Changes of tracer distributions in the doubled CO 2 climate

Changes in tracer distributions in the troposphere and stratosphere are calculated from a control and doubled CO 2 climate simulation run with the Goddard Institute for Space Studies Global Climate Middle Atmosphere Model. Transport changes are assessed using seven on‐line tracers. Results show that interhemispheric transport is reduced by 5% along with a reduction in the Hadley circulation. Tropical transport from the troposphere into the stratosphere increases by some 30% associated with an increase in the stratospheric subtropical residual circulation. The tropical pipe becomes significantly more leaky, and greater transport into the lowermost stratosphere in the subtropics appears to be occurring, possibly in conjunction with a poleward shift in wave energy convergences. An increase in the high‐latitude lower stratosphere residual circulation reduces the stratospheric residence time of extratropical injections such as bomb 14 C by 11%. Vertical mixing within the troposphere by convection increases, reducing low level concentrations of tracers. The Hadley cell change is affected by the latitudinal gradient of tropical warming. The high‐latitude lower stratosphere residual circulation change depends on the latitudinal gradient of the extratropical warming. Increased penetrating convection to the upper troposphere and the intensified residual circulation in the tropical upper troposphere/lower stratosphere appear to be the most robust of these results, with a magnitude that depends upon the degree of tropical warming. The consequence of this circulation change is to increase trace gas concentrations in the stratosphere and to decrease them in the troposphere for those species that have tropospheric sources.