3 Frequency analysis of the post-1982 data

A Fourier frequency analysis of the data shown in Fig. 5 (omitting the Percy & Welch data) was carried out in the frequency range 0.0001-0.075 cycles-per day (cd^-1) using the Period software (Sperl 1998). The amplitude spectrum and the spectral window are given in Fig. 8. Almost no power is visible in the period range 15-50 days (see Sect. 4), but there appear a number of strong peaks at frequencies below 0.006 cd^-1, the strongest being at 0.00030 cd^-1 ( $P=\sim 3100$ d) with a corresponding maximum in the spectral window, the second and third-strongest at 0.0057 (P=175 d) and 0.0020 (P=500 d), respectively. The group of the 100d-type variations is represented by peaks between 0.011 and 0.014 cd^-1. A peak at 0.0006 cd^-1(P=1600 d) seems to correspond to the period of the wave-like pattern seen in Fig. 5.