Impact of nonzero intercept gas transfer velocity parameterizations on global and regional ocean–atmosphere CO₂ fluxes

Carbon dioxide (CO₂) fluxes between the ocean and atmosphere (FCO₂) are commonly computed from differences between their partial pressures of CO₂ (∆pCO₂) and the gas transfer velocity (k). Commonly used wind-based parameterizations for k imply a zero intercept, although in situ field data below 4 m s⁻¹ are scarce. Considering a global average wind speed over the ocean of 6.6 m s⁻¹, a nonzero intercept might have a significant impact on global FCO₂. Here, we present a database of 245 in situ measurements of k obtained with the floating chamber technique (Sniffle), 190 of which have wind speeds lower than 4 m s⁻¹. A quadratic parameterization with wind speed and a nonzero intercept resulted in the best fit for k. We further tested FCO₂ calculated with a different parameterization with a complementary pCO₂ observation-based product. Furthermore, we ran a simulation in a well-tested ocean model of intermediate complexity to test the implications of different gas transfer velocity parameterizations for the natural carbon cycle. The global ocean observation-based analysis suggests that ignoring a nonzero intercept results in an ocean-sink increase of 0.73 Gt C yr⁻¹. This corresponds to a 28% higher uptake of CO₂ compared with the flux calculated from a parameterization with a nonzero intercept. The differences in FCO₂ were higher in the case of low wind conditions and large ∆pCO₂ between the ocean and atmosphere. Such conditions occur frequently in the Tropics.