Growth rate σ_r versus vertical wavenumber k_z at k_x = 0.445 for different Péclet numbers Pe: Pe = ∞ (black solid line), Pe = {100,10,5,1,0.5,0.1,0.01} (gray solid lines) and other parameters: panel a: N = 2, N/f = ∞, panel b: N = 2, N/f = 0.1, panel c: N = 0.5, N/f = 1. Dashed lines denote the growth rate for unstratified case N = 0 at panel a: f = 0, panel b: f = 20, panel c: f = 0.5. Arrows indicate the direction of decreasing Pe.