夏季长江中下游和华南两类雨型的环流特征及预测信号

Analysis of Atmospheric Circulation and Prediction Signals for Summer Rainfall Patterns in Southern China

利用中国南方66站降水观测资料和NCEP/NCAR再分析资料,采用经验正交函数分解(EOF)、合成分析和相关分析等方法,对夏季长江中下游和华南两类雨型进行了划分,对比分析了两类雨型同期大气环流和前期海温及环流的差异,以探讨两类雨型的形成机制及前期预测信号。结果表明:20世纪80年代之前华南型出现的频次较高,之后长江中下游型出现频次增多;长江中下游型年西太平洋副热带高压(副高)偏强偏西偏南,东亚夏季风(EASM)偏弱,副热带西风急流位置偏南,乌拉尔山阻塞高压(乌阻)和鄂霍次克海阻塞高压(鄂阻)较强,欧亚中高纬以经向环流为主,冷暖空气在长江中下游辐合,导致长江中下游降水偏多;华南型年大气环流与长江中下游型年大体相反,登陆华南的台风偏多,冷暖空气在华南地区辐合,导致华南地区降水偏多;其中副高的脊线位置和中高纬阻塞强弱是长江中下游型和华南型形成的关键因素。两类雨型前期海温分析表明,长江中下游型年,前冬赤道中东太平洋和印度洋偏暖,为典型的东部型El Ni?o,副热带南印度洋偶极子(SIOD)呈负位相,春季El Ni?o衰减,SIOD负位相也减弱,但印度洋持续增暖;华南型年,前冬和春季的海洋演变与长江中下游型年大体相反;关键区域海温与长江中下游夏季降水(YRR)和华南夏季降水(SCR)的年际关系存在年代际变化,YRR和SCR与前冬Ni?o3.4指数、SIOD指数和春季热带印度洋全区一致海温模态(IOBW)指数的相关关系在80年代之后逐步减弱,这主要是由于这三个关键海温指数与EASM及副高脊线的相关关系在80年代之后逐步减弱;两类雨型前期大气环流差异分析表明,春季大气环流的差异性要比前冬显著,长江中下游型年,春季副高、南海副高、马斯克林高压(马高)、澳大利亚高压(澳高)均偏强,大西洋欧洲区极涡强度偏弱,北太平洋涛动(NPO)呈正位相;华南型年春季的关键环流系统异常不明显,仅大西洋欧洲区极涡强度偏强,NPO呈负位相。前期海温演变及春季大气环流关键系统的异常可以作为两类雨型年的一些预测信号。

Based on analysis of monthly average precipitation data collected at 66 stations of the China Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research（NCEP/NCAR）, summer rainfall in southern China is classified into Yangtze-River Pattern（YRP） and South China Pattern（SCP） in this study. Atmospheric circulations and differences in earlier period SST and atmospheric circulations corresponding to the two rainfall patterns are analyzed to investigate the formation mechanisms and predictors for the above two rainfall patterns. Results show that the SCP occurrence frequency was higher than that of the YRP before the 1980 s, while the YRP occurrence frequency increased after the 1980 s. During the YRP years, the western Pacific subtropical high（WPSH） strengthened and shifted southwestward; the East Asian summer monsoon（EASM） weakened, accompanied by a southward shift of the subtropical westerly jet and stronger Ural blocking high（UB） and Okhotsk blocking high（OB）. Under such a circulation pattern, the high-latitude Eurasia was under control of meridional circulations; cold and warm airmasses converged over the lower Yangtze River Valley, leading to abundant precipitation in this region. The atmospheric circulation pattern in the SCP years was almost opposite to that in the YRP years. Coupled with more landing typhoons in the SCP years, cold and warm airmasses often converged over South China, causing more precipitation in this region. Among all the influential factors, the location of the WPSH ridge-line and the intensity of the middle-and high-latitude blocking highs are two key factors that determine the YRP and SCP rainfall patterns. In addition, an analysis of sea surface temperature（SST） indicate that the pre-winter SSTs during the YRP years were warmer than normal in the central and eastern equatorial Pacific, which corresponded to the typical Eastern-Pacific type of El Ni？o, and the pre-winter subtropical South Indian Ocean Dipole（SIOD） was in negative phase. In contrast, the above two phenomena weakened in the spring of the YRP years when a continuous increase in SST could be observed in the Indian Ocean. The earlier period SST anomalies in the SCP years were also nearly opposite to that in the YRP years. Moreover, the correlations between summer rainfall in the middle and lower reaches of the Yangtze River（YRR） or in South China（SCR） with the pre-winter SST anomaly indices of Ninor3.4 and SIOD and with the spring Indian Ocean basin-wide（IOBW）, both weakened after the 1980 s, which is mainly attributed to the weakening of correlations between SSTs over the three key regions and the EASM and the ridgeline of the WPSH. Comparisons of earlier period atmospheric circulations for the two rainfall patterns show that significant differences in the atmospheric circulation between the YRP and SCP years mainly occurred in the spring. During the YRP years, the WPSH, the South China Sea subtropical high, the Mascarene high and Australian high all were stronger than that in the normal years, while the Euro-Atlantic polar vortex was weaker than that in normal years, and the NPO was in positive phase. For the SCP years, no significant key atmospheric circulations anomalies could be observed in early spring except that the Euro-Atlantic polar vortex was stronger than normal and the NPO was in negative phase. The abnormal signals of these key atmospheric circulation systems can be used as predictors for the two rainfall patterns.