(a)
(b)
Figure 2: The spectral decomposition of the (a) entire 67-year time series of the PNA index and (b) a subset for 1000–1500 waves corresponding to our time frame. The abscissa is wave number 1000 to 1500, and the ordinate is spectral power. The (a) green dashed and (b) blue dotted lines are the 95% confidence level using a red or white noise spectrum, respectively.