The role of extreme precipitation in driving the humidification of northwest China from 1961 to 2020

Northwest China is a typical arid and semi-arid region, and a critical climate-sensitive and vulnerable area. Over the past few decades, the region has experienced a significant humidification trend. Understanding the characteristics and trends of precipitation and atmospheric water cycles in this region is crucial for predicting the future evolution of this humidification process. Based on observational and reanalysis data, this study categorizes precipitation events in Northwest China from 1961 to 2020 into 20 class intervals. The analysis reveals that over 70 % of the total increasing trend in precipitation is due to the upper 10 % of daily precipitation events, and the rise in extreme precipitation frequency accounting for most of the observed changes, in which the contribution rate of the increase in frequency is approximately 10 times that of the increase in intensity. A 15-day backward moisture tracing analysis indicates that approximately 69 % of the moisture in the region originates from terrestrial evaporation, and 21 % contributed by local evaporation within Northwest China. Compared to light precipitation events, strong precipitation events are associated with more substantial external moisture transport, higher regional recycling ratios, and greater precipitation efficiency. Further analysis shows that over the past 60 years, the residence time of moisture associated with the precipitation events in Northwest China is 8.6 days. The extreme events above the 95th precipitation percentile have a mean moisture residence time of about 5 days, with an increasing trend that is mainly driven by the establishment and enhancement of the cross-equatorial moisture transport channel from the Indian Ocean. Concurrently, the decline in moisture contributions from terrestrial sources in the westerly belt, combined with strengthened regional evaporation, has further improved precipitation efficiency. These factors have led to a significant increase in the number of days with extreme precipitation events in Northwest China, serving as a primary driver of the humidification trend of this region.