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Hall, T.M., and D.W. Waugh, 1997: Tracer transport in the tropical stratosphere due to vertical diffusion and horizontal mixing. Geophys. Res. Lett., 24, 1383-1386, doi:10.1029/97GL01289.
Observations of linearly increasing (e.g., SF6) and periodically varying (e.g., H2O+2CH4) 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 D) and advection (at rate Q) through isentropic surfaces, and mixing of extra-tropical air into the tropics (with relaxation time T). 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 D = 0.3 K2/day (0.01 m2/s), T = 1.3 years, and Q = 0.5 K/day (0.3 mm/s). These values produce profiles for CO2 in reasonable agreement with aircraft observations. However, a large range of D results in equally good agreement, although T and Q are more tightly constrained. In the lower tropical stratosphere, vertical diffusion appears to play little role in transporting tracer.
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