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Journal of Tropical Oceanography >

2018 , Vol. 37 >Issue 3: 19 - 25

DOI: https://doi.org/10.11978/2017079

Orginal Article

Analysis of sub-mesoscale dynamic processes in the periphery of anticyclonic eddy in the northern South China Sea

  • ZHENG Ruixi , 1, 2 ,
  • JING Zhiyou , 1 ,
  • LUO Shihao 1, 2
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  • 1. State Key Laboratory of Tropical Oceanography (South China Sea Institute of Oceanology, Chinese Academy of Sciences), Guangzhou 510301, China
  • 2. University of Chinese Academy of Sciences, Beijing 100049, China;
Corresponding author: JING Zhiyou. E-mail:

Received date: 2017-07-14

  Request revised date: 2017-09-18

  Online published: 2018-05-03

Supported by

National Natural Science Foundation of China (41776040, 41230962)

Foundation of State Key Laboratory of Tropical Oceanography (LTOZZ1701).

Copyright

热带海洋学报编辑部
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Abstract

Mesoscale energy can be effectively extracted from geostrophic flows via sub-mesoscale processes and forward cascade to smaller dissipation scales. These ubiquitous sub-mesoscale processes in the upper ocean play an important role in the transport of mass and energy, mesoscale variability and re-stratification of mixed layer. Using the high-resolution (500-m) ROMS results, we preliminarily analyze the sub-mesoscale dynamic processes of typical anticyclonic eddy in the northern South China Sea in winter. Our results show that the strong lateral buoyancy gradient at eddy periphery can efficiently reduce the Ertel potential vorticity of the front, which exacerbates frontal instabilities and is favorable for the development of symmetric instability (SI). In this case, one of the most important mechanisms is the frontogenesis for the generation of frontal SI. Furthermore, sub-mesoscale processes and associated instabilities can trigger a strong vertical secondary circulation across the front. The vertical velocity is up to 95 m·d-1, suggesting significant vertical exchanges of mass and energy in the mixed layer.

Key words： northern South China Sea; anticyclonic eddy; front; sub-mesoscale process; symmetric instability

Cite this article

ZHENG Ruixi , JING Zhiyou , LUO Shihao . Analysis of sub-mesoscale dynamic processes in the periphery of anticyclonic eddy in the northern South China Sea[J]. Journal of Tropical Oceanography, 2018 , 37(3) : 19 -25 . DOI: 10.11978/2017079

近年来, 随着卫星遥感技术、现场观测和数值模式的发展, 活跃在上层海洋的次中尺度(sub- mesoscale)过程 (空间尺度约为100m~10km, 时间尺度约为1天)逐渐引起了科学家们的关注(Haine et al, 1998; Capet et al, 2008a; Thomas et al, 2008, 2013; Taylor et al, 2009; D'Asaro et al, 2011; Gula et al, 2016; Mcwilliams, 2016; Zhong et al, 2017)。由于受地转约束, 中尺度过程在运动学上表现为准二维运动; 而与中尺度地转过程不同, 次中尺度过程具有较大的罗斯贝数(Ro~Ο(1))和相对较小的瑞查德森数(Ri~Ο(1)), 相对涡度ζ接近甚至超过行星涡度f, 在运动学上表现为完全的三维运动。多个海域的现场观测和数值研究结果表明, 次中尺度过程引起的垂向速度可达到20~100m·d-1, 比中尺度过程高一个量级; 且次中尺度过程能将上边界层的能量耗散率提高1~2个量级, 有效地重新再分配上层海洋的浮力分布, 并显著增强大气强迫和海洋内部之间的垂向浮力和动量通量、营养盐输运以及能量交换(Lévy et al, 2001; Mcgillicuddy et al, 2003; D'Asaro et al, 2011; Gula et al, 2014; Omand et al, 2015; Brannigan, 2016; Zhong et al, 2017)。
次中尺度过程在海洋多尺度能量串级过程中也起着重要的媒介作用。次中尺度过程及其不稳定能有效地从中尺度地转剪切中汲取能量, 并通过垂向次级环流的浮力再分配, 使得混合层再层化, 从而释放平衡态的地转剪切动能和锋面有效位能, 并最终向小尺度湍流混合串级能量(Boccaletti et al, 2007; Capet et al, 2008b; Fox-Kemper et al, 2008; D'Asaro et al, 2011; Thomas et al, 2013; Omand et al, 2015; Bachman et al, 2017; Stamper et al, 2017)。另一方面, 次中尺度不稳定从地转剪切获取的能量能通过近惯性运动向深海传递, 进而影响大尺度环流, 在气候系统中具有重要的积分作用(Joyce et al, 2013; Holmes et al, 2014; Haney et al, 2015; Zhang et al, 2015)。
理论研究和大涡模拟结果表明, 海洋锋面海域易于触发多种类型的非地转不稳定过程, 例如对称不稳定(Stone, 1966; Taylor et al, 2009; Thomas et al, 2013)、反气旋非地转不稳定(Mcwilliams et al, 2004), 以及K-H不稳定(Kelvin-Helmholtz instability)(Stone, 1966; Taylor et al, 2009)等。这些非地转不稳定过程能显著增强局地混合、垂向输运并加深混合层深度(D'Asaro et al, 2011; Omand et al, 2015; Thomas et al, 2016)。其中, 对称不稳定是最不稳定模态(Stone, 1966; Fox-Kemper et al, 2008; Thomas et al, 2013), 能联合次级K-H不稳定将锋面有效位能和地转剪切动能串级至三维湍流和小尺度混合过程, 在中尺度过程变异和中尺度到小尺度的正向能量串级中扮演着非常重要的角色(Stone, 1966; Haine et al, 1998; Taylor et al, 2009; Thomas et al, 2013, 2016; Haney et al, 2015; Stamper et al, 2017)。
南海是西北太平洋最大的陆架边缘海, 其复杂的地形特征、显著的季风强迫和不均匀的热盐强迫以及黑潮入侵等动力因素决定了南海有着丰富的中尺度涡旋和海洋锋面(Wang et al, 2003; Tian et al, 2006; Liu et al, 2010, 2012; Nan et al, 2011; Jing et al, 2015, 2016; Zhang et al, 2016)。高分辨率的卫星遥感和数值模拟及现场观测结果表明, 中尺度涡边缘和锋面海域往往伴随着次中尺度过程和混合增强现象(刘国强, 2011; Liu et al, 2015; Jing et al, 2016; Zhang et al, 2016; 罗士浩 等, 2016; Zhong et al, 2017; 冀承振 等, 2017)。由于次中尺度过程具有相对较小的时间和空间尺度, 受资料分辨率等因素的限制, 目前人们对南海次中尺度现象的动力过程尚缺乏深入的理解。因此, 本文基于ROMS (Regional Ocean Modeling System)区域海洋数值模式, 通过高分辨率嵌套模拟并结合理论分析, 在前人研究基础上对南海北部中尺度涡边缘次中尺度过程的动力学特征及非地转不稳定进行分析与探讨。

1 模式配置和卫星遥感数据

1.1 卫星遥感数据介绍

本文使用的高度计资料为法国国家空间研究中心AVISO (Archiving Validation and Interpolation of Satellite Oceanographic Data, 网址为http://www. aviso.oceanobs.com/html)提供的TP/Jason1和ERS/ENVISAT网格化融合的海面高度异常(sea level anomaly, SLA)产品, 空间分辨率为0.25°×0.25°, 时间分辨率为日平均。本文对所选2005—2015年冬季(12月至次年2月, DJF)的SLA数据进行了30~90天的带通滤波以获取季节内信号, 然后计算得到涡动能(eddy kinetic energy, $EKE=\frac{1}{2}(u'^2_{g}+v'^2_{g})$, 其中$u'^2_{g}$和 $v'^2_{g}$分别为纬向和经向地转流异常)。南海北部2005—2015年冬季平均涡动能分布(图1)显示, 南海北部涡动能高值中心主要位于吕宋海峡西北部, 表明冬季该海域中尺度涡旋较为活跃, 与前人研究结果一致 (Wang et al, 2003; Chen et al, 2009; Du et al, 2016), 因此本文选取南海东北部典型中尺度涡进行分析与研究。
Fig. 1 Spatial distribution of the mean eddy kinetic energy in the South China Sea based on the daily AVISO SLA data of winters (Dec., Jan., Feb.) from 2005 to 2015. Topography is shown by the gray isobaths at 200, and 1500 m, respectively

图1 2005—2015年冬季南海平均涡动能空间分布
深灰线分别为200、1500m等深线

1.2 模式配置与计算

本文利用ROMS区域海洋数值模式, 通过在线嵌套(online nesting)方法, 对南海北部冬季反气旋涡旋进行了高分辨率的模拟。图2所示的南海海域为第一层嵌套区域R2(102°—127°E, 2°12′—25°36′N),水平分辨率为1.55km; 第二层嵌套区域R3范围为109°—120°E、15°—24°N, 水平分辨率提高至500m(如图2中黑框所示)。地形数据使用NOAA提供的ETOPO2数据, 分辨率为2′, 各模型在垂向上均分为60层, 并在表层和底边界层进行适当的加密。本文首先对最外层大区域模式(95°—170°E, 10°—45°N)诊断计算20年, 得到基本稳定的结果后, 对R2区域进行了为期2年的在线嵌套模拟, 最后对R3区域进行第二层在线嵌套模拟, 从而获得能够刻画次中尺度过程的高分辨率模拟结果。本文利用第二层嵌套(空间分辨率为500m, 时间间隔为2h)的模拟结果来对南海北部典型中尺度涡边缘的次中尺度过程进行分析与研究。
Fig. 2 Topography of the South China Sea used in the first nested model domain. Gray isobaths show the depth at 200, and 1500 m, respectively. The online second nested domain is delineated by the black box

图2 南海海底地形与第一层模式嵌套区域
黑框为模式第二层嵌套区域, 深灰线分别为200、1500m等深线

2 结果分析

2.1 涡旋边缘锋面及次中尺度现象

R3区域的罗斯贝数(Rossby number) $R_0=\frac{\xi}{f}$(相对涡度$\xi=\frac{\partial v}{\partial x}-\frac{\partial u}{\partial y}$, u、v分别为流速u的纬向和经向分量, f为科氏参数)分布图显示(图3), 南海北部上层海洋广泛存在着Ro绝对值接近或大于1(即相对涡度ζ大于当地行星涡度f)的涡丝状结构, 其中分布在中尺度涡旋边缘的细长涡丝表明该海域有次中尺度过程发生。进一步的位势密度分布(图4)显示, 该涡旋边缘尤其是涡旋南侧存在显著的锋面结构。为分析锋面的结构特征, 本文计算了锋面海域的Okubo-Weiss参数$W=S^2-\xi ^2$(其中水平应变率$S=\sqrt{(\frac{\partial u}{\partial x}-\frac{\partial v}{\partial y})^2+(\frac{\partial u}{\partial y}-\frac{\partial v}{\partial x})^2}$)和锋面强度$F=\frac{|\nabla_{h}b|^2}{2}$(其中$b=-g\frac{\rho}{\rho_0}$为浮力,ρ为密度, ρ0为参考密度, g为重力加速度, $\nabla_{h}b$为水平浮力梯度)(Gula et al, 2016)。当W<0时, 表明相对涡度在局地动力特征中占主导地位; 当W>0时, 局地水平剪切强于相对涡度, 说明该区域具有较强的水平应变率。如图5所示, 本文研究的涡旋中心区域W值接近-2×10-9s-2, 而涡旋边缘细长锋面区域Okubo-Weiss参数数值为正, 最大可达3×10-9s-2(图5a); 且该区域锋面强度F远强于涡旋内部, 接近1×10-13s-4(图5b), 表明涡旋边缘的锋面区域具有显著的水平剪切和水平浮力梯度, 存在较强的锋生作用。
Fig. 3 Spatial distribution of the Rossby number with horizontal velocity (vector) at 5-m depth in the northern South of China Sea. Topography is shown by the black isobaths at 200 and 1500 m, respectively. The black box denotes the eddy region

图3 南海北部(图2黑框区域)5m层罗斯贝数空间分布
黑线分别为200、1500m等深线; 矢量为5m层水平流速, 黑框为涡旋区域

Fig. 4 Horizontal distribution of potential density with horizontal velocity (vector) at 5-m depth in the mesoscale eddy. Topography is shown by the black thick isobaths at 200 and 1500 m, respectively. The black box denotes the front region

图4 中尺度涡旋区域5m层位势密度水平分布
黑线分别为200、1500m等深线; 灰细线代表位势密度, 间隔为0.1kg∙m-3; 黑框为锋面区域; 矢量为5m层水平流速

Fig. 5 Spatial structure of Okubo-Weiss parameter (a) and frontal sharpness (b) at 5-m depth in the front. Topography is shown by the black isobath at 1500 m

图5 锋面区域5m层Okubo-Weiss参数(a)和锋面强度(b)水平分布
黑线为1500m等深线

2.2 次中尺度不稳定及其垂向次级环流

在混合层锋面海域, 较弱的垂向层化、强水平浮力梯度以及不均匀的热力和风应力强迫等条件均有利于诱发次中尺度不稳定(Thomas et al, 2008)。其中对称不稳定是增长速度最快的不稳定模态, 也是最重要的模态(Stone, 1966, 1970; Hoskins, 1974; Thomas et al, 2016)。为进一步诊断分析次中尺度过程引起的对称不稳定, 本文计算了Ertel位涡(Ertel potential vorticity, EPV)和理查德数(Richardson number)Ri。其中, Ertel位涡作为动力学上的示踪剂, 是流体稳定性分析的重要参数之一, 定义为
$q=\omega \cdot \nabla_{h}b=(f \hat{k}+\nabla \times u) \cdot \nabla_{h}b$
其中$ω=f+ζ$ 为绝对涡度, $\hat{k}$为z方向单位向量。理论研究结果表明, 在斜压流中对称不稳定发生时, Ertel位涡与科氏参数f符号相反(Stone, 1966, 1970; Hoskins, 1974; Thomas et al, 2013)。根据公式(1), Ertel位涡可以分解为两项, 即
$q=q_{v}+q_{h}$
其中, 第一项为与绝对涡度和层化相关的垂向分量
$q_{v}=(f+\xi)N^2$
第二项是与水平浮力梯度有关的斜压分量
$q_{h}=(\frac{\partial u}{\partial z}-\frac{\partial w}{\partial x})\frac{\partial b}{\partial y}+(\frac{\partial w}{\partial y}-\frac{\partial v}{\partial z})\frac{\partial b}{\partial x}$
其中 $N^2=\frac{\partial b}{\partial z}=-\frac{g}{\rho_0} \frac{\partial \rho}{\partial z}$为浮力频率, w为垂向速度。根据准地转理论, 地转速度ug≈u满足热成风平衡, 且垂向流速的水平剪切相对于水平流速的垂向剪切要小很多, 则上式可以简化为
$q_{h} \thickapprox=f|\frac{\partial u_g}{\partial z}|^2=-\frac{1}{f} \frac{|\nabla_{h} b|^2}{2}$
在北半球, 由于科氏参数f是正值, 所以qh恒为负值, 表明流体斜压性会减小Ertel位涡, 有利于对称不稳定的发生。理查德数Ri是判断流体稳定性的另一个重要参数(Stone, 1966, 1970)。在地转流中, 其定义如下:
$Ri=\frac{fN^2}{-q_h} \thickapprox= Ri_{g}=\frac{N^{2} f^{2}}{|\nabla_{h} b|^2}$
其中, Rig为地转理查德数。理论研究表明, 对称不稳定在Ri<1时可能发生, 并在0.25<Ri<0.95时具有最快增长速度(Stone, 1966)。而参数 $\phi_{Ri}=tan^{-1}(\frac{-1}{Ri})$可以将-∞<Ri<∞转换为有限区间-180°<ϕRi<180°。Thomas等(2013)研究发现, 对于反气旋涡, 在-180°<ϕRi< -135°时能发生重力不稳定; 当-135°<ϕRi<-90°时, 对称不稳定和重力不稳定均可能发生; 当-90°<ϕRi<-45°时, 可能只激发对称不稳定; 当-45°<ϕRi<ϕc时, 对称不稳定和惯性不稳定都会被触发; 当ϕc<ϕRi<0°时, 流体处于准稳定态, 其中临界值$\phi_{c}=tan^{-1}(\frac{\xi_{g}}{f})$, $\xi_{g}$为地转位涡。
由于锋面区域的弱层化结构(图6b), Ertel位涡垂向分量qv相对较小; 同时锋面较强的水平浮力梯度(图5b)使得水平斜压分量qh绝对值大于垂向分量qv, 导致涡旋边缘的Ertel位涡小于0, 甚至可达到-1×10-8s-3(图6a)。由Ertel位涡的水平分布(图6a)可以看出, 锋面的Ertel位涡低值区主要呈带状分布, 宽度为10~30km, 长度约为200km, 与理论计算的-90°<ϕRi<-45°区域较为一致。而跨锋面的Ertel位涡垂向结构则显示, 锋面区域负位涡自海表可延伸至密度跃层, 取决于锋面影响深度(图6b)。因此, 涡旋边缘锋面的存在有利于触发次中尺度对称不稳定。由于本文研究的锋面区域位于开阔海域且伴随着强锋生作用, 而锋生作用能加剧锋面的水平拉伸, 增强锋面水平浮力梯度, 并减小Ertel位涡, 所以锋生作用可能是诱发中尺度涡边缘对称不稳定主要原因之一。
Fig. 6 Horizontal (a) and across-front (b) distributions of EPV. (a) The vectors show the horizontal velocity at 5-m depth. The region of -90°<ϕRi<-45° is denoted by the thin gray contours. Topography is shown by the thick black isobaths at 200 and 1500 m, respectively. The across-front section is indicated by the pink line. (b) Density is shown by the black contours

图6 Ertel位涡5m层水平分布(a)和跨锋面垂向分布(b)
图a中细灰线为-90°<ϕRi<-45°区域边界; 断面如图中粉线所示; 矢量为5m层水平速度; 粗等值线分别为200、1500m等深线。图b中黑线为等密度线

次中尺度过程及其不稳定能够驱动较强的非地转垂向次级环流, 从而对混合层浮力进行有效的再分配(Taylor et al, 2010; D'Asaro et al, 2011; Thomas et al, 2013; Haney et al, 2015)。为定量探讨跨锋面的垂向次级环流, 本文采用Omega方程诊断计算得到的垂向速度来进行分析。由于涡旋区域中尺度背景场较强, 为凸显次中尺度的信号, 图7中的速度场已扣除了7天平均的背景场。如图7a所示, 涡旋区域垂向速度w′呈现正负交替的分布特征, 表明广泛存在跨锋面的垂向对流; 在锋面区域, 向下的垂向速度明显增大, 说明锋面区存在显著的下降流。跨锋面的结果显示(图7b), 在地转作用下, 锋面海域存在顺锋面的强流, 同时在纬向上有着较强的速度剪切; 在跨锋面方向上, 表层海水在锋面区域辐聚下沉, 在近混合层底向两侧辐散上升, 形成了两个典型的垂向次级环流, 最大垂向速度可达95m·d-1, 影响深度可至80m。因此, 广泛存在于中尺度涡边缘的次中尺度动力过程及其非地转斜压不稳定, 能够显著贡献于上层海洋的垂向热盐和浮力交换, 以及为上层海洋浮游生物生长提供营养盐补给。
Fig. 7 Frontal spatial distribution of vertical velocity w′ at 50-m depth (a), and cross-front structure of vertical secondary circulation (b). Topography is shown by black isobaths of 1500 m in (a). (b) The along-front velocity u′ (shading) is positive for westward velocity. The vectors show lateral velocity v′ (m·s-1) and vertical velocity w′ (m·d-1). Potential density is denoted by black contours

图7 锋面50m层垂向速度(a)和跨锋面垂向次级环流(b)
图a中黑线为1500m等深线。图b中黑线为等密度线; 填色为纬向速度u′, 均为西向流; 矢量为侧向速度v′ (m·s-1)和垂向速度w′ (m·d-1)

3 结果与讨论

本文利用高分辨率ROMS数值模拟结果, 初步分析了南海北部冬季典型中尺度涡边缘的次中尺度不稳定及其垂向次级环流的动力过程。模拟结果显示, 南海北部普遍存在着次中尺度现象, 尤其在涡旋和锋面海域。本文通过对涡旋锋面的分析发现, 涡旋边缘具有显著的侧向剪切与拉伸以及较强的水平浮力梯度; 锋生作用能够有效地减小Ertel位涡, 并触发对称不稳定。同时, 次中尺度过程及其非地转不稳定能够驱动跨锋面的垂向次级环流, 显著增强上边界层与海洋内部之间的物质能量及动量交换, 并对中尺度过程变异、上层海洋营养盐供给等产生重要影响。本文只是对模拟结果的初步分析, 下一步我们希望能够结合观测资料, 对次中尺度过程的动力机制、能量串级、生态环境效应等科学问题进行进一步的研究。

The authors have declared that no competing interests exist.

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