Natural variability in air–sea gas transfer efficiency of CO2

The flux of CO 2 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 CO 2 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 CO 2 fluxes.