We decomposed the space-time distribution of sunspot areas
over 1759 Carrington rotations by applying the natural orthogonal
composition technique, whose primary characteristics
is the identification of the most suitable set of basis orthogonal
functions (the empirical orthogonal functions − φk). Results
obtained by using this technique confirm previous findings of
a T1 ∼ 11 yr periodicity (found in the first two PCs), which
characterizes the long-term evolution of the solar cycle. A less
prominent periodicity was found to correspond to the second
harmonic (T2 ∼5.5 yr). However, while the periodicity at T1
agrees with similar results for the solar magnetic cycle (Mininni
et al. 2002, 2004; Lawrence et al. 2005), the one at T2 emerges
only after studying the 11-year cycle.