Equatorial power spectrum of latitudinal velocity v_θ near the surface r = 0.95 R_⊙. The spectra are computed in a frame rotating at Ω₀/2π = 431.3 nHz. The blue solid line represents the differential rotation rate at the surface ω = m(Ω_sf − Ω₀) where Ω_sf = ⟨Ω(0.95 R_⊙, π/2)⟩. The red points denote the theoretical dispersion relation of the sectoral Rossby modes, ω = −2Ω_sf/(m + 1)+m(Ω_sf − Ω₀).