Understanding summer monsoon variability in Northern India through isotopic signatures of precipitation water and water vapor isotopes

Northern India receives rainfall from dual precipitation systems: Indian Summer Monsoon (ISM) and the Westerlies. Isotopic analysis of precipitation water and water vapor isotopes from the region can serve as a tracer to identify the moisture source and atmospheric phenomenon associated with the advection of moisture-laden air parcel. Water isotopic ratios (δ¹⁷O, δ¹⁸O) and secondary parameters (d-excess and ¹⁷O-excess) provide critical insights in understanding the monsoon dynamics of the region. ¹⁷O-excess enables us to estimate relative humidity conditions at the source region. This study presents the first continuous record of triple oxygen isotopes in the North Indian region (a low-latitude region but still having high heat exchange capacity due to extensive glacial mass).

Event based rainwater samples have been collected using a standard rain collector and high-resolution isotopic data of atmospheric vapor has been acquired using Picarro-L2140i installed at Manali station for June-July-August-September (JJAS) 2024. Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back trajectory analysis suggests that moisture responsible for rainfall in Manali originates from Bay of Bengal, Arabian Sea, Westerlies and some local moisture sources. Specific humidity changes plotted along the back trajectories carry vital information regarding moisture percentage calculations from various sources. Local Meteoric Water Line (LMWL) have also been generated for the region which suggests significant influence of continental recycling and evaporative enrichment. Quantitative estimation of moisture contribution from various sources and effects of local meteorological parameters (e.g. wind speed & direction, relative humidity, temperature, rainfall amount) on isotopic values of atmospheric water observed in this study will be discussed at the time of presentation.