THE CHARACTERISTICS OF LOW-FREQUENCY OSCILLATIONS OVER THE SOUTH CHINA SEA

Using 1975-1993 (with 1978 missing) data of the outgoing longwave radiation (OLR), characteristics of seasonal variation of low-frequency oscillations in the South China Sea and its relation to the establishment and activity of the summer monsoon there are studied. As is shown in the result, the low-frequency oscillation in the South China Sea is much stronger in the period of summer monsoon than in that of winter monsoon and the summer monsoon there usually begins to set up in a negative phase of the first significant low-frequency oscillation for the early summer. The study also reveals that the circulation for the low-frequency oscillation during the summer monsoon in the Sea is embodied as north-south fluctuations of the ITCZ and east-west shifts of western ridge point of the West Pacific subtropical high, suggesting close correlation between the low-frequency oscillation and the active and break (decay) of the South China Sea monsoon. In the meantime. the work illustrates how the low-frequency oscillation in the South China Sea are superimposed with the seasonal variation of the general circulation. so that the summer inonsoon covers the establishment of the I st, intensification of the 2nd and 3rd the low-frequency oscillations and decay of the 4th oscillation.

Overview of the South China Sea Monsoon Experiment

The present paper gives an overview of the key project ' South China Sea Monsoon Experiment (SCSMEX)' operated by the Ministry of Science and Technology of China during the period of 1996-2001. The SCSMEX is a joint atmospheric and oceanic field experiment which aims to better understand the onset, maintenance, and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May-August 1998, a large amount of meteorological and oceanic data was acquired, which provides excellent datasets for the study of the SCS monsoon and the East Asian monsoon and their interaction with the ocean. The preliminary research achievements are as follows. (1) The earliest onset of the Asian monsoon over the SCS and Indo-China Peninsula has been well documented. Prom the viewpoint of the synoptic process, its onset is closely related to the early rapid development of a twin cyclone to the east of Sri Lanka. The conceptual modei of the SCS monsoon onset in 1998 was put forward. The 50-year time series of the SCS monsoon onset date was also made. (2) Two major modes, namely the 30-60-day and 10-20-day oscillations were ascertained. The influences of the abnormal SCS monsoon on the precipitation over eastern China and its modes were identifled. A strong (weak) monsoon over the SCS usually leads to less (more) precipitation over the middle and lower reaches of the Yangtze River basin, and more (less) precipitation in North China. (3) During the monsoon onset over the SCS, a wide variety of organized mesoscale convective systems (MCSs) were observed by a Doppler radar array deployed over the northern SCS. The relationship between large-scale circulations and MCSs during the monsoon onset process in 1998 was clearly revealed. It was suggested that there is a kind of positive feedback mechanism between large-scale circulations and MCSs. (4) The SST over the SCS during the early period influences the timing of the monsoon onset date and the monsoon's intensity. During the monsoon onset, the ocean undergoes a process of energy release through air-sea interaction. During the break phase of the SCS monsoon, the ocean demonstrates the process of energy re-accumulation. Obvious differences in the air-sea turbulent flux exchange between the southern and northern parts of the SCS due to different characteristic features of the atmosphere and sea structure were observed in those regions. (5) The verification of impact of intensive observations on the predictive performance is made by the use of regional models. The air-sea coupled regional climate modei (CRCM) was also developed under the SCSMEX Project . The simulation of the oceanic circulation in 1998 produced with the modei was well compared with the observations.

South China Sea Monsoon Experiment(SCSMEX) and the East Asian Monsoon

The SCSMEX is a joint atmospheric and oceanic experiment by international efforts, aiming at studying the onset, maintenance, and variability of the South China Sea （SCS） summer monsoon, thus improving the monsoon prediction in Southeast and East Asian regions. The field experiment carried out in May-August 1998 was fully successful, with a large amount of meteorological and oceanographic data acquired that have been used in four dimensional data assimilations by several countries, in order to improve their numerical simulations and prediction. These datasets are also widely used in the follow-up SCS and East Asian monsoon study. The present paper has summarized the main research results obtained by Chinese meteorologists which cover six aspects： （1） onset processes and mechanism of the SCS summer monsoon; （2） development of convection and mesoscale convective systems （MCSs） during the onset phase and their interaction with large-scale circulation; （3） low-frequency oscillation and teleconnection effect; （4） measurements of surface fluxes over the SCS and their relationship with the monsoon activity; （5） oceanic thermodynamic structures, circulation, and mesoscale eddies in the SCS during the summer monsoon and their relationship with ENSO events; and （6） numerical simulations of the SCS and East Asian monsoon.

南海季风试验与东亚夏季风

南海季风试验是一次国际性大气与海洋的联合试验 ,旨在更好地了解南海季风的爆发、维持与变化 ,以改进东亚和东南亚地区的季风预报。 1998年 5～ 8月进行的外场试验取得了圆满成功 ,获得了大量气象与海洋资料。不少国家对这些资料进行四维资料同化 ,并改进数值模拟和预报 ;同时也为东亚与南海地区季风的研究提供了必要的资料集。文中总结了中国科学家在这方面的主要研究成果 ,共包括 6个方面 :(1)南海夏季风的爆发过程与机理 ;(2 )南海季风爆发过程中对流与中尺度系统的发展及其与大尺度环流的相互作用 ;(3)低频振荡与遥相关作用 ;(4 )南海海 气通量的测量及其与季风活动的关系 ;(5 )夏季风时期南海海洋的热力结构、环流和中尺度涡旋及其与ENSO事件的关系 ;(6 )南海与东亚季风的数值模拟。

南海表层水温低频振荡的基本特征

使用1958－1987年COADS资料对南海全区表层水温做了分析。结果发现，南海表层水温存在20－40个月的低频振荡，其中以准两年周期最显著，，其次是3－4年．这种振荡主要发生在深水海盆区，并且与季风异常有着十分密切的关系。

南海地区大气30～60天低频振荡及其对南海夏季风的可能影响

本文利用1979~2008年美国国家环境预报中心（NCEP）和美国国家大气研究中心（NCAR）第二套再分析资料、美国国家海洋和大气管理局（NOAA）的向外长波辐射（OLR）资料及1979~2007年全球降水资料（CMAP）,分析了南海地区热带大气的低频振荡及其对南海夏季风的可能影响。结果表明,夏半年南海地区存在显著的30~60天的周期振荡。当30~60天低频振荡处于活跃位相时,南海及其周围地区的低层大气为低频西南风,南海和菲律宾北部为低频气旋流场且为正的位涡度,对应着增强的南海夏季风槽和南海夏季风;当30~60天低频振荡处于不活跃位相时,情形正好相反。进一步的研究揭示出,夏半年30~60天低频振荡变化的空间型与夏半年平均场的年际变化的空间分布非常相似,并且南海及其附近地区的30~60天低频振荡活动的年际变化对夏半年平均场的年际变化有显著的贡献。强、弱南海夏季风年30~60天低频振荡活动的比较也说明,强的南海夏季风年30~60天低频振荡活跃状态发生的概率大于不活跃状态发生的概率,而弱的南海夏季风年则是不活跃状态发生的概率大于活跃状态发生的概率。因此,南海地区30~60天低频振荡对南海夏季风很可能有重要影响,当30~60天低频振荡的活跃状态处于主导时,南海夏季风往往会偏强;反之,如果不活跃状态处于主导时,南海夏季风往往会偏弱。

南海地区降水的时空特征

文中利用美国 NCEP重分析资料中的 1 979～ 1 995年 1 7a逐旬的全球降水资料 ,采用小波分析方法分析了南海地区降水的多时间层次和多空间层次结构 ,研究了南海季风的爆发及时间演变 ,探讨了南海季风爆发的机制。结果表明 :( 1 )南海季风爆发于 5月中旬 ,季风爆发过程实际上是小范围 ( 32个经度 )降水向大范围 ( 64个经度 )降水调整的过程 ,一旦出现较强的大范围降水 ,并到达南海地区 ,就爆发了南海季风 ,调整完毕则是印度季风和东亚季风的相继爆发。( 2 )在 1 0°N以北的地区 ,季风最早发生在南海 ,然后逐渐西移到印度 ,达到印度季风最盛期后 ,迅速东撤。( 3)南海地区可分为 3个区域 :北部 ( 2 0～ 2 2°N)、中部 ( 1 0～ 2 0°N)和南部 ( 1 0°N以南 )。南海雨季主要发生在 1 0°N以北的北部和中部 ,北部雨季是平稳增强的单峰型 ,而中部雨季是突发性的 ,雨季内降水起伏较大。( 4 )南海季风区有很强的年变化 ,30～ 60 d和 2 0～30 d的变化也比较显著 ,还有 3个月左右的周期变化。除年振荡以外 ,各种周期振荡随时间变化较大 ,在雨季表现得最强烈。( 5)南海季风的爆发与 2 0～ 30 d和 30～ 60 d两种低频振荡有关。

2005～2010年南海海平面异常升高的特征与机制

利用高分辨率卫星高度计观测海平面异常（SLA）数据, 结合潮位站观测资料, 研究2005-2010 年南海海平面异常升高的空间分布特征.结果表明, 南海海平面显著上升, 平均速率为17mm·a^–1, 上升速率在空间分布上存在显著差异.速率最大的海域出现在南海中西部, 大于30mm·a^–1, 而南海南部海域上升速率则相对较小, 大约为12mm·a^–1, 潮位站的观测结果与卫星高度计观测结果一致.1993-2004 年期间南海海平面线性变化主要与海面经向风场变化导致的海面温度变化密切相关; 2005-2010 年3 次强La Nina 事件导致南海呈显著的正SLA; 此外, 受南方涛动、北太平洋的年代际振荡（PDO）等的低频变化影响, 南海中部海盆呈显著的东风异常, 由此在南海中西部海域产生负的风应力旋度, 是该海区海平面显著上升的另一重要因素.

2008年广东强龙舟水与准10天振荡

采用小波分析、相关分析、Lanczos时间滤波器等方法分析了2008年广东强龙舟水与大气低频振荡的关系。结果表明,强龙舟水期间广东逐日降水存在显著的准7～12天周期振荡即准10天振荡,广东上空的逐日500 hPa高度场存在显著的8～13天周期振荡、南海北部平均的逐日850 hPa纬向风场存在显著的准12天周期振荡。广东前汛期降水与南海北部季风的关系最密切,其次为影响广东上空的高空槽。从中太平洋开始向西传播的准10天振荡在菲律宾东侧强对流区得到加强并继续向西传播,与从南海南部北传并加强的季风相结合,导致强季风涌的出现,从而导致强龙舟水期间的2次强降水过程。利用典型位相的合成分析,对广东强龙舟水期间2次强降水过程不同位相的大气环流场的共同演变特征进行了分析,它们可为广东强降水过程尤其是龙舟水的中期预报提供参考依据。

南海夏季风的低频特征

利用１９７５－１９９３年（其中１９７８年缺）的向外长波辐射（ＯＬＲ）资料，分析了南海低频振荡的季节变化特征及其与南海夏季风建立和活动的关系。结果表明：南海低频振荡在夏季风期间比冬季风期间明显较强，南海夏季风一般在初夏第一个较强低频振荡的负值位相开始建立；南海夏季风期间低频振荡的环流实体是ＩＴＣＺ的南北振荡和西太平洋高压西脊点的东西摆动，低频振荡与南海季风的活跃和中断（或减弱）密切相关；南海低频振荡与大气环流的季节变化相叠加，造成夏季风经历了第一个低频振荡的建立，到第二、三个振荡的加强，再到第四个振荡的减弱这种演变过程。

青藏高原夏季风和南海夏季风低频振荡的关系

利用1948-2010年NCEP/NCAR全球大气逐日平均的再分析资料分析了青藏高原夏季风和南海夏季风大气低频振荡的可能关系。结果表明,夏半年高原地区和南海地区季风均存在明显的30~50天的振荡周期,并且两者在这个振荡周期上存在明显的位相关系,即南海夏季风的低频振荡比青藏高原夏季风提前约3/4个位相,对500 h Pa和850 h Pa低频风场的研究也得出同样的结果。两者存在明显位相关系的原因之一可能是3月下旬开始南海向青藏高原地区的低频输送。

南海表层水温甚低频振荡的动力学机制

用一个简单的海洋模式，研究了东亚季风异常对南海表层水温（ＳＳＴ）的影响，发现南海表层水温的甚低频振荡是东亚季风异常风应力强迫所致．这种甚低频振荡主要反映在深水区，其空间分布呈椭圆形，几乎与南海海盆形状一致。从海盆中央到边缘振幅逐渐减少，ＳＳＴ的甚低频振荡是非行进波，无明显传播特征。

东亚梅雨季低频波波包传播特征

利用1976-2016年41 a NCEP/NCAR再分析日平均资料和中国2426站20～20时日降水量资料,使用带通滤波以及波包传播的诊断方法分析了10～20 d低频波波包分布和传播特征,并进一步研究了典型梅雨异常年(1998年和2002年)梅雨季低频扰动能量的传播特征,提高对10～20 d低频扰动能量的认识,进而提高中期延伸期的天气预报水平。研究表明,南半球低频波波包值高于北半球;低频波波包呈经向传播,冬季低频波波包高值中心基本位于高纬度地区,到了夏季,高纬度地区低频波波包向低纬度赤道方向移动。梅雨典型异常年波包的纬向和经向传播表明,丰梅年长江中下游地区扰动能量纬向上主要受来自太平洋中部西传累积的影响,经向上受来自蒙古高原向南传播累积的影响。枯梅年则正好相反。