Seasonal changes in baroclinic wave activity during northern autumns (Ls = 195-225°) and winters (Ls = 280-300°) are studied with a Martian general circulation model. For the weak dust load, the simulated s = 2 harmonic with 3.1 sols period dominates near the surface, while s = 1 (5.5 sols period) is more prominent at higher altitudes during autumns. In winters, the s = 1 wave has stronger amplitudes and a deep (up to 0.1 mb) vertical structure. These simulations are consistent with observations from MGS. We provide a simple explanation for the planetary wave changes in terms of seasonal variations of the background zonal mean fields. In particular, the observed and simulated significant reduction of the baroclinic wave activity during strong solstitial season dust storms occurs due to the stabilization of the jet stream with respect to baroclinic disturbances and due to the associated weakening of the wave excitation.