孟加拉湾季风爆发对南海季风爆发的影响Ⅰ:个例分析

IMPACTS OF THE ONSET OF THE BAY OF BENGAL MONSOON ON THE ONSET OF THE SOUTH CHINA SEA MONSOON. PART I: A CASE STUDY

利用南海季风试验分析场和NCAR向外长波辐射通量(OLR)资料研究了1998年孟加拉湾季风和南海季风爆发期间副热带环流的大尺度和天气尺度特征,探讨了孟加拉湾季风爆发与南海季风爆发之间的物理联系及孟加拉湾季风气旋的对流凝结潜热释放对副热带高压“撤出”南海的影响。结果表明,1998年5月爆发的东亚季风展现出典型的从孟加拉湾地区东传发展到南海地区的过程。随着孟加拉湾季风爆发和对流活动增强、北移,南海北部出现了低层西风和对流活动,领先于副热带高压在南海地区减弱和撤退。结果还显示南海北部地区的对流凝结加热有助于该地区经向温度梯度的反转,在热成风关系的制约下南海上空副热带高压脊面的垂直倾斜由冬季型转向夏季型,季风爆发。

SCSMEX assimilative analysis fields and NCAR interpolated outgoing longwave radiation (OLR) have been employed to describe large scale and synoptic features of subtropical circulation during the Bay of Bengal (BOB) monsoon and South China Sea (SCS) monsoon onsets in 1998. The physical linking between the BOB monsoon onset and SCS monsoon onset as well as the impacts of convective condensation heating associated with the BOB monsoon onset on the withdrawal of low level subtropical anticyclone is investigated. Results show that the Asian monsoon onset occurring during May 1998 exhibits a typical eastward development from the BOB region to the SCS domain. The vigorous convection over the BOB during and after the BOB monsoon onset results in the development of westerlies and convection over the north part of SCS. It is shown that the condensation heating is helpful in the overturning of the meridional gradient of temperature over the SCS. As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS.