East Asian Precipitation δ18O Relationship With Various Monsoon Indices

Chinese speleothem oxygen isotope (delta O-18) variations have been widely interpreted as recording the evolution of the Asian Monsoon. However, calibration of delta O-18 to monsoon intensity has not yet been carried out in a quantitative way. To understand the climatic significance of delta O-18, we measured monthly precipitation delta O-18 data at Heshang Cave, China, where speleothem delta O-18 records were previously obtained. We examined the influence of local climate and large-scale atmospheric circulation on the delta O-18 by correlating to local meteorological data and various monsoon indices. We find neither a significant amount effect nor temperature effect, and therefore suggest that local climate is not the primary driver of delta O-18 variability. On seasonal timescales, monthly delta O-18 is significantly correlated to all monsoon indices, especially the Indian Monsoon (IM) and Western North Pacific Monsoon (WNPM) indices, proving delta O-18 responds dominantly to the tropical monsoon circulation. On interannual timescales, there are significant negative correlations between annual delta O-18 and the WNPM and IM monsoon indices defined by zonal winds, reflecting the impact of the El Nino-Southern Oscillation on delta O-18. Similar correlations between delta O-18 and precipitation amount in these monsoon regions lend further support to this finding. As the driving force of seasonal and interannual monsoon changes can be compared to variations on orbital and suborbital scales, respectively, we argue that speleothem delta O-18 in southern China is influenced by both external forcing and internal variability. Solar radiation plays an indispensable role on the orbital and seasonal scale delta O-18 variations, whereas ocean-atmosphere interactions are dominant on suborbital timescales.