Left: amplitude of the meridional velocity fluctuations Fourier transform in the (k_x-time) plane for model HighRes, spatially averaged over the region |y| < 5 × 10⁷ m. Contours are for , , and . The region surrounding the maximum is filled in gray. Right: same as the left panel, but showing the time evolution of particular modes: k_x = 5 (thick solid line), k_x = 7 (dashed line,) and k_x = 9 (dot-dashed line). The straight dashed line represents an exponential growth with a timescale of one day.