TRACER TRANSPORT IN THE TROPICAL STRATOSPHERE DUE TO VERTICAL DIFFUSION AND HORIZONTAL MIXING

Observations of linearly increasing (e.g., SF6) and periodically varyi ng (e.g., H2O+2CH(4)) long-lived tracers in the lower stratosphere pro vide independent constraints on theories of transport in the region. T aken 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 rel axation time tau). Using a one-dimensional diffusive-advective model o f 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 approxim ate to 0.3 K-2/day (K-z approximate to 0.01 m(2)/s), tau approximate t o 1.3 years, and Q approximate to 0.5 K/day (<(omega)over bar> approxi mate to 0.3 mm/s). These val ues produce profiles for CO2 in reasonabl e agreement with aircraft observations. However, a large range of K-th eta results in equally good agreement, although tau and Q are more tig htly constrained. In the lower tropical stratosphere, vertical diffusi on appears to play little role in transporting tracer.