1990s年代际转型前后南海季风系统的季节变化

Seasonal Variability of the South China Sea Monsoon System Before and After the Decadal Shift in 1990s

利用多变量经验正交分解(MV-EOF)等方法,研究了在季节变化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。第一模态表现为冬夏反位相的年周期变化,但爆发早年夏季风持续时间略长于爆发晚年,空间上都反映了南海中央海盆区的夏季强降水和850 hPa上南海北部的气旋性环流异常,但夏季风爆发早年中国华南沿海降水加强而南海南部降水偏少。相应的大范围环流场上主要反映了南海夏季风爆发后进入盛夏时节亚太地区大范围的环流特征,南海夏季风爆发偏早年索马里越赤道气流偏强,东亚季风槽位置偏北,爆发偏晚年则相反。第二模态反映了南海季风系统春秋反位相的季节变化,且秋季的振幅更强,空间降水场上对应着秋季华南沿海和南海北部与南海中南部北旱南涝的跷跷板式分布,850 hPa风场上则主要表现为异常的东北季风,该模态时空特征表明南海夏季风爆发偏早年的秋季,冬季风建立也偏早,越南及周边地区的降水偏多。相应的大范围环流场上则主要反映了冬季风的环流特征,在南海夏季风爆发偏早年的秋季,菲律宾以东的热带对流减弱,PJ波列增强,爆发晚年则相反。

Based on multi-variables empirical orthogonal function（MV-EOF） analysis,spatial and temporal patterns of the South China Sea（SCS） monsoon system in seasonal cycle are analyzed.Results show that the onset of SCS summer monsoon appears a decadal shift around 1993—1994 from later onset before 1993 to earlier than normal after 1994.The first leading mode shows a winter-summer out-of-phase mode,or so-called solstitial mode.The summer monsoon duration in early onset years lasts longer than that in late onset years.Spatial patterns in early and late onset years all appear strong rainfall in summer in the central basin of the SCS and cyclonic circulation over the north of SCS at 850 hPa.Stronger rainfall appears along the coast of the South China and dryer situation over the south of the SCS during the early summer monsoon onset years.Corresponding large-scale circulation relative to the first mode shows a strong Somali cross-equatorial flow and northward of monsoon trough in early onset years,and vice versa in late onset years.The second mode appears a spring-autumn out-of-phase mode,or so called equinoctial asymmetric mode.Spatial pattern appears a north-south see-saw precipitation pattern and northeasterly at 850 hPa over the SCS.This mode indicates that the winter monsoon onset earlier in the early summer monsoon onset years,with richer rainfall in Vietnam.Corresponding large-scale circulation shows weaker convection east of Philippine and a stronger PJ teleconnection wavetrain in early onset years,and vice versa in late onset years.