中文摘要：

基于1998—2011年的TRMM卫星逐日降水资料、NCEP/NCAR大气再分析资料以及向外长波辐射（OLR）资料,利用小波分析、Butterworth带通滤波、EOF分析以及合成分析的方法,对秋季（9—10月）南海北部地区（10°—24°N,100°—120°E）持续性强降水的低频特征进行分析。结果表明,秋季南海北部降水存在显著的准双周振荡特征,主要表现为南海北部地区一致变化型和南海东北部地区和中南半岛沿海地区反相变化型两个模态。其中,第一模态的低频降水主要集中在中南半岛沿岸附近的南海中西部,第二模态则主要集中在南海东北部。进一步对与这两种模态相关联的大气环流和OLR的低频场进行合成分析。结果表明,第一模态的对流层低层准双周低频信号主要来源于南半球近赤道上空,该地区异常强对流活动产生的波列和气旋性环流越赤道进入南海并加强传播至南海北部。而第二模态则来源于南海中南部和吕宋岛东侧西太平洋上空,异常强对流和气旋性环流的合并加强并传播至南海东北部。

英文摘要：

Based on the 1998-2011 daily mean rainfall data from TRMM satellite,the NCEP/NCAR reanalysis data and the OLR data,the low frequency characteristics of persistent severe rain over the northern of the South China Sea（SCS）（10°-24°N,100°-120°E） in autumn（September and October） are investigated by applying the methods of wavelet,Butterworth band filter,EOF and the composite analysis.The results show that the autumn rainfall over the northern SCS presents the characteristic of quasi-biweekly oscillation.There are two dominant modes,the first EOF mode depicts a consistent precipitation anomaly over the northern SCS and the second EOF mode describes a precipitation oscillation between the northeast of the SCS and the coast of the Indo-China peninsula.The low frequency precipitation of the first mode mainly concentrates in the midwest of the SCS near the coast of Indo-China peninsula and the second mode mainly concentrates in the northeast of the SCS.The atmospheric circulation and the OLR patterns associated with these two modes are investigated through the composite analysis method.It shows that the quasi-biweekly oscillation signal under the lower troposphere of the first mode sources from the Southern Hemisphere near the equator and the low frequency signal of the second mode mainly originates from the south-central region of the SCS and the Western Pacific on the eastern side of Luzon Island.In the first mode,the cross-equatorial wave train of abnormal strong convective activities and cyclonic circulation enter the SCS and strengthen the dissemination to the northern part of the SCS.And for the second mode,the abnormally strong convection and cyclonic circulation merger to strengthen and spread to the northeastern part of the SCS.