Tree-ring stable carbon isotope-based April–June relative humidity reconstruction since ad 1648 in Mt. Tianmu, China

Based on accurate dating, we have determined the stable carbon isotope ratios (δ¹³C) of five Cryptomeria fortunei specimens from Mt. Tianmu, a subtropical area in southern China. The five δ¹³C time series records are combined into a single representative δ¹³C time series using a “numerical mix method.” These are normalized to remove temporal variations of δ¹³ C in atmospheric CO₂ to obtain a carbon isotopic discrimination (Δ¹³C) time series, in which we observe a distinct correlation between Δ¹³C and local April to June mean relative humidity (RH_AMJ) (n = 64, r = 0.858, p OpenSPiltSPi 0.0001). We use this relationship to reconstruct RH_AMJ variations from ad 1648 to 2014 at Mt. Tianmu. The reconstructed sequence show that over the past 367 years, Mt. Tianmu area was relatively wet, but in the latter part of the twentieth century, under the influence of increasing global warming, it has experienced a sharp reduction in relative humidity. Spatial correlation analysis reveals a significant negative correlation between RH_AMJ at Mt. Tianmu and Sea Surface Temperature (SSTs) in the western equatorial Pacific and Indian Ocean. In other words, there is a positive correlation between tree-ring δ¹³C in Mt. Tianmu and SSTs. Both observed and reconstructed RH_AMJ show significant positive correlations with East Asian and South Asian monsoons from 1951 to 2014, which indicate that RH_AMJ from Mt. Tianmu reflects the variability of the Asian summer monsoon intensity to a great extent. The summer monsoon has weakened since 1960. However, an increase in relative humidity since 2003 implies a recent enhancement in the summer monsoon.