Tree-ring δ¹⁸O, a window into the summer hydroclimatic variations in the Central Water Tower of China

The Qinling-Bashan Mountains (QBMs) serves as an important boundary between southern and northern China and is dubbed China’s Central Water Tower (CCWT). However, understanding the spatiotemporal characteristics of summer hydroclimatic variations within the CCWT and their underlying causes has been challenging by a lack of long-term, high-resolution data. Here, we present a two-century-long summer relative humidity (RH_JJA) reconstruction for the southern CCWT using tree-ring δ¹⁸O records. This reconstruction explains 43.60 % of the instrumental RH_JJAvariance and, for the first time using proxy data, highlights that two notorious mega-droughts—“the Dingwu drought” and “the 1940–1943 drought”—also significantly impacted the study area. The identified three dry periods (1850–1859, 1920–1943, 1966–1982 CE) and three wet periods (1861–1875, 1885–1898 and 2009–2013 CE) in our reconstruction largely align with broader CCWT hydroclimatic oscillations, indicating decadal synchronicity. However, an annual RH_JJAdiscrepancies was found between the northern and southern CCWT during 1943–1953 CE. A dry-warm/wet-cold pattern in the CCWT suggests that future warming may exacerbate dry conditions. The study demonstrates that the hydroclimatic variations in the CCWT are primarily driven by the Asian summer monsoon (ASM), with water vapor transported by the Indian summer monsoon (ISM) playing a dominant role. These hydroclimatic changes are further modulated by the El Niño-Southern Oscillation (ENSO). This new reconstruction is pivotal for comprehending the impacts of climate change, managing water resources, and safeguarding ecological systems within the CCWT and other monsoon regions.