Seasonal Variations of Stratospheric Age Spectra in GEOSCCM

The stratospheric age spectrum is the probability distribution function of the transit times since a stratospheric air parcel had last contact with a tropospheric boundary region. The age spectrum provides valuable information on stratospheric transport timescales such as the mean age, modal age, spectral width, and stratospheric decay rate. However, knowledge of the age spectrum's seasonal variability is very limited. In this study, we investigate the seasonal variations of stratospheric age spectra using the pulse tracer method in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). The differences between the age spectrum and the Boundary Impulse Response (BIR), which is the direct product of the pulse tracer method, are clarified. We introduce a simplified method to reconstruct seasonally varying age spectra from seasonally varying BIRs. Age spectra in the GEOSCCM have significant seasonal variations throughout the stratosphere. The largest seasonal changes occur in the lowermost and lower stratosphere and the subtropical overworld. Up to 40% differences between the individual month and annually averaged mean age are commonly found in these regions. The modal ages and spectral shapes demonstrate even bigger changes in the polar stratosphere. The seasonal variations of the age spectra reflect the seasonal evolution and relative importance of the slow Brewer-Dobson circulation and the fast isentropic mixing. The seasonal and interannual variations of the BIRs are also investigated. Our results clearly show that computing an ensemble of seasonally dependent BIRs is necessary in order to capture the seasonal and annual mean properties of the stratospheric age spectra.