Mesospheric gravity waves at Saskatoon (52°N), Kyoto (35°N), and Adelaide (35°S)

The gravity waves with periods of 10 min to 8 hours in the mesosphere (67–80 km) are studied using observations from the Adelaide MF radar (35°S), Saskatoon MF radar (52°N), and the middle and upper atmosphere (MU) radar at Shigaraki, near Kyoto (35°N). The seasonal variations of the gravity wave intensity deduced from the horizontal wind velocities showed semiannual variations with solstitial maxima and equinoctial minima at all three locations, but the month of maximum intensity in the year depends on the locations and wave frequency. The kinetic energy due to gravity waves showed similar amplitude between Kyoto and Adelaide, but smaller at Saskatoon, which agrees well with the results of the preliminary comparison using a limited frequency range and limited observation periods by Nakamura et al. [1993a, b]. The horizontal propagation direction of these gravity waves showed small seasonal variations and significant dependency with wave periods at Saskatoon but showed more frequent variation without a large frequency dependency at Kyoto and Adelaide. These characteristics indicate that at the lower latitudes gravity wave activity is larger, as pointed out by Tsuda et al. [1994] from various observational techniques. Also, the frequent changes of the gravity wave intensities at the lower latitudes suggest that there are strong influences of the background wind variation in the middle atmosphere such as semiannual oscillation, mesospheric semiannual oscillation and other wind variations in the low latitudes.