This study adopted the stalagmite sample TK22-1 collected from Tiankeng Cave in Youyang Autonomous County, Chongqing, China, as the research object. A high-resolution δ¹⁸O sequence spanning the past century was reconstructed through a combination of high-precision ²³⁰Th dating and stalagmite laminae counting. This study reveals that the δ¹⁸O values of stalagmite TK22-1 exhibited a range of -6.03‰ to -7.29‰, with a mean value of -6.57‰. The positive shifts of δ¹⁸O (peaks) correspond well with recorded major drought events in the Sichuan-Chongqing region, such as the 2006 summer drought and the 1960 drought disaster. Correlation analyses show that δ¹⁸O variations exhibit a significant positive correlation with annual mean temperature but significant negative correlations with both annual mean humidity and the number of annual precipitation days. Although negatively correlated with total annual precipitation, this relationship does not reach statistical significance. Notably, the δ¹⁸O record closely corresponds to local drought events, indicating that regional hydrological conditions-such as temperature, the number of precipitation days, and humidity-are the dominant factors influencing stalagmite δ¹⁸O values.

The nearly 100-year stalagmite δ¹⁸O time series shows distinct phases: δ¹⁸O remained consistently negative from 1900 to 1910, shifted to positive between 1910 and 1960, briefly decreased from 1960 to 1970, and have exhibited a consistently positive trend since 1970. Principal component analysis demonstrates that the positive δ¹⁸O trend in stalagmites from the Chinese monsoon region is spatially widespread, with a significant weakening observed after 1980. The observed weakening of the Asian Summer Monsoon (ASM) results from a combination of natural variability, such as 11-year solar cycles and Pacific Decadal Oscillation phase shifts, and anthropogenic forcing including greenhouse gases and aerosols. The primary driving mechanisms including ENSO regime transitions, weakening of Atlantic Meridional Overturning Circulation (AMOC) and Pacific Walker Circulation (PWC), and aerosol effects.

This trend is significantly associated with global rise in temperature, an increased frequency of extreme ENSO events, and the weakening of both the AMOC and the PWC. Wavelet analysis identifies significant ENSO-related periodicities (2 years to 7 years) in the δ¹⁸O record. Between 1960 and 2000, the ENSO system progressively shifted toward El Niño-like states, with Central Pacific (CP) El Niño events suppressing monsoon intensity. The weakening of AMOC reduces land-sea thermal gradients, while the weakening of PWC alters zonal sea surface temperature gradients; together, these changes diminish monsoon dynamics.