(a) Optimal wavenumbers k_z, c from (87) (solid lines) and k_z, c0 from (90) (dashed lines) as a function of Re at 2Ω = 1, N = 4, and k_x = 0 for colatitudes θ: 30° (blue), 45° (red), and 60° (green). Filled circles denote the critical Reynolds numbers. (b,c) Critical Reynolds number Re_c as a function of the absolute Coriolis parameter 2Ω for N = 4, and (b) Pr = 1, (c) Pr = 10⁻⁶ at different colatitudes θ: 0° (black), 30° (blue), 45° (red), and 60° (green) from numerical results (crosses) and asymptotic results of Re_c (solid lines in b) from (88) and Re_c, 0 (dashed lines in c) from (91).