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东亚夏季风环流和雨带的季节内变化
引用本文:苏同华,薛峰.东亚夏季风环流和雨带的季节内变化[J].大气科学,2010,34(3):611-628.
作者姓名:苏同华  薛峰
作者单位:中国科学院大气物理研究所国际气候与环境科学中心,北京,100029;中国科学院研究生院,北京,100049;中国科学院大气物理研究所国际气候与环境科学中心,北京,100029
基金项目:国家自然科学基金资助项目40821092, 中国科学院知识创新工程项目KZCX2-YW-217
摘    要:基于常规气象要素资料及变差度方法, 分析了东亚夏季风环流的演变特征, 发现东亚地区在夏季期间存在两次明显的次季节突变, 主要表现为西太平洋副热带高压 (副高) 的两次东退北跳, 第一次是在6月中旬, 第二次是在7月下旬。由于副高与雨带密切相关, 雨带在演进过程中也呈现出两次明显的突跳, 分别对应于江淮流域至日本一带梅雨期以及中国华北和东北雨季的开始。较第一次北跳而言, 副高的第二次北跳更为明显。副高的第一次北跳主要受南海地区对流活动加强的影响, 而第二次北跳则是暖池对流活动与高纬地区环流共同作用的结果。暖池地区向东北方向传播的Rossby波列以及高纬地区东传的Rossby波通过锁相作用使得副高强烈北跳。此外, 副高与其西部边缘凝结潜热的相互作用导致副高发生季节内的低频振荡。 风场变差度的分析表明, 高纬地区对流层中低层环流的调整随着夏季季节进程逐渐减弱, 这与中高纬地区温差的变化有关。而高纬地区高层环流的调整在夏季后半期随着高度的增加却逐渐增强, 这与高层环流从夏到冬的季节变化有关。从风场相似度的变化上还可以看到, 副高第二次北跳后东亚地区呈现出明显不同的环流状态。 南半球环流对于南海及暖池地区对流活动的增强有重要影响。6月中旬, 南海与暖池地区对流活动的增强是由于南海西边界西风加强并向东扩展造成的, 这与马斯克林高压 (马高) 的加强密切相关。而在7月中旬, 澳大利亚高压 (澳高) 的增强使其东北部的越赤道气流加强, 南半球大量冷空气侵入到暖池地区, 加强了暖池地区的不稳定性以及低层的辐合, 从而使暖池地区的对流活动增强。但在夏季前半期, 暖池对流活动也可调制澳高强度与其东北部越赤道气流强弱的关系, 使得二者呈现出相反的变化趋势。南半球冬季期间, 澳高在振荡中减弱, 这与澳洲大陆下垫面温度及上游马高的能量频散有关, 前者影响澳高的变化趋势 (减弱), 而后者影响澳高的低频振荡。

关 键 词:东亚夏季风  次季节突变  副高  对流活动  变差度

The Intraseasonal Variation of Summer Monsoon Circulation and Rainfall in East Asia
SU Tonghua and XUE Feng.The Intraseasonal Variation of Summer Monsoon Circulation and Rainfall in East Asia[J].Chinese Journal of Atmospheric Sciences,2010,34(3):611-628.
Authors:SU Tonghua and XUE Feng
Affiliation:1.International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029; Graduate University of Chinese Academy of Sciences, Beijing, 1000492.International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029
Abstract:Based on routine meteorological data and the method of the normalized finite temporal variation (NFTV), the evolution features of East Asian summer monsoon (EASM) circulation are analyzed. It is found that there exist two distinct subseasonal abrupt changes in East Asia during the summertime, characterized primarily by two eastward contractions and northward jumps of the western Pacific subtropical high (WPSH), one of which is in middle June and the other is in late July. Because of the close relationship between the WPSH and rainfall distributions in East Asia, the rain belt in East Asia exhibits two obvious northward jumps in the seasonal evolution as well, corresponding to the beginning of the Meiyu period from the Yangtze-Huaihe valley to Japan and the rainy season in North China and Northeast China, respectively. Compared with the first jump, the second jump of the WPSH is much more evident. The first jump is mainly caused by the enhancement of the convective activities in the South China Sea (SCS) while the second jump is influenced by both the convective activities over the western Pacific warm pool (WPWP) and the circulation systems in high latitudes. Through the phase-locking of the northeastward propagation of the Rossby wave trains from the WPWP and the downstream propagation of the Rossby waves in high latitudes, both the convective activities over the WPWP and the circulation systems in high latitudes play a key role in the second northward jump of the WPSH. In addition, the interactions between the WPSH and the release of the latent heat on its western edge lead to the intraseasonal low-frequency oscillation of the WPSH.  The analyses of NFTV indicate that the adjustment of the low-level circulation in high latitudes tends to weaken gradually with the seasonal evolution, which is related to the temperature difference between middle and high latitudes. By contrast, the adjustment of the high-level circulation tends to intensify with the altitude during the latter half of the summertime. Besides, the evolution of the similarity also shows that the atmospheric circulation in East Asia exhibits a distinctly different state after the second jump of the WPSH. The Southern Hemisphere circulation plays an important role in the enhancement of the convective activities in the SCS and the WPWP. In middle June, the enhancement of the convective activities in these two regions is due to the intensification and eastward extension of the westerly on the western edge of the SCS, which is deeply involved with the Mascarene high (MH). In middle July, the enhancement of the Australian high (AH) leads to the intensification of the cross-equatorial flow on its northeastern edge, and a large amount of cold air from the Southern Hemisphere invades into the warm pool region, which increases the atmospheric instability and the low-level convergence over this region. As a result, the convective activities in the warm pool are enhanced. In the first half of the summertime, however, the relationship between the intensity of the AH and the cross-equatorial flow on its northeastern edge can be modulated by the convective activities in the WPWP, resulting in an opposite trend between the AH and the associated cross-equatorial flow. During the austral wintertime, the AH tends to weaken with the low-frequency oscillation, which is affected by both the surface temperature in Australia and the energy dispersion of the upstream MH. The weakening trend of the AH is influenced by the former while the MH plays a dominant role in the low-frequency oscillation of the AH.
Keywords:East Asian summer monsoon  subseasonal abrupt change  the western Pacific subtropical high  convective activity  normalized finite temporal variation
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