Kinetic effects during ocean evaporation: observed relationship with wind speed

Water vapor has a fundamental role in weather and climate, being the strongest natural greenhouse gas in the Earth’s atmosphere. The main source of water vapor in the atmosphere is ocean evaporation, which transfers a large amount of energy via latent heat fluxes. In the past, evaporation was intensively studied using stable isotopes because of the large fractionation effects involved during water phase changes, providing insights on processes occurring at the air-water interface. Current theories describe evaporation near the air-water interface as a combination of molecular and turbulent diffusion processes into separated sublayers. The importance of those two sublayers, in terms of total resistance to vapor transport in air, is expected to be dependent on parameters such as moisture deficit, temperature and wind speed. Non-equilibrium fractionation effects in isotopic evaporation models are then expected to be related to these physical parameters. In the last 10 years, several water vapor observations from oceanic expeditions were focused on the impact of temperature and wind speed effect, assuming the influence of those parameters on non-equilibrium fractionation in the marine boundary layer. Wind speed effect is expected to be small on total kinetic fractionation and was discussed at length but was not completely ruled out. With a gradient-diffusion approach (2 heights above the ocean surface) and Cavity Ring-Down Spectroscopy we have estimated non-equilibrium fractionation factors for ¹⁸O/¹⁶O during evaporation, showing that the wind speed effect can be detected and has no significant impact on kinetic fractionation. Results obtained for wind speeds between 0 and 10 m s^-1 in the North Atlantic Ocean are consistent with the Merlivat and Jouzel (1979) parametrization for smooth surfaces (mean ε₁₈=6.1‰). A small monotonic decrease of the fractionation parameter is observed as a function of 10 m wind speed (slope  ≅ 0.15 ‰ m^-1 s), without any evident discontinuity. However, depending on the data filtering approach it is possible to highlight a rapid decrease of the kinetic fractionation factor at low wind speed (≤ 2.5 m s^-1). An evident decrease of fractionation factor is also observed for wind speeds above 10 m s^-1, allowing to hypothesize the possible effect of sea spray in net evaporation flux. Considering the average wind speed over the oceans, we conclude that a constant kinetic fractionation factor for evaporation is a more simple and reasonable solution than a wind-speed dependent parametrization. 

 

Merlivat, L., & Jouzel, J. (1979). Global climatic interpretation of the deuterium‐oxygen 18 relationship for precipitation. Journal of Geophysical Research: Oceans, 84(C8), 5029-5033.