| ZHANG Ningning,LAN Jian,MA Jie,CUI Fengjuan. 2016. The shallow meridional overturning circulation in the South China Sea and the related internal water movement. Acta Oceanologica Sinica, 35(7):1－7 |
| The shallow meridional overturning circulation in the South China Sea and the related internal water movement |
| 南海浅层经向翻转环流及相关的内部水体流动 |
| Received:May 20, 2015 Revised:July 14, 2015 |
| DOI：10.1007/s13131-016-0900-3 |
| Key words:South China Sea shallow meridional overturning circulation Ekman transport subduction upwelling |
| 中文关键词: 南海 浅层经向翻转环流 Ekman输运 潜沉 上升流 |
| 基金项目:The Strategic Priority Research Program of the Chinese Academy of Sciences under contract No. XDA11010302; the National Natural Science Foundation of China under contract Nos 41276011, 41521091 and U1406401; the Research Project of Ministry of Education of China under contract No. 113041A. |
| Author Name | Affiliation | E-mail | | ZHANG Ningning | Physical Oceanography Laboratory, College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China | | | LAN Jian | Physical Oceanography Laboratory, College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China | lanjian@ouc.edu.cn | | MA Jie | Physical Oceanography Laboratory, College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China | | | CUI Fengjuan | North China Sea Data and Information Service, North China Sea Branch, State Oceanic Administration, Qingdao 266061, China | |
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| Abstract: |
| The structure of the annual-mean shallow meridional overturning circulation (SMOC) in the South China Sea (SCS) and the related water movement are investigated, using simple ocean data assimilation (SODA) outputs. The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale, which consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow, with a strength of about 1×106 m3/s. The formation mechanisms of its branches are studied separately. The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m. The annual-mean Ekman transport across 18°N is about 1.2×106 m3/s. An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework. An annual subduction rate of about 0.66×106 m3/s is obtained between 17° and 20°N, of which 87% is due to vertical pumping and 13% is due to lateral induction. The subduction rate implies that the subdution contributes significantly to the downwelling branch. The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward. The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents. Significant upwelling mainly occurs off the Vietnam coast in the southern SCS. An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×106 m3/s, of which a large portion is contributed by summer upwelling, with both the alongshore component of the southwest wind and its offshore increase causing great upwelling. |
| 中文摘要: |
| 利用SODA数据，本文对南海浅层经向翻转环流的结构及相关的水体流动进行了研究。从年平均时间尺度来看，南海400米以上存在着显著的顺时针的经向翻转环流，强度约为1×106 m3/s。它的下沉支在南海北部，次表层向南流动，上升支约位于10°N处，表层为北向流动。进一步，本文对各分支的形成原因进行了研究。南海区域年平均风应力的纬向分量主要是向西的，因此在50米以上可以引起向北的Ekman输运，通过18°N的年平均北向Ekman输运可达1.2×106 m3/s。根据拉格朗日的观点，通过估计从混合层进入温跃层的水体体积，得到南海北部17°至20°N处年平均的潜沉率约为0.66×106 m3/s，其中87%是由垂向抽吸造成的，13%是由水平流动造成的，该潜沉率的值表明潜沉对浅层经向翻转环流下沉支的形成有重要作用。潜沉水质点的运动轨迹表明，部分水体从二月的混合层底潜沉后，会随着南海西边界流向南流动，最远可达11°N，然后转而向北流动。冬季混合层底的水平流速场及经向流速的断面也证实了次表层的南向流动主要发生在南海的西边界流区。水体的上升流动主要发生在南海南部的越南沿岸，10°至15°N之间的年平均水体上升率最大约为0.7×106 m3/s，其中大部分是由夏季的上升流动造成的。夏季西南季风的经向分量会引起离岸流，进而引起沿岸上升流；并且西南风随着离岸距离的增加而逐渐增强，相应的正风应力旋度会进一步加剧上升流动。 |
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