A quantitative study on the role of gravity waves in driving the tropical Stratospheric Semiannual Oscillation

p Using a coordinated experimental observation under Middle Atmospheric Dynamics (MIDAS) program, an extensive study is carried out to quantify the role of gravity waves in driving the tropical Stratospheric Semiannual Oscillation (SSAO). Rayleigh lidar observations of middle atmospheric temperature over Gadanki (13.5 degrees N, 79.2 degrees E) and rocketsonde wind measurements over Trivandrum (8.5 degrees N, 76.9 degrees E) during the period November 2002 to June 2005 are used for the present study. Gravity waves with periods ranging from 2-4 hours and 0.5-1 hour are found to be dominant in the middle atmosphere throughout the observational period. The altitude profiles of momentum fluxes of gravity waves having these time periods are estimated and their seasonal variations are studied, which showed semiannual variation with maximum around equinoxes and minimum around solstitial months. The mean flow acceleration estimated from the divergence of momentum flux of gravity waves is compared with the mean flow acceleration observed using rocket measured zonal winds during three different cycles of SSAO. This comparison provided an opportunity to quantify the contribution of gravity waves toward generation of SSAO, which is found to be similar to 30-50% of the observed acceleration during the evolution of the westerly phase of the SSAO. The present observations showed that the contribution of the gravity waves toward the westerly phase of SSAO varies significantly from cycle to cycle. The significance of the present results lies in quantifying the gravity wave-mean flow interaction during both easterly and westerly phases of SSAO for the first time over this tropical latitude.