Sea-air CO2 fluxes and carbon transport: A comparison of three ocean general circulation models

Many estimates of the atmospheric carbon budget suggest that most of the si nk for CO2 produced by fossil fuel burning and cement production must be in the Northern Hemisphere. Keeling et al. [1989] hypothesized that this asym metry could be explained instead by a northward preindustrial transport of similar to1 Pg C y(-1) in the atmosphere balanced by an equal and opposite southward transport in the ocean. We explore this hypothesis by examining t he processes that determine the magnitude of the preindustrial interhemisph eric flux of carbon in three ocean carbon models. This study is part of the first stage of the Ocean Carbon Model Intercomparison Project organized by International Geosphere Biosphere Programme Global Analysis, Interpretatio n, and Modelling Task Force, We find that the combination of interhemispher ic heat transport (with its associated carbon transport), a finite gas exch ange, and the biological pump, yield a carbon flux of only -0.12 to +0.04 P g C y(-1) across the equator (positive to the north). An important reason f or the low carbon transport is the decoupling of the carbon flux from the i nterhemispheric heat transport due to the long sea-air equilibration time f or surface CO2. A possible additional influence on the interhemispheric exc hange is oceanic transport of carbon from rivers.