Quantification of gravity wave forcing in driving the stratospheric Quasi-Biennial Oscillation

Using long-term Rayleigh lidar temperature observations during April 1998 - April 2005 in the lower stratospheric region, momentum flux divergence of the gravity waves of periods 0.5-1 and 2-4 hours are estimated over Gadanki ( 13.5 degrees N, 79.2 degrees E), India. Monthly mean zonal winds from NCEP/NCAR reanalysis data over Gadanki at two pressure levels 10 and 20 hPa are used to study the characteristics of the Quasi-Biennial Oscillation ( QBO) and its month-to-month acceleration. This observed mean flow acceleration is compared with that estimated from the momentum flux divergence of gravity waves, which led to a better insight into the role of gravity waves in driving the QBO. The present analysis reveals that the contribution of gravity waves towards the westerly phase of QBO varies from similar to 10-60% while that during easterly phase from similar to 10-30%. The present study thus quantifies the role of gravity waves in driving the easterly and westerly phases of QBO.