The Indian Summer Monsoon (ISM) exhibits a strong interplay between rainfall variability and the isotopic composition of precipitation, particularly the δ¹⁸O signature. While numerous studies have examined the spatial and temporal patterns of monsoon precipitation isotopes, high-temporal-resolution observations (e.g., daily or sub-daily) remain scarce, limiting our understanding of rapid isotopic responses to synoptic events such as cyclones, monsoon onset, and active–break phases. In this study, we analysed daily rainfall samples collected at Varanasi (25.32°N, 82.97°E) in the Ganga Basin during the Indian summer monsoon season (JJAS) of 2015 and 2016. Rainfall at Varanasi is strongly influenced by synoptic-scale monsoonal variability, which is reflected in distinct isotopic signatures during alternating active and break phases. Our results confirm the presence of the amount effect in this region across varying spatial scales. We also demonstrate the influence of a cyclonic event, which resulted in pronounced δ¹⁸O depletion. Furthermore, interannual variability in the relationship between d-excess and δ¹⁸O suggests differences in moisture sources and post-condensation processes. Overall, the findings highlight interannual variations in isotopic sensitivity to monsoonal circulation anomalies and underscore the utility of δ¹⁸O as a tracer of monsoon variability.
Datye et al. (Sun,) studied this question.