Interaction of the Monsoon Trough and Western Disturbance Ignites Multiday Extreme Rainfall Event in July 2023 over North India

Abstract The present paper aims to investigate the atmospheric processes that led to extreme rainfall resulting in destructive flash flooding and loss of life and property in the northwestern region of India on 8–9 July 2023. Exceptionally heavy precipitation occurred in the western Himalaya and adjoining regions, leading to widespread disruption of communication, electricity, and inundation of houses. Flash floods, combined with debris flows, caused massive devastation in northern India, with the overall death toll surging to 91. Records from automatic weather stations highlight the severity of the event, with several stations breaking 20-yr records. This study examines the synoptic drivers, dynamic and thermodynamical features associated with the rainfall episode. Prior to the extreme precipitation, the monsoon trough migrated northward, and its interaction with an incoming western disturbance (WD) passing over northwest India led to deep tropospheric instability over the western Himalaya. The dynamic coupling of the monsoon trough with an unusually strong and slow-moving WD, caused by an extratropical cutoff low, facilitated moisture exchange, which further intensified, resulting in a severe rainstorm. The high moisture convergence was driven by extremely high amounts of moisture flux directed from the Bay of Bengal and the Arabian Sea toward the western Himalaya by the combined monsoon trough–WD system. Alongside this, an intense jet streak associated with the WD led to additional ageostrophic forcing over the western Hindukush Himalayas from 7 July, creating conducive conditions for deep ascent. Vorticity and moisture budget analyses highlight the main dynamical drivers (horizontal vorticity advection and vertical moisture transport) of the event.