Large-scale atmospheric circulation control on stable water isotopes in precipitation over the northwestern and southeastern Tibetan Plateau

Abstract. The mid-latitude westerlies and South Asian Summer Monsoon (SASM) are two major atmospheric circulation systems influencing the Tibetan Plateau (TP). We report a seven-year (2007/2008–2013/2014) dataset of δ¹⁸O in precipitation (δ¹⁸O_p) collected at three stations. Taxkorgan (TX) and Bulunkou (BLK) are located on the northwestern TP where westerly winds dominate while Lulang (LL) is situated on the southeastern TP where the SASM dominates. δ¹⁸O in precipitation (δ¹⁸O_p) in northwestern TP varies with surface temperature (T) throughout the study period, and is depleted in ¹⁸O in precipitation during June to September when the monsoonal circulation enters the TP. Integration with model outputs suggests that large-scale atmospheric circulation plays a major role in isotopic seasonality in both regions. A teleconnection between precipitation on the northwestern TP and the El Niño-Southern Oscillation (ENSO) warm phase is suggested by changes in the relationship between δ¹⁸O and δD (e.g., reduced slope and weighted d-excess) in precipitation samples. These observations are indicative of a weakening of the mid-latitude westerly jet allowing local processes in the continental interior to become more dominant, thereby increasing the contribution of secondary evaporation from falling raindrops and kinetic fractionation. Under the conditions of a high Northern Annular Mode (NAM) the westerly jet is intensified over the southeastern TP which enhances local evaporation and continental recycling as revealed by a lower δD-δ¹⁸O slope and intercept, but higher d-excess average in contemporaneously collected precipitation samples. The significant correlation between T and δ¹⁸O_p in the northwestern TP during various composite periods highlights a variation from 0.39 ‰ / ℃ (ENSO warm) to 0.77 ‰ / ℃ (high NAM), attributable to decreased (increased) water vapor availability over the northwestern TP during the ENSO warm (strong positive NAM) phase. ENSO cold and strong negative NAM phases show analogous effects on atmospheric circulation over both regions.