Relationship between reductions of summer precipitation in North China and atmospheric circulation anomalies

Based on NCEP/NCAR data and summer rainfall data from China National Climate Center (NCC) , by using trend analysis and composited analysis methods, investigated the relationship between the reduction of summer rainfall in North China and northern hemisphere's circulation anomalies. The results show that summer rainfall in North China had a significant decreasing tendency, reducing 15mm averaged for every 10yr, a climate sudden change happened in 1965, since then the precipitation reduced more obviously. The northern hemisphere atmospheric circulation had a clear interdecadal variations after 1965. First, at 500hPa height field in the summer during the period of 1951�`1965, there was a shallower polar vortex, deeper trough over the European and Lake Baikal, the Urals a strong high-pressure ridge which formatted a "-,+,-" Eurasian teleconnection pattern,with outstanding meridional circulation; While in 1966�`2008, there was a deeper polar vortex, shallower trough over Europe and Lake Baikal, the Urals high-pressure ridge is weaker which formatted a "+,-,+" Eurasian teleconnection pattern, with outstanding zonal circulation. Second, at 500hPa temperature field in the summer during the period of 1951�`1965, the Mongolian cold trough located northerly and westerly, with cold air often affected North China region; In 1966�`2008, the Mongolian cold trough location shifted southeastward to the Yellow River loop nearby, with cold air often affected eastern and southern regions to the North China. Third, at sea level pressure field in the summer during the period of 1951�`1965 Mongolia low pressure was unusually large and strong ,in 1966�`2008 Mongolian low pressure weakened obviously. Fourth, at 850hPa wind field in the summer during the period of 1951�`1965, the East Asian summer monsoon was as strong as the Indian summer monsoon, extending to the Northeast China. Meanwhile there is an obvious cyclonic circulation in Mongolia area which resulted in a convergence of wind direction near the Yellow River Loop; In 1966�`2008, the East Asian summer monsoon in the sub-tropical areas significantly weakened and rarely crossed the Yangtze River to arrive in North China, producing a convergence of wind speed in the Yangtze River Basin. Meanwhile Mongolia cyclonic circulation disappeared, the convergence of the wind direction near the Yellow Loop very weak. The rainy nature in North China in the summer during the period of 1951�`1965 as a result of the effective water vapor transmission through East Asian summer monsoon and the convergence near the Yellow Loop caused by the wind direction and in the Yangtze River Basin in the summer during the period of 1966�`2008 caused by the wind speed convergence of the East Asia summer monsoon were different. So summer precipitation reduction in North China is due to atmospheric circulation anomalies in the northern hemisphere caused by global changes.