Atmospheric control on isotopic composition and d-excess in water vapor over ocean surface

For decades, stable isotopes of water have been used as proxies to infer the variation of the hydrological cycle. However, it is still not clear how various atmospheric processes quantitatively control kinetic fractionation during evaporation over the ocean. Understanding kinetic fractionation is important in that the interpretation of the isotopic composition record preserved in ice cores and precipitation relies in part on the isotopic information at the moisture source. In addition, the isotopic composition of vapor contains information about variation of atmospheric processes such as turbulence and change in moisture source region which is useful for studying meteorological processes and climate change.;In this study the isotopic composition of water vapor in the marine boundary layer (MBL) over the ocean was investigated using a combination of a newly developed marine boundary layer (MBL) model and observational data. The new model has a more realistic MBL structure than previous models and includes new features such as vertical advection of air and diffusion coefficients that vary continuously in the vertical direction. A robust linear relationship between deltaD and delta18O was found in observational oceanic water vapor data and the model can well capture the characteristics of this relationship. The individual role of atmospheric processes or variables on deltaD, delta18O and d-excess was quantitatively investigated and an overview of the combined effect of all the meteorological processes is provided. In particular, we emphasize that the properties of subsiding air (such as its mixing ratio and isotopic values) are crucial to the isotopic composition of surface water vapor.;Relative humidity has been used to represent the moisture deficit that drives evaporative isotopic fluxes, however, we argue that it has serious limitations in explaining d-excess variation as latitude varies. We introduce a new quantity Gd=SST-Td, the difference between the sea surface temperature (SST) and dew point (Td) at the measurement height (20m). Gd represents the dew point temperature gradient between sea surface and 20m; it better explains the variation of d-excess than relative humidity. Finally, we study the difference of isotopic composition and d-excess of water vapor between Arctic and subtropical regions. The results highlight the relationship between climate and isotopic ratios of moisture sources at different regions. Model results show that deltaD of Arctic oceanic surface water vapor is much more sensitive to variability in mixing ratio of subsiding air than that of subtropics. The Tropospheric Emission Spectrometer (TES) six years (2004--2009) observational data shows a significant decreasing trend of deltaD immediately above the MBL. The observed decrease of deltaD may be a result of the concurrent moisture gain of the Arctic upper troposphere.