2H/H and 18O/16O Non-Equilibrium Fractionation Factors for Ocean
Evaporation in the North-West Atlantic Region
Abstract
Isotopic evaporation models, such as the Craig-Gordon model, rely on the
description of non-equilibrium fractionation factors that are, in
general, poorly constrained. To date, only a few gradient-diffusion type
measurements have been performed in ocean settings to test the validity
of the commonly used non-equilibrium fractionation factor
parametrizations for ocean evaporation. In this work we present six
months of water vapor isotopic observations collected from a
meteorological tower located in the northwest Atlantic Ocean (Bermuda)
with the objective of estimating the best non-equilibrium fractionation
factors (k, ‰) for ocean evaporation and their dependency on wind speed.
Gradient-diffusion measurements are sensitive enough to resolve
non-equilibrium fractionation factors during evaporation and provide
mean values of k18= 5.2±0.6 ‰ and k2= 4.3±3.4 ‰. In this study, we
furthermore evaluate the relationship between k and 10-m wind speed over
the ocean. Such relationship is expected from current evaporation theory
and from laboratory experiments made in the 1970s, but observational
evidence is lacking. We show that (i) sensitivity of k to wind speed is
small, in the order of -0.16 to 0.20 ‰ s/m for k18, and (ii) there is no
empirical evidence for the presence of a discontinuity between smooth
and rough wind speed regime during isotopic fractionation, as proposed
in earlier studies. Instead, k18 monotonically decreases within our
observed wind speed range [0 – 10 m/s]. Implications for using such
k values in modelling ocean vapor d-excess are briefly discussed.