亚洲—太平洋夏季风系统的基本模态特征分析

亚洲—太平洋季风区各季风子系统间的相互作用对季风区甚至全球的气候变化都有着显著的影响.整个亚洲—太平洋夏季风系统都处于高层辐散、低层辐合的庞大辐散环流中,从高层辐散中心流出的三支气流分别对推动印度夏季风、东亚副热带夏季风和南海夏季风起着重要的作用,很好地表现了亚洲—太平洋夏季风系统的整体性特征.季风区多种气象要素的基本模态在年代际和年际尺度上都表现出较为一致的变化特征:年代际尺度上亚洲—太平洋夏季风系统整体呈现减弱趋势；年际尺度上存在准2年和准4年的两个周期,其中准2年振荡特征表现为若印度西南季风偏强,则印度季风雨带偏强偏北,导致印度大陆中北部地区降水偏多；同时,由于西太平洋副热带高压的北移和偏强的印度西南季风显著向东延伸,10°N~30°N范围内的西北太平洋地区则表现为异常的气旋性环流,而30°N~50°N之间为反气旋性环流异常,对应东亚夏季风偏强,季风雨带能够北推至我国华北地区.也就是说,当亚洲夏季风中某一季风子系统表现为异常偏强时,另一季风子系统在这一年中也将表现为异常偏强,反之亦然.准2年的振荡周期可能是亚洲—太平洋夏季风系统的一种固有振荡,它从年际尺度上反映了亚洲—太平洋夏季风受热带太平洋—印度洋海温的强迫表现出明显的整体一致特征.

Analysis of the Leading Modes of the Asian-Pacific Summer Monsoon System

The interactions among the Asian—Pacific monsoon subsystems have significant impacts on the climatic regimes in the monsoon region and even the whole world.The Asian—Pacific summer monsoon system is under the control of the huge divergent circulation system which is divergent at the upper level and convergent at the lower level.Three branches of the air flows from the divergent center at the upper level play significant roles in the promotion of the Indian summer monsoon(ISM),the East Asian subtropical summer monsoon(EASM),and the South China Sea summer monsoon(SCSSM),respectively.It is one of the perfect performances of the holistic characteristics of the Asian—Pacific summer monsoon system.The leading modes of a couple of meteorological elements in the Asian—Pacific monsoon region have shown a more consistent characteristics on both the interdecadal and interannual timescales:the decreasing trend emerges in the Asian—Pacific summer monsoonon the interdecadal timescale during recent 55 years;while on the interannual timescale,the subsystems of the Asian-Pacific summer monsoon have synchronous variation withquasi-two-year and quasi-four-year cycles.The quasi-two-year oscillation of the monsoon system gives expression that when the ISM is anomalously strong,the monsoon rainfall belt will be strong and more northward,bringing more precipitation in the northern-central Indian continent.Because of the western Pacific subtropical high(WPSH)’s northward shift and the ISM’s anomalously eastward extension in the same year,an anomalous cyclone circulation occurs over the western North Pacific in 10°N-30°N and another anomalous anticyclone circulation in 30°N-50°N.It is indicated that the EASM is anomalously strong,and can push the monsoon belt northward to northern China.That is to say,when one of the Asian-Pacific summer monsoon subsystems is much stronger than normal,the other monsoon subsystem in the same year will be also stronger than normal,but both of them will become weaker than normal in the next year.The tropospheric biennial oscillation(TBO) may be an inherent cycle in the Asian-Pacific summer monsoon system.It reflects the significant holistic characteristics of the Asian-Pacific summer monsoon system on the interannual timescale under the response of the tropical Pacific—Indian Ocean temperature forcing.