南海夏季风强度年代际变化基本特征

Interdecadal Variabilities of SCS Summer Monsoon Intensity

利用美国国家环境预报中心/美国国家大气研究中心(NCEP/NCAR)的再分析资料,通过合成分析、对比分析等方法,对南海夏季风的年代际变化的划分、不同年代际阶段平均场的差异及其成因进行研究。分析表明:南海夏季风强度具有显著的年代际变化特征,在20世纪70年代末出现了年代际时间尺度的转折,可将其分成两个阶段,1960—1976年(简称第一阶段)和1980—1998年(简称第二阶段),第二阶段与第一阶段相比,南海地区夏季西南风强度显著减弱,年际变化的方差显著变大,变化周期变短,但是南海中南部地区的上升运动却有所加强。夏季,从低层到对流层高层,中国大陆上空的气温显著降低,海洋上空的气温有所升高,在热力作用下,导致大陆中低层位势高度增加比海洋上大,形成大陆地区反气旋性环流加强,从而减弱了南海中北部地区的西南风。从辐散风场来看,赤道东太平洋地区海温显著增加可能对南海中南部地区上升运动的加强起着重要的作用。

In order to diagnose the interdecadal variabilities of South China Sea （SCS） summer monsoon activities and reveal the inner relationships of the sea surface temperature and the general circulation anomalies to the interdecadal variabilities of SCS summer monsoon activity, by using NCEP/NCAR reanalysis grid data, the interdecadal phases of SCS summer monsoon are divided, the differences of the average fields in the different interdecadal phases and their causes are studied with composite and comparison analysis. The results show that the intensity of SCS summer monsoon is characterized by intedecadal variability that has an abrupt jump occurring around 1976. It can be divided into two phases. The first phase is from 1960 to 1976 and the second phase is from 1980 to 1998. Comparing the southwesterly over SCS in the second phase with that in the first phase, it is found that the average intensity is weaker, the amplitude of annual variation is greater, the periods are shorter and the ascending flow over the south and center of SCS is stronger in the second phase than that in the first phase. The temperatures in whole troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in the increase of geopotential height in lower and middle levels in troposphere over China continent, and it is more than that over the ocean by the thermodynamic forcing process. The weakening of the pressure gradient between the continent and ocean can result in the enhancement of an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. The temperatures in the troposphere in summer drop over the China continent and rise over the surrounding ocean, leading in geopotential height in lower and middle levels of troposphere over China continent increasing more than that over ocean through the thermodynamic forcing process. The weakening of pressure gradient between the continent and ocean can trigger an anticyclonic anomalous circulation over the continent, leading to the weakening of the southwesterly over the north and center SCS. As shown in the distribution of divergence wind field and vertical motion, it implies that the remarkable increase of sea surface temperature over the eastern equatorial Pacific Ocean may play an important role in enhancing the ascending motion over southern and center of SCS.