Controls of precipitation δ18O on the northwestern Tibetan Plateau: A case study at Ngari station

The shifting atmospheric circulation between the Indian monsoon and the westerlies on the northwestern Tibetan Plateau (TP) influences precipitation as well as precipitation isotopes. Isotopic records will therefore show historical fluctuations. To understand better the factors controlling present day precipitation delta O-18 values on the northwestern TP, we made continuous observations of precipitation isotopes at Ngari station from 2010 to 2013. The drivers of precipitation delta O-18 were investigated using analyses of their statistical relations with temperature, precipitation amount, relative humidity, and convective activities based on outgoing longwave radiation (OLR) data from NOAA satellites, and downward shortwave radiation (DSR) data collected at the Ngari automatic weather station. Atmospheric circulation patterns from NCAR reanalysis, and moisture transport paths of individual events derived from the HYSPLIT model using NCEP data, were also used to trace moisture sources. The results of our study include: (1) The slope and intercept of the Local Meteoric Water Line (LMWL) at Ngari (delta D = 8.51 delta O-18 + 11.57 (R-2 = 0.97, p < 0.01)) were higher than for the Global Meteoric Water Line (GMWL), indicating drier local climatic conditions; (2) Precipitation delta O-18 values showed a weak "temperature effect" and a weak "precipitation amount effect" at Ngari; and (3) Convection (or temperature patterns) integrated over several days (0-20) preceding each event were determined to be the main driver of precipitation isotopic values in monsoon (or non-monsoon) season. The longer (shorter) periods of tau(m) days when correlation coefficients between precipitation delta O-18 and OLR were at their maxima (minima) indicate deep convective activities (shorter moisture transportation pathways) in August (June, July, and September). (C) 2017 Elsevier B.V. All rights reserved.