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

中文关键词:  东亚夏季风  东北区  建立标准

英文关键词:Asian summer monsoon  Northeast China  Criterion

中文摘要:

      文中利用中国东北区80个测站,1980年以来近20a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850hPa候平均资料,作沿122.5°E时间纬度剖面图,将θ_se的336K和南风4m/s等值线同时越过40°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θ_se的336K等值线永久撤离到40°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θ_se的336K和南风4m/s等值线没有同时北移越过40°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1980～2000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41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45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1980,1983,1987,1991,1993和1997年,与国内学者定义的弱夏季风年亦基本一致。通过分析东亚夏季风在中国东北区建立和不建立的典型年份流场和加热场,发现两者差异显著:建立的年份西北太平洋副热带高压偏西和偏北(大约7个经、纬距),东亚中高纬度地区没有阻塞形势;不建立年则相反。夏季强季风年(1994年)的200hPa加热场正距平中心偏向南印度洋赤道地区, 西太平洋暖池相对是冷源,中国东部沿海地区辐散风V_x距平梯度较大,而没有建立夏季风的1980年北半球海陆热力差异相对减小,低层更加不明显。

英文摘要:

      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. Apentad is defined as the withdraw of East Asian summer monsoon from Northeast China when the pent adaveraged 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-est ablishment 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 45t h pentad (3rd in A ugust). 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 westw ard 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 tow ards tropical South India Ocean, West Pacific warm poolis 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.