Ecological environment in the arid areas of Northwest China is very vulnerable. Ecological vulnerability is considered to be one of the important indicators to measure the status and evolutions of the regional ecological environment and has become a key measure in the study of ecological change. However, studies on the ecological vulnerability in the whole arid areas of Northwest China on the county scale are currently sparse. Here, according to vegetation, land use type, topography, climate and socio-economic data, we present the spatial and temporal evolution of ecological vulnerability in the arid areas of Northwest China over the period 2000-2018 by structuring a pressure (P)-sensitivity (S)-restoration (R) ecological vulnerability evaluation index system and using an ordered weighted average (OWA) model. Our results suggested that the overall ecological vulnerability in the arid areas of Northwest China was dominated by the severe level from 2000 to 2018 with an average multiyear ecological vulnerability index of 0.48, which appeared a slight downward trend with the implementation of ecological restoration measures. Ecological vulnerability exhibited a significant stepped differentiation feature, and the eastern and western regions were markedly lower than that of the central region. Gravity center of the ecological vulnerability index has a significant spatial difference, which developed in the shape of Z direction between Heshuo county and Tuokexun county. Our study revealed the dynamic changes of ecological vulnerability at the county scale and provided decision-making information for the formulation of targeted ecological vulnerability management measures. Under the background of climate change and the new normal of social economy, the dynamic monitoring of ecological vulnerability and effective identification of vulnerability factors still require in-depth research.

Investigation of extreme precipitation events in arid and semiarid regions, especially for occurrence time and the associated circulation mechanisms, is vital to support the forecasting of and the advanced response to resultant disasters. In this study, the spatiotemporal pattern of occurrence time of extreme precipitation and atmospheric circulation mechanisms in the Arid Region of Northwest China (ARNC) were analyzed using two indicators (precipitation concentration degree and period) and the climate diagnosis method. Results showed that the significant scattered pattern of extreme precipitation occurrence time (EPOT) in Northern Xinjiang and the postponed pattern of maximum extreme precipitation occurrence (MEPO) from southern to northern Xinjiang are consistent with the input pathway of the Arctic air mass. During the anomaly dispersion year of EPOT and the anomaly delay year of MEPO, the Arctic air mass carried sufficient water vapor is transported to ARNC for triggering extreme precipitation events. Meanwhile, the pattern of concentration-dispersion-concentration in eastern ARNC demonstrates interaction between the westerlies and the summer monsoon. Sufficient water vapor is transported to southwestern ARNC by the southwest monsoon during the anomaly delay year of MEPO and the anomaly concentration year of EPOT. The findings of this study suggest that invasion of the Arctic air mass and the summer monsoon could influence extreme precipitation in ARNC.