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

摘要

Whereas data on deuterium (D) and oxygen (~(18)O), 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 (δ~(18)O_(vapor). δDvapor) and rain (δ~(18)O_(rain), δD_(rain)) 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 δ~(18)O_(vapor), δD_(vapor), respectively, and 46‰ in deuterium excess (δD - 8. δ~(18)O) of local atmospheric water vapor (ⅱ) δ~(18)O_(vapor), δD_(vapor). δ~(18)O_(rain) and δD_(rain) exhibit correlated variations during the monsoon season (mid-June to mid-September): the daily variations of δ~(18)O_(vapor) and δD_(vapor) 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.