Fig. 5.

The ENSO frequency cascade: deterministic transfer of power from interannual to higher frequencies. (A) Power spectral density for the normalized 1958–2013 HadISST1 N3.4 index (black line) and the anomalous JRA-55 surface stream function NWP-AC(t) index (orange line) using the Welch method. (B) FFT power spectral density for the ENSO forcing of each individual frequency experiment (thin black lines), and the anomalous ensemble mean circulation index $\overline{\text{NWP}-\text{AC}\left(\text{t}\right)}$ for each individual frequency experiment (thin blue lines). Power spectral density of the anomalous ensemble mean circulation index $\overline{\text{NWP}-\text{AC}\left(\text{t}\right)}$ of the 1958–2013 experiment using the Welch method (red line). All of the Welch power spectra are scaled by the same factor to approximately match the power estimate of the FFT spectra for better visualization.