东亚夏季风活动与东亚高空西风急流位置北跳关系的研究

The Relationship between East Asian Summer Monsoon Activity and Northward Jump of the Upper Westerly Jet Location

利用美国NCEP/NCAR再分析资料 ( 1 980～ 1 999年 )探讨了东亚夏季风活动的两个重要事件 ,即南海夏季风爆发和江淮流域梅雨起始 ,与东亚高空西风急流位置北跳的关系。系统的分析研究表明 ,东亚高空西风急流在由冬向夏的转变过程中一般存在着两次向北突跳现象 ,并与东亚夏季风活动有密切关系。第一次东亚高空急流的北跳 (由 2 5～2 8°N跳到 30°N以北 )平均发生在 5月 8日左右 ,比南海夏季风爆发日期 (平均为 5月 1 5日 )早 7天左右 ;高空急流位置的北跳是中高纬度大气环流系统减弱北退的表现 ,它为热带环流和系统的向北推进提供了条件 ,从而有利于南海夏季风的爆发。第二次东亚高空急流的北跳 (由 32°N左右北跳到 35°N以北 )平均发生在 6月 7日左右 ,先于江淮流域梅雨起始时间 (平均在 6月 1 8日左右 ) 1 0天左右 ,它是梅雨起始的前期征兆。高空西风急流的两次北跳分别与亚洲大陆南部地区对流层中上层 ( 5 0 0～ 2 0 0hPa)经向温度梯度的两次逆转 (反向 )有关 ,在由冬到夏的季节转换中 ,由于大陆加热较快 ,导致对流层中上层大气在 5～ 2 5°N间的经向温度梯度发生反向 (逆转 ) ,通过地转适应使流场向气压场 (温度场 )调整 ,从而高空急流位置北跳。数据分析还发现 ,东亚高空急流位置的第一次北跳有时?

It is studied by using NCEP/NCAR reanalysis data (1980-1999) that the relationship between the northward jumps of the upper westerly jet location and two important events of the East Asian summer monsoon activity the onset of summer monsoon over the South China Sea(SCS) and the beginning of the Meiyu rainfall in the Changjiang River and the Huaihe River basins. The systematic analyses show that the upper westerly jet has northward jump phenomenon for two times during the transforming process from winter to summer and they are closely related to summer monsoon activities. The northward jump of the upper westerly jet in East Asia for the first time (jet location from 25-28°N to the north of 30°N) occurred about on 8 May on average, it is about 7 days earlier than the onset date of the SCS summer monsoon (mean date is 15 May). The northward jump of upper westerly jet in East Asia for the second time (jet location from 32°N to the north of 35°N) occurred about on 7 June on average, it is about 10 days earlier than the beginning date of the Meiyu rainfall in the Changjiang River and the Huaihe River basins and can be the forewarning of Meiyu rainfall beginning. This two-time northward jumps of the upper westerly jet location in East Asia are related to two-time reverses of meridional temperature gradient in the upper-middle troposphere (500-200 hPa). During the seasonal transforming the continent was heated quickly, so that the meridional temperature gradient in the upper-middle troposphere will be reversed in 5-25°N in South Asia. Then through the geostrophic adjustment, the flow field adjusts to the pressure field (temperature field) and it will lead to the northward jumps of the upper westerly jet location. The analyses also show that sometimes the enhancement and northward moving of the upper westerly jet in the southern hemispheric subtropics can also influence the first northward jumps of the upper westerly jet location in East Asia.