Seasonal incursion of Indian Monsoon humidity and precipitation into the southeastern Qinghai-Tibetan Plateau inferred from tree ring δO18 values with intra-seasonal resolution

•A clear intra-seasonal cycle was detected in Abies georgei tree-ring δ18O.•The intra-seasonal variation was mainly controlled by precipitation δ18O.•A similar seasonal δ18O pattern occurs in other parts of the Indian Monsoon region.•Thus, intra-seasonal tree-ring δ18O can reconstruct monsoon incursions.

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 (δ18O) of Smith fir (Abies forrestii var. georgei) growing at two different altitudes (high-elevation site: 29°39′N, 94°42′E, 4200 m a.s.l.; low-elevation site: 29°39′N, 94°43′E, 3800 m a.s.l.) on a SE-facing slope in the Sygera Mountains, southeastern Qinghai-Tibetan Plateau. Mechanistic modeling suggests that δ18O 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 δ18O 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.