华南前汛期开始日期的年际与年代际变化机制

The first rainy season over South China is a regional, explosive, and continuous heavy rainfall process. The onset date of the first rainy season over South China (ODFRSSC) is the beginning of first precipitation concentration period during one year, and its yearly variation exhibits significant interannual and interdecadal oscillation.The change of ODFRSSC is related to anomalous north wind in lower level over East China. The interannual variation of ODFRSSC is significant relationship with SST anomalies during previous winter in Equatorial East Pacific, north Pacific and tropical northwest Pacific east of Philippine; and its interdecadal variation is significant relationship with SST anomalies during previous year in tropical northeast Pacific, east of Australia and Indonesia. These SST anomalies can possible affect anomalous north wind in lower level over East China by local Rossby response, Hadley circulation or teleconnection composed by Walker circulation and Hadley circulation. The important physical process of interannual and interdecadal variation of ODFRSSC will be explored by empirical analysis of observed data and model simulation.This proposal can not only intensify the scientific understanding of ODFRSSC but also estabilish solid physical basis for prediction of ODFRSSC.

华南前汛期是一个区域性、爆发性和连续性的强降水过程，华南前汛期开始日期是年内第一个降水集中期之始，其逐年演变有显著的年际变化和年代际变化。本项目拟采用多尺度的观点研究华南前汛期开始迟早年际变化和年代际变化的物理机制。华南前汛期开始迟早与中国东部存在北风异常有关。华南前汛期开始日期年际变化与前期冬季赤道东太平洋、北太平洋中部及菲律宾以东热带西北太平洋海温显著相关；其年代际变化与前一年的热带东北太平洋、澳大利亚以东及印度尼西亚附近海温显著相关。这些区域的海温异常可能通过局地Rossby波响应、Hadley环流或由Walker环流和Hadley环流组成的遥相关来影响中国东部北风异常。将用观测资料分析和模式模拟两种技术手段来揭示出对华南前汛期开始日期年际和年代际变化有重要影响的物理过程。本项目不仅加深对华南前汛期开始变化的科学理解，还会把华南前汛期开始预测问题置于坚实的物理基础之上。

本课题首先客观确定了能表征华南前汛期降水特征的关键区（106-120ºE,21-26ºN），并重新定义了华南前汛期开始日期。该定义不仅可以较好地反映华南前汛期降水开始前后降水的爆发性与持续性,而且可以反映华南前汛期降水的准双周振荡特征。研究结果表明，来自印度半岛北部和副高边缘转向的西南风向华南地区输送暖湿气团，给华南地区提供对流不稳定能量；随着对流不稳定层逐渐向上发展，产生有组织的深对流，华南地区出现爆发性、持续性的强降水过程，华南进入前汛期。冬季至3月(4月)华南地区出现异常偏南(北)风是华南前汛期开始偏早(晚)的关键。西风漂流区和赤道东太平洋的冬季SSTA是华南前汛期开始日期年际变化的前兆信号。西风漂流区和印度洋近赤道海区的前一年SSTA是华南前汛期开始日期年代际变化的前兆信号。资料分析和模式模拟都表明，冬季赤道中东太平洋海温正(负)异常通过PNA遥相关影响北太平洋中部气旋式(反气旋式)环流，进而影响西风漂流区海温负(正)异常。春季时西风漂流区海温负(正)异常通过影响北太平洋上空气旋式(反气旋式)环流，减弱 (增强)东亚大槽，影响华南的冷空气活动偏弱（强），华南处于异常偏南(北)风控制，增加(减少)了气候态偏南风向华南的暖湿气团输送，使华南前汛期开始偏早(晚)。