Power spectrum of observed (a) ENSO and (b) PDO indices from HadISST for the period 1900–2014. (c–h) Same as (a,b), but from the different CESM model experiments with coupled ocean, i.e. Natural, FixSolar and NOQBO. The thick black lines indicate the best fit based on a first-order autoregressive model, whereas the dashed black lines indicate the 95% confidence level. The dashed vertical lines indicate time periods of 9 and 13 years. Influences from volcanic aerosols have been removed by a linear regression before applying the FFT analysis.