Seasonal incursion of Indian Monsoon humidity and precipitation into the southeastern Qinghai-Tibetan Plateau inferred from tree ring δ18O values with intra-seasonal resolution

Incursion of the Indian Monsoon flow onto the Qinghai-Tibetan Plateau is an important regional moisture-delivery mechanism, but the causes of intra-seasonal and longer-term variations are poorly understood. We observed a clear intra-seasonal cycle in the oxygen isotope composition (delta O-18) of Smith fir (Abies forrestii var, georgei) growing at two different altitudes (high-elevation site: 29 degrees 39'N, 94 degrees 42'E, 4200 m a.s.l.; low-elevation site: 29 degrees 39'N, 94 degrees 43'E, 3800 m a.s.l.) on a SE-facing slope in the Sygera Mountains, southeastern Qinghai-Tibetan Plateau. Mechanistic modeling suggests that delta O-18 of precipitation, which in turn depends on relative humidity and precipitation amount, is important in controlling the intra-seasonal tree-ring isotopic cycle, and we speculate that the same controls also operate on interannual and longer time scales. If so, multi-century delta O-18 records with intra-seasonal resolution may enable us to infer long-term variations in the incursion of the Indian Monsoon onto the Qinghai-Tibetan Plateau. (C) 2016 Elsevier B.V. All rights reserved.