海洋环流与波动重点实验室知识库

东海毗邻的西北太平洋及其陆架边缘海是全球最重要的半日内潮和全日内潮源区之一。东海陆坡-陆架区由于复杂的地形，强盛的正压潮汐和层结变化的水体，因而成为内潮能量生成的重要区域。此外，东海陆架沿岸存在复杂多变的背景动力场，可能存在由陆坡传入的内潮，且处于内潮演变发展的关键纬度范围，是研究内潮的时空变化和演变过程的理想海区。由于没有针对性的系统观测, 先前的研究多关注东海陆坡内潮，缺乏针对东海近岸陆架内潮的系统性研究工作。因此，开展东海陆架内潮的时空变化特征及其驱动机制研究，不仅可以丰富并加深陆架区域内潮的理论认知，也可进一步为东海陆架区域物质输送和生态过程变化提供动力学支撑。

本文基于东海陆架近岸四套潜标观测，结合数值模式和通用海洋资料数据，系统研究了东海内潮的基本特征，时空分布和演变，垂向模态结构，内潮变化的调制因子，以及依赖于纬度的两种非线性相互作用，得到如下主要结果：

1．揭示了东海西南陆架区域内潮的时空变化特征，调制因子和源地。半日内潮和全日内潮明显表现出不同的时间变化特征。层结和背景流场是影响内潮时间变化的两个重要因素。其中，层结增强，尤其是陆架区底部层结增强，显著影响陆架区内潮生成，可以解释半日内潮在观测后半段的增强现象。背景流带来的相对涡度调制了全日内潮临界纬度效应的作用范围和影响时间。当流场涡度减小，临界纬度的影响减弱，更多陆坡区的全日内潮入侵陆架。同时，背景流显著调制内潮能量传递过程，其引起的水平和垂向剪切分别促使内潮能量向更高模态和更高频率转移。此外，通过计算复合潮成分群速度，衡量了不同层结和环流背景下全日内潮和半日内潮大小潮相位传播时间。采用垂直方向上的射线追踪，剖析了内潮在陆架陆坡区的传播反射情况，确定了潜标点半日内潮和全日内潮的能量来源，即半日内潮可能产生于较近的陆架源区，而全日内潮可能来自较远的陆坡源区。

2. 为了对东海陆架地区内潮的时间变化和空间分布特征有更为整体的认识，我们对陆架地区四套潜标数据进行分析，并借助数值模型对整个东海地区的内潮进行模拟。四套潜标处均存在半日内潮和全日内潮，其中半日内潮流速更强，是我们主要关注的成分。跃层数量和位置可影响内潮的跨零点位置和数量，进而影响内潮垂向空间结构。根据数值模拟结果，揭示了东海半日内潮和全日内潮的源区分布。尽管东海陆坡区生成的内潮能量占主导地位，但向近岸方向传播的能量较弱且传播距离非常有限，难以影响到陆架地区。因此对于陆架区，局地内潮源区分布和能量传播更为重要。四个潜标点处的半日内潮多为局地或附近区域生成，这也解释了观测到的半日内潮大-小潮周期与局地正压潮的相位相对应的现象。尽管东海陆架地区生成的内潮能量弱，但能量生成后几乎全部耗散于此，对近岸地区的局地湍流混合具有重要意义。

3. 探究了两种纬度依赖的内潮非线性相互作用。一种是全日内潮的临界纬度效应，当高于临界纬度时，全日内潮会通过与半日内潮相互作用产生更强的高阶谐波向高频传播能量。另外，全日内潮也会具有强的高波数能量，指示着局地失稳耗散的过程。另一种非线性作用是半日内潮的参数次谐波不稳定（PSI）机制，它是一列低波数半日内潮向两列垂直传播方向相反的高波数半频率内波转化的三波共振机制。我们在东海陆架近岸区首次观测到PSI过程并获得充分的证据。近惯性频段内波的时间变化与半日内潮变化较为一致，在特定深度处分离出了半日内潮的经PSI过程转化得到的次谐波M₁谱峰，而且在半日母波和子波的交叉频段具有较高的双相干谱峰，意味着PSI这种三波共振机制的发生，而且生成的次谐波进一步向高频转化。

The East China Sea (ECS), as a part of the Northwest Pacific Ocean and its adjacent marginal seas, are one of the most important source areas of both semidiurnal and diurnal internal tides. The ECS shelf-slope region features complicated topography, strong barotropic tides and varied stratifications, therefore it become the critical region of internal tides generation. For a long time, the former research has been concerned only with the characteristics of the off-shore internal tidal branch of the ECS slope, and there are relatively few systematic studies that focus on the internal tides of the ECS shelf region. Additionally, ECS shelf also characterizes as complex and vigorous background dynamics system, possibly exists inshore internal tides from slope, locates in the range of critical latitude for the evolution and transfer of internal tides, which is an ideal region to investigate the spatiotemporal variability and evolution of internal tides. Due to the lack of systematic observations, most of the previous studies have focused on the ECS continental slope internal tides, there is a lack of systematic research on the coastal internal tides at the ECS shelf. Therefore, the study of the spatiotemporal characteristics and driving mechanisms of the internal tides at the ECS shelf can not only enrich the theoretical knowledge of the internal tides in the shoal water, but also support for the changes of mass transport and ecological processes in the ECS shelf.

In this paper, four sets of mooring observations associated with numerical simulation and general ocean datasets at the ECS shelf were used to systematically study the internal tides, in terms of their basic characteristics, spatiotemporal distribution and evolution, vertical modal structure, modulation factors of variation, and two kinds of latitude-dependent nonlinear interactions. The main conclusions of this thesis are as follows:

I. The spatiotemporal variation, source region and modulation factors of internal tides at the southwestern ECS shelf are revealed. Semidiurnal and diurnal internal tides clearly exhibited different temporal variation. Stratifications and background currents are two critical factors affecting the internal tides. The intensified stratification, especially the near-bottom stratification, significantly affected the generation at the shelf, it can explain the enhancement of the semidiurnal internal tide in the latter half of the record. The relative vorticity brought by the background current modulates the extent and duration of the diurnal critical latitude effect. When the vorticity decreases, the effect of critical latitude weakens, and more diurnal internal tides generated on the slope intrude the shelf. Meanwhile, the background currents can significantly modulate the energy transfer of internal tides, and its induced horizontal and vertical shear contribute to the transfer of internal tidal energy to higher modes and higher frequencies, respectively. In addition, by calculating the group velocity of the compound tidal constituents couples, the propagation time of the internal tides spring-neap phase under the different background stratification and current are measured. The propagation and reflection of the internal tide are profiled by the internal tide ray tracing in the vertical direction. The source regions of semidiurnal and diurnal internal tides at the mooring location are discussed, and it is concluded that the semidiurnal internal tides may be generated from the closer shelf source region, while the diurnal internal tides may come from the more distant continental slope.

II. In order to fully understand the temporal variation and spatial distribution of the internal tides at the ECS shelf, we analyze four sets of moorings data and simulate the internal tide in the whole ECS area with numerical models. Semidiurnal and diurnal internal tides exist at all four sets of moorings, where the semidiurnal internal tides are stronger and are our main concern of interest. The numbers and locations of pycnocline can influence the numbers and location of zero-across layer, and further influence the vertical structures of internal tides. Based on the numerical simulation results, we reveal the source region and distribution of the internal tides in the ECS. Although the slope area is the major generation source of internal tides, the energy propagating shoreward is weak and the propagation distance is very limited, and it is difficult to affect the shelf area. Therefore, for the shelf area, the distribution of local internal tidal source region and energy propagation are more important. Semidiurnal internal tides are mostly generated at or near the local area, which explains the observed semidiurnal spring-neap cycles corresponding to the phase of local barotropic tides. Although the energy of the internal tide generated in the East China Sea shelf area is weak, almost all of the energy generated is dissipated locally, which is important for the regional turbulent mixing in the coastal area.

III. Two different latitude-dependent nonlinear interactions of the internal tide in ECS are investigated. One of the interactions is the critical latitude effect of diurnal internal tides, which generates stronger higher-order harmonics to transfer energy to high frequencies by interacting with the semidiurnal internal tide where it is poleward of the critical latitude. In addition, the diurnal internal tides also intensified the energy in high wave number, indicating the process of local instability to dissipation. Another nonlinear interaction is the parametric subharmonic instability (PSI) of the semidiurnal internal tide, a triad-wave resonance mechanism that transforms one set of semidiurnal internal tide with low wave number to two sets of high wave number and equivalent upward and downward internal waves half of its original frequency. We observed PSI for the first time and obtained sufficient evidence in the shallow water of the ECS shelf. The temporal variation of the near-inertial band internal waves is more consistent with the semidiurnal internal tides counterpart, and the subharmonic M1 spectral peak resultant from PSI of semidiurnal internal tide is separated at some certain depths, and there is a high bi-spectral peak in the crossover frequency band of the parent and daughter waves, implying the occurrence of triad-wave resonance mechanism PSI, and the generated subharmonic waves have a tendency to high frequency via other nonlinear coupling.

摘  要... I

Abstract III

目  录... VII

图表目录... XI

第1章  引言... 1

1.1  研究背景... 1

1.1.1  内潮介绍... 1

1.1.2  内潮研究意义... 2

1.2  研究现状... 4

1.2.1  东海潮汐特征和水文环境... 4

1.2.2  东海内潮空间分布特征... 6

1.2.3  东海内潮动力和能量演变过程... 8

1.3  科学问题和研究内容... 10

第2章  数据与研究方法... 12

2.1  观测与数据资料... 12

2.1.1  潜标观测数据与处理... 12

2.1.2  其他数据... 13

2.2  谱分析方法... 14

2.2.1  功率谱... 14

2.2.2  频率-波数谱... 18

2.2.3  速度的垂向分解... 18

2.3  正压潮体积力的计算... 19

2.4  临界纬度和有效纬度... 19

2.5  ROMS模式介绍... 20

2.5.1  模式控制方程... 20

2.5.2  模式边界条件... 21

2.5.3  模式网格... 23

2.6  斜压能量方程... 27

第3章  东海西南陆架内潮的时空演变特性和调制因子... 29

3.1  引言... 29

3.2  本章研究结果... 30

3.2.1  背景正压潮... 30

3.2.2  内潮的谱特征和潮流椭圆... 32

3.2.3  全日内潮和半日内潮的不同时间变化... 33

3.2.4  内潮生成源区... 37

3.2.5  背景环流调制的内潮模态贡献率... 39

3.2.6  全日内潮的临界纬度效应... 42

3.2.7  高频内波和内潮能量串级... 43

3.3  结果与讨论... 45

3.3.1  全日内潮和半日内潮的观测证据和模态特征... 45

3.3.2  内潮的传播特征及源区分析... 46

3.4 本章小结... 52

第4章  东海陆架浅水区内潮特征与能量分布... 53

4.1  引言... 53

4.2  模型配置与验证... 54

4.2.1  数值模型配置... 54

4.2.2  模型正压潮验证... 56

4.3  本章研究结果... 57

4.3.1  正压潮观测与TPXO潮汐参数... 57

4.3.2  潜标处层结条件... 60

4.3.3  斜压潮特征... 61

4.3.4  内潮源区和能量传播... 65

4.3.5  内潮引起的波动... 68

4.3.6  内潮能量耗散特征... 70

4.4  总结与讨论... 72

第5章  东海内潮依赖于纬度的非线性相互作用机制... 74

5.1  引言... 74

5.2  本章研究结果... 76

5.2.1  全日临界纬度效应... 77

5.2.2  临界纬度效应下的内潮频率与波数特征... 78

5.2.3  半日内潮的参数次谐波不稳定... 80

5.3  总结与讨论... 85

第6章  结论和展望... 89

6.1  本文的主要结论... 89

6.2  本文的主要创新点... 90

6.3  未来工作展望... 91

参考文献... 93

致  谢... 105

作者简历及攻读学位期间发表的学术论文与研究成果... 105