Vertical wavelengths of propagation of the 12-hour oscillation of the upper atmosphere have been optically determined from measurements of horizontal motions derived from the Doppler shifts of [OI> at 557 nm and OH(6-2) at 840 nm and from the [OI> and OH(8,3) heights of emission measured by the WINDII experiment. These determinations, at 43.98¿ S, 170.42¿ E, have established the existence of a large, brief, yet persistent, enhancement in the vertical wavelength of propagation near the austral autumn equinox. The typical vertical wavelengths of oscillation during the year are in the range 20 km to 40 km, except during the brief period near the austral autumn equinox when they increase to about 100 km. These findings indicate a change in the atmospheric structure at those heights, favoring the presence of longer vertical wavelength modes near the austral autumn equinox. ¿ American Geophysical Union 1995