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

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Abstract
Recent comparisons of modeled and observed stratospheric mean age show large differences between model predictions and data as well as between individual models, indicating large variations (and deficiencies) in model transport. We explore here the sensitivity of the mean age to different components of the transport by examining the effect of varying the transport parameters within a two‐dimensional model. The mean age is shown to be sensitive to changes in advective circulation and diffusion coefficients, with the sensitivity being largest for changes in the circulation strength. In most cases the magnitudes, but not the orientation, of the mean age isopleths change. However, if the horizontal mixing is made very small within middle latitudes or large within low latitudes, large changes occur in the orientation of mean age isopleths. The effects of these transport changes on chemically active long‐lived tracers are also examined. It is found that the lower stratospheric concentrations are relatively insensitive to the transport changes if these changes do not alter the general shape of mean age isopleths. However, significant changes occur when the transport parameters are modified so as to change the orientation of the mean age (and long‐lived tracer) isopleths.