Climatological studies of the quasi 16-day oscillations in the mesosphere and lower thermosphere at Yamagawa (31.2° N, 130.6° E), Japan

The 16-day wave climatology at Yamagawa (31.2degrees N, 130.6degrees E) is investigated by analyzing the mesosphere and lower thermosphere (MLT) wind data collected with the recently installed MF radar. We have utilized the continuous data gathered during the five-year period 1996-2000. The wave climatology clearly shows some seasonal variations. The period of late autumn-spring is marked with larger wave activity, with the strongest waves being observed in the winter months. The maximum amplitude observed at Yamagawa is about 20 m/s, which is comparatively larger than the amplitudes observed at mid-latitude stations. The height dependence of the 16-day wave suggests that the maximum amplitude is observed at altitudes below 80 km. The summer months are characterized with much weaker wave activity. The vertical wavelength appears to be larger in the winter months and shorter in the summer months. The present analysis again confirms that the 16-day wave is highly sensitive to the background mean winds. Eastward motion of the background winds is a more favourable condition for the 16-day wave penetration to the MLT heights. The wave features show some signs of interannual variability. Overall, the observed features of the 16-day wave at Yamagawa, which is located at the edge of the subtropical latitudes, show some correspondence with the results reported for mid-latitude stations.