Abstract
Semiannual variations in the temperature and zonal wind have been found to be the dominant oscillations of the circulation in the tropical upper stratosphere and mesophere. Many hypotheses have been presented in an attempt to explain these semiannual variations. In this study a number of these hypotheses are examined through the use of a diagnostic numerical model for the zonally symmetric flow. It is found that a semiannually varying momentum source is required to drive the zonal wind oscillation. Although the precise form of this momentum source is not determined, some evidence is presented relating this source to the eddy momentum flux by tidal motions in the region above 40 km.
