Carbon isotope evidence for the latitudinal distribution and wind speed dependence of the air-sea gas transfer velocity

The air-sea gas transfer velocity is an important determinant of the exchange of gases, including CO2, between the atmosphere and ocean, but the magnitude of the transfer velocity and what factors control it remains poorly known. Here, we use oceanic and atmospheric observations of C-14 and C-13 to constrain the global mean gas transfer velocity as well as the exponent of its wind speed dependence, utilizing the distinct signatures left by the air-sea exchange of (CO2)-C-14 and (CO2)-C-13. While the atmosphere and ocean inventories of (CO2)-C-14 and (CO2)-C-13 constrain the mean gas transfer velocity, the latitudinal pattern in the atmospheric and oceanic C-14 and C-13 distributions contain information about the wind speed dependence. We computed the uptake of bomb C-14 by the ocean for different transfer velocity patterns using pulse response functions from an ocean general circulation model, and evaluated the match between the predicted bomb C-14 concentrations and observationally based estimates for the 1970s-1990s. Using a wind speed climatology based on satellite measurements, we solved either for the best-fit global relationship between gas exchange and mean wind speed or for the mean gas transfer velocity over each of 11 ocean regions. We also compared the predicted consequences of different gas exchange relationships on the rate of change and interhemisphere gradient of C-14 in atmospheric CO2 with tree-ring and atmospheric measurements. Our results suggest that globally, the dependence of the air-sea gas transfer velocity on wind speed is close to linear, with an exponent of 0.5 +/- 0.4, and that the global mean gas transfer velocity at a Schmidt number of 660 is 20 +/- 3 cm/hr, similar to the results of previous analyses. We find that the air-sea flux of C-13 estimated from atmosphere and ocean observations also suggests a lower than quadratic dependence of gas exchange on wind speed.