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 (δ 13 C) of five Cryptomeria fortunei specimens from Mt. Tianmu, a subtropical area in southern China. The five δ 13 C time series records are combined into a single representative δ 13 C time series using a “numerical mix method.” These are normalized to remove temporal variations of δ 13 C in atmospheric CO 2 to obtain a carbon isotopic discrimination (Δ 13 C) time series, in which we observe a distinct correlation between Δ 13 C and local April to June mean relative humidity ( RH AMJ ) (n = 64, r = 0.858, p < 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 δ 13 C 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.