高层辐散对热带季风发展的作用

Effects of Divergent Fields in the Upper Troposphere to the Development of Summer Monsoons in East Asia

采用欧洲中心格点资料分析发现,最初影响东亚的热带季风是源于75°～90°E地区的近赤道强西风。当6月这支强西风出现季节性北移后,东亚季风的来源可以追溯到索马里越赤道气流。对流层上层热带东风带上的高层辐散,对低层近赤道西风向北伸展起触发作用;而副热带西风带上的高层辐散,有利于低层倒槽的发展,从而引导热带季风继续北上。由于高原地区副热带环流的经向发展,促成东、西风带上辐散作用的连结,可能是引导低层热带季风伸向东亚的一种机制。随着热带季风的到来,华南前汛期盛期暴雨具有热带季风雨的性质。造成暴雨的降水切变线是一种夏季风天气系统。水汽输送计算表明,华南前汛期盛期降水的水汽,主要是由这支源于近赤道强西风的热带季风所输送的。

An analysis of ECMWF data shows that the tropical summer SW monsoon, which firstly reaches East Asia in Mid-May,comes from the strong equatorial westerlies(SEW)near 75°～90°E.Only after the northward migration of the SEW flows happens in June,can the origin of summer monsoons over East Asia be traced back to the cross-equatorial flows near Somali. The divergent flow field in the tropical easterly of the upper troposphere stimulates the northeastward stretch of the SEW.Also the divergent flow field in the subtropical westerly and the inverted-trough underneath urge the SW mon- soon spreading over the Indo-China Peninsula and southwestern China.Thus the connection of divergent fields in the easterly and westerly,which is caused by a meridional development process of the general circulation over the Tibetan Plateau, might be a possible mechanism steering the tropical monsoon in the lower tropos- phere towards East Asia. Associated with the approaching of tropical monsoons,the precipitation in South China becomes tropical monsoonal.The wind shear line is the major system causing heavy rain,and it is different from the ITCZ,which also exists over the South China Sea in summer.The computation of water vapour shows that during thè heavy rain period of the pre-typhoon rainy season in South China most of water vapour is transported by the tropical monsoon originated near the equator.