Fluid field dissolution shows that maintenance and evolution of the mean circulation can be studied by analyzing kinetic energy equlibrium. 500-mb pentad height data from 1972 through 1980 are used for calculating mean zonal kinetic energy, eddy kinetic energy and the rate of their conversion. The results indicate that the inter relations between the three parameters for kinetic energy of the mean pentad circulation as well as their seasonal and other periodical variations are analogous to, but smaller than, those of the instantaneous circulation. Analyzing the sequence of the parameters by the power and cross spectra yields main periodic changes of quasi-two-week oscillation. It is then found from the wave spectrum and mechanism of kinetic energy parameters that such oscillation is behavior of extra-long-wave activity in energy cycle. The primary process consists in the peak value of kinetic energy conversion occurring 3—4 days after the eddy energy has attained a peak one and in another 3—4 days the zonal average kinetic energy exhibits the same one.