Diagnostic analyses are done for the equilibrium of enstrophy and its nonlinear cascade in the wavenumber domain for each season and different latitude zones by using the 500 hpa pentad average data for 1979. Results indicate that in winter and spring super-long waves (wavenumber 1-3) and Rossby long waves(wavenumber 5-6) are suppliers of enstrophy for the other waves and the zonal flows in middle and higher latitudes. In summer, Rossby waves are no longer the enstrophy "source", which shifts to shorter waves. Meanwhile, the wavenumber 4 component, which is the enstrophy "sink" in winter and spring, now becomes the "source" In autumn, the wavenumber 4-5 components are the "source", a transitional pattern from summer to winter superlong waves are the "source" in all seasons but their distribution differs with latitudes The width is only 20 latitudes, in about 37.5-57.5°N in winter and shifting northward about 10-15 latitudes in summer. In the subtropics south of 35°N, super-long waves are the enstrophy "sink" in almost all seasons.