The imprint of surface fluxes and transport on variations in total column carbon dioxide

They therefore provide fundamentally different information than observations made within the boundary layer, which reflect the combined influence of large-scale and local fluxes. Observations of both (CO₂) and CO₂ concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in (CO₂) in the Northern Hemisphere midlatitudes through horizontal advection.

Rather than obscure the signature of surface fluxes on atmospheric CO₂, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in (CO₂) from covariations in (CO₂) and potential temperature, θ, a dynamical tracer, on synoptic timescales to evaluate surface flux estimates commonly used in carbon cycle models. We find that simulations using Carnegie Ames Stanford Approach (CASA) biospheric fluxes underestimate both the (CO₂) seasonal cycle amplitude throughout the Northern Hemisphere midlatitudes and the meridional gradient during the growing season.