东亚地区夏季风爆发过程

On the Process of Summer Monsoon Onset over East Asia

利用中国１９４站 １９６１～１９９５年日降水资料及 ＮＣＥＰ １９７９～１９９７年候格点降水资料，探讨了亚洲地区自春到夏的雨季开始分布。结果表明，东亚地区自春到夏存在副热带季风雨季开始和热带季风雨季开始。前者于４月初开始于华南北部和江南地区，随后向南和向西南扩展，于４月末扩展到华南沿海和中南半岛，这个雨带主要是冷空气和副热带高压西侧转向的ＳＷ风以及南亚地区冬春副热带南支西风槽中西风汇合而形成的，是副热带季风雨季开始。后者是南海热带季风爆发后使原来由江南移到华南沿岸的副热带季风雨带随副热带高压北进而北进，前汛期雨季进入盛期，江南出现第二次雨峰，形成梅雨期和江淮及华北雨季。同时，热带季风雨带也自东向西传播到达南亚地区而形成热带季风雨季。还讨论了 １９９８年东亚地区夏季风爆发过程，指出南海夏季风爆发期的季风由副高北侧形成的新生气旋进入南海造成南海中部西风和南海越赤道气流转向的ＳＷ季风加强汇合而形成，因而是东亚季风系统中环流系统季节变化造成的，和印度季风无关。在南海季风爆发期阿拉伯海仍由副热带反气旋控制．南亚仍是上述副热带反气旋北侧ＮＷ风南下后转向的偏西副热带气流所控制，索马里低空急流仍未爆发，赤道西风并未影响南海。

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 and NCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997, the distri- bution of rainy season onset over Asian area from spring to summer is discussed The results show that there exists two stages of rainy season onset over East Asia region from spring to summer-rainy season onset accompanying subtropical monsoon and tropical monsoon respectively. The former started over north of South China and south of the Yangtze River basin in early April and them extended southward to South China coastal regions and westsouthward to the Indo-China Peninsula by the end of April. The corresponding rain belt is mainly formed by the convergence of cold air and the recurved southwesterly flow from the western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurred during winter to spring over South Asia. We usually call it as 'starting period of pre-flood rainy season over South china'. So it should belong to the onset of subtropical monsoon rainy season. The latter is formed in the process of subtropical monsoon rainy belt over inshore regions of South China Sea originally came from the Yangtze River basin advancing with northward shift of subtropical high after the onset of tropical monsoon over South China Sea. The pre-flood rainy season over South China region then came into mature period and the second peak of rainy season appeared, 'plum season', rainy season over the Yangtze River basin-the Huihe River ba- sin and North China then formed consequently. Meanwhile the tropical monsoon rain belt has propa- gated westward to South Asia and formed tropical monsoon rainy season. The process of summer tropi- cal monsoon onset over South China Sea in 1998 is also discussed it indicated that the monsoon during summer tropical monsoon onset over South China Sea is the result of the westerly flow over middle of South China Sea from the new generated cyclone formed in north subtropical high entering into South China Sea converged with the tropical southwesterly flow recurved by the 'intensified cross-equatorial flow'.Therefore the monsoon onset over SCS is mainly caused by the seasonal variation of circulation system in East Asian monsoonal system but has no relationship with Indian monsoon. During monsoon onset over South China Sea, Arabian Sea is still controlled by subtropical anticyclone, South Asia is controlled by the recurved westerly flow of the south-moving northwesterly flow from north part of pre-mentioned subtropical anticyclone, and the Somali low-level jet has not outbroken yet. The equa- torial westerly over Southern Asia does not affect South China Sea.