Frequencies of the retrograde-propagating inertial modes in the CZ (dashed coloured; the same as in Fig. 4) overplotted with the dispersion relations of the RZ Rossby modes (solid grey; given by Eq. (14)). The left and right panels correspond to the modes with north–south symmetric and antisymmetric vorticity, respectively. Open circles mark the subset of RZ Rossby modes that can form mixed modes with the CZ inertial modes.