Age of air in a "leaky pipe" model of stratospheric transport

We discuss the characteristics of analytical solutions for the mean age of air in a simple, one-dimensional conceptual model of stratospheric transpor t. This "leaky pipe" model is a version of the "tropical pipe" model which has been modified to include detrainment of surf zone air into the tropics. We examine the dependence of the mean age on advection, diffusive mixing, and quasi-horizontal mixing between the surf zones and the tropics. In the absence of vertical diffusion across tracer mixing ratio isopleths the age difference between the tropics and midlatitudes does not depend on mixing a cross the edge of the tropics, but only on advection. The age difference gi ven by this nondiffusive limit is an upper bound on the tropical-midlatitud e age difference. This work provides some insight into the results of recen t modeling studies that have used the mean age as a diagnostic of stratosph eric transport.