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 (K₆₆₀). The first order driver for K₆₆₀ 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 K₆₆₀. In this study we measured K₆₆₀ 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 K₆₆₀. At a moderate wind speed of 7 m s^-1, K₆₆₀ associated with high GTE exceeded K₆₆₀ with low GTE by 30% in the mean. The sensitivity of K₆₆₀ 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 K₆₆₀. Neglecting these variations could result in biases in the computed air-sea CO₂ fluxes.