The Effect of Monsoon Circulation on the Stable Isotopic Composition of Rainfall

Understanding the factors that control the variability of oxygen isotopic ratios (O-18) of Indian Summer Monsoon (ISM) rainfall (O-18(p)) is of vital importance for the interpretation of O-18(p) derived from climate proxies (e.g., speleothem and tree ring cellulose) of this region. Here we demonstrate the importance of moisture transport pathways on spatiotemporal variations of ISM O-18(p) using a new set of daily observations from central and northern India and previously reported data aided by simulations from an isotope-enabled General Circulation Model. O-18-depleted rain events are characterized by a higher number of air parcel back trajectories through the Bay of Bengal branch of moisture transport, while those through the Arabian Sea branch are associated with O-18 enriched rain events. This effect is observed on intraseasonal to interannual timescales in the long-term observations at New Delhi as well. Thus, the shift in moisture transport regimes must be considered when interpreting O-18(p) from climate proxies of the ISM region.