On review of the theories of wave-mean flow interaction and non acceleration, theorem, and the corresponding numerical analyses, the dynamical mechanism of variation of westerly index is discussed. It is pointed out that the first and active causes of the time variation of westerly index are tne internal forcing induced by wave-mean flow interaction and external forcing due to the existence of sources of momentum and heat, and its second and Passive-cause is the mean meridional circulation induced by the above-mentioned internal and external forcing. The possibility of forecasting westerly index by employing highly simplified dynamic systems is also discussed.