Tm. Hall et Dw. Waugh, TIMESCALES FOR THE STRATOSPHERIC CIRCULATION DERIVED FROM TRACERS, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 102(D7), 1997, pp. 8991-9001
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.