Analysis of residual mean transport in the stratosphere 1. Model description and comparison with satellite data

A coupled two-dimensional model of the dynamics, chemistry, and radiation of the stratosphere is described. The effects of Rossby wave mixing are parameterized by externally specified coefficients Kyy, which are used consistently in the zonal mean momentum equation and the diffusion term of the tracer transport equation. Rossby wave mixing is reduced in the tropics. Transport between the tropics and midlatitudes is controlled by advection in the model. It is shown that subtropical tracer gradients [e.g., Trepte and Hitchman, 1992; Randel et al., 1993] are produced as a consequence of the difference between advective timescales in the tropics and the diffusive time scale in midlatitudes. In the annual mean, the flow of air is directed from the tropics toward midlatitudes. However, advection between tropics and midlatitudes takes place in both directions at stratospheric altitudes at any given time. Calculated temperatures show some discrepancies with observations in the polar regions. Distributions of long-lived tracers are compared with observations made by instruments aboard the Upper Atmospheric Research Satellite and data from the Atmospheric Trace Molecule Spectroscopy Experiment. Tropical and midlatitude profiles calculated by the model are in reasonable agreement with observations. The sensitivity of calculated temperatures and distributions of long-lived tracers to the choice of large-scale diffusion coefficients, Kyy, is shown to be small due to the dual role of Kyy in determining diffusive tracer transport and providing a forcing term for the mean circulation [Holton, 1986].