Tropospheric Mean Age Responses to Tropical Circulation Changes Using GEOS-5 Simulations

The mean age since air was last at the Northern Hemisphere midlatitude surface is a fundamental property of tropospheric transport. Recent comparisons among chemistry climate models, however, reveal that there are large differences in the mean age among models and that these differences are most likely related to differences in tropical (parameterized) convection. Here we use simulations of the Goddard Earth Observing System Model Version 5 (GEOS-5) to explore the sensitivity of the mean age to changes in the tropical circulation. Tropical circulation changes are forced by prescribed localized off-equatorial warm sea surface temperature anomalies that (qualitatively) reproduce the convection and circulation differences among the comprehensive models. Less idealized simulations using realistic boundary conditions but different convective parameterizations are also used. Idealized chemical species subject to prescribed OH loss are integrated in parallel in order to illustrate the impact of tropical transport changes on interhemispheric constituent transport.