1998 SCSMEX期间亚洲30－60天低频振荡特征的分析

对１９９８年 ５－８月南海季风试验（ＳＣＳＭＥＸ）期间东亚地区 ８５０ ｈＰａ中低纬环流指数、东亚季风指数和长江中下游降水进行了Ｍｏｒｌｅｔ 小波分析,结果表明在此期间这些要素均有明显的３０－６０天周期低频振荡。在此基础上对 ５－８月每隔 ５天的 ８５０ ｈＰａ低频流场进行分析,结果表明：（１）１００°－１５０°Ｅ间东亚从中国东中部大陆经南海和西太平洋的南北半球中明显的存在一个以３０－６０天低频荡为特征的东亚季风低频环流系统,东亚季风活动主要受东亚季风系统中低频活动影响;（２）５月第５候南海热带季风爆发、６月中旬长江中下游人梅及产生大暴雨以及７月中旬以后的该地区大暴雨均与低频气旋带在该地区活动有关,而８月长江上游大暴雨则与低频反气旋伸人到大陆有关;（３）ＳＣＳＭＥＸ期间东亚低频振荡系统的源地有二个,即南海赤道和北半球中太平洋中高纬。南海低频系统向北传播,而中高纬低频系统自东北向西南传播为主。长江中下游６、７月二次大暴雨均与上述二个低频气旋系统自热带向北和中高纬向西南传播并于长江中下游汇合有关;（４）５－８月间东亚季风系统中有二次低频气旋带和二次低频反气旋带活动,这些低频环流系统的活动与印度季风低频环流系统活动并无明

Analysis of the Characteristics of 30-60 Day Low-Frequency Oscillation over Asia during 1998 SCSMEX

The wavelet analysis is performed of the mid- and low-latitude circulation index at 850 hPa over East Asia, the East Asian monsoon index and the precipitation over the middle and lower reaches of the Yangtze River during 1998 South China Sea Monsoon Experiment (SCSMEX) from May to August. Analysis shows that distinct 30-60 day low-frequency oscillation (LFO) exists in all of the above elements during the exper iment period. Analysis of low-frequency wind field at 850 hPa from May to August with 5 days interval is performed in this paper. Analysis results reveal that: (1)A low-frequency monsoon circulation system over East Asia, characterized by distinct 30-60 day low-frequency oscillation, exists over 100°-lS0°E of East Asian area from the middle and eastern parts of China continent and the South China Sea to the western Pacific in both the Northern and Southern Hemisphere. The activity of East Asian monsoon is mainly affected by the low-frequency systems in it; (2) All of the tropical monsoon onset over the South China Sea in the fifth pentad of May, the beginning of the Meiyu period and heavy rainfall over the middle and lower reaches of the Yangtze River in mid-June and the heavy rainfall after mid-July are related to the activity of low-frequency cyclone belt over the region, whereas the torrential rainfall over the upper reaches of the Yangtze River in August is associated with the westward propagation of low-frequency anticyclone into the mainland; (3) There are two sources of low-frequency oscillation system over East Asia during SCSMEX. i.e. the equatorial South China Sea (SCS) and mid-high latitudes of the middle Pacific in the Northern Hemisphere. The low-frequency system over SCS propagates northward while that in mid-high latitudes mainly propagates from northeast to southwest. Both of the heavy rainfall over the middle and lower reaches of the Yangtze River in June and July are associated with the northward propagation of the above-mentioned SCS low-frequency systems from the tropical region and the southwestward propagation from mid-high latitudes respectively and their convergence in the middle and lower reaches of the Yangtze River; (4) There are two activities of iow-frequency cyclone and anticyclone belt each in the East Asian monsoon system during May to August. However the activity of these low-frequency circulation systems is not clearly relevant to the low-frequency circulation system in the Indian monsoon system. This means that the low-frequency circulation systems in Indian monsoon and East Asian monsoon are independent of each other. The concept previously put forward by Chinese scholars that the East Asian monsoon circulation system (EAMCS) is relatively independent monsoon circulation system is testified once more in the summer 1998.