Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability

Radiosonde observations made at Cocos Islands (12 degrees S, 97 degrees E) in the Indian Ocean between September 1992 and June 1998 are used to study seasonal and interannual variations in gravity wave activity in the lower stratosphere (1825 km). The islands are located in a region of generally strong convection that occurs at all times of the year, with the period of strongest convective activity between December and July (wet season). The prevailing zonal winds during the observational period and height range are westward with a quasi-biennial oscillation (QBO) superimposed. Time series of wave energy show that largest wave amplitudes occur during the wet season when convection is strongest, but a QBO-like variation is also apparent. Maximum energy densities of about 25 J kg-l occur early 1993, 1995, and 1997 at the times when the westward shears are largest. Wave energy is found to be propagating upward, and in the horizontal there is considerable azimuthal anisotropy, with predominate eastward propagation against the prevailing wind. Upward fluxes of zonal momentum flux (<(u'w')over bar>) are estimated by combining the temperature and wind information. Fluxes show a similar temporal behavior to the energy. The motion and temperature fields are dominated by waves with vertical wavelengths similar to 2 km. Using a Stokes parameter analysis, it is found that the intrinsic frequencies are, on average, 2-3 times the inertial frequency, corresponding to intrinsic periods of 20-25 hours. Horizontal wavelengths between 200 and 2000 km are inferred, with a mean value of about 1000 km. The mean intrinsic phase speeds are about 10 ms(-1), but ground-based phase speeds are centered on 0 ms(-1).