Spatial structure of the 12-hour wave in the Antarctic as observed by radar

We present radar measurements of the 12-hour wave, a zonal wavenumber 1 wes tward propagating wave that exists in the southern polar mesopause region w inds (Hernandez er al., 1993; Forbes et at, 1995). MF radar measurements of the horizontal winds at McMurdo (77.8 degrees S, 166.67 degrees E) show th at the 12-hour wave is highly seasonal, occurring during the austral summer solstice. During these seasonal occurrences, the wave is highly intermitte nt with amplitude peaks of greater than or similar to 30 m s(-1). The burst -like occurrences of large 12-hour wave amplitudes are highly correlated be tween the zonal and meridional direction. The diurnal tide over McMurdo has a more constant amplitude, but it is also an almost exclusively summertime phenomenon. Inertia-gravity wave activity is evident at periods less than 12 hr during the austral winter months. The weakening of gravity wave activ ity during the summer is probably due to critical layer filtering by the zo nal mean wind, 12-hour wave and diurnal tide which are all strong during th is season. The 12-hour wave is confined in height to the vicinity of the ze ro crossing in the zonal winds above the westward jet. Extreme distortion i s observed in the vertical phase fronts of the 12-hour wave which could sig nify either refraction or in situ forcing. The distortion in the phase fron ts and localization of the 12-hour wave in time and height is apparently re sponsible for departures in period from the nominal 12 hours. We do not fin d the wave period to be systematically different from 12 hours. The associa tion of the 12-hour wave events with shear in the mean wind suggests that r efractive effects could conceivably cause a dilation in wave amplitude. How ever, the shear is of the opposite sign to cause this dilation unless the w ave originates at higher altitudes and propagates downward into the mesosph ere. Investigations are made of the zonal structure of the 12-hour wave by comparing phases of the 12-hour wind component between McMurdo and the dyna sonde at Halley (75.8 degrees S, 26.4 degrees W). The phase is found to be stable and consistent with a westward propagating zonal wavenumber 2 struct ure during seasons when the 12-hour wave is weak. The migrating semidiurnal tide evidently dominates during these times of the year. During seasons wh en the 12-hour wave amplitude is large, the zonal structure is highly unsta ble and there is not an obvious dominant zonal wavenumber.