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Huang Xiaolong,Jing Zhiyou,Zheng Ruixi,Cao Haijin. 2020. Dynamical analysis of submesoscale fronts associated with wind-forced offshore jet in the western South China Sea. Acta Oceanologica Sinica, 39(11):1-12
Dynamical analysis of submesoscale fronts associated with wind-forced offshore jet in the western South China Sea
南海西部风驱离岸急流次中尺度锋面的动力学分析
Received:April 13, 2020  
DOI:10.1007/s13131-020-1671-4
Key words:submesoscale fronts  enhanced vertical velocity  energy transfer  offshore jet  western South China Sea
中文关键词:  次中尺度锋面  增强的垂直速度  能量传递  离岸急流  南海西部
基金项目:This work is supported by the Chinese Academy of Sciences under contract Nos ZDBS-LY-DQC011, ZDRW-XH-2019-2 and ISEE2018PY05; the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) under contract No. GML2019ZD0303; the National Natural Science Foundation of China under contract Nos 41776040 and 92058201; the Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. OCFL-201804; the State Key Laboratory of Tropical Oceanography under contract No. LTO1907; the Guangzhou Science and Technology Project under contract No. 201904010420.
Author NameAffiliationE-mail
Huang Xiaolong State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
University of Chinese Academy of Sciences, Beijing 100049, China 
 
Jing Zhiyou State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China 
jingzhiyou@scsio.ac.cn 
Zheng Ruixi State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
University of Chinese Academy of Sciences, Beijing 100049, China 
 
Cao Haijin College of Oceanography, Hohai University, Nanjing 210024, China  
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Abstract:
      This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea (WSCS), using satellite observations, a 500 m-resolution numerical simulation, and diagnostic analysis. Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(~10) km are characterized with one order of Rossby (Ro) and Richardson (Ri) numbers in the WSCS. This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts. The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water. By this way, the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity (PV) and triggering frontal symmetric instability (SI) at the submesoscale density front. The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100) m/d. Additionally, the estimate of the down-front wind forcing the Ekman buoyancy flux (EBF) is found to be scaled with the geostrophic shear production (GSP) and buoyancy flux (BFLUX), which are the two primary energy sources for submesoscale turbulence. The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities. In this content, submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale. These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.
中文摘要:
      本文利用卫星观测资料和500 m分辨率数值模拟结果,结合理论分析,对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明,南海西部(WSCS)存在侧向尺度为O(1-10)km的次中尺度锋面,在地转和非地转运动的共同作用下,次中尺度密度锋面具有一阶Rossby(Ro)和Richardson(Ri)数。锋面诊断结果显示,沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送,有效地在跨锋面方向将表层冷水平流输送至暖水侧,导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡(PV),表明该密度锋面易受次中尺度对称不稳定(SI)的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度,其最大值可达100 m·d-1。能量评估结果表明,次中尺度湍流的两个主要能量源,即地转剪切项(GSP)和垂向浮力通量(BFLUX)在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下,大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此,次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量,可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。
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