南海夏季风活动指标的定义及应用

DEFINITION AND APPLICATION OF INDICES FOR THE SOUTH CHINA SEA SUMMER MONSOON ACTIVITIES

为了分析南海夏季风活动不同阶段的大气环流特征,引入南海区域（105-120°E,5-20°N）平均高（200hPa）低（850hPa）层风场和向外长波辐射（OLR）作为南海夏季风指数。分析结果表明这些指数的组合可以较好地反映南海夏季风季节内以下时间尺度的活动情况。当南海地区低层平均为西南风、高层为东北风且OLR异常（OLRa）小于零时,南海夏季风处于活跃期,此时副高远离南海,南海区域对流强盛,有明显的季风槽;当南海地区低层为西南风,高层为东北风,但是OLRa大于零时,南海夏季风处于不活跃阶段,此时副高远离南海,虽然南海地区对流不活跃,但是季风环流依然存在且向北扩展,使得华南-江南对流活跃;当南海地区风场为其他情况时,此时不论对流强弱,南海夏季风处于中断期,南海或者受副高控制,或者受热带气旋影响,季风环流在南海地区中断。利用定义的南海夏季风活动指标对2011年和2012年南海夏季风活动进行分析,结果指出这两年南海夏季风活跃期较长,季节内对流北传事件一般发生在南海夏季风活跃期或活跃期向非活跃期的转换期,而中断期即使有强对流发生,也不会向北传播。分析了这两年中断和不活跃情况下的大气环流分布,进一步验证了定义的南海夏季风活动指标的实用性。

In order to analyze the characteristics of South China Sea summer monsoon （SCSSM） in different stages, SCSSM indices of South China Sea （SCS） regional （105 -120 °E, 5 -20 °N） 200 and 850 hPa winds and outgoing longwave radiation （OLR） are introduced. The results indicate that the combined index can represent the SCSSM activities on the intraseasonal and synoptic scales. When southwesterly and OLR anomaly （OL- Ra） are less than zero, the SCSSM is in an active phase. At the stage a western Pacific Subtropical high （WP- SH） stays far away from the SCS where the lower-level southwesterly and high-level northeasterly prevail and convections are strong with an evident monsoon trough. As the SCSSM indices are southwesterly and larger-than- zero OLRa, the SCSSM is in an inactive phase. At this time, the WPSH is still far away from the SCS. Though there are relatively weak convections in the SCS region, the monsoon circulations maintain and extend north- wards, which excites the convections of the South China and the south of the Yangtze River. When there is no southwesterly in the SCS, the SCSSM is in a break phase no matter what magnitude the OLR is in. When the SCS region is controlled by the WPSH or the tropical cyclone, and the monsoon circulations are broken in the SCS. The combined index is used to analyze the activities of SCSSM in 2011 and 2012. The results show a rela- tively long active phase. In general, intraseasonal northward propagation of convection occurs in the active phase or conversion period for the active to inactive phase rather than in the break phase.