Parameter study (2D model) for different values of the superadiabaticity δ, at fixed ν_t = 100 km² s⁻¹. The modes are those shown in Fig. 1. The spiral patterns in u_ϕ of the m = 1 high-latitude and m = 2 critical-latitude modes are sensitive to a small change in δ. To obtain a pattern consistent with the observations, δ < 2 × 10⁻⁷ is implied. The case δ = 10⁻⁶ is excluded by both the eigenfunctions and the eigenfrequencies. The m = 3 equatorial Rossby mode is almost independent of δ because it is nearly purely horizontal (quasi-toroidal).