Stratospheric residence time and its relationship to mean age

Transport inaccuracies in stratospheric models are a major source of uncertainty in predicting the environmental impact of trace gases with stratospheric source, such as emissions from high-flying aircraft. Because there are no observed tracers of stratospheric aircraft emissions, a direct evaluation of the models' transport of the emissions is not possible. Here we examine the relationship between the stratospheric residence time tau(R) for tracers of midlatitude stratosphere source and the mean age Gamma, a transport diagnostic for which observations are available and have been used to evaluate stratospheric models. Employing a representation of the stratosphere that includes the basic kinematic features of the global circulation, we find that tau(R) and Gamma are correlated over a range of plausible circulations, but that the correlation is imperfect. A major (but not sole) limitation on Gamma as a proxy for tau(R) is the midlatitude tropopause height Z(M) Elevating Z(M) reduces tau(R) significantly, while Gamma is only weakly affected. The relationship between tau(R) and Gamma seen among the participants of a recent stratospheric model intercomparison is consistent with this analysis.