Long-Term Changes in Stratospheric Age Spectra in the 21st Century in GEOSCCM

One robust model result on stratospheric circulation change in the 21st century is the decrease of the mean age of air, which indicates an acceleration of stratospheric transport. The mean age, however, only gives partial information on transport times. The complete information on transit times is included in the age spectrum. In this study, in order to improve understanding of changes in stratospheric transport in the 21st century, we investigates the long-term changes in the age spectrum using simulations of the Goddard Earth Observing System Coupled Chemistry Climate Model (GEOSCCM). The age spectrum is calculated using the so-called “pulse tracer” experiment (Hall and Waugh, 1997). Pulse-tracer simulations were initiated at different times throughout a 100-year simulation (2000-2099) and a total of 50 age spectra are obtained.

Changes in the age spectra in the 21st century in the GEOSCCM are characterized by decreases in the modal age (the spectral peak time), the mean age (the first moment), the spectral width (the second moment), and the long-time decay rate (exponential decay rate in the tail region of the spectrum). The decrease of the modal age in the tropical stratosphere can be accounted for by the strengthening of the Brewer-Dobson circulation. Outside the tropics, the decrease of the modal age is also closely connected to the increase of the Brewer-Dobson circulation. Strong correlation between the mean age and the rate of tropical upwelling is also identified, consistent with previous studies. Interestingly, it is found that the decrease of the decay rate (or a shorter tail) makes significant contributions to the decrease of the mean age. Examining the probability distribution functions of N2O and CH4 shows that mixing between the tropics and the midlatitudes remains about the same in the 21st century, but mixing in the high latitude lower stratosphere increases. These results suggest that the younger mean age in the tropics and midlatitudes at the end of the 21st century is mainly caused by a faster Brewer-Dobson circulation.