A Puzzling Quasi-Periodic Variability in the Tropical Middle Atmosphere

The Quasi-Biennial Oscillation and the Semiannual Oscillation have been identified to be the leading modes of variability in the tropical middle atmosphere. With reanalysis data and independent rocket soundings from a low latitude site, we report the existence of yet another variability in the tropical lower mesosphere which is primarily evident as easterly bursts in zonal winds during the months of May-July. It occurs with a variable interval of 2-5 yrs in the late 20th century and 7-9 yrs in the early 21st century. These Quasi-Periodic Easterly Bursts are found to have remote influences on the Antarctic polar vortex as well as residual circulation in the lower mesosphere. We identify a potential causative mechanism for the easterly bursts that involve enhanced cross equatorial advection of momentum as well as gravity wave drag. A close association with Quasi Biennial Oscillation winds is observed, however, cause of the observed periodicity remains elusive. Circulation in the tropical middle atmosphere is distinctively different from that of the higher latitudes in that there exists long-period oscillations chiefly driven by a broad spectrum of atmospheric waves. These oscillations that dominate the wind variability here, include the Quasi-Biennial Oscillation (QBO) and the Semiannual Oscillation (SAO) with approximate periods of 28 and 6 months, respectively. With the help of reanalysis data sets and independent rocket soundings from a low latitude site, we identify the existence of a hitherto unrecognized variability in the tropical middle atmosphere. The newly identified variability seems to have association with the Southern Hemispheric polar vortex as well as the transport circulation in the lower mesosphere. Further, an attempt is made to identify the causative mechanism using the framework of interaction of large-scale planetary waves with the mean flow. A new pattern of variability in the tropical middle atmosphere is identified The observed variability is associated with stronger polar vortex and an enhanced residual circulation in the Southern Hemispheric mesosphere A close connection with the presence of an extended layer of westerlies in the Quasi Biennial Oscillation regime is observed