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 excees (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.