南海环流数值模拟和南海及邻近海域对南海季风爆发的影响

【作者】 白学志；

【导师】 胡敦欣；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 物理海洋学， 2002， 博士

【摘要】 本文分两部分，第一部分主要探讨了南海及邻近海域对南海夏季风爆发的影响，第二部分用较高分辨率的POM模式模拟了南海月平均环流。在第一部分里，利用历史海洋和大气资料分析了南海季风建立的大尺度背景场和南海季风爆发前后的大气环流演变特征、探讨了南海季风爆发的年际变化与热带海洋海温异常的关系；利用1998年南海季风试验中的现场观测资料，分析了南海季风爆发前后南海一个测站（6°15’N,110°E）的海面热通量和SST的变化特征和变化机制；并用区域气候模式对南海海洋在南海季风爆发中的作用作了数值实验研究，得到了以下几点主要结果： 1、南海季风爆发前，孟加拉湾和中南半岛的对流和降水释放的潜热，会促使200hPa南亚高压向中南半岛西北部移动，对南海季风的爆发可能具有触发作用。2、南海季风爆发早晚与前期（特别是春季）大气环流异常有很好的关系，前期大气环流异常基本上决定了南海季风爆发的早晚。印度洋上的赤道西风和越赤道气流异常对南海季风爆发的影响显著。3、热带海温异常可以通过影响前期季风区的异常环流来影响南海季风的爆发，一种可能过程是：关键海区的海温异常（正或负）所引起的对流异常（上升或下沉），会使得亚洲季风的横向（东西）和侧向（南北）两个辐散环流分量出现异常，从而引起南海季风的提前或推迟爆发。与南海季风爆发有显著相关的海区往往位于这两个季风环流分量的上升或下沉区。赤道中东太平洋可以通过Walker的异常影响南海季风的爆发。4、合成分析表明，南海海温北高南低的分布对应着季风爆发早，整个南海海温偏高时，对应着季风爆发晚。但从相关分析来看，前期南海海温异常与南海季风爆发早晚的关系并不显著。仅在季风爆发前4侯，南海的南部才出现较显著的正相关（海温高，季风爆发晚），这种正相关很可能是一种响应信号，而不具有主导作用。5、1998年南海季风爆发前，南海南部站点（6°15’N,110°E）SST急剧升高；季风爆发后，SST持续下降。爆发前的增温主要原因有两个：一是风速降低导致的潜热输送的减小；二是风速降低导致的海洋混合过程减弱，出现海表薄层跃层。风速的变化在其中起了重要作用。爆发后的降温主要是由云量增加导致的短波辐射的大幅度降低引起的；南海季风爆发前，海面净得热，海面净热通量的变化主要由潜热输送的变化引起的。爆发后，海面净失热；在整个观测时段，海面净热<WP=3>通量对SST的变化有决定性的作用。但各分量对SST变化的相对贡献有所不同，对SST变化有主要贡献的只有潜热和短波辐射。并且在季风爆发前后又有所不同。具体来说，南海季风爆发前，潜热对SST变化贡献最大，短波辐射次之。季风爆发后，短波辐射对SST变化贡献最大，潜热次之。海洋动力过程中，水平平流对SST变化的影响很小夹卷过程是对SST变化的一种重要修正，它是导致几次短暂降温的主要原因。但它不频繁发生。6、区域气候模式模拟结果表明，南海增温可以使南海季风的爆发提前并使季风的爆发现象更明显，它主要通过以下三种途径影响南海季风的爆发：南海增温可在其西侧的中南半岛、孟加拉湾产生气旋差值环流，使得位于该处的印缅槽和季风低压加强,促使印度洋上的赤道西风加强北抬，使得南海季风提前爆发。 通过加强南海与其以南地区的热力差异，促使越赤道气流的提前出现；南海海温增加，会导致南海地区的大气湿度增加，湿度增加会导致大气出现不稳定，有利于对流的出现和季风的爆发。南海海温的作用与合成分析的结果是不一致的，在实际的大气环流异常中，南海海温的作用很可能被掩盖。在第二部分里，利用较高分辨率的POM模式模拟了南海的月平均环流较好地模拟出了南海环流的基本特征，并得到了以下几点有意义的结果：1、黑潮流套常年存在，但有明显的季节变化：冬季较弱，春季有所加强，夏、秋季流套向南海的西伸较显著，平均达到117(-118(E,8、9月份可以达到116(E附近。模拟出了黑潮分离流环的形成和脱离过程。 2、在一年的大部分月份里没有黑潮的直接分支进入南海，在黑潮流套比较强的月份，看不到明显的分支，但在10、11、12月、1月这几个黑潮流套比较弱的月份，则有明显的分支进入南海。3、冬季有一部分黑潮水进入台湾海峡；而夏季台湾海峡中的北向流水主要来自南海暖流水的一部分，没有明显的黑潮水流入。4、夏季吕宋海峡的流向在垂向上有差异，在海峡南部，上层为流入，下层为流出；北部上层为流出，下层为流入；与上层的流速相比，下层要弱得多。5、模拟出了黑潮主轴东侧的暖涡和台湾岛东南角的深层气旋涡。更多还原

【Abstract】 The thesis consists of two parts. The first part deals with influences of the South China Sea and its adjacent oceans on onset of South China Sea summer monsoon (SCSSM). In the second part, an ocean model POM with higher resolution is used to simulate the ocean circulation in the South China Sea.In the first part, atmospheric and oceanic datasets are used to analyse the large-scale circulation background during SCSSM set up and the evolutionary characteristics of atmospheric circulation before/after summer monsoon onset in the South China Sea. The relationship between the interannual variability of SCSSM onset and SST anomaly (SSTA) in the tropical oceans is also discussed. Observational data in situ taken in SCSMEX98 are used to analyse the variation of SST and surface heat flux at (6(15’N,110(E) during the onset period of SCSSM in 1998. Numerical study is also performed with a regional climate model (NCAR/ReGcm2) to study the impact of the South China Sea on SCSSM onset. The results obtained are as follows: 1. Before SCSSM onset, latent heat flux released by convection and precipitation over the Bay of Bangle and Indo-China Peninsula made the South Asian High at 200hPa shift to the northwest of Indo-China peninsula, which may trigger onset of the SCSSM. 2. A close relationship exists between SCSSM onset and the atmospheric circulation anomalies in spring, which basically control the early/late onset of the SCSSM. The impacts of the anomalies of equatorial westerly over the Indian Ocean and cross-equator flow on the SCSSM are significant.3. SSTA in the adjacent tropical oceans can affect SCSSM onset, by the way of affecting the atmospheric circulation over the monsoon region. One possible process is as follows: positive (negative) SST anomaly in the key regions gives rise to positive (negative) convection anomalies, inducing anomalies of the two divergent cells (longitudinal/latitudinal) of the Asian monsoon, with the result of earlier (later) onset of the SCSSM. The key regions close correlating to onset of SCSSM are always in the ascending or descending areas of the two components of the monsoon circulation. The mid- and eastern Pacific can affect onset of SCSSM through changing the Walker circulation.4. Composite analysis shows that the pattern with positive SSTA in the north of SCS and negative in the south corresponds to earlier onset of SCSSM, while the pattern with positive SSTA in the whole SCS basin is associated with later onset of SCSSM. But there is no significant correlation between the previous (April) SSTA<WP=5>and the onset time of the SCSSM from correlation analysis, only when 4 pentads before onset of SCSSM appears prominent positive correlation over the south of the SCS. This positive correlation may be a response signal and does not mean a leading role for SCSSM onset. 5. Before onset of the SCSSM in 1998, SST at the southern station (6°15’N,110°E) increased abruptly, and decreased continuously after that. There are two main reasons for the increasing of SST before SCSSM onset. One is due to decrease of latent heat flux transition induced by weaker winds. The other reason is owing to weakening of oceanic mixing process as a result of wind speed decrease. The change of wind speed plays an important role in SST increasing. The decreasing of SST after SCSSM onset is mainly because the increased cloud leads to the depression of the short wave radiation largely. Before (after) monsoon onset, sea surface gains (loses) heat by transition of latent heat flux. During the observation period, the net heat flux through sea surface plays a leading role in variation of SST, but other components play different roles in SST change. In details, the latent heat made significant contribution to SST change before SCSSM onset, and the short wave radiation played a secondary role. After monsoon onset, the short wave radiation contributed more to the variation of SST than latent heat. In the process of ocean dynamics, advection term is too small and negligible. Entrainment process also plays更多还原