Low-frequency variability and mean circulation of the tropical stratosphere from UARS data

The low frequency variability in the equatorial middle atmosphere is dominated by the semiannual (SAO) and quasi-biennial (QBO) oscillations. Although the general characteristics and forcing mechanisms of these oscillations are thought to be known, these oscillations are still not well reproduced in modeling experiments of the middle atmosphere. This thesis uses the relatively long period (1992-1996) of UARS measurements and UKMO assimilated model data to examine the SAO in the upper stratosphere and the QBO throughout the stratosphere. The SAO in zonal wind is found to be latitudinally asymmetric about the equator with strongest amplitudes in the Southern Hemisphere subtropics. The latitudinal asymmetry of the SAO in zonal wind is diagnosed using the TEM momentum equation and UKMO data. The asymmetry is mostly due to the different phasing of the peak advective forcing by the mean meridional circulation in each hemisphere. The contribution of gravity waves to the zonal wind SAO are also explored using the forcing from a linear gravity wave ray tracing model. Gravity waves are found to be the primary source of westerly momentum in each hemisphere during fall and contribute to the latitudinal asymmetry of the SAO by having more easterly forcing in the Southern Hemisphere during summer compared to in the Northern Hemisphere during summer.; Long-lived trace gas mixing ratios measured by UARS are used to obtain mean transport velocities in the tropical stratosphere using a simplified budget equation. Time scales for quasi-isentropic mixing into the tropical lower stratosphere from the extratropics are estimated using the tracer data and residual vertical velocities. These calculations suggest the most substantial barrier to mixing is present between 18-25 km and varies in strength with season and phase of the QBO. This result justifies the use of Plumb's "Tropical Pipe" model (1996) which is used to calculate bulk vertical velocities and air mass fluxes across tracer isolines in the lower stratosphere.