海洋环流与波动重点实验室知识库

北太平洋低纬度西边界流系的湍流混合过程在维系北太平洋作为全球热盐环流的一个重要的上升分支中起着举足轻重的作用，同时也调节着热带西太平洋温跃层的深度和梯度，在厄尔尼诺与南方涛动（ENSO）的发生、发展及全球气候变化中起着重要作用。了解北太平洋低纬度西边界流系湍流混合的时空变异特征及机制，能够有效的量化海洋垂向输送过程，改进和调整混合参数化方案，提高海洋、气候模式的数值模拟和预测能力。然而，由于该区域直接的湍流观测非常稀少，我们对其湍流混合物理过程和机制的认识非常有限，因此，气候模式中对该区域的混合参数化方案仍然缺失了一些重要的的物理过程。

本文基于近几年在北太平洋低纬度西边界流系海域获取的直接湍流观测资料及同期的水文观测数据，对北太平洋低纬度西边界流区域上层海洋的湍流混合的时空分布特征及变异机理开展了较为系统的分析，得到的主要创新性成果如下：

（1）发现了存在于西赤道太平洋海域的一种大尺度海流能量耗散驱动湍流混合的新路径，即赤道太平洋温跃层对称不稳定诱发的湍流混合现象。

根据2017年在130ºE断面北赤道流(NEC)和北赤道逆流(NECC)交汇区(2-9ºN)的自由落体式微结构剖面仪的直接观测，结合同步的温盐、溶解氧和流速观测，在NECC温跃层区域发现了比背景场高1-3个量级的湍流混合。进一步通过能量分析发现湍流剖面仪观测的湍动能耗散率与地转剪切生产率基本一致，符合对称不稳定过程的能量来源；同时，从Ertel位涡分析得到的平衡理查德森数也满足对称不稳定发生的判断标准。由于对称不稳定的发生通常是由于地转流的强垂向剪切（伴随着强水平密度梯度）引起，传统上认为该过程一般发生在海表面或地形边界层附近，因为在这两种区域风应力和地形摩擦作用可提供对称不稳定产生所需的反气旋式位涡。我们观测到的这一对称不稳定过程发生在远离海表和底地形作用的大洋内区的赤道温跃层内，从而揭示了一种全新的大尺度环流能量耗散的新路径。

该对称不稳定过程发生的机制在于，2017年秋季观测期间，新几内亚沿岸潜流异常强劲，导致它的深层流核可以携带更多的次表层南太平洋负位涡水跨过赤道，并被较强的哈马黑拉涡携带到达北赤道逆流流轴附近，在此处NECC较强的垂向地转剪切作用下，产生对称不稳定诱发湍流混合。本文还通过与前人在2014年夏季同一个断面的湍流剖面仪观测比较显示，该区域大尺度环流的变异主要受不同ENSO状态的影响：2017年秋季观测期间，西太处于拉尼娜状态，新几内亚沿岸潜流和哈马黑拉涡的增强，有利于更多来自南太平洋的负位涡水被携带至NECC区域，为对称不稳定的发生提供有利条件；而2014年夏季观测期间西太处于厄尔尼诺状态，新几内亚沿岸潜流和哈马黑拉涡较弱，南太平洋水跨赤道的北向入侵较弱，难以到达NECC区域。由于对称不稳定过程是一种自我阻尼的过程，必须有持续的反气旋式位涡水的持续补给才能维持其发生。因此，2014年观测中就没有观测到对称不稳定过程。

由于130ºE断面NECC流域既是南、北太平洋热带环流的闭合区和印尼贯穿流（ITF）的发源地，也是西太平洋暖池所在地，其海洋温跃层的结构和变异在ENSO的发生、发展乃至全球气候变化中都起着非常重要的作用。本文新发现的温跃层内存在的这一对称不稳定过程诱发湍流混合的机制对该海域温跃层结构变异有着重要作用。鉴于这一湍流混合机制发生的两个关键条件就是南、北半球正、负位涡水跨赤道运动和足够强的垂向地转剪切，而在赤道区域，观测和模式研究都表明普遍存在着交错的次表层环流结构和跨赤道水交换，因此，有理由推测该次表层对称不稳定机制引起的湍流混合可能普遍存在于赤道海域的温跃层内。因此，该成果对于完善我们对该区域湍流混合过程与机理的认识，提高耦合模式模拟和预测精度都具有非常重要的作用。

(2) 评估了Gregg-Henyey-Polzin(GHP)、Mackinnon and Gregg (MG)和Thorpe尺度方法这三种常用混合参数化方案在北太平洋低纬度西边界流区域的适用性，并指出MG参数化方法更能准确表达该海域湍流混合，并基于此方法揭示了2015-2017年间该区域湍流混合的变异特征与机制。

由于北太平洋西边界流系处于全球最强的年际异常信号ENSO的发源地，这里的海洋状态具有很强的年际变化，因此湍流混合在不同的ENSO年份表现出显著差异。由于直接的湍流观测比较费时且成本较高，难以实现大面积的同步获取，目前数据积累还比较稀少。相比之下，常规的温度、盐度、流速观测资料积累较多，因此，学者们利用温盐流数据，通过各种参数化方法来实现对湍流混合的估算。但不同的参数化方法估算的精度可能在不同的海域存在较大差异，因此本文依据2017年在北太平洋西边界流系获取的直接湍流观测数据对目前较为常用的细尺度GHP参数化方法、MG参数化方法和Thorpe尺度方法进行比较分析，并对它们的适用性进行评估。

发现MG参数化方法能够很好地估算北赤道低纬度西边界流系区域的湍流混合，该方法估算的扩散率（κ_ρ）和观测的差异在5倍（0.5个量级）以内的结果占94%,差异在2倍以内的结果占25%,估算的湍流混合率（κ_ρ）在水平和垂向分布上呈现出与微结构湍流剖面仪观测基本一致的空间分布特征。GHP参数化估算的κ_ρ与观测差异在5倍以内的结果占57%, 2倍以内的结果占10%。基于Thorpe尺度方法估算的κ_ρ与观测值差异要比MG和GHP方法都大，在5倍到10倍之间，且空间分布上与观测结果差异也较大。对比北赤道低纬度西边界流系区域湍流混合参数化方法的结果表明MG参数化方法最优,GHP参数化方法其次, Thorpe尺度方法相对GHP和MG参数化方法较差。

因此我们利用MG参数化方法基于2015-2017年期间西北太平洋三个航次在130°E断面2°N-14°N获取的CTD资料对海洋上800米的湍流混合的时空特征进行了分析。结果显示，北赤道低纬度西边界流系区域由MG参数化方法得到的ε在130°E断面的时间分布呈现2015厄尔尼诺发展期和2017拉尼娜峰值期该区域的湍流混合明显高于2016年航次结果。ε空间分布大致呈现随深度增加而减弱的结构。经向的ε空间分布显示出6°N以南和8°N以北较高，在6°N-8°N也就是棉兰老冷涡中心所在位置明显偏低，表明该中尺度冷涡旋边缘湍流混合增强。在2°N -4°N之间对应的哈马黑拉暖涡区域在2017年明显增强，与直接的微结构剖面仪观测结果一致。

（3）揭示了北太平洋低纬度西边界流系中的纬向流NEC上层湍流混合日变化过程与机理。

根据2018年4月在西太NEC上160m开展的连续25小时自由落体微结构剖面仪观测，并针对不同的动力深度将NEC垂向分割为表面处于混合中的水层（Mixing Layer）、混合层（Mixed Layer）、过渡层（Transition Layer）和温跃层的湍流混合进行了详细分析。结果显示了NEC表面处于混合中的水层厚度约40m，其湍动能耗散率最高且存在明显的日变化特征，ε 在10⁻⁸~10⁻⁷ W kg^-1的量级；混合层大约在40-110m之间，其湍流动能耗散率 ε 在10⁻⁸~10⁻⁹ W kg^-1的量级，混合层之下 ε 与混合层相当，同时存在由于局部剪切不稳定产生的一些斑块状的高湍动能耗散率区域，其量级可达10⁻⁸~10^−7.5 W kg^-1。过渡层内的湍动能耗散率与其剪切（S2）和浮力频率具有显著相关，其关系可以用ε~S^-0.40N^0.20来表达。

观测期间还捕获到了在较淡的热带表层水（TSW）和其下较咸的北太平洋热带水（NPTW）之间的障碍层，其平均厚度约11m,夜晚最大厚度可达20m。一个西向移动的反气旋式涡旋的平流效应影响，其携带的淡水侵蚀导致障碍层在午后消失。除了湍流混合，双扩散过程也对NEC上层的垂向混合起着重要作用。

Turbulent mixing in the North Pacific low-latitude western boundary current system (LLWBCs) is not only very important in maintaining its role as the upwelling branch of global thermohaline circulation, but also regulates the tropical western Pacific thermocline depth and gradient, playing crucial role in the occurrence, development of the El Niño-Southern Oscillation (ENSO) and global climate change.Understanding the spatio-temporal variability and mechanism of turbulent mixing in the North Pacific LLWBCs can efficiently quantify the ocean vertical transport process, improve and adjust the mixing parameterization scheme, hence promote the simulation and prediction ability of ocean and climate models.However, due to the scarcity of direct turbulence observations in this region, our understanding of turbulent mixing physical processes and mechanisms is very limited. Therefore, some important physical processes are still missing in the mixing parameterization scheme of this region in climate models.

Based on the direct microstructure measurements and sychroneous hydrological observations conducted in the tropical Western Pacific in recent years, this paper systematically analyzed the spatio-temporal distribution, the variation, and associated mechanisms of turbulence mixing in the upper ocean in the North Pacific LLWBCs. The main innovative results obtained are as follows:

(1) This study discovers a new route of energy dissipation for large-scale ocean currents driving turbulent mixing in the Western Equatorial Pacific, which is turbulent mixing induced by symmetric instability (SI) in the equatorial Pacific thermocline.

Based on direct microstructure measurements and sychroneous temperature, salinity, dissolved oxygen, and velocity observations conducted in fall 2017 by a free-fall microstructure profiler (MSP) at the convergence region (2-9ºN) of the North Equatorial Current (NEC) and the Northern Equatorial Countercurrent (NECC) along the 130ºE section, 1-3 orders of magnitude higher turbulent mixing level in the thermocline (150m-300m) compared with that of the background field was observed. Further energy analyses show that the observed dissipation rate of turbulent kinetic energy is basically consistent with the geostrophic shear production rate, which is consistent with the energy source of SI processes. Meanwhile, the balanced Richardson number obtained from Ertel potential vorticity (PV) analyses also meet the criterion of the onset of SI. As SI is induced by strong vertical shear (with strong horizontal density gradient) of geostrophic currents, traditionally, it is thought to be occur at the sea surface or near the topographic boundary layers, where the anticyclonic vorticity needed for the onset of SI can be generated by wind stress or friction effect. By contrast, the observed SI occurs in the equatorial thermocline in the interior ocean away from the influence of the surface and bottom topography, thus revealing a new route for energy dissipation  of large-scale ocean currents.

The mechanism of SI observed in our study lies in the enhancement of the New Guinea Coastal UnderCurrent (NGCUC) during our MSP observations, which transports more subsurface South Pacific water crossing the equator in the thermocline, entering into the enhanced Halmehera Eddy (HE), and flowing further northward arriving the NECC region. Under the strong vertical shear of the NECC in the thermocline and combined with supply of the anticyclonic PV water from the South Pacific, SI occurs in the thermocline. We also compare our microstructure measurements with previous observations conducted in summer 2014 along the same section, which suggests that turbulent mixing level here is highly modulated by the  large-scale ocean currents varied significantly during different ENSO states. The western Pacific was in a La Niña state in fall 2017, when the NGCUC and HE were both strengthened with more South Pacific negative PV water being transported northward arriving the NECC region, preconditioning for the onset of SI. However, in summer 2014, the western Pacific was in an El Nino state, both the NGCUC and HE were weakened, so it’s hard for the South Pacific water to intrude into the NECC in the thermocline. Since SI is a self-damped process, the continuous supply of anticyclonic PV water is necessary to maintain its occurrence. Therefore, no SI process was observed in the 2014 observations.

As the NECC region along the 130ºE section is not only the closed region of North and South Pacifc tropical gyres, the source region of the Indonesian Through Flow (ITF), but also inhabits the westerm Pacific warm pool, its structure and variation of the thermocline play a very important role in the occurrence and development of ENSO and global climate change. The newly discovered mechanisms of turbulent mixing induced by SI play an important role in setting the structure of the thermocline in this area. Given the two key conditions for the occurrence of this turbulent mixing mechanism are the corss-equatorial transport of positive and negative PV in the northern and southern hemispheres and the strong enough vertical geostrophic shears, it is reasonable to speculate that turbulent mixing caused by the SI may be prevalent in the thermocline in global equatorial thermocline, as observations and model studies have shown that there are vigorous undercurrents and cross-equatorial water exchanges in equatorial oceans. Therefore, the results are very important to improve our understanding of turbulent mixing process and mechanism in this region, and to improve the accuracy of coupling model simulation and prediction.

(2) The applicability of Gregg-Henyey-Polzin(GHP) fine scale parameterization, Mackinnon and Gregg (MG) parameterization, and Thorpe scale methods in the North Pacific LLWBCs is evaluated. The MG parameterization shows more consistent results with the direct MSP measurements. The interannual variability of turbulent mixing and associated mechanism in this region during 2015-2017 are investigated.

It is found that the turbulence mixing estimated by MG parameterization method could well estimate the turbulent mixing of the observed results. Although the diapycnal diffusivity estimated by the MG parameterization method is generally weaker than the observation results, the difference between the estimation and observation results within a factor of 2 accounts for 25% and a factor 5 accounts for 94%, showing the same distribution characteristics with the horizontal and vertical distribution of the diapycnal diffusivity observed by the microstructural turbulence profilers. The diapycnal diffusivity showing the spatial distribution is quite different upper from the 26.5 σθ. It is in higher value about O (10^–3) m² s^-1 in south of 6°N than the north of 6°N. The diapycnal diffusivity is in high value under the 26.5 σθ, corresponding to the ME dome. Based on the GHP parameterization method, the results of GHP parametric estimation and observation were within a factor of 2 accounts for 10% and a factor of 5 accounting for 57%. When the Thorpe scale method was used to estimate the diapycnal diffusivity of the Western boundary current system of the North Pacific Ocean, 90% of the difference between estimation and observation was within a factor of 5-10, and more than 0.5-1 order of magnitude, showing the spatial distribution is quite different from the observed results, but showing the same distribution characteristics with the horizontal and vertical distribution of the diapycnal diffusivity rate observed by the microstructural turbulence profilers.Comparing the results of turbulent mixing parameterizations in the Western boundary current system of the North Pacific Ocean, showing that the MG parameterization method is the best, the GHP parameterization method is the second, and the Thorpe scale method is inferior to the GHP and MG parameterization methods.

Therefore, we use MG parameterization method to analyze the variation characteristics and mechanism of turbulent mixing at 130ºE sections between 2°N and 14°N during 2015-2017 upper 800 meters in the Northwest Pacific low-latitude western boundary current system, based on the CTD data. According to the results of MG parameterization, ε is in higher due to the developing 2015 EI Niño event and 2017 La Niña event than 2016. The ε decreases by the depth increases, and is in high value both in the south of 6°N and the north of 8°N. And the ε is in low value about the center of ME, shows the high level of turbulent mixing. The enhancement of HE due to the developing La Niña event in 2017 also resulted in a 2015-2017 maximum TKE here in 2017, corresponding to the high TKE of MG parameterization and observation.

(3) The diurnal variation process and mechanism of turbulent mixing on the upper NEC layer of the zonal flow in the western boundary current system at low latitudes in the North Pacific are revealed.

Based on 25-hour continuous free-fall microstructural profiler observations performed at 160m on the Western Pacific NEC in April 2018, at different dynamic depths, NEC is vertically divided into Mixing Layer, Mixed Layer, Transition Layer and thermocline Layer. The results show that the thickness of water layer on NEC surface is about 40m, and the dissipation rate of turbulent kinetic energy is the highest and the diurnal variation is obvious. The ε ranges from 10⁻⁸ to 10⁻⁷ W kg^-1.In the mixed layer, the dissipation rate of turbulent kinetic energy is 10⁻⁸~10⁻⁹ W kg^-1. In the mixed layer, the dissipation rate of turbulent kinetic energy is about 10⁻⁸~10^−7.5 W kg^-1. In the mixed layer, the dissipation rate of turbulent kinetic energy is similar to that of the mixed layer. Transition layer in the turbulent kinetic energy dissipation rate and shear (S2) and buoyancy frequency has significant correlation, the relationship can use ε~S^-0.40N^0.20to express.

The barrier layer between the paler tropical surface water (TSW) and the saltier north Pacific tropical water (NPTW) below was also captured during the observation period, with an average thickness of about 11m and a maximum thickness of 20m at night. The barrier disappears in the afternoon due to the advection effect of a westerly moving anticyclonic eddy carrying fresh water erosion. In addition to turbulent mixing, double diffusion processes also play an important role in vertical mixing in the upper layers of NEC.

目  录

第1章  绪论. 1

1.1  研究意义和背景. 1

1.2  国内外研究进展. 2

1.2.1  北太平洋低纬度西边界流系湍流混合空间结构的研究. 2

1.2.2  北太平洋低纬度西边界流系湍流混合时间演化的研究. 5

1.3  本文研究的内容. 7

1.3.1  对称不稳定的观测. 7

1.3.2  2015-2017年不同ENSO状态下的垂直混合过程和湍流估算方法的对比  7

1.3.3  海洋上层湍流混合日变化. 7

第2章  资料和方法. 9

2.1  航次观测资料. 9

2.1.1  130°E断面航次水文观测资料和湍流混合观测资料. 9

2.1.2  130°E断面湍流混合观测资料. 10

2.1.3  北赤道流2018年25小时连续水文气象和湍流混合观测资料. 11

2.2  其他数据资料. 15

2.2.1  高度计数据. 15

2.2.2  HYCOM GOFS 3.1 数据. 16

2.2.3  欧洲中期天气预报中心（ECMWF）数据. 16

2.3  参数化方法. 16

2.3.1  GHP细尺度参数化方法. 16

2.3.2  Thorpe尺度方法. 17

2.3.3  MG参数化方法. 18

2.3.4  双扩散参数化方法. 18

2.4  能量生产率估计方法. 19

2.5  判断低纬度区域纬向地转流对称不稳定的准则. 19

2.6  浮力通量. 20

2.7  障碍层的判别方法. 21

第3章  北太平洋低纬度西边界流系湍流混合的新机制. 23

3.1  SI上层海洋垂直结构和NWEPO温跃层中增强的湍流混合. 27

3.2  观测证明海洋内部次表层SI 33

3.3  本章小结. 43

第4章  北太平洋低纬度西边界流系湍流混合年际变异及混合参数化方法评估  45

4.1  观测结果. 46

4.1.1  水文观测结果. 46

4.1.2  北太平洋低纬度西边界流系湍流混合海洋观测特征. 51

4.2  参数化结果. 52

4.2.1  湍流混合扩散率的水平比较和垂向分布. 52

4.2.2  混合参数化方案的比较. 56

4.3  基于参数化方法湍流混合的时空特征. 67

4.3.1  湍流混合时空结构. 67

4.3.2  双扩散时空结构. 72

4.4  本章小结. 76

第5章  北太平洋低纬度西边界流系湍流混合日变化. 79

5.1  气象和水文条件. 79

5.2  结果分析. 81

5.2.1  障碍层的演化. 81

5.3.2  湍流特征. 87

5.3.3  双扩散. 96

5.3  本章小结. 99

第6章  结论和展望. 103

6.1  主要结论与创新点. 103

6.2  展望. 106

参考文献. 109

致谢. 123

作者简历及攻读学位期间发表的学术论文与研究成果. 124