Parameter study (2D model) for different values of the turbulent viscosity ν_t, for a convection zone that is adiabatically stratified (δ = 0). The modes are those shown in Fig. 1. The frequencies of the m = 1 high-latitude mode and the m = 2 critical-latitude modes are sensitive to the choice of ν_t. The smaller values of ν_t (≤100 km² s⁻¹) give a better agreement with the observed frequencies (respectively −86.3 nHz and −73.4 nHz). The m = 3 equatorial Rossby modes is essentially insensitive to ν_t.