Power spectra of horizontal velocities near the surface extended to negative azimuthal orders (m <  0). (a) Equatorial power spectrum of north-south symmetric component of v_θ near the top boundary r = 0.95 R_⊙, which is the same as Fig. 7b. The power is normalized at each m. Shown in red points are the frequencies of the mixed Rossby modes obtained from our linear analysis. The blue line represents the advection frequency of the equatorial differential rotation, m[Ω(0.95 R_⊙,π/2)−Ω₀]. (b) Equatorial power spectrum of north-south antisymmetric component of v_ϕ near the top boundary r = 0.95 R_⊙.