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

Abstract

Interannual and day-to-day variations of gravity wave activities in the lower stratosphere were examined using operational rawinsonde data over eastern part of Japan (27°N-45°N, roughly aolng∼140°E) during 1989-95. The gravity-wave activities were quantified by variances of temperature and zonal wind fluctuations with vertical scales of 2-6km.
Active gravity waves were confirmed to appear around the 20km altitude level over Akita (∼40°N) every winter throughout the seven year period. It was found that the gravity wave activities observed in this region have remarkable interannual and day-to-day variations. The interannual variation of temperature variance at about the 20km level over Akita in winter shows ∼1.5 times increase from 1990/91 to 1994/95 together with the increase in the background zonal wind. On the other hand, a clear relationship was not observed between the temperature variance and the background Väisälä-Brunt number.
The day-to-day variation of the gravity wave activity in the same region (around the 20km level over Akita in winter) is characterized by intermittent 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 20km level was intensified when zonal wind near the surface is strong. The similar features were also found at other stations over the northeastern and northern part of Japan. The results suggest that the fluctuations that appear around 20km over northeastern and northern Japan every winter, are due to mountain waves excited by strong zonal wind flowing over north-south running mountain ranges.