A Study on Anomalous Activities of East Asian Summer Monsoon during 1999
A Study on Anomalous Activities of East Asian Summer Monsoon during 1999
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DOI：
10.2151/jmsj.79.1119
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发表时间：
2001-12
影响因子：
3.1
通讯作者：
Yihui Ding;Ying Sun
中科院分区：
文献类型：
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作者：
Yihui Ding;Ying Sun
利用NCEP-NCAR再分析资料，对1999年东亚夏季风异常活动进行了研究，发现如下特征：1)南海中国夏季风开始于5月底，明显晚于常年(5月中旬)，有3个活跃期和2个休止期；2)中国东部夏季风或南风向北推进明显较弱，最北位置为32°N，长江以北地区季风气流不能建立，导致水汽输送很弱，造成北部持续干旱，中国，相应地，中国东部季节性强降水带的静止位置明显偏南，最北位置位于32°N附近，整个夏季不出长江；4)来自孟加拉湾和热带印度洋的低层西南风气流在南海夏季风异常活动中起着非常重要的作用。南海季风的三个活跃期几乎与这股气流的异常东延同步，5)7月初以后，随着西北太平洋副热带高压(WNPH)(110-150°E，10-30°N)迅速东移到日本以东地区，低层西南气流表现出明显的东移，其前锋达到160-170°E，导致主要水汽输送带东移，日本周边长时间出现异常暴雨。研究了1999年夏季风异常活动背后的原因。从热力条件看，今年夏季风爆发较晚可能与青藏高原上空热源发展较晚、热源强度减弱有关，导致高原上空大气与周边地区大气热力反差较小。这可能导致高原东侧和东南侧异常东风的盛行，不利于东亚夏季风的正常发展。异常东风阻挡了夏季风西南风的持续北上，使西南气流长期局限在低纬地区。
Using the NCEP-NCAR reanalysis datasets, the anomalous activities of East Asian summer monsoon for 1999 have been studied, with the following characteristic features identified: 1) the South China Sea (SCS) summer monsoon began at the end of May, which was much later than the normal year (mid-May), with three active and two break phases observed, 2) the northward advance of the summer monsoonal southwesterlies or southerlies in East China was markedly weak with the northernmost position of 32°N, and the monsoonal airflow could not establish itself in the areas north of the Yangtze River, thus leading to a very weak moisture transport and the condition of prolonged drought in North China, 3) correspondingly, the stationary position of seasonal heavy rainfall-belt in East China was considerably farther south than normal, also with the northernmost position being near 32°N, never going beyond the Yangtze River for the whole summer, 4) the low-level southwesterly (SW) airflow from the Bay of Bengal and the tropical Indian Ocean played a very important role in the abnormal activities of the SCS summer monsoon. Three active periods of the SCS monsoon were almost concurrent with the anomalous eastward extension of this airflow, and 5) after the beginning of July, as the Western North Pacific subtropical high (WNPH)(110-150°E, 10-30°N) rapidly shifted eastward up to the region east of Japan, the low-level southwesterly airflow assumed a marked eastward migration with its leading front reaching the longitudes of 160-170°E, thus bringing about the major moisture transport belt to shift eastward, and anomalous heavy rainfalls to occur around Japan for a long time. The reasons behind these anomalous activities of the 1999 summer monsoon have been examined. From the viewpoint of thermal conditions, the late onset of the summer monsoon this year was likely to be related to late development and reduced intensity of heat source over the Tibetan Plateau, which resulted in the smaller thermal contrast between the atmosphere over the plateau and the atmosphere over its surrounding areas. This could lead to prevalence of the anomalous easterlies in the eastern and southeastern flanks of the plateau, which was not favorable to the normal development of the East Asian summer monsoon (EASM). The continuous northward advance of summer monsoonal southwesterlies was blocked by the anomalous easterlies, so that the SW airflow was confined in lower latitudes for a long time.