Timescales for the stratospheric circulation derived from tracers

A class of atmospheric constituents used as tracers of stratospheric flow are chemically long-lived in comparison with timescales of interest and exhibit gradients due primarily to time-dependent mixing ratios at the tropopause. In general, the stratospheric transport properties derived from such tracers depend on the nature of their time variation. To explore the relationship between timescales associated with the propagation of tracer mixing ratio signals and bulk transport properties, we have used two three-dimensional chemical transport models to simulate the age spectrum (the distribution of transit times of mass present in air parcels) of the stratosphere. From the age spectrum the stratospheric response to any time-varying tropospheric forcing may be deduced. The modeled age spectra are broad, indicating a range of transit times present in air parcels, and asymmetric, so that the mean transit time, called the mean age, is much larger than the modal transit time. Furthermore, the phase lag time of an oscillating signal (e.g., the annual CO2 cycle and lower stratospheric H2O) and the mean age are not timescales characterizing the same transport property. Periodic signal phase lags do not represent ''mean transit times'' and, in general, cannot be readily related to bulk transport properties. However, if the age spectrum is peaked enough, as it appears to be in the lower tropical stratosphere, periodic signal phase lags approximate the modal time.