Compound Changes in Precipitation Intensity, Dryness Conditions, and Heatwaves in Southwest China: Unified Facts and Implications ^※

In recent decades, global precipitation patterns have undergone dramatic shifts, characterized by an increase in heavy precipitation and a decrease in light precipitation. The reshuffling of precipitation of different intensities not only threatens water resources but also has the potential of exacerbating compound risks such as concurrent droughts and heatwaves. Whether such changes have occurred in Southwest China, the source of hydropower of China and a densely populated area, remains unknown. Based on station observations of daily meteorological variables, this study analyzes changes in precipitation intensity spectrum and further explores the implication of the change in compound droughts and heatwaves in Southwest China. We report remarkable compounding changes in the top 10% heavy precipitation, bottom 10% light precipitation, dry days, droughts, and heatwaves during 1961–2018 in this region. Specifically, in Southwest China, the overall trend in precipitation distribution is marked by increased concentration and growing aridity; the amount of precipitation resulted from heavy precipitation events has increased, but the heavy precipitation frequency remains stable, whereas light precipitation events have significantly decreased, accompanied with increases in the number of dry days (daily precipitation < 0.1 mm day⁻¹) by 9%. Also notable are the synchronized increases in dry spells (periods of at least 5 consecutive dry days), drought days [standardized precipitation evapotranspiration index (SPEI) ≤ −1.0] and heatwave days (days of daily maximum temperature > 90 percentile threshold for ≥ 3 days) by 16%, 13%, and 24% respectively during 1961–2018 in Southwest China. These compounding changes translate into higher risks of concurrent droughts and heatwaves, posing greater threats to energy, ecology, and human health against their occurrence in isolation.