Long-period wind oscillations in the mesosphere and lower thermosphere at Yamagawa (32°N,13°E), Pontianak (0°N, 109°E) and Christmas Island (2°N, 157°W)

Abstract

From daily mean winds observed with the MF radar in the mesosphere and lower thermosphere (80–100 km) at Yamagawa (32°N,130°E) in 1996–1997, we have analyzed periodic oscillations of zonal and meridional wind velocity with a period of 4–10 days and 12–20 days. These oscillations are interpreted as planetary wave or Rossby normal mode. Such oscillations have also been observed by other MF radars located in equatorial regions: Pontianak (0°N,109°E) and Christmas Is. (2°N,157°W). Enhanced wave activity sometimes continued simultaneously a few tens of days at three sites: moreover, the envelope of the intensity of these waves was generally similar among the different coordinates. Therefore, these oscillations are interpreted as global scale planetary waves, several of which have the characteristics of Rossby normal modes. In order to identify the zonal wave number and latitude structure for each wave event, we have analyzed the coherence and phase difference of wind oscillations with periods of 4–10 days among the three radar sites. Oscillations at the period of 4–10 days are explained by 5-day and 4-day waves which are the equatorially symmetric first mode having zonal wave number 1 and 2, respectively. In particular, a coherent wave event was identified in three observation periods. From the middle of April to the middle of May, 1996, the 4–10 day oscillation can be interpreted as the 5-day wave of the Rossby normal mode. However, from the middle of July to the end of August, 1996, the oscillation seems to be a manifestation of the 4-day wave. On the other hand, from the end of September to the end of October 1996, the oscillation could not be simply explained as a normal mode. A similar analysis was applied to 12–20 day wind oscillations. The latitude and longitude structure of the 12–20 day oscillation from the middle of January to the end of March, 1996, seems to be explained by that for a 16-day wave, having zonal wave number 1, which is called the equatorially symmetric second Rossby normal mode. From July to the middle of October, 1996, we were unable to clearly identify a specific mode for the 12–20 day oscillations. Amplitudes of the 12–20 day oscillation were large during days 185–285, 1996, at Yamagawa when these waves were enhanced at Pontianak and Christmas Is. This result suggests that an interhemispheric penetration of the 16-day wave occurred through a weak mean wind region over the tropics.