Nighttime warming amplifies the synergistic effects of snowmelt and rain on floods in arid northwestern China

Nighttime warming profoundly affects the hydrological cycle in arid northwestern China. However, it remains unclear why nighttime warming triggers floods. Given the increasing severity of extreme floods, elucidating the impacts of nighttime warming on floods is crucial for disaster mitigation. Herein, we combined the sliding window with the XGBoost algorithm to analyse the dependence of the frequencies of spring and summer floods on nighttime warming in arid northwestern China from 1980 to 2019. Our results indicate that nighttime warming remarkably intensified spring and summer floods in this region. Crucially, driven by pronounced nighttime warming, summer floods primarily occurred in the central and south areas of arid northwestern China, rising (about 2.24 times per decade) more rapidly than spring floods, especially after the 2010s. Across multiple basins, the average impact of nighttime warming on floods was 79.52% greater than that of daytime warming. In spring, when nighttime temperature approached 0 °C, multi-day precipitation accumulation exceeded 5.12 mm and snowmelt accumulation surpassed 61.25 mm, the synergistic interaction between snowmelt and rainfall considerably amplified flood occurrence in the Ertis river basin, Junggar and Yili endorheic basin and Tarim endorheic basin. In summer, the combination of nighttime high temperatures (≥5.63 °C), snowmelt and extreme precipitation in the Tarim endorheic basin, Qaidam endorheic basin and Hexi Corridor-Alax endorheic basin resulted in a greater number of floods than in spring. Our results reveal the critical thresholds and mechanisms through which nighttime warming triggers floods in snowmelt-dominated basins, highlighting the importance of improving the monitoring of nighttime catastrophes.