A monsoon-related 174-year relative humidity record from tree-ring δ¹⁸O in the Yaoshan region, eastern central China

The authors present a time series of tree-ring cellulose stable oxygen isotope ratios (δ¹⁸O) constructed using a numerical mixing method, from five Pinus tabulaeformis from Yaoshan, eastern central China. The time series is synchronous with the observed April-to-September precipitation δ¹⁸O time series records from surrounding stations, implying that the tree-ring δ¹⁸O signals reflect Asian summer monsoon-related precipitation. Moreover, significant correlations were detected between the tree-ring δ¹⁸O records and central-eastern Pacific sea surface temperature (SSTs)/Asian summer monsoon indices. This suggests that the Yaoshan tree-ring δ¹⁸O time series offers a promising means to study past Asian summer monsoons and ENSO variability. The time series is significantly correlated with the mean relative humidity from April to September (RH_AS) (r=-0.638, n=52, p<0.0001) over the Yaoshan region. The authors assign a transfer function to the reconstruct the RH_AS time series from 1835 to 2008. The mean value of RH_AS over the period 1870-1890 is the lowest observed value during the past 174years, implying severe drought at that time. From 1930 to 2008, an obvious drying trend in the Yaoshan region is observed. Particularly from 1985 to 2008, frequent, extremely dry years occurred, with few extremely wet years. The drying trend is consistent with decreasing Asian summer monsoon precipitation over the Loess Plateau over the past 60years. The acidification implied by this data extends well beyond the vicinity of Yaoshan and may be expected to impact the entire monsoonal region in northern China. A multi-taper spectral analysis and an ensemble empirical mode decomposition EEMD of the time series indicate 14.6-12.4 and 6.4-2.3 year cycles in the reconstructed RH_AS time series during the past 174years, which correspond with solar cycles and ENSO cycles, respectively.