Middle atmosphere age of air in a globally balanced two-dimensional model

We extend the concepts of the mean age derived from the age spectrum and th e age of air derived from the time lag in the middle atmosphere by specifyi ng the tracer source gases uniformly in the lower troposphere. The concepts are illustrated by use of both a time-independent one-dimensional diffusiv e model and a globally balanced two-dimensional middle atmosphere model. We quantitatively examine several factors that may cause the difference betwe en the mean age and the age of air. It is found that the mean age and the a ge of air can differ by any amount in general for a time-dependent transpor t operator. For a time-independent transport operator the age of air derive d by integrating a model for a finite time is less than the mean age. The a ge of air at the stratopause is about 5 to 6 years in our globally balanced two-dimensional middle atmosphere model. In the mesosphere the age of air changes with season significantly due to the reversal of the mesospheric me ridional circulation between the summer and the winter. With a chemical lif etime of SF6 above 30 km being similar to 38 years our two-dimensional mode l shows no signs of overestimate of age of air for SF6 at 30 km. It is also demonstrated that stratospheric mean age is sensitive to the horizontal mi xing in the troposphere. Weak horizontal mixing in the troposphere will red uce the mean age in the upper stratosphere but increase the mean age in the high-latitude lower stratosphere. The former is caused by diminishing the recycling of air parcels between the troposphere and stratosphere, and the latter is caused by minimizing the portion of tracers directly transported upward from the high-latitude troposphere into the stratosphere.