Lagrangian quantification of atmospheric moisture sources for extreme rainfall events over India during the 2023 summer monsoon.

Extreme rainfall during the Indian Summer Monsoon (ISM) accounts for approximately 27 % of the total seasonal rainfall. Most of this moisture is transported from the Indian Ocean. Amid ongoing warming of the Indian Ocean, 2023 stood out as one of the warmest monsoon years on record. The year 2023 exhibited the highest frequency, with 20 extreme rainfall events, most of which occurred during July and August. These conditions make 2023 a pivotal year for studying the intensification of extreme rainfall events in a warming climate. These unprecedented monsoon extremes are likely influenced by anomalously high Sea Surface Temperature (SST) relative to the past decade. This study quantifies moisture sources for 20 extreme rainfall events during the record-warm 2023 monsoon using a 3D Lagrangian framework (UTrack) with vertical-level moisture release, realistic vertical mixing, and fine-scale temporal resolution (0.1h). Using daily gridded precipitation data (1990-2023) and hourly ERA5 reanalysis meteorological data, we employed the Lagrangian model to quantify moisture transport. Our findings reveal significant spatial variability in moisture contributions from the remote as well as local moisture sources throughout the JJAS period. Among the different moisture source regions, the Western Indian Ocean was the largest contributor, accounting for 33.2 % of total moisture during these events. Meanwhile, the Ganga Basin and Bay of Bengal contributed 11.4 % and 10.5 %, respectively. Local sources, including the Bay of Bengal, the Ganga Basin, and the Arabian Sea, supplied almost 30 % of the total moisture, emphasizing their critical role. This study highlights regional contrasts in moisture source behavior during extreme events. It also provides insights that can enhance regional climate modeling and improve forecasts of extreme rainfall.