Natural variability in air-sea gas transfer efficiency of CO2

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 similar to 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.