Long-term changes in stratospheric age spectra in the 21st century in the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM)

In this study we investigate long-term variations in the stratospheric age spectra using a 21st century simulation with the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). Our purposes are to characterize the long-term changes in the age spectra, and identify processes that cause the decrease of the mean age in a changing climate. Changes in the age spectra in the 21st century simulation are characterized by decreases in the modal age, the mean age, the spectral width, and the tail decay timescale throughout the stratosphere. Our analyses show that the decrease in the mean age is caused by two processes: the acceleration of the residual circulation that increases the young air masses in the stratosphere, and the weakening of the recirculation that leads to a decrease of the tail of the age spectra and a decrease of the old air masses. Weakening of the stratospheric recirculation is also strongly correlated with the increase of the residual circulation. One important result of this study is that the decrease of the tail of the age spectra makes an important contribution to the decrease of the mean age. Long-term changes in the stratospheric isentropic mixing are also investigated. Mixing increases in the subtropical lower stratosphere, but its impact on the age spectra is smaller than the increase of the residual circulation. The impacts of the long-term changes in the age spectra on long-lived chemical tracers are also investigated.