In this study, energy spectrum of the large-scale atmospheric motions is examined in the framework of the 3D normal mode decomposition. The horizontal scale of disturbance is measured by the phase speed of a Rossby mode c. According to the analysis result for the barotropic component of the atmosphere, we obtain a characteristic energy spectrum with distinct slopes for the turbulence and wave regimes separated by the spherical Rhines speed. In order to explain the observational finding that the energy spectrum is proportional to c², we put forward a hypothesis based on the criterion of Rossby wave breaking such that the local meridional gradient of potential vorticity becomes negative, ∂q/∂y <0, somewhere in the domain. With a constant m describing a total mass of the atmosphere for unit area, we have shown that the barotropic energy spectrum of the general circulation E can be represented as E=mc².