Abstract: A purely tonal current is very rare in the atmosphere. In the mean maps planetary waves are very clear. These mean waves indicate that in the atmosphere there exist large-scale disturbances whose geographic positions are fairly fixed. The analysis of the spectra of the atmospheric waves proves this inference. Thus the basic state of the atmospheric circulation should be a wave-shaped westerly current, but not a purely tonal one. This paper thus investigates the dynamics of disturbances in wave-shaped westerlies. The equation for the disturbed motion shows that in such a basic current the disturbances of different wave lengths mutually interact. This does not happen in a purely tonal basic current.Several rules governing the mutual interaction between the disturbances of different wave lengths are given. The evolution of an initially single-wave disturbance in the wave-shaped basic westerlies is also studied. The theoritical calculations give several phenomena which agree very well with the observations. It is especially worth mentioning that the deepening process of a trough is very like that of the difluent trough of Bjerknes.