Power spectra of south-north travel-time shifts measured in the frame rotating at Ω_eq/2π = 453.1 nHz. Panels a–c: from three time periods. Panel d: mean spectrum from an average of the three (seven years each). The blue lines highlight the dispersion relation of the classical Rossby waves described in Eq. (1) with ℓ = m and Ω = Ω_eq. The orange ellipse marks the excess low-frequency power at low m and the purple ellipse marks the spectral leakage from the low-frequency power. The gray scale is the same for the four panels and is shown in the color bar on the right. For clarity, the spectra are rebinned in frequency by a factor of three, such that the frequency resolution is 3/T = 13.6 nHz.