Quasi-stationary planetary waves exhibit different seasonal behaviour in the two winter stratospheres. Whereas, in a climatological sense, wave amplitudes are large throughout northern winter, in the Southern Hemisphere there is a climatological minimum in midwinter. It is suggested here that the southern hemisphere behaviour is basically linear, the midwinter minimum arising from the opacity of the strong westerlies of southern midwinter to stationary wave propagation. On the other hand, it is further suggested that, in the northern hemisphere winter, the westerlies are prevented from becoming so strong (in a climatological sense) by the action of the waves themselves on the mean state and that the penetration of large-scale waves into the midwinter northern stratosphere thus depends on a nonlinear feedback process. Preliminary tests of this hypothesis are conducted, using a highly truncated beta-plane model of the stratospheric flow.