Stable isotope variations in monsoon precipitation on the Tibetan Plateau

Abstract

Precipitation samples were collected in a large-scale region (more than 2,000 km from the north to the south of the Tibetan Plateau) and a middle scale region (with an area about 9,000 km2) in the Tibetan Plateau to study the impact of monsoon precipitation on stable isotopes. In the large-scale region, samples of precipitation and river water were collected at several stations from the north to the south along the Qinghai-Tibet Highway on the plateau. Results show that the Indian monsoon not only influences the spatial variation of stable isotope in precipitation, but also the relationship between δ18O and δD. Deuterium excess (d) in areas where precipitation is directly controlled by monsoon precipitation is lower, while in the northern Tibetan Plateau, d is in higher value due to the influence of continental air mass and strong local hydrological recycling. A detailed study on oxygen stable isotope in precipitation was carried out on the middle scale in the Nagqu Basin area. The temporal variation of δ18O in daily precipitation at 6 sites shows quite similar trends, indicating that δ18O in precipitation in the middle Tibetan Plateau is mainly controlled by large-scale synoptic condition, rather than the local meteorological factors. The spatial variation of δ18O in precipitation in the basin agrees with the Tibet-scale spatial variation trend and local hydrological recycling. Temporal variations of δ18O in precipitation also show a close relation to the movements of monsoon precipitation. The onset of monsoon in the earlier summer leads to low δ18O value. There is a negative relation between δ18O and SWI (south wind index). High SWI, which represents intensive transport of moisture from south with the monsoon movement, is coincided with lower δ18O values, while low SWI, which represents moisture from inland air mass or local evaporated moisture, is associated with higher δ18O values. ©2001, Meteorological Society of Japan.