Planetary Waves - Major Forcing Agent in Generating Stratospheric and Mesospheric Quasi Biennial Oscillation

Quasi Biennial Oscillation (QBO) and Semi Annual Oscillation (SAO) are dominant natural oscillations in the equatorial lower stratosphere and mesosphere and are believed to be generated by the wave-mean flow interaction. The characteristics of stratospheric QBO (SQBO) and mesospheric QBO (MQBO) and SAO are studied by making use of the monthly mean zonal and meridional winds derived from four decades of balloon (1970-2010) and rocket (1971-2007) flights and seven years of SKiYMET meteor radar (2004-2011) observations over an equatorial station Thiruvananthapuram. The year-to-year variation and interrelationships between these oscillations are also studied. The quantitative estimation of forcing towards the mean flow acceleration producing the maximum phase of SQBO during the four-decade period shows 70-80% contribution from planetary waves, while during minimum phase of SQBO the contribution is 30-35%. It is also found that the contribution by planetary waves towards the mean flow acceleration producing MQBO maximum during the seven-year period is around 30-90% and during MQBO minimum phase it is 30-60%. The vertical structure of these oscillations present over the equatorial station is also derived.