Normalized power spectra of zonally-averaged data at four selected latitudes for Flh (a) and Fsh (b).
Dashed vertical lines mark the range of frequencies corresponding to periods of 25–30 days. In (a), all peaks in this range lie above the upper 95% prediction bounds (Supplementary Fig. 16); in (b), only the peak for the 50°S-spectrum. See Supplementary Fig. 18 for corresponding spectra of other variables.