The Circulation Anomalies Pattern Causing The Persistent Drought In North China And Its Relation To The Thermal Variability In The Arid Region In Northwest China

Due to importance of persistent droughts in North China after 1977, the circulation anomalies pattern causing the persistent droughts in North China and its relation to the thermal variability in the arid region of Northwest China are analyzed by using observed data, NCEP/NCAR's and ERA-40 reanalysis data with the analyses of observational fact, dynamical diagnoses and numerical simulations. The analyzed results are as follows:(1) Interdecadal variability of summer precipitation in China is very obvious. From 1977 to 2000, summer precipitation obviously decreased and the persistent droughts occurred in North China and the Yellow River basin, but summer precipitation greatly increased in the Yangtze River valley and Northwest China.(2) The warm high ridge in mid-and-lower troposphere over North China, the pressure ridge over Okhotsk sea and the western Pacific subtropical high are major circulation system influencing summer climate anomaly in North China. Moreover, after 1977, a teleconncetion wave-train is opposite to that before 1977 occurred in the lower troposphere over Eurasian continent and a altitude-longitude circulation over the arid region of Northwest China and the monsoon region of the eastern China is also opposite to that before 1977. After 1977, a anti-cyclonic anomaly circulation and the northerly wind anomalies obviously appeared at 700hPa over North China, and the descent flow, divergence of water vapor transports, and the southward shift of the westerly wind over North China. These are not helpful to summer monsoon rainfall and caused the decrease of summer precipitation in North China, and thus, the persistent droughts occurred.(3) The summer circulation anomalies over North China are obviously associated with the thermal variability in the arid and semi-arid regions of Northwest China. The Ts-Ta anomalies in spring and summer in the arid and semi-arid regions of Northwest China were enhanced from 1977, especially the spring sensible heating were obviously intensified from 1977. It is shown from the correlation analysis that when the spring sensible heating is anomalously strong, the summer monsoon rainfall may be below normal in North China and above normal in the Yangtze River valley. Moreover, it is revealed from the analyzed results that the stronger spring sensible heating transport in the arid and semi-arid region of Northwest China from 1977 have led to an obvious anomaly of ascend flow and an anomaly of descent flow, which is not helpful to the summer monsoon rainfall in North China. Therefore, the decrease of summer rainfall in North China was caused and the persistent droughts occurred there.(4) The impact of the spring sensible heating anomaly on the potential vorticity(PV) of East Asian circulation anomaly is analyzed by using the theory on the slant potential vorticity development and it is shown that since the Ts-Ta and sensible heating in the arid and semi-arid region of Northwest China were enhanced from 1977, but the air temperature obviously decreased in upper troposphere over these regions, and the anomaly of ? ??θp became negative, and the anomaly of vertical component of PV also became negative after 1977. This PV change was favorable for the intensification of cyclonic circulation anomaly in the arid region of Northwest China, and the intensification of ascent flow anomaly, and the increase of summer precipitation in this region, and it was also helpful to the developing of altitude-longitude circulation over the eastern and western parts of China. Just due to the developing of this circulation, it was caused the positive anomaly of ? ??θp in the lower troposphere over North China from 1977, which is favorable for not only the intensification of the anti-cyclonic circulation anomaly over North China, but also the intensification of the descent flow anomaly. Thus, the decrease of summer rainfall in North China was caused and the persistent droughts occurred in this region.(5) The results of simulated with RegCM3 region climate model show that when the spring sensible heating is intensified, summer rainfall in the southern part of North China will decrease, but it will increase in the Yangtze River valley and Northwest China, and the descent flow anomaly will be enhanced, which is not favorable for the increase of summer rainfall in North China.