Simulated changes in atmospheric transport climate

Atmospheric "transport climate'' characterizes how trace gases are distribu ted by and within the atmosphere, on average, as a consequence of the inter action of atmospheric flow with tracer sources and sinks. The change in tra nsport climate under global warming is investigated using passive tracers. Experiments with constant localized surfaces sources, pulsed sources, and p ulsed boundary conditions are analyzed using a Green-function approach in c onjunction with a climatological budget calculation. Under climate warming, interhemispheric exchange times, mixing times, and m ean transit times all increase by about 10%. The main transport pathway bet ween the hemispheres via the "tracer fountain'' at the ITCZ is suppressed. Generally less vigorous flow manifests itself in higher tracer burdens in t he source hemisphere and in downwind plumes of enhanced mixing ratio close to the sources; these increases are also about 10%. Resolved advection and subgrid transport do not cooperate for all sources in enhancing the near-so urce mixing ratio. The warmer climate has a reduced cross-tropopause gradie nt, primarily due to a slightly higher tropopause, which results in a reduc tion of about 25% in the average tropospheric tracer mixing ratio, and a co rresponding enhancement in the stratosphere. A global variance budget shows increased mean and transient tracer variance due to increased generation f rom strengthened mean gradients near the source and weakened eddy and subgr id transport.