“…Once the dark material reaches the midlatitude, the meridional shear of the zonal wind associated with the midlatitude jet stretches the dark feature in the northeast‐southwest (northwest‐southeast) direction in the southern (northern) hemisphere, forming the Y shape. Since the meridional velocity amplitude of the Rossby wave is comparable to the mean meridional velocity, which is estimated to be 1–5 m/s (e.g., Lee, Imamura, et al, ; Lee, Titov, et al, ; Rossow et al, ; Takagi et al, ), the poleward flow undergoes large oscillation, thereby creating the undulation of the boundary between the dark region on the low‐latitude side and the bright region on the high‐latitude side (Figure ). Furthermore, since the velocity of the poleward advection of the dark material is variable depending on the phase relationship between the Kelvin wave and the Rossby wave, the slope of the tilted dark band becomes variable as seen in the mosaic images given by Del Genio and Rossow ().…”