Relationship between summer precipitation reduction in north China and atmospheric circulation anomalies in Northern Hemisphere.

Based on NCEP/NCAR reanalysis data and summer rainfall data from the 20 stations in North China,by using trend analysis and composite analysis methods,the relationship between the summer rainfall reduction in North China and northern hemisphere's circulation anomalies was investigated.The results show that summer rainfall in North China presents a significant decreasing tendency from 1951 to 2008 with an average reduction of 13.4 mm in every 10 years.This trend has been even more significant since 1965.There are obvious differences in northern hemisphere atmospheric circulation before and after 1965.Firstly,at 500 hPa height field in the summer during the period of 1951—1965,there were a shallower polar vortex and deeper trough over the European and Lake Baikal and a strong high-pressure ridge over the Ural mountain,which formed a -+- Eurasian teleconnection pattern resulting in outstanding meridional circulation;however,in 1966—2008,there were a deeper polar vortex and shallower trough over Europe and Lake Baikal and a weaker high-pressure ridge over the Ural mountain,which formed a +-+ Eurasian teleconnection pattern resulting in outstanding zonal circulation.Secondly,at 500 hPa temperature field in the summer during the period of 1951—1965,the Mongolian cold trough located northerly and westerly with cold air often affecting North China region while in 1966—2008,the Mongolian cold trough location moved southeastward to the Yellow River Loop area with cold air often affecting eastern and southern regions of North China.Thirdly,at sea level pressure field in the summer during the period of 1951—1965,Mongolia low pressure was unusually large and strong but obviously weakened in the summer of 1966—2008.Fourthly,at 850 hPa wind field in the summer of 1951—1965,the East Asian summer monsoon extended to Northeast China.Meanwhile,there was an obvious cyclonic circulation in Mongolia area,which resulted in a wind direction convergence near the Yellow River Loop;in the summer of 1966—2008,the East Asian summer monsoon in the subtropical areas significantly weakened and rarely went over the Yangtze River to arrive in North China.It produced a convergence of wind speed in the Yangtze River Basin.At the same time,Mongolia cyclonic circulation disappeared and the convergence of the wind direction near the Yellow River Loop area became very weak.The rainy season in North China in the summer of 1951—1965 was a result of the effective water vapor transmission through East Asian summer monsoon and the wind direction convergence near the Yellow River Loop area.The rainy season in the Yangtze River Basin in the summer of 1966—2008 was caused by the wind speed convergence of the East Asian summer monsoon.There is a significantly obvious relationship between the summer precipitation reduction in North China and the circulation anomalies in northern hemisphere.