Power spectrum of south-north travel-time shifts computed in the Earth’s frame. The blue line highlights the dispersion relation of sectoral modes of classical Rossby waves in this frame. The mode frequencies (red circles) are shifted by m × 421.41 nHz when measured in the Earth’s frame. The low-frequency power from active regions or convection (near zero frequency at low m in Figs. 6 and 7) is also shifted to m × 421.41 nHz (green crosses) when measured in the Earth’s frame.