TIMESCALES FOR THE STRATOSPHERIC CIRCULATION DERIVED FROM TRACERS

A class of atmospheric constituents used as tracers of stratospheric f low are chemically long-lived in comparison with timescales of interes t 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 p ropagation of tracer mixing ratio signals and bulk transport propertie s, we have used two three-dimensional chemical transport models to sim ulate the age spectrum (the distribution of transit times of mass pres ent in air parcels) of the stratosphere. From the age spectrum the str atospheric response to any time-varying tropospheric forcing may be de duced. The modeled age spectra are broad, indicating a range of transi t times present in air parcels, and asymmetric, so that the mean trans it time, called the mean age, is much larger than the modal transit ti me. Furthermore, the phase lag time of an oscillating signal (e.g., th e annual CO2 cycle and lower stratospheric H2O) and the mean age are n ot timescales characterizing the same transport property. Periodic sig nal 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 tr opical stratosphere, periodic signal phase lags approximate the modal time.