Climatic Significance Of Stable Oxygen In Tree Rings Over The Yunnan-Guizhou Plateau

The Asian summer monsoon(ASM),the strongest regional component of the global monsoon systems,is the dominant large-scale circulation process that affects summer climate dynamics in Monsoon Asia.ASM transports large quantities of heat and water vapor to Inner Asia during the boreal summer,exerting a considerable and long-lasting influence on Asian natural ecosystems and cultural development.Due to insufficient temporal coverage of the available instrumental data,various paleoclimatic proxies were utilized to develop a long-term history of ASM variability and explore ASM's effects on different areas.Thereinto,stable oxygen isotope ratios in tree-ring cellulose(?¹⁸O)are considered as an indispensable paleoclimatic proxy that can exactly reflect ASM-related hydroclimate variability due to the unique advantage of precise dating,high temporal resolution,broad spatial distribution,successive sequence,and high sensitivity to monsoon rainfall changes.Previous tree-ring?¹⁸O-based climatic studies were concentrated in the areas that are controlled by the two main subsystems of ASM,namely the Indian summer monsoon(ISM)and East Asian summer monsoon(EASM)regions.In the ISM-EASM transitional zone,however,few studies have reported ASM signals identification based on tree-ring?¹⁸O records.Current studies of isotopic dendroclimatology over the ISM-EASM transitional zone have paid specific attention to the identification of climatic factor and climate reconstruction either from individual sites or a single tree species,there lacks a comprehensive climate-proxy relationship based on a regional-scale tree-ring?¹⁸O network development.This study is carried out in the Yunnan-Guizhou Plateau(YGP),a core region in the ISM-EASM transitional zone.Here,an annually resolved tree-ring network consisting of eight?¹⁸O records was developed across YGP for investigating their temporal variability and spatial pattern over the past 150 years.The relationships between the tree-ring?¹⁸O records and regional climate were explored to identify spatiotemporal patterns and climatic forcing of the tree-ring?¹⁸O variability.Furthermore,we examined the linkages of tree-ring?¹⁸O records with observational rainfall?¹⁸O records and existing tree-ring?¹⁸O records in the ISM and EASM regions to comprehensively clarify the climatic significance of the tree-ring?¹⁸O records on different timescales and reveal moisture sources and transport processes in the ISM-EASM transitional zone.This study provides a convincing hydroclimatic interpretation of large-scale paleo-?¹⁸O records and promote an integrated understanding of monsoon hydrological cycles in Monsoon Asia.Meanwhile,the findings of this study also provide a paleo-isotopic insight into monsoon-related moisture transport,which will benefit further development of isotope-based hydroclimatic models on a larger spatiotemporal scale.The main conclusions of this study are summarized as follows:(1)Tree-ring?¹⁸O variability in YGP mainly reflected local hydroclimate variability and large-scale circulation processes.Local hydroclimate(especially summer relative humidity)affected isotopic components of source water and fractionation process of oxygen isotope during tree growth.Large-scale ocean-atmosphere interaction modes across the Indo-Pacific region controlled ASM intensity that regulates the moisture transport process from the tropical oceans to inner Asia,which generally led to the alteration of the amount of monsoon rainfall in upstream regions(the Indochina Peninsula)and the study region,and further affected tree-ring?¹⁸O variations over YGP.(2)Interannual changes in tree-ring?¹⁸O over YGP showed a spatial coherence,which was caused by the synchronous hydroclimate variability and moisture transport process under the control of ISM.Regional synchrony of summer relative humidity between the adjacent sites modulated the inter-site coherence of tree-ring?¹⁸O records.Synchronous changes in monsoon rainfall?¹⁸O contributed considerably to the spatial coherence of the tree-ring?¹⁸O records over YGP.Furthermore,the ISM-related moisture transport from the Indian Ocean to the Indochina Peninsula and YGP resulted in the consistency in temporal changes of tree-ring?¹⁸O records in YGP and the Indochina Peninsula.(3)The east-west(divided by 105°E)asynchronized pattern of tree-ring?¹⁸O records in the ISM-EASM transitional zone was primarily attributed to the different moisture sources and transport pathways to respective regions under the control of the distinct summer monsoon systems.Tree-ring?¹⁸O records in the western part reflected ISM-related moisture transport that originates from the Indian Ocean and through the Indochina Peninsula and to YGP.While tree-ring?¹⁸O records in the eastern part revealed the moisture sources from both the Indian Ocean and South China Sea,which was modulated by the collective effects of ISM and EASM.Moreover,the eastern tree-ring?¹⁸O records can reflect the interannual variability in regional precipitation seasonality(the ratio of monsoon rainfall and non-monsoon rainfall).(4)Temporal variation and spatial distribution of the tree-ring?¹⁸O records in tropical and subtropical regions of Monsoon Asia were able to trace the moisture sources of monsoon rainfall at the tree-ring sites.Spatial distribution pattern of tree-ring?¹⁸O values resembled that of monsoon rainfall?¹⁸O values,which showed a geographically depleted trend from coastal to inner areas.Tree-ring?¹⁸O records in the northern Indian subcontinent,the Indochina Peninsula,and South China commonly indicated a moisture source signal from the southwest,while tree-ring?¹⁸O records in the southeastern Tibetan Plateau reflected the stronger local moisture signal.In addition,large-scale tree-ring?¹⁸O records mainly revealed ASM intensity and interannual variability of El Ni?o-Southern Oscillation(ENSO).