【作者】 周连童；

【导师】 黄荣辉；

【作者基本信息】 中国科学院研究生院（大气物理研究所） ， 气象学， 2007， 博士

【摘要】 鉴于我国华北地区从1977年之后发生持续性干旱的严重性,本研究利用我国气候资料、ERA-40和NCEP/NCAR的再分析资料从观测事实、动力理论和数值模拟分析了引起华北地区持续干旱的环流异常型及其与西北干旱区热力变化的关联。分析结果如下:(1)我国夏季降水在1976年前后发生了一次明显的跃变,从1977年到2000年,华北地区和黄河流域夏季降水明显减少,并导致了持续性干旱的发生;而西北地区和长江流域夏季降水明显增加。(2)华北地区上空对流层中下层发生的暖高压、鄂霍兹克海地区的高压脊和西太平副热带高压是影响华北地区夏季气候变异的主要环流系统。并且,从1977年之后,欧亚大陆对流层下层环流异常分布产生了一个与1977年之前的分布相反的遥相关波列,且我国东部季风区与西部干旱区的纬圈环流也于1977年之前的分布相反。这就是从1977年之后,华北地区上空700hPa出现反气旋型环流异常,并出现明显的偏北风异常,且下沉气流异常加强,水汽输送出现辐散,且200hPa西风带偏南,华北上空西风减弱,这些都不利于华北地区夏季降水,造成华北地区自1977年之后降水偏少,发生了持续性的干旱。(3)华北地区夏季环流异常与西北干旱、半干旱区的热力变化有很明显的关联。西北干旱、半干旱区春、夏季地气温差异常从1977年之后都增强了,特别是春季感热输送从1977年之后明显增强,从相关分析可知,当西北地区春季感热输送异常偏高时,华北地区夏季降水偏少,而长江流域夏降水偏多;并且,分析结果还揭示了从1977年之后西北干旱、半干旱区春季感热输送偏强,这导致了西北地区夏季上空出现明显的上升气流异常,而华北地区上空夏季出现下沉气流异常,这个下沉气流的加强不利于华北地区夏季降水,故引起华北地区夏季降水的减少,并发生持续性的干旱。(4)利用倾斜位涡理论分析西北干旱区春季感热异常对于东亚大气环流位涡异常的影响,表明:由于西北干旱区地气温差(Ts-Ta)和感热输送自1977年之后明显增强,而此地区对流层上层温度却明显下降,这种热力变化使得此地区pθ从1977年之后变成负距平,并且,低层垂直位涡分量也随之变成负距平,这种位涡变化有利于西北干旱区气旋性环流异常的加强,并造成上升气流异常的加强和夏季降水的增多,从而有利于纬圈环流的发展。正是由于此纬圈环流的发展,导致了华北地区上空对流层低层pθ从1977年之后由负距平变成正距平,这不仅有利于华北地区上空反气旋式环流异常的加强,而且还有利于下沉气流异常的加强,从而导致华北地区夏季降水偏少,发生持续性干旱。(5)RegCM3模拟结果表明了,当西北地区春季感热增强时,我国华北地区南部夏季降水偏少,而长江流域和西北地区夏季降水偏多。并且在华北上空下沉气流异常加强,这不利于华北地区夏季降水偏多。更多还原

【Abstract】 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.更多还原