Asian Summer Monsoon-Related Relative Humidity Recorded by Tree Ring δ¹⁸O During Last 205 Years

To deeply understand the Asian summer monsoon (ASM) strength variation in its fringe region, we established a 205-year stable oxygen isotope (δ¹⁸O) chronology based on four individual Pinus tabulaeformis Carr. from Ordos Plateau, China. First, we found that δ¹⁸O in tree rings inherits the signals of summer (June to August) monsoonal precipitation δ¹⁸O (δ¹⁸O_P). Second, relative humidity (RH) was observed to have a large effect on the δ¹⁸O of pine trees. The tree ring δ¹⁸O series significantly correlates with the observed local RH from June to August (RH_JJA) with r = −0.65 (N = 56, p < 0.001). The RH over Ordos Plateau is greatly influenced by ASM-related precipitation. As such, it could well reflect the variations of ASM intensity. Thus, we reconstructed the RH_JJA over Ordos, the center of the ASM fringe region (ASMFR), for the last two centuries. Our reconstructed RH_JJA series has a large geographical and spatial representativeness, which is well compared with other precipitation reconstructions in ASMFR and jointly reflects ASM variations in the past. The RH_JJA reconstruction showed that the 1870s–1890s were the wettest years in the entire reconstruction, while the middle-late 1920s were the driest. From 1930 to 2012, the RH of the study area continuously decreased. We found significantly negative correlations between RH_JJA and the Central Pacific sea surface temperature. This finding means that if the El Niño–Southern Oscillation of the Central Pacific continues to increase in the future, the aridification in the ASMFR will be intensified.