Natural variability in air–sea gas transfer efficiency of CO2

The flux of CO2between the atmosphere and the ocean is often estimated as the air–sea gas concentration difference multiplied by the gas transfer velocity (K660). The first order driver forK660over the ocean is wind through its influence on near surface hydrodynamics. However, field observations have shown substantial variability in the wind speed dependencies ofK660. In this study we measuredK660with 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 CO2using a Segmented Flow Coil Equilibrator system. GTE varied by 20% during the transect, was distinct in different water masses, and related toK660. At a moderate wind speed of 7 m s−1,K660associated with high GTE exceededK660with low GTE by 30% in the mean. The sensitivity ofK660towards 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 inK660. Neglecting these variations could result in biases in the computed air–sea CO2fluxes.