The crossover of the most unstable mode of |$m=1$| from the regime of |$|\omega _{A\theta }/\Omega | \ll 1$| to that of |$|\omega _{A\theta }/\Omega | \sim 1$| in the case of |$Rb=-1/2$|⁠. The maximum growth rate occurs at |$k \to \infty$| (solid line) in the former regime but at |$k=0$| in the latter (dashed line), and they are simultaneously shown as a function of |$\omega _{A\theta }/\Omega$|⁠. The flow is Keplerian (⁠|$Ro=-3/4$|⁠). In both limits, the maxima are taken at |$|\alpha |=1$|⁠.