Interannual and day-to-day variations of gravity wave activity in the lower stratosphere over the eastern part of Japan observed in winter 1989-95

Interannual and day-to-day variations of gravity wave activities in the low er stratosphere were examined using operational rawinsonde data over easter n part of Japan (27 degrees N-45 degrees N, roughly along similar to 140 de grees E) during 1989-95. The gravity-wave activities were quantified by var iances of temperature and zonal wind fluctuations with vertical scales of 2 -6 km. Active gravity waves were confirmed to appear around the 20 km altitude lev el over Akita (similar to 40 degrees N) every winter throughout the seven y ear period. It was found that the gravity wave activities observed in this region have remarkable interannual and day-to-day variations. The interannu al variation of temperature variance at about the 20 km level over Akita in winter shows similar to 1.5 times increase from 1990/91 to 1994/95 togethe r with the increase in the background zonal wind. On the other hand, a clea r relationship was not observed between the temperature variance and the ba ckground Vaisala-Brunt number. The day-to-day variation of the gravity wave activity in the same region (a round the 20 km level over Akita in winter) is characterized by intermitten t appearance with a periodicity of several days. Cross-correlation analyses between the temperature variance and the background zonal wind velocity at each altitude level, showed clearly that gravity wave activity around the 20 km level was intensified when zonal wind near the surface is strong. The similar features were also found at other stations over the northeastern a nd northern part of Japan. The results suggest that the fluctuations that a ppear around 20 km over northeastern and northern Japan every winter, are d ue to mountain waves excited by strong zonal wind flowing over north-south running mountain ranges.