Gravity waves in the upper mesosphere over Antarctica: Lidar observations at the South Pole and Syowa

Lidar observations of the mesospheric Na layer were made at the south pole (90 degrees S) and Syowa (69 degrees S) during the winters of 1990 and 1985, respectively. These observations are used to characterize the gravity wave activity in the upper mesosphere at both sites. Strong wave activity is observed throughout the winter at both the south pole and Syowa and shows remarkable similarity with observations from several midlatitude and low-latitude sites. The quasi-monochromatic gravity waves exhibit the same general relationships between their wavelengths, observed periods, and amplitudes as observed at lower latitudes. The average growth length of these waves is approximately 26 km, indicating that the wave field at both Antarctic sites is strongly influenced by dissipation and saturation processes. The spectra and variances of the density perturbations associated with quasi-random wave field at the south pole are reported. The vertical wavenumber and temporal frequency spectra follow power-law shapes. The mean index of the vertical wavenumber spectrum is -2.4, and the mean characteristic wavelength is 14 km. The mean index of the temporal frequency spectrum is -1.7. The mean density variance at the south pole is (5.7%)(2) and is similar in magnitude to that observed at a variety of lower-latitude sites. With no tropospheric convection during the polar night and little orographic forcing over the relatively featureless Antarctic plateau, these observations suggest that nonlinear processes, rather than the source characteristics, primarily determine the characteristics of the gravity wave field in the upper mesosphere. These observations show two other distinct features. The mean Na layer over Antarctica is significantly lower and broader (centroid height approximate to 90 km and rms width approximate to 4.8 km) than at lower latitudes, reflecting the stronger downwelling and warmer winter temperatures in the mesopause region at high latitudes. Strong coherent oscillations were observed in the bottomside density contours of the Na layer with periods close to the inertial period. These oscillations were also observed in OH airglow measurements and appear to be associated with planetary scale waves.