Sensitivity of mean age and long-lived tracers to transport parameters in a two-dimensional model

Recent comparisons of modeled and observed stratospheric mean age show larg e differences between model predictions and data as well as between individ ual models, indicating large variations (and deficiencies) in model transpo rt. We explore here the sensitivity of the mean age to different components of the transport by examining the effect of varying the transport paramete rs within a two-dimensional model. The mean age is shown to be sensitive to changes in advective circulation and diffusion coefficients, with the sens itivity being largest for changes in the circulation strength. In most case s the magnitudes, but not the orientation, of the mean age isopleths change . However, if the horizontal mixing is made very small within middle latitu des or large within low latitudes, large changes occur in the orientation o f mean age isopleths. The effects of these transport changes on chemically active long-lived tracers are also examined. It is found that the lower str atospheric concentrations are relatively insensitive to the transport chang es if these changes do not alter the general shape of mean age isopleths. H owever, significant changes occur when the transport parameters are modifie d so as to change the orientation of the mean age (and long-lived tracer) i sopleths.