Abstract: The vorticity equation and adiabatic equation are converted into a set of nonlinear ordinary differential equations which have been integrated on the electronic computer by using the Runge-Kutta method. The results obtained show that the basic current and disturbances constrain each other, that the basic current alters quasi-periodically with periods of 10 to 13 days, and that the rate of change of basic current in each cycle is .quite different. The disturbances of wave number six vary sinusoidally with a period of 11 to 13 days, the evolution of the disturbances of wave number four is complicated, and the disturbances of wave number two has a period of variation of about 3 days. T'he disturbances of wave number six develop or damp at the upper and lower levels simultaneously, but the evolution of the disturbances of wave number four or two at the upper and lower levels have the opposite trend. Finally, the results obtained show that the solutions of different initial values possess quite different mean periods of index cycles.