Natural variability in air–sea gas transfer efficiency of CO2

Abstract The flux of CO2 between the atmosphere and the ocean is often estimated as the air–sea gas concentration difference multiplied by the gas transfer velocity (K 660). The first order driver for K 660 over the ocean is wind through its influence on near surface hydrodynamics. However, field observations have shown substantial variability in the wind speed dependencies of K 660. In this study we measured K 660 with the eddy covariance technique during a ~ 11,000 km long Southern Ocean transect. In parallel, we made a novel measurement of the gas transfer efficiency (GTE) based on partial equilibration of CO2 using a Segmented Flow Coil Equilibrator system. GTE varied by 20% during the transect, was distinct in different water masses, and related to K 660. At a moderate wind speed of 7 m s−1, K 660 associated with high GTE exceeded K 660 with low GTE by 30% in the mean. The sensitivity of K 660 towards GTE was stronger at lower wind speeds and weaker at higher wind speeds. Naturally-occurring organics in seawater, some of which are surface active, may be the cause of the variability in GTE and in K 660. Neglecting these variations could result in biases in the computed air–sea CO2 fluxes.