南海夏季风爆发迟早与赤道纬向风关系的诊断研究

Diagnosis of Relationships between the Onset of South China Sea Summer Monsoon and the Equatorial Zonal Wind

利用ECMWF和NCEP(1958~1999年)的再分析资料,研究南海夏季风与前期气象要素的关系,结果表明:南海夏季风建立日期与当年2月份赤道印度洋地区的纬向风存在高层(200~100 hPa)为正、低层(1000~700 hPa)为负的显著相关分布,类似于偶极子的分布特征;而在赤道中太平洋地区(160°E^160°W)则存在高层为负、低层为正的另一个显著相关"偶极子"。分析这种相关特征的持续性,发现上述形势从2月份到5月份一直存在,且赤道印度洋地区高低层相反的相关分布从3月份开始逐渐东移,到5月份维持在印度洋东部至南海一带。从5月份的相关系数分布图可发现,南海地区低层为负相关,高层为正相关,说明低层西风异常、高层东风异常的赤道纬向风分布有利于南海夏季风早爆发。针对南海夏季风建立日期与赤道地区(10°S^10°N平均)纬向风的"偶极子"型相关分布特征,定义了一个用于诊断南海夏季风爆发迟早的前期因子。该因子与大部分学者定义的南海夏季风建立日期存在着显著的相关,说明该因子对南海夏季风爆发迟早有一定的反映能力和预测作用。根据Gill(1980年)理论分析发现,上述2月份赤道地区纬向风异常是同期赤道印度洋-大陆桥地区异常强对流活动造成热带赤道大气环流显著异常变化的结果。

Using two kinds of the monthly mean reanalysis data （from 1958 to 1999） of the ECMWF and NCEP/ NCAR, correlation between the South China Sea （SCS） summer monsoon onset dates and the preceding meteorological fields is analyzed. It is found that there exists a resembling ＂dipole＂ of the correlation between the onset dates and the equatorial zonal wind, which is positive in upper layers （200- 100 hPa） and negative in lower layers （1000- 700 hPa） over the equatorial Indian Ocean. Another ＂dipole＂ is found over the equatorial middle Pacific Ocean （negative correlation in upper layer and positive in lower layer） in February. These patterns persist from February to May with the correlation coefficient ＂dipole＂ drifting slowly from the Indian Ocean to eastern regions, finally standing over the eastern Indian Ocean and SCS. By studying the distribution of correlation coefficient in May, it is found that the negative correlation areas correspond to anomalous westerly wind in lower layers and positive correlation area corresponds to anomalous easterly wind in upper layers over SCS regions, which is favorable for the early onset of SCS summer monsoon. A preceding factor for diagnosing the onset of SCS summer monsoon is defined according to the correlation coefficient ＂dipole＂ characteristic. The factor is actually the vertical shear of mean zonal wind over the equatorial （10°S- 10°N） Indian Ocean （50°E- 90°E） and that over the Pacific Ocean （160°E- 160°W）. If the factor has positive （negative） value, the anomalous Walker circulation （anti-Walker circulation） exists over the equatorial region, the SCS summer monsoon onset is earlier （later） in February. The high correlation coefficient between the preceding factor and most of summer monsoon onset dates indicates this factor can diagnose and forecast SCS summer monsoon onset. The farther research shows that the preceding factor which explains the equatorial zonal wind anomalies is corresponding to some kinds of OLR anomalies in the tropic region in February. In early （late） monsoon years, there is the negative （positive） anomaly of OLR over the equatorial eastern India Ocean and Sumatra regions, which will firstly do favor to form two anomalous barometric low （high） centers over equatorial eastern IO and anomalous barometric low （high） belt over the equatorial western Pacific Ocean, and then induce anomalous west （east） wind over the equatorial India Ocean and anomalous east （west） wind over the equatorial Pacific Ocean in February. This diagnostic result can be explained by Gill＇s research.