Natural variability in air–sea gas transfer efficiency of CO(2)

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.