Initial Examination of Interdecadal Shifts and Causes of Summer and Autumn Dryness and Wetness in Northwest China over Recent Decades

Using observational and reanalysis data from 1961 to 2014, the spatial and temporal characteristics of dry and wet changes during summer and autumn in Northwest China (35°N–50°N, 75°E–95°E) were investigated. The authors examined the contributions of evapotranspiration and precipitation to the linear trend of dryness and wetness, and the large-scale atmospheric circulations and water vapor budget associated with interdecadal dryness and wetness patterns were also investigated. Results indicate that drought variability in summer and autumn in Northwest China was the most significant across all four seasons, with the highest probability of drought occurring during these two seasons. However, a remarkable increasing trend in wetness was observed in Northwest China from 1961 to 2014, with evapotranspiration and precipitation playing crucial roles in the region’ s rising humidity. Both increasing precipitation and decreasing evapotranspiration positively contributed to the wetness trend in Northwest China. The combined contribution rate of these two trends was 93.4% in summer and 67.5% in autumn. An interdecadal shift from dry to wet conditions occurred in summer and autumn around 1987 over Northwest China. Since then, interdecadal humidification has been primarily influenced by changes in evapotranspiration, driven by reduced surface wind speed, which positively contributed to the area’s humidification. The anomalous convergence of water vapor transport flux resulted in increased precipitation in Northwest China. Water vapor diagnostic analysis further revealed that the increase in precipitation primarily originated from enhanced local evaporation, with an 80% contribution rate, signifying that local evaporation is a critical water vapor source for precipitation. Additionally, the summer water vapor advection term was positive, indicating strengthened water vapor flux convergence, which positively contributed to the increase in precipitation. This contribution was mainly due to the dynamic component related to wind speed. In contrast, after 1987, the decline in net radiation flux and surface wind speed together led to reduced evapotranspiration in Northwest China during autumn, causing interdecadal humidification in the region.