Sources of water vapor and their effects on water isotopes in precipitation in the Indian monsoon region: a model-based assessment

Climate records of ratios of stable water isotopes of oxygen (delta O-18) are used to reconstruct the past Indian monsoon precipitation. Identifying the sources of water vapor is important in understanding the role of monsoonal circulation in the delta O-18 values, to aid in monsoon reconstructions. Here, using an isotope-enabled Earth system model, we estimate the contributions of oceanic and terrestrial water vapor sources to two major precipitation seasons in India-the Southwest monsoon and the Northeast monsoon, and their effects on the delta O-18 in precipitation (delta O-18(p)). We find that the two monsoon seasons have different dominant sources of water vapor because of the reversal in atmospheric circulation. While Indian Ocean regions, Arabian Sea, and recycling are the major sources of the Southwest monsoon precipitation, North Pacific Ocean and recycling are two crucial sources of Northeast monsoon precipitation. The delta O-18(p) of the Southwest monsoon precipitation is determined by contributions from the Indian Ocean sources and recycling. Despite reduced precipitation, more negative delta O-18(p) values are simulated in the Northeast monsoon season due to larger negative delta O-18(p) contributions from the North Pacific. Our results imply that changes in atmospheric circulation and water vapor sources in past climates can influence climate reconstructions using delta O-18.