Journal of Geophysical Research, 104, 30559-30569, 1999
Sensitivity of mean age and long-lived tracers to transport coefficients in two-dimensional models
Shuhua Li,
Cooperative Research Centre for Southern Hemisphere Meteorology, Monash
University, Clayton, Australia
Darryn W. Waugh,
Department of Earth and Planetary Science, Johns Hopkins
University, Baltimore, MD
Abstract
Recent comparisons of modeled and observed stratospheric mean age show large differences between models 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 coefficients within a two-dimensional model. The mean age is shown to be sensitive to changes in advective circulation or diffusion coefficients, with the sensitivity largest for changes in the circulation strength. In most cases the magnitude 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 there are large changes in the orientation of 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 age isopleths. Significant changes occur only when the transport coefficients are modified so as to change the orientation of the mean age (and chemical tracer) isopleths.