Dynamics of the Tropical Middle Atmosphere: A Tutorial Review*

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Abstract
The general circulation of the tropical stratosphere, mesosphere and lowermost thermosphere is discussed at a tutorial level. Observations of the quasi‐biennial and semiannual oscillations by both in situ and satellite techniques are first reviewed. The basic dynamics controlling the zonal‐mean component of the circulation are then discussed. The role of radiative diabatic cooling in constraining the zonal‐mean circulation in the middle atmosphere is emphasized. It is shown that the effectiveness of this radiative constraint is reduced at low latitudes, allowing for the sustained mean flow accelerations over long periods of time characteristic of the quasi‐biennial and semiannual oscillations in the tropics. The current view is that the dominant driving for the equatorial mean flow accelerations seen in the middle atmosphere derives from vertically‐propagating waves. This process is illustrated here in its simplest context, i.e. the Plumb (1977) model of the interaction of monochromatic internal gravity waves with the mean flow (based on earlier work of Lindzen and Holton, 1968; Holton and Lindzen, 1972). It is shown that the dynamics illustrated by this simple model can serve as the basis for an explanation of the quasi‐biennial oscillation. The paper then describes some of recent developments in the theory of the quasi‐biennial and semiannual oscillations, including aspects related to the interaction between tropics and midlatitudes in the middle atmosphere. The paper concludes with a discussion of the effects of the long period dynamical variations in the tropical circulation on the chemical composition of the stratosphere.