Multivariate driver analysis and moisture attribution of the December 2023 Tuticorin floods

Between 17^th -18^th December 2023, Tuticorin district and adjoining regions in Southern India experienced an exceptional extreme precipitation event, receiving approximately 950 millimetres of rainfall within 24 hours, leading to severe inundation and extensive losses to agriculture and infrastructure. The fact that the amount of rainfall received on a single day exceeded the average annual rainfall over Tuticorin makes this event particularly noteworthy. This study investigates the hydrological and meteorological drivers responsible for this rare extreme event using high resolution reanalysis datasets and India Meteorological Department 0.25° gridded precipitation data. The influence of Integrated water Vapour Transport (IVT), along with other dynamic factors such as atmospheric instability and total column water vapour on the extreme precipitation is assessed using a factor combination methodology based on conditional probability. Additionally, to reveal the moisture sources for this event, the backward trajectory of moisture particles was traced using HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Results reveal that all three factors exceeded their 99^th percentile thresholds, with their peaks occurring one day prior to the rainfall maximum, indicating a strong preconditioning of the atmosphere for extreme convection. HYSPLIT results confirmed sustained moisture influx from the Bay of Bengal and equatorial Indian Ocean up to seven days before the event. A comparative evaluation across El Niño years (1991, 1997, 2005, 2015, and 2023) showed that only the 2023 event exhibited concurrent extremes in all parameters. These findings underscore the compound nature of the 2023 Tuticorin flood and highlight the need for integrated moisture diagnostics in predicting future extreme rainfall events over peninsular India.