东亚夏季风在中国东北区建立的标准、日期及其主要特征分析

THE STUDY OF THE ON-SET CRITERION AND THE DATE OF EAST ASIAN SUMMER MONSOON IN NORTHEAST CHINA AND ITS MAIN CHARACTERISTIC ANALYSIS

文中利用中国东北区 80个测站 ,1980年以来近 2 0a逐日降水和同期NCEP/NCAR逐日再分析资料 ,提出了东亚夏季风在中国东北区建立和撤退的标准 ,即取 85 0hPa候平均资料 ,作沿 12 2 .5°E时间 纬度剖面图 ,将θse的336K和南风 4m/s等值线同时越过 4 0°N的候定义为东亚夏季风在中国东北区建立的日期 (候 ) ,持续及累积的总候数为影响时间 ;该年θse的 336K等值线永久撤离到 4 0°N以南的候为东亚夏季风从中国东北区撤退的日期 ;如果某年θse的 336K和南风 4m/s等值线没有同时北移越过 4 0°N的候出现 ,确定为东亚夏季风在中国东北区没有建立的年份。文中相应给出 1980～ 2 0 0 0年建立、持续及累积和撤退的时间年历表 ,经计算东亚夏季风在中国东北区建立的平均日期为第 4 1候 ( 7月第 5候 ) ,累积平均影响候数为 3.6候 ( 18d左右 ) ,开始撤退的平均日期为第 4 5候( 8月第 3候 ) ;东亚夏季风在中国东北区建立前后的候降雨量增加和撤退前后候降雨量递减十分显著 ,建立时水汽场由辐散转变为辐合。文中还定义了中国东北区强夏季风的年份 (建立的时间早 ,且累积候≥ 4候的年份 )为1981,1988,1990和 1994年 ,与国内学者定义的强夏季风年基本一致 ;在中国东北区没有建立东亚夏季风的年份为1980 ,1983,1987,1991,

In this paper, the criterion of establishment and withdrawal of East Asian summer monsoon in Northeast China is suggested by using NCEP/NCAR reanalysis daily date and daily precipitation from 80 observation stations of Northeast China for the past 20 years. When the pentad-averaged contours of θ se 336 K and 4 m/s south wind at 850 hPa along 122.5°E time-latitude cross section are across 40°N together, the pentad is defined as the established time of East Asian summer monsoon in Northeast China. The total pentads of persistency and accumulation are effective days. A pentad is defined as the withdraw of East Asian summer monsoon from Northeast China when the pentad-averaged θ se 336 K contours withdraw to the south of 40°N. If the pentad-averaged contours of θ se 336 K and 4 m/s south wind are not across 40°N at the same time, it is defined that this is an un establishment year of East Asian summer monsoon in Northeast China. The time table of establishment, persistency, accumulation and withdraw of East Asian summer monsoon in Northeast China in 1980～2000 is given. We find that the established time is the 41th pentad (5th pentad in July), accumulated effective time is 3.6 pentads (about 18 days), and withdraw time is the 45th pentad (3rd in August). The precipitation increases (decreases) clearly around the establishment (withdrawal) time of East Asian summer monsoons in Northeast China as the change of water vapour convergence. The strong summer monsoons defined in this paper, such as 1981, 1988, 1990, and 1994, are basically accordance with others defined. The streamline and heating fields in the years of East Asian summer monsoon's establishment and un establishment in Northeast China are analysed, and the difference is significant. In the establishment year, the Northwest Pacific High is westward and northward (about 7 longitude and latitude) and no blocking high in East Asia mid high latitude. It is opposite in the un establishment year. In 1994, a strong summer monsoon year, the positive anomaly heating center at 200 hPa is towards tropical South India Ocean, West Pacific warm pool is a relative cool area, and the anomaly divergent wind V x gradient is strong along the East China coast. But in 1980, an un establishment summer monsoon year, the thermal difference between sea and land in North Hemisphere is relative weak, and more un clearer in low tropopause.