Seasonal variations in the isotopic composition of near-surface water vapour in the eastern Mediterranean

Although the isotopic composition of precipitation is widely used in global climate change studies, use of water vapour isotopes is considerably more limited. Here we present the results from 9 yr of atmospheric vapour measurements in the Eastern Mediterranean, at a site in Israel. The measurements show a strong mean seasonal cycle of about 4‰ in ¹⁸O (peaking around July). This seasonality could not be adequately explained by changes in surface interactions or in air mass trajectories, as usually invoked for variations in local precipitation. We could explain this cycle only as a combination of three components: (1) rainout effects; (2) temperature and relative humidity control of the initial vapour and (3) seasonal variations in the vertical mixing across the top of the planetary boundary layer. This last component is emphasized in the current study, and it was shown to be a significant factor in the seasonal cycle features. The measurements were also compared with an isotope-enabled GCM (CAM2) run, which exhibited a markedly different seasonal cycle. Such comparisons with vapour isotopes data could help in constraining models better.