What Drives Regional Variation in Global Ocean-Atmosphere CO2 Fluxes?

Oceanic and atmospheric carbon reservoirs are tightly linked by air-sea exchange of carbon dioxide. Regional and seasonal variations in the CO₂ flux reflect the balance of drivers such as surface heat (solubility) forcing, freshwater fluxes, biological sources and sinks associated with photosynthesis and respiration, and upwelling of biologically regenerated dissolved inorganic carbon. Here, we present a comprehensive, mechanistic diagnostic framework that quantifies the relative contributions of these processes to the total air-sea CO₂ flux. We test the framework using simulations with a global circulation-biogeochemistry model (MITgcm) where the “true” CO₂ fluxes are known. Generally, the leading order balance is between the CO₂ fluxes driven by regional surface heat fluxes and opposed by a combination of biologically-driven carbon uptake and disequilibrium-driven carbon outgassing, with the disequilibrium dominating at mid- to high latitudes and biological uptake dominating at low latitudes. Only minor modifications are due to the net effect of freshwater fluxes on surface salinity, alkalinity and carbon concentrations. We then apply the diagnostic suite to climatological observations and map the relative contributions of the physical and biological drivers in setting the regional patterns of air-sea CO₂ fluxes.