海洋环流与波动重点实验室知识库

内潮是在世界各大洋普遍存在的一种中、小尺度的海洋动力过程，是能量由大尺度向小尺度传递的关键一环。内潮长距离传播和破碎引起的混合重新分配海洋中的动量、热量和营养物质，是调节水团形成和大尺度环流变异性的重要因素。内潮与变化的海底地形密切相关，西太平洋具有全球最复杂的地形条件，也是内潮最强源区之一，为开展深海复杂地形下的内潮能量研究提供了理想条件。以往大部分研究更关注内潮二维分布或上层海洋，对于深海内潮及耗散的认识仍然不够。因此本文关注具有复杂地形的马里亚纳地区和卡罗琳海盆，基于MITgcm数值模型，并结合高度计结果、混合细尺度参数化估计和现场观测数据，研究了内潮的能量分布和传播特征，并讨论了深海尤其是海沟地区内潮能量的传递过程。

马里亚纳地区具有典型的沟弧盆地形体系。基于数值模拟，马里亚纳地区生成的内潮能量为8.35 GW，其中两个海脊贡献了81%。马里亚纳海盆和海沟对内潮生成贡献很小，但它们对能量辐射和耗散有着重要的调制作用。在马里亚纳海脊西侧，从海脊辐射出的内潮向西可传播1800 km并跨过帕劳海脊，传播过程中在海盆内汇聚形成能量高值区，同时两个海脊中部的共振增强了西传的内潮能量。在东侧，源于马里亚纳海脊的内潮可以传播至马里亚纳海沟，并在深海中耗散能量。直接计算的耗散揭示了耗散在垂直方向的多层分布，与细尺度参数化估计较为一致。然而能量平衡方法得到的耗散在参数化中，通常假设耗散率随着与海底的距离增加呈指数衰减，与观测结果不一致。研究结果揭示了马里亚纳地区内潮三维辐射路径和耗散的复杂性，可为海洋环流模型参数化和耗散垂直结构函数的确定提供一定的参考。

卡罗琳海盆位于马里亚纳地区南部，尽管海盆内地形平坦，但周围存在海山、海脊、海沟等多种复杂地形。不同于马里亚纳地区，卡罗琳海盆更接近赤道，内潮受赤道流系的影响，能量和机制可能存在不同特征。研究结果表明，M₂内潮的源区都分布在海盆周围的海山和海脊处。包括北部的帕劳群岛、雅浦海山和卡罗琳海脊，东部的穆绍海脊，南部的极乐鸟湾，同时东西卡罗琳海盆分界处Eauripik海隆也可提供少部分内潮能量。在赤道流系的影响下，有两支相对较强的内潮射线向西南方向长距离传播，传播过程中环流的折射效应使内潮射线向北偏转。内潮耗散较强的区域除了分布于内潮源区之外，内潮传播路径也对应着耗散的高值。西卡罗琳海盆和帕劳群岛西侧由于地形平坦且内潮能量弱，是耗散最弱的区域。海沟处由于层化极弱，不能提供内潮生成的能量。由于卡罗琳海盆的三个海沟地形狭窄，源区产生的内潮射线无法直接传入海沟，因此流速和耗散都很弱。由于地形特征不同，不同海沟所在的深渊区能量分布和耗散也存在差异。

赤道地区内潮普遍较弱，无法长距离传播。但我们在卡罗琳海盆的研究中发现尽管穆绍海脊M₂内潮的能量生成只有0.51 GW，但辐射出的M₂内潮能够以较弱的能量传播很远距离，到达卡罗琳海盆的西边界。海脊、海沟和海隆的地形作用和赤道极弱的地转效应的共同作用是其长距离的原因。同时南赤道流的折射进一步促进了其向西传播。内潮的长距离传播带来了远区的耗散，表现为上层强，随深度增加而减弱。相比之下，由于层化的垂向差异，混合表现出不同特征，上层扩散系数最弱，约为10^-6 m²/s，与前人得出的赤道地区温跃层弱混合的结论一致；随着深度的增加，内潮混合增强，海底附近的扩散率甚至达到10^-4 m²/s。计算的扩散系数在大部分区域与直接湍流观测和细尺度参数化一致，但在科氏参数f接近零的区域，细尺度参数化是不适用的。该研究讨论了赤道内潮对耗散和混合的贡献，对海洋气候模式参数化方案的改进具有重要意义。

Internal tide is a kind of mesoscale dynamic process which exists in the world's oceans. It is a key link of energy transfer from large scale to small scale. The long-distance propagation of internal tide and the mixing caused by internal tide breaking redistribute energy, heat and nutrients in the ocean, and thus internal tide is an important factor regulating water mass formation and the variability of large-scale circulation. Internal tide energy is closely related to the steep topography. The Western Pacific has the most complex topographic conditions in the world and is one of the strongest sources of internal tides, which provides conditions for the research of tidal energy under the complex topography of deep sea. Most of the previous studies focused on the two-dimensional distribution of internal tides or the upper ocean, but the understanding of the deep-sea internal tides and their dissipation is still insufficient. Therefore, this paper focuses on the Mariana region and the Caroline Basin with complex topography. Based on numerical simulations and observations, we study the energy distribution and propagation characteristics of internal tides, and discuss the tidal energy transfer process in the deep sea, especially in the trenches.

The Mariana area has a typical trench-arc-basin system, and both the east and west ridges in this area are internal tidal sources. Based on the numerical simulation, the total M₂ barotropic-baroclinic conversion rate in the Mariana region is 8.35 GW, of which two ridges contribute 81%. The Mariana Basin and Mariana Trench contribute little to internal tidal generation, but they play an important role in modulating energy radiation and dissipation. The internal tide radiating from the Mariana ridge can spread westward for 1800 km and cross the Palau Ridge. During the propagation, tidal beams converge in the basin to form an energy focusing region. Meanwhile, the resonance in the middle part of the two ridges enhances the internal tide energy spread westward. In the east side, internal tides originating from the Mariana Ridge can travel to the Mariana Trench and dissipate energy in the deep sea. The direct calculation of the dissipation reveals the vertical multilayer distribution of the dissipation in the whole water column, which is in good agreement with the fine-scale parameterization estimation. However, in the parameterization of the dissipation inferred by the energy balance, it is usually assumed that dissipation rate decays exponentially with increasing distance from the seafloor, which is inconsistent with the observation results.

The Caroline Basin is located south of the Mariana region. Although the topography in the basin is flat, it is surrounded by seamounts, ridges, trenches and other complex topography. Different from the Mariana region, the Caroline Basin is closer to the equator, and the internal tide is influenced by the equatorial current system, which may bring the different characteristics of energy and dynamics. The sources of M₂ internal tides are distributed in seamounts and ridges around the basin. The sources include the Palau Islands, Yap Seamounts and Caroline Ridge in the north, Mussau Ridge in the east, the Cenderawasih Bay in the south, and the Eauripik Rise at the boundary between the east and west Caroline basins, which also provides a small amount of tidal energy. Under the influence of equatorial current system, there are two relatively strong internal tide beams propagating southwest for a long distance, and the refraction effect of circulation deflects the internal tide beams northward. The areas with strong internal tide dissipation are not only distributed in the source region, but also the propagation paths. The west Caroline Basin and the west side of the Palau Islands are the weakest areas of dissipation due to flat topography and weak tidal energy. The trench is weakly stratified that it cannot provide generated tidal energy. Due to the narrow topography of the three trenches in the Caroline Basin, the internal tidal beams generated in the source region cannot directly propagate to the trenches, so the velocity and dissipation are very weak. Therefore, the energy distribution and dissipation in abyssal trenches are different due to the different topography.

Internal tides are generally weak in equatorial regions and tidal beams cannot travel long distances. However, in our study of the Caroline Basin, we found that although the M₂ internal tide generation in the Mushau Ridge is only 0.51 GW, the radiated internal tide beam can travel a long distance with relatively weak energy, and reach the western boundary of the Caroline Basin. The long-distance propagation can attributed to the combined effect of topography including ridge, trenches and rise, and the weak geostrophic effect in the equatorial region. Meanwhile, the refraction of the South Equatorial Current further contributes to its westward propagation. The long distance propagation of the internal tide leads to the remote dissipation, which was strong in the upper layer and weakened with the increase of depth. The mixing characteristic is opposite, the diffusivity is weaker in the upper layer, with the value about 10^-6 m²/s, which is consistent with the previous conclusion of weak mixing in the equatorial thermocline. But as the depth increases, the tidal mixing intensifies and the diffusivity near the seafloor even reaches 10^-4 m²/s. The calculated diffusivities are consistent with direct turbulence observations and fine-scale parameterization in most regions, but the fine-scale parameterization is not applicable near the equator where the Coriolis parameter f is close to zero. This study discusses the contribution of equatorial internal tides to dissipation and mixing, which is of great significance for the improvement of parameterization schemes in ocean and climate models.

第1章  引言... 1

1.1  内潮介绍及研究意义... 1

1.2  内潮研究现状... 3

1.2.1  主要研究手段... 3

1.2.2  全球内潮研究进展... 6

1.2.3  深海内潮和混合研究... 10

1.3  问题的提出... 12

第2章  研究方法... 15

2.1  MITgcm模式介绍... 15

2.1.1  模式控制方程... 15

2.1.2  模式边界条件... 16

2.1.3  模式网格... 17

2.2  内波本征方程... 19

2.3  内潮能量方程... 20

第3章  马里亚纳沟弧盆地区内潮的三维传播和耗散特征... 25

3.1  引言... 25

3.2  研究方法... 27

3.2.1  模型设置... 27

3.2.3  模式验证... 29

3.3  结果分析... 30

3.3.1  内潮源区分布... 30

3.3.2  内潮长距离传播和能量汇聚... 33

3.3.3  内潮能量辐射机制... 35

3.3.4  内潮耗散水平特征... 41

3.3.5  内潮耗散的垂向多层分布... 42

3.3.6  马里亚纳海沟展望... 45

3.4  总结与讨论... 46

3.5  敏感性实验... 47

第4章  卡罗琳海盆赤道流系影响下的内潮能量收支... 51

4.1  引言... 51

4.2  方法... 52

4.2.1  模型设置... 52

4.2.1  模型验证... 54

4.3  结果分析... 55

4.3.1  M₂内潮源区和能量分布... 55

4.3.2  内潮能量传播特征... 57

4.3.3  环流对内潮的调制作用... 59

4.3.4  内潮耗散的水平分布特征... 61

4.3.5  内潮能量垂向传播和耗散... 62

4.4  总结与讨论... 64

第5章  赤道地区长距离传播的内潮射线与潮致混合... 67

5.1  引言... 67

5.2  结果验证... 68

5.3  沿赤道传播的内潮射线... 71

5.3.1  内潮弱生成和长距离传播... 71

5.3.2  内潮长距离传播的影响因素... 73

5.4  耗散与混合... 74

5.4.1  耗散与混合的水平分布... 74

5.4.2  耗散与混合的垂向分布... 75

5.5  总结与讨论... 78

第6章  结论与展望... 81

6.1  主要结论... 81

6.2  本文创新点... 81

6.3  工作展望... 82