Monsoon onset signal in the stable oxygen and hydrogen isotope ratios of monsoon vapor

•Early detection of monsoon onset using stable isotopic composition of atmospheric vapour appears possible.•The assumption of isotopic equilibrium between rain water and vapour during the monsoon is tested.•Important data set for the validation of satellite observations of vapour isotopes in the atmosphere.•Rare high resolution vapour isotopic data over Indian monsoon region.

Whereas data on deuterium (D) and oxygen (18O), effective tracers of hydrological processes, are available for global precipitation, such data on atmospheric water vapor are limited. With the advent of satellites capable of measuring D in atmospheric water vapor, mesoscale moisture transport processes (e.g. monsoons) could be detected early provided the signal is well above the noise. Our results of daily measurements of stable oxygen and hydrogen isotopic compositions of atmospheric water vapor (δ18Ovapor, δDvapor) and rain (δ18Orain, δDrain) during 2007–2008 CE over Ahmedabad, India, show that this is indeed so: (i) the onset of monsoon is marked by a dramatic decrease of ∼2.9‰ and ∼60‰, in δ18Ovapor, δDvapor, respectively, and 46‰ in deuterium excess (δD – 8. δ18O) of local atmospheric water vapor (ii) δ18Ovapor, δDvapor, δ18Orain and δDrain exhibit correlated variations during the monsoon season (mid-June to mid-September): the daily variations of δ18Ovapor and δDvapor are higher during large rain events, with sharp negative excursions from mean values. Deuterium excess of vapor and rain are indistinguishable from each other, within their natural variability, consistent with the achievement of isotopic equilibrium between them.