We examine the stability of two-layer geostrophic flows with large displacement of the interface. The depth of the upper layer is assumed much smaller than the total depth of the fluid. It is proven that all westward and weak eastward flows are stable with respect to disturbances whose wavelengths is of the order of, or longer than the width of the flow. It is further demonstrated that westward flows are (locally) unstable with respect to short disturbances.