Seasonal Variations of Age Spectrum of Air in the Lower Stratosphere

An age spectrum of stratospheric air is the probability distribution function of elapsed times for air parcels to be transported from a tropospheric source region to a stratospheric sample region. The age spectrum is a useful tool to understand and quantify stratospheric transport processes. This work investigates the seasonal variations of age spectrum, a topic that has received little attention in previous studies. Age spectra are computed from the Goddard Earth Observing System Coupled Chemistry-Climate Model (GEOSCCM) Version 2 using pulse tracer experiments for releases in different seasons and years. The model age spectra in the extratropical lower stratosphere have significant seasonal variations, but show only small interannual variability. In general, age spectra derived from the pulse tracer released in the boreal winter have younger mean and modal age in the northern extratropics than those released in the boreal summer. The largest seasonal variations of age spectra are found in the Arctic lower stratosphere. These results reflect the seasonal cycle of fast quasi-horizontal transport in the tropical transition region and its interaction with the Arctic polar vortex. The results suggest that the large seasonal variability of the age spectrum in the Arctic lower stratosphere arises because the extent of penetration of the fast isentropic transport into the northern high latitudes is determined by the timing relative to the development of the Arctic polar vortex. Quasi-horizontal transport plays a less important role in the age spectrum in the southern extratropical lower stratosphere due to weaker Rossby wave activity and a more isolated Antarctic polar vortex.