Observed and Attributed Increase in Western Disturbance Induced Monsoon Extremes in a Warming Climate

Western disturbances (WD) and Indian Summer Monsoon (ISM) are two key atmospheric phenomena driving the complex hydroclimate over India, subsequently influencing the winter and monsoon season of the subcontinent. In recent decades, the WDs have been increasingly observed in the monsoon season, potentially enhancing their interaction with the Indian Summer Monsoon. This interaction is prone to high-impact precipitation extremes (such as the Uttarakhand flood 2013, Himachal Flood 2023) over Northern India and the Himalayan region, potentially impacting the water security and disaster preparedness of the region. Despite their importance, the role of climate change in modulating these WD–ISM precipitation extremes remain underexplored. To address this crucial gap, the study investigates the shift in WD occurrences during the monsoon season and their co-occurrence with extreme precipitation events for the period 1950-2024 over the Hindu Kush Himalayan region using multiple precipitation datasets, including ERA5, CPC, MSWEP and the WD track dataset (developed by Hunt et al. 2018). Additionally, a probabilistic climate attribution framework has been applied in this research using CMIP6 model simulations to compare the occurrence of such extremes in factual (recent) and counterfactual (pre-industrial) climate conditions. Preliminary findings indicate a significant increasing observed trend of ~0.054 events per year (p=0.002) in the occurrence of WD-associated monsoon extreme systems during 1950–2024, with a more pronounced rise in recent decades. Subsequently, a significant upward tendency (p<0.001) has been observed in total annual precipitation from WD-associated extreme grids for the same. The findings emphasise the growing importance of the interaction between Western Disturbances and the Indian Summer Monsoon in shaping regional water security and highlight the need for enhanced disaster preparedness in a warming climate.

Keywords: Western Disturbances (WD), Indian Summer Monsoon (ISM), Hindu Kush Himalayan Region (HKH), Climate Attribution, Climate Change.