Panel a: contours of the maximum growth rate σ_max in the parameter space of (k_x, k_z) for f = 0.5, N = 1, Pe = 0.1. Panels b and c: corresponding growth rates σ_r versus vertical wavenumber k_z at panel b: k_x = 0 and panel c: k_x = 0.445. For clarity, only the first four eigenvalue branches are plotted. The dashed line in panel c denotes the growth rate for rotating case f = 0. Symbols denote the parameters for eigenvalues in Fig. 2 and eigenfunctions displayed in Fig. 4.