A model-based assessment of the relative contribution of different regions to water isotopes in precipitation over India

Climate records of ratios of stable isotopes of water (18O) are used to reconstruct the past Indian monsoon precipitation. For this purpose, an assessment of the factors that control the present-day distribution of water isotopes in the monsoon precipitation (18Oprecip) is necessary to interpret these records. In this study, we estimate the relative contribution of different water vapor sources (oceanic and land water recycling) to the present-day distribution of 18Oprecip in the Indian land region. For this, we use water vapor and isotope tagging in the isotope-enabled earth system model iCESM1.2. We choose two major precipitation seasons for our study the Southwest monsoon [SW, June to September], the major contributor of annual precipitation in the region, and the Northeast monsoon [NE; October to December] that is important for the annual precipitation in the southern peninsular Indian region. Our results indicate that iCESM1.2 can reproduce the seasonal distribution of precipitation and 18Oprecip in the Indian region. The two monsoon seasons have different major sources of water vapor because of the reversal in circulation between these two seasons. South and the central Indian Ocean, Arabian Sea, and Indian land recycling are the major contributing regions of water vapor for the Southwest monsoon precipitation over India. Because of the monsoon circulation reversal, the North Pacific, Indian land recycling, the Bay of Bengal, and the Arabian Sea are the major contributors of water vapor for Northeast monsoon precipitation. More negative 18Oprecip values are simulated over the Indian monsoon region in the Northeast monsoon season relative to the Southwest monsoon season despite lower precipitation rates. This is due to larger negative contributions from remote sources, such as the North Pacific and the Bay of Bengal during the Northeast monsoon season.