Moisture source signals preserved in a 242-year tree-ring δ18O chronology in the western Himalaya

Abstract

The oxygen isotope ratio (δ¹⁸O) of tree-ring cellulose is known to be a reliable proxy for hydroclimate in monsoon Asia. However, tree-ring δ¹⁸O data are still spatially and temporally limited, so a denser tree-ring network in Asia is required to better understand the dynamics of the monsoon circulation and its past variability. Here, we present a 242-year δ¹⁸O chronology calculated from five silver fir trees collected in the western (Indian) Himalaya, a region located on the northwestern periphery of the summer monsoon incursions. Response analyses using regionalized climatic data revealed that tree-ring δ¹⁸O is controlled by hydroclimatic variables, including precipitation, relative humidity, and the drought index, during the summer monsoon season. In addition, spatial correlation analyses with gridded climatic parameters showed that the strongest correlations of tree-ring δ¹⁸O are not observed with the climatic parameters at the sampling site, but with those in a region several hundred kilometers to the southwest, indicating that water vapor originating in the Arabian Sea is transported to the study site. Based on these results, we reconstructed the self-calibrating Palmer Drought Severity Index (scPDSI) for the summer monsoon season (June–September) over the past 242 years (1767–2008 CE), using a linear regression model that accounts for 45.0% of the actual scPDSI variance. Our chronology showed significant correlations with other tree-ring δ¹⁸O data from Nepal and Bhutan, indicating that common signals related to the moisture supply from the Bay of Bengal are also recorded in the present reconstruction. However, the tree-ring record from India often showed weak correlations with that from Bhutan, especially when the summer monsoon was relatively weak. This result, together with the fact that the water vapor at the tree site was also derived from the Arabian Sea, implies that a weaker monsoon circulation enhances the flux of Arabian Sea moisture and reduces the flux of Bay of Bengal moisture to the study region.