Evidence that RK modes accelerate superrotation in a nonlinear axisymmetrically forced run with Ro_T = 10 and T_rad = 25. Left: cospectrum of upper-level EMFC in (latitude, phase speed) space. The solid blue line shows the upper-level zonal-mean zonal wind. The dashed black line shows the phase speed associated with a constant frequency ω = 0.23 and wavenumber k = 1. Right: upper-level zonal geopotential (shading) and zonal wind (arrows) filtered for k = 1 and ω = 0.23.