Biorthogonal decomposition techniques unveil the nature of the irregularities observed in the solar cycle.

We present a biorthogonal decomposition of the temporal and latitudinal distribution of sunspots recorded since 1874. We show that the butterfly diagrams can be interpreted as the result of approximately constant amplitudes and phases of two oscillations with periods close to 22 years. Our analysis reveals clear evidence of the absence of low-dimensional chaos, at least for the time scales that can be analyzed with this database. This result suggests that the spatiotemporal irregularities observed in the solar cycle are due to the superposition of regular structures with a stochastic background.