Deciphering the variability in air-sea gas transfer due to sea state and wind history

Understanding processes driving air-sea gas transfer and being able to model both its mean and variability are critical for studies of climate and carbon cycle. The air-sea gas transfer velocity (<i>K</i> ₆₆₀) is almost universally parameterized as a function of wind speed in large scale models-an oversimplification that buries the mechanisms controlling <i>K</i> ₆₆₀ and neglects much natural variability. Sea state has long been speculated to affect gas transfer, but consistent relationships from in situ observations have been elusive. Here, applying a machine learning technique to an updated compilation of shipboard direct observations of the CO₂ transfer velocity (<i>K</i> _CO2,660), we show that the inclusion of significant wave height improves the model simulation of <i>K</i> _CO2,660, while parameters such as wave age, wave steepness, and swell-wind directional difference have little influence on <i>K</i> _CO2,660. Wind history is found to be important, as in high seas <i>K</i> _CO2,660 during periods of falling winds exceed periods of rising winds by ∼20% in the mean. This hysteresis in <i>K</i> _CO2,660 is consistent with the development of waves and increase in whitecap coverage as the seas mature. A similar hysteresis is absent from the transfer of a more soluble gas, confirming that the sea state dependence in <i>K</i> _CO2,660 is primarily due to bubble-mediated gas transfer upon wave breaking. We propose a new parameterization of <i>K</i> _CO2,660 as a function of wind stress and significant wave height, which resemble observed <i>K</i> _CO2,660 both in the mean and on short timescales.