Tracer transport in the tropical stratosphere due to vertical diffusion and horizontal mixing

Observations of linearly increasing (e.g., SF6) and periodically varying (e.g., H2O+2CH(4)) long-lived tracers in the lower stratosphere provide independent constraints on theories of transport in the region. Taken together, these data allow separation of the roles of diffusion (with coefficient K-theta) and advection (at rate Q) through isentropic surfaces, and mixing of extra-tropical air into the tropics (with relaxation time tau). Using a one-dimensional diffusive-advective model of the tropical stratosphere, which allows relaxation of mixing ratios to extra-tropical values, we obtain solutions for periodic and linear tracers. Fitting the solutions to observations yields K-theta approximate to 0.3 K-2/day (K-z approximate to 0.01 m(2)/s), tau approximate to 1.3 years, and Q approximate to 0.5 K/day (<(omega)over bar> approximate to 0.3 mm/s). These val ues produce profiles for CO2 in reasonable agreement with aircraft observations. However, a large range of K-theta results in equally good agreement, although tau and Q are more tightly constrained. In the lower tropical stratosphere, vertical diffusion appears to play little role in transporting tracer.