The middle atmosphere (stratosphere and mesosphere) is the region of the atmosphere between about 16 and 100 km altitude. Studies of dynamical and chemical processes in this region have greatly expanded in recent years owing to the impact of human activities on the stratospheric ozone layer, and the coupling between stratospheric changes and surface climate. A primary area of emphasis is study of the dynamical interactions between the troposphere and the stratosphere, including the transfer of momentum and trace constituents across the tropopause. This effort requires understanding of the influence of both large- and small-scale wave motions on the momentum balance and mass circulation of the middle atmosphere.
The prevailing winds in the atmosphere near the equator above about 17 km are known to undergo a slightly irregular oscillation from strong westerlies to strong easterlies roughly every other year. This Quasi-biennial Oscillation (QBO) has been observed to dominate the winds in this region of the atmosphere for at least the last 6 decades and is characterized by a downward propagation of the westerly and easterly jets that form.