Seasonal Variations of Stratospheric Age Spectra in GEOSCCM
There are many pathways for an air parcel to travel from the troposphere to the stratosphere, each of which takes different time. The distribution of all the possible transient times, i.e. the stratospheric age spectrum, contains important information on transport characteristics. However, it is computationally very expensive to compute seasonally varying age spectra, and previous studies have focused mainly on the annual mean properties of the age spectra. To date our knowledge of the seasonality of the stratospheric age spectra is very limited. In this study we investigate the seasonal variations of the stratospheric age spectra in the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). We introduce a method to significantly reduce the computational cost for calculating seasonally dependent age spectra. Our simulations show that stratospheric age spectra in GEOSCCM have strong seasonal cycles and the seasonal cycles change with latitude and height. In the lower stratosphere extratropics, the average transit times and the most probable transit times in the winter/early spring spectra are more than twice as old as those in the summer/early fall spectra. But the seasonal cycle in the subtropical lower stratosphere is nearly out of phase with that in the extratropics. In the middle and upper stratosphere, significant seasonal variations occur in the sUbtropics. The spectral shapes also show dramatic seasonal change, especially at high latitudes. These seasonal variations reflect the seasonal evolution of the slow Brewer-Dobson circulation (with timescale of years) and the fast isentropic mixing (with timescale of days to months).