By analyzing reanalysis/assimilation data and CCSM simulations, we propose that the ENSO cycle consists of two different oscillators: a central Pacific oscillator which is primarily forced by atmospheric forcing and an eastern Pacific oscillator which is resulted from the air-sea interaction involving the thermocline variation in the cold tongue. The warm and cold phases of these two oscillators are not exactly symmetric in their spatial structures. The asymmetric parts result in a net ENSO forcing to the basic state which gradually shifts the Pacific Walker circulation eastward or westward from its normal location. As the basic state change, the ENSO alternates between the eastern Pacific oscillator and the central Pacific oscillator. These ENSO-basic state interactions give rise to a 12-15-year modulation cycle of ENSO intensity. The decadal modulation cycle can be identified from not only the reanalysis data, but also proxy-reconstructed ENSO indices and CCSM simulations. The physical mechanism behind the modulation is further examined with both reanalysis and CCSM experiments.