The shallow meridional overturning circulation in the South China Sea and the related internal water movement

• Published in: Acta oceanologica Sinica Vol. 35; no. 7; pp. 1 - 7
• Main Authors: Zhang, Ningning, Lan, Jian, Ma, Jie, Cui, Fengjuan
• Format: Journal Article
• Language: English
• Published: Beijing The Chinese Society of Oceanography 01.07.2016; Springer Nature B.V; Physical 0ceanography Laboratory, College of Physical and Environmental 0ceanography, 0cean University of China, Qingdao 266100, China%North China Sea Data and Information Service, North China Sea Branch, State 0ceanic Administration, Qingdao 266061, China
• Subjects: Annual; Boundary currents; Climatology; Data assimilation; Data collection; Downwelling; Earth and Environmental Science; Earth Sciences; Ecology; Ekman transport; Engineering Fluid Dynamics; Environmental Chemistry; General circulation models; Heat; Internal water; Marine; Marine & Freshwater Sciences; Mixed layer; Ocean circulation; Oceanic analysis; Oceanography; Offshore; Salinity; Subduction; Surface flow; Thermocline; Upper bounds; Upwelling; Velocity; Velocity distribution; Velocity sections; Water circulation; Western boundary currents; Wind; Wind stress; 上升流; 南海北部; 水运动; 浅层; 混合层; 环流结构; 翻转; 速度场; South China Sea; China; ISEW, South China Sea; ISEW, Vietnam; South China Sea; subduction; shallow meridional overturning circulation; Ekman transport; upwelling
• ISSN: 0253-505X; 1869-1099
• DOI: 10.1007/s13131-016-0900-3

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 1x 108 ma/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 m^3/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 ma/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 1 I~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 upwelfing rate between 10° and 15°N is 0.7×108 ma/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.