Re-examination of age-of-air in the middle atmosphere

We extend the concepts of the age spectrum, mean-age, and the age-of-air in the middle atmosphere by specifying the tracer source gases uniformly in the lower troposphere. The concepts are illustrated by use of both a steady one-dimensional diffusive model and an improved globally balanced two-dimensional middle atmosphere model where the time integration of geopotential tendency is performed explicitly. We quantitatively examine several factors that may cause the difference between the age-of-air derived from the time lag and the mean-age derived from the age spectrum. It is found that the age-of-air derived from a finite time of model integration is less than the mean-age. The age-of-air at the stratopause is about 5 to 6 years in a 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 meridional circulation between the summer and the winter. It is also demonstrated that stratospheric mean-age is sensitive to the horizontal mixing in the troposphere. Weak horizontal mixing in the troposphere will reduce the mean-age in the upper stratosphere but increase the mean-age in the high-latitude lower stratosphere for a source tracer located at the equatorial lower troposphere.