1990s年代际转型前后南海季风系统的气候季节内振荡(CISO)特征

利用多变量经验正交分解(MV-EOF)等方法,研究了在季节内振荡尺度上南海季风系统的时空分布特征。结果表明:南海夏季风的爆发时间在1993/1994年前后存在显著的年代际转型,由爆发偏晚转变成爆发偏早。第一模态反映了南海夏季风爆发时季风系统的时空特征,转型前后特征类似,降水场自赤道向北依次呈现干-湿-干交替分布的特征,南海中心为异常气旋。相应的大范围环流场主要反映了转型前的偏晚年,南海夏季风槽位置偏南,转型后的偏早年,南海夏季风槽位置偏北。第二模态体现了南海季风系统夏季的时空特征,转型前后共同特征表现为南海地区夏季北湿南干的南北偶极子降水分布及南海中心区的异常西风。相应的大范围环流场主要反映了南海季风活动与东亚季风呈现反位相的特点,且对流信号向北传播。转型前的偏晚年,季风活动受准双周振荡控制,对流信号由西北方向传入南海;转型后的偏早年,季风活动以30～60天振荡为主,对流信号由东南方向传播至南海。

CLIMATOLOGICAL INTRAEASONAL OSCILLATION OF THE SOUTH CHINA SEA MONSOON SYSTEM BEFORE AND AFTER THE DECADAL SHIFT IN 1990S

Based on multi-variables empirical orthogonal function (MV-EOF) analysis, spatial and temporal patterns of the South China Sea (SCS) monsoon system in intraseasonal oscillation are analyzed. Results show that the onset of SCS summer monsoon shows a decadal shift around 1993/1994 from later-than-normal onset before 1993 to earlier-than-normal onset after 1994. The first leading mode shows similar characteristics during the onset of SCS summer monsoon (SCSSM) before and after the decadal shift. The spatial distribution of precipitation in early and late onset years shows a pattern of alternatively dry and wet from the equator to the north, and an anomalous cyclonic circulation is located north of the SCS at 850hPa. Corresponding large-scale circulation shows that relative to the first mode the SCSSM trough is southward in late onset years before the shift and it is northward in early onset years after the shift. The second mode reflects the characteristics during the summer after the onset of SCSSM. The second spatial distribution shows a north-south see-saw precipitation pattern and westerly anomaly at 850hPa over the SCS. Corresponding large-scale circulation shows northward propagation of convection signals and an out-of-phase relationship between SCSSM and EASM. Quasi-biweekly oscillation dominates the monsoon activities in late onset years and convection signals spread into the SCS from the northwest before the shift, while early onset years, after the shift, are controlled by 30-60-day oscillation and the convection signals enter the SCS from the southeast.