Interdecadal Variability of Summer Precipitation in Northwest China and Associated Atmospheric Circulation Changes

Daily precipitation data from 149 rain gauge stations in China and NCEP–NCAR reanalysis data during 1961–2018 are used to investigate the interdecadal variability of summer precipitation in Northwest China and related causes. The results suggest that, on the interdecadal timescale, Northwest China shifts into a rainy period from the year 1987, with an increase in the precipitation amount and intensity; an increase in the probability of moderate rain, heavy rain, torrential rain, and extremely heavy rain; and a decrease in the probability of light rain. More than 60% of the increase in precipitation can be attributed to rainfall with intensity above the grade of heavy rain. The associated interdecadal variability of atmospheric circulations over midlatitude Eurasia in summer is examined and it is found that the interdecadal variability is mainly characterized by the Silk Road pattern (SRP), with a cyclonic circulation anomaly and an anticyclonic circulation anomaly over central Asia and Mongolia, respectively; enhanced ascending motion and atmospheric instability in Northwest China; and strengthened easterly winds caused by the Mongolian anticyclonic anomaly along the northern boundary of the Tibetan Plateau. On the south side of the Mongolian anticyclone, the water vapor transported from the Pacific and Indian Oceans as well as the South China Sea to Northwest China by easterly winds increases significantly, providing the main water vapor source for the increase in precipitation in Northwest China on the interdecadal timescale. The transition of the Atlantic multidecadal oscillation to a positive phase may be the main cause of the interdecadal transition of the SRP to a positive phase, resulting in the interdecadal increase in summer precipitation in Northwest China.