LAUSR

Spatial and temporal patterns in precipitation indices across Northwest China (NWC) during 1960–2011 were investigated using 11 previously defined indices. Results show that more than 50% of stations for wet-related indices exhibit increasing trends and the largest increases appear in winter, while 72.1% of stations display decreasing trends for consecutive dry days. The observing sites with positive trends are mainly distributed over mid-western NWC and northwestern Xinjiang, while the declining trends are concentrated in eastern NWC. Very wet days account for 23% of the total rainfall on average, combined with similar patterns of spatial distribution between annual total rainfall and precipitation extremes, suggesting that extreme, rather than moderate, precipitation moderates the variations in total rainfall. After the mid-1980s, the NWC exhibits a wetting tendency, and precipitation extremes tend to increase with larger magnitudes and fluctuations. During 1987–2011, the prevailing eastward wind field anomalies at 850 hPa and statistically significant increasing trends of geopotential height are found over the Eurasian continent from spring to autumn, together with enhanced anticyclonic circulation near 47° N and 100° E, providing favorable conditions for the occurrence of extreme precipitation events over NWC. While the circulation pattern in winter is distinct compared with other seasons, a convergence between eastward wind anomalies and southeasterly winds favors significantly positive increases in precipitation extremes. The well-organized wave train-like structures originated from the North Atlantic travel through Europe and central Asia and impact the precipitation extremes over NWC from spring to autumn during 1987–2011. Similar to wind field anomalies, the southeasterly water vapor fluxes are remarkable in winter. Generally, the approximately opposite circulation patterns dominate during 1960–1986 in comparison with those during 1987–2011.