Considerable influences of recycled moistures and summer monsoons to local precipitation on the northeastern Tibetan Plateau

Abstract

Located within the transition zone between the monsoon and westerlies circulation system, the northeastern Tibetan Plateau (NETP) is a critical region for understanding the alternative roles of various atmospheric moisture sources in local precipitation and the resultant hydrological divergences. Yet, determination of contributions of the recycled moisture and the summer monsoonal flows to the local precipitation remain ambiguous. In this study, daily time series precipitation isotope data for three consecutive years (2016–2018) at two representative sites (Huangyuan (HY) and Gangcha (GC)) on the NETP were analyzed. The results reveal remarkable seasonal variations in the precipitation isotopes (δ2H and δ18O) on the NETP, which were alternatively influenced by local climate conditions and the transitions of moisture sources. Much lower slopes (<8) and intercepts (<10) of the daily-based Local Meteoric Water Lines (LMWLs) at both sites suggested that sub-cloud evaporation of raindrops and monsoonal activities exerted significant influences in local precipitation. Local climatic conditions partially modulated the isotopes of precipitation and during entire study period at HY; however, they exhibited weak impacts on the isotopes of precipitation at GC, indicating different controlling factors and diverse moisture sources between the two sites. Combined the isotopic data and the HYSPLIT model results, we found that the westerly, the recycled moisture, and the summer monsoon circulation alternatively influenced the isotopic composition of precipitation between June and August. The recycled fractions accounted for 0%-11.6% (monthly mean: 5.1%–7.0%) and 0%–20.2% (monthly mean: 3.1%–7.3%) of the monthly precipitation at HY and GC, respectively. Notably, we demonstrated that both the East Asian Summer Monsoon (EASM) and the Indian Summer Monsoon (ISM) reached the NETP and contributed considerable moistures to the local precipitation. As many as 87.0 mm and 62.2 mm of precipitation were generated from monsoonal water vapor in 2016, which accounted for 14.96% and 9.93% of the annual total amounts at HY and GC. These findings provide a better understanding of the interactions between atmospheric water vapor and precipitation and the roles of monsoonal activities in local climates, both of which are beneficial for interpreting paleo-climate reconstruction on the TP.