Unstable relationship between tree-ring δ¹⁸O in the transitional zone of the Asian summer monsoon and the Indian summer monsoon

The Asian Summer Monsoon system (ASM) affects a large population in the monsoonal Asia. Previous studies have thoroughly discussed the temporally unstable response of tree-ring oxygen isotope (δ¹⁸O) to the regional climate variables and ocean–atmosphere interaction modes, however, the response to the ASM remains ambiguous. In this study, an annually resolved tree-ring δ¹⁸O chronology is developed from eastern Yunnan-Guizhou Plateau, in the transitional zone of the ASM. Based on the results from response analyses, the climatic controls of tree-ring δ¹⁸O is considered to be the moisture-related variables of the monsoon season. Spatial correlations suggest that in the study area, the water vapor is mainly originated from the Bay of Bengal and transported by the Indian Summer Monsoon (ISM). Furthermore, the temporal stability in relationships between the tree-ring δ¹⁸O and the ISM variability are explored. Our findings suggest that on interannual to decadal timescales, the tree-ring δ¹⁸O is significantly anti-correlated, but temporally unstable with respect to the ISM. The strength of interannual relationship is affected by the ISM intensity, which is modulated by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variability. Moreover, the strength of decadal relationship is associated with the phase shift of the Pacific Decadal Oscillation (PDO). Additionally, the decreasing trend of tree-ring δ¹⁸O after the late 1970 s that diverges from the simultaneous normal level of the ISM is caused by the isotopically-depleted trend in upstream precipitation of the moisture source. The opposite patterns of correlations between the tree-ring δ¹⁸O and the ENSO and IOD in recent decades demonstrate that the regional hydroclimate in the study region has been undergoing significant secular variations that are characterised by an increasing effect of the IOD and a decreasing effect of the ENSO. The findings from this study further improve our understanding of the tree-ring δ¹⁸O response to the ASM, at multi-time scales, and shed light on the impact of the ASM on regional hydroclimate variability, across the transitional zone of the ASM.