Scale Dependence of Atmospheric Carbon Dioxide Transport

The ability to quantify and model atmospheric carbon dioxide transport represents a strong source of uncertainty within carbon science. Transient eddies, or synoptic systems, produce strong poleward carbon dioxide concentration flux. This occurs because these systems concentrate high carbon dioxide concentration gradients along cold frontal boundaries and transport gases poleward along warm conveyor belts. The strength of these gradients range from 10-20 ppm of carbon dioxide across the boundary, but the high-gradient region is typically embedded in clouds so satellites are obstructed from view. Conventional global models are unable to resolve clouds and vertical transport along fronts. Cloud-resolving regional models can reproduce clouds and frontal boundaries, but are regionally contained and too computationally expensive to run globally. We investigated the effect of parameterized convection versus resolved cloud-scale transport on poleward carbon dioxide transport in a global model. We ran the Community Atmosphere Model 6 (CAM6) and the super-parameterized CAM6 (SP-CAM6) from the Community Earth System Model 2 for 3 years using 1.9 by 2.8 degree spatial resolution. CAM6 uses parameterized sub-grid scale convection to advect tracers in the vertical. SP-CAM6 uses identical large-scale tracer advection, but replaces the parameterized convection and turbulence with a sub-grid scale 2-D "curtain" of high-resolution cloud resolving models in each grid cell. We hypothesize that transport statistics in SP-CAM6 will be intermediate between global and regional models due to the resolved cloud-scale transports. We compared both the seasonal cycle of carbon dioxide transport and the partitioning of the carbon transport budget between the models. We found that both models produce a similar meridional transport contribution overall, but the partitioning between the overturning and transient components were different depending on whether or not clouds were explicitly resolved. This study is part of a multi-model intercomparison of synoptic carbon dioxide transport that includes evaluation against observational data from the ACT-America field campaign.