The "Age" of Stratospheric Air

The "age" of a parcel of stratospheric air is a measure of the mean time since the air within the parcel left the troposphere. Age can be indirectly measured by determining the time lag between stratospheric and surface concentrations of linearly growing, chemically inert, tracers such as SF6 and CO2.

Model simulation of age
Recently, as a part of NASA's "Models and Measurements II" exercise, a suite of stratospheric models used in ozone assessments were subjected to a series of tests, one of which was a simulation of age. Unlike simulations of chemical tracers, a simulation of age is unambiguously a test of model transport. Most of the models produced lower stratospheric age that is too young compared with observations, suggesting that their transport is too vigorous.

The distribution of age
Age can be viewed as a tracer of stratospheric motions, just like long-lived chemical tracers such as N2O and, as such, its spatial distribution is governed by the same transport processes. Because of its simplicity as a tracer, however, this structure can be used directly to infer some gross aspects of stratospheric transport.

A simple "tropical pipe" model has been unsed to analyze the factors that control the structure of age in the stratosphere. There is downwelling within the rapidly mixed midlatitude "surf zone", upwelling in the tropics, and entrainment and detrainment across the subtropical interface between the two. Vertical mixing is neglected. Surfaces of constant age are discontinuous across the subtropical interface, being higher in the upwelling air of the tropics.

From this analysis, we conclude that: (1) The age difference across the subtropical interface depends only on the gross upwelling rate in the tropics; rather surprisingly, it does not depend on exchange across the interface; (2) Without any interaction with the tropics, age would decrease with altitude in the extratropical stratosphere. Observations indicate otherwise; the implication is that there is vigorous exchange of air across the subtropical interface in the lower stratosphere.