Global-scale transport of carbon dioxide in the troposphere

An atmospheric transport model was used to examine the roles of variously scaled atmospheric transport processes (Lagrangian mean motions, large-scale eddies, and parameterized vertical diffusion and convective transport) in the spatiotemporal distributions of tropospheric carbon dioxide (CO2). The mean and eddy transports were analyzed using the mass-weighted isentropic zonal mean. We found several differences in the dominant transport processes for tropospheric CO2 distributions between the extratropics of both hemispheres and the tropics. (1) In the northern extratropics in boreal autumn to spring, CO2 emitted by anthropogenic and biospheric sources is uplifted and dispersed through quasi-isentropic eddy mixing associated with baroclinic waves and accumulates in the extratropical low-isentropic troposphere (in cold pocket'' below 300K). (2) High-CO2 air is transported from the northern extratropics into the tropics through low-level mean meridional flow. It is uplifted together with CO2 emitted by tropical vegetation through deep convection and diabatic eddies in the tropics during boreal winter to spring. (3) During summer at the northern midlatitudes, the low mixing ratio of CO2 produced by biospheric uptake is uplifted into the upper troposphere by convection and is strongly isolated from the lower latitudes. (4) The CO2 emitted in the Northern Hemisphere and the tropics is transported into the Southern Hemisphere via the tropical upper troposphere due to eddy mixing during boreal winter to spring and mean divergent flow of Hadley circulation during boreal summer. (5) In the Southern Hemisphere, an upward gradient of CO2 forms by upper tropospheric southward advection during boreal spring-autumn.