华南夏季风降水开始日的异常与前冬大气环流和海温的关系

利用1951～2004年NCEP/NCAR再分析资料及ERSST海温资料，研究了华南夏季风降水开始日期的变化特征及其与前期冬季大气环流和海温的关系。小波分析表明，夏季风降水开始日期具有明显的年际和年代际变化，年际变化以准2～3年变化为主，年代际变化周期约16年。华南夏季风降水开始偏早年在大气环流上的前兆信号表现为前期冬季乌拉尔山阻塞高压偏强、东亚大槽较深，阿留申低压偏强，冷空气活动偏强。冬季，鄂霍次克海附近的海温异常为华南夏季风降水开始迟早有物理意义的、稳定正相关前兆信号。合成分析表明，冬季鄂霍次克海附近SST正异常时，5月100 hPa青藏高压偏东偏北偏弱，异常偏西风控制华南；850 hPa环流在华南表现为东北风，华南受冷空气影响为主，华南夏季风降水开始日期偏晚。相反时，若冬季鄂霍次克海附近SST负异常，5月100 hPa青藏高压偏西偏南偏强，异常偏东风控制华南；850 hPa环流在华南表现为偏南风，华南受热带系统影响为主，华南夏季风降水开始日期偏早。并提出冬季中高纬度地区冷空气活动影响华南夏季风降水开始日异常的物理机制。

Variation of Starting Date of Summer Monsoon Rain in South China and Its Relation to Atmospheric Circulation and SST in the Preceding Winter

The variation characteristics of starting date of summer monsoon rain in South China and its relation to atmospheric circulation and SST in the preceding winter are studied using NCEP/NCAR reanalysis data and ERSST(Extended Reconstruction of global Sea Surface Temperatures) data during 1951-2004.The wavelet analysis exhibits significant interannual and interdecadal variation in the sequence of SDSMR(Starting Date of Summer Monsoon Rain).The interannual variation is dominated by quasi-bi-triennium periodic oscillation and the period of interdecadal variation is about 16 years.But the interdecadal variation became weaker from the early 1990s.The Ural Mountains blocking high is stronger,the East Asia trough is deeper,the Aleutian low is stronger and the cold air activity is stronger during the preceding winter for years of earlier SDSMR.The sea surface temperature(SST) anomalies near the Okhotsk Sea are stable precursory signal with positive correlation for SDSMR and can last for winter and spring.The composite analysis shows that the Tibetan high is weaker and is located east and north,anomalous westerly controls South China at 100 hPa in May,and the SDSMR is later when the SST anomalies near the Okhotsk Sea is positive.The northeast wind prevails in South China at 850 hPa meanwhile,so the cold air dominates South China.Contrarily,when the SST anomalies near the Okhotsk Sea is negative,the Tibetan high is stronger and is located west and south,anomalous easterly controls South China at 100 hPa in May and the SDSMR is earlier.The southerly prevails in South China at 850 hPa,so the tropical circulation system dominates South China.The precursory signals for SDSMR in atmospheric circulation and SST are consistent.A possible physical process is as follows.The stronger cold air during winter increases the total wind speed and cools SST near the Okhotsk Sea through enhanced surface evaporation.The negative SST anomalies during winter in the Okhotsk Sea make the Okhotsk Sea blocking high weaken during spring,East Asia trough becomes weaker,and hence the cold air activity affecting eastern China during spring becomes weaker.So South China is easily affected by the tropical synoptic system,SDSMR is earlier,and vice versa.