Quasi-biennial oscillation in sunspot activity

A 233-year series of monthly means of relative sunspot numbers for the period 1749-1981 is analyzed. Different statistical methods are used: autocorrelation, power spectrum and complex demodulation analysis. It is found that there exists a quasi-biennial oscillation with a basic period of 25.6 months. The lifetime of the quasi-biennial oscillation is one solar cycle and contains five cycles of oscillations and, for most solar cycles, the oscillation is formed at the beginning of the solar cycle and is destroyed at the end. The time variations of amplitude and oscillation period during the solar cycle are analyzed as well.