Climatic Controls on Water Vapor Deuterium Excess in the Marine Boundary Layer of the North Atlantic Based on 500 Days of in Situ, Continuous Measurements

Abstract:

Continuous, in situ measurements of water vapor isotopic composition have been conducted in the North Atlantic, Bermuda Islands (32.26°N 64.88°W) between November 2011 and June 2013, using a cavity-ringdown-spectrometer water vapor isotope analyzer and an autonomous self-designed calibration system. Meticulous calibration allows us to reach an accuracy and precision on 10 minute average of d¹⁸O, dD, and d-excess of respectively 0.14‰, 0.85‰, and 1.1‰, verified using two parallel instruments with independent calibration. As a result of more than 500 days with 6-hourly data the relationships between deuterium excess, relative humidity (rh), sea surface temperature (SST), wind speed and direction are assessed. From the whole dataset, 84% of d-excess variance is explained by a strong linear relationship with relative humidity. The slope of this relationship (-42.6 ± 0.4 ‰ per % (rh)) is similar to the theoretical prediction of Merlivat and Jouzel (1979) for SST between 20ºC and 30ºC. However, in contrast with theory, no effect of wind speed could be detected on the relationship between d-excess and relative humidity. Separating the dataset into winter, spring, summer, and autumn seasons reveals different linear relationships between d-excess and humidity. Changes in wind directions are observed to affect the relationships between d-excess and humidity. The observed seasonal variability in the relationship between d-excess and relative humidity underlines the importance of long-term monitoring to accurately separate signals of local evaporation from signals associated with moisture advection.

Steen-Larsen, H. C., Sveinbjörnsdottir, A. E., Peters, A. J., Masson-Delmotte, V., Guishard, M. P., Hsiao, G., Jouzel, J., Noone, D., Warren, J. K., and White, J. W. C.: Climatic controls on water vapor deuterium excess in the marine boundary layer of the North Atlantic based on 500 days of in situ, continuous measurements, Atmos. Chem. Phys., 14, 7741-7756, doi:10.5194/acp-14-7741-2014, 2014.