanism is observed in which the core of the growing vortex is separated from the vorticity source at the interface. A secondary core develops and an altered vortex pairing interaction is observed. A spatial linear stability analysis reveals that one of two modified Kelvin–Helmholtz modes is dominant, resulting in the centre of the instability being offset from the density interface into the slower moving stream. Digital particle imaging velocimetry (DPIV) measurements are presented along with flow visualization which indicate that the mechanism is a result of the offset in the vortex core from the source of vorticity at the interface combined with the effects of spatial acceleration and buoyancy. The mixing induced by the interfacial instabilities is such that a sharp density interface remains near the high-momentum stream, with a low-gradient region extending into the low-momentum stream.