海洋环流与波动重点实验室知识库

  西太平洋具有复杂的三维环流结构和丰富的多尺度变化特征。作为连接不同大洋水团、控制海洋物质能量循环的“大动脉”，西太平洋环流一直是物理海洋学研究的热点。其中，北赤道流作为北太平洋西边界流的起源，以及热带环流和亚热带环流的分界线，在西太平洋的水体平衡和热量收支中发挥着重要的作用。近年来的潜标观测研究揭示了北赤道流区海流的季节内变化表现出表层强化和次表层强化的特征，其分布具有纬度依赖性，表层强化的季节内变化一般发生在较高纬度（13°N以北），而次表层强化的季节内变化则通常发生在较低纬度（13°N以南）。潜标ADCP观测还显示，在北赤道流轴线上（13°N，130°E），表层强化的季节内变化和次表层强化的季节内变化同时存在，而该现象被以往研究忽视，其动力机制也不清楚。针对这一现象，本文进一步结合了卫星高度计数据和OFES模式的输出结果，研究了这两种同时存在却不同类型的季节内变化信号的基本特征，探究了其动力机制。

       基于潜标观测的流速剖面，首先，本文研究了北赤道流区季节内变化信号的基本特征。结果表明，在北赤道流区同时存在两种不同类型的季节内变化信号，其中较为高频的信号，周期约为45天，表现为表层强化的特征，与第一斜压模有关；而较为低频的信号，周期约为85天，表现为次表层强化的特征，可能与第二斜压模有关。进一步分析卫星高度计和OFES模式输出结果发现，表层强化的季节内变化信号传播速度约为-0.15m/s ，波长约为618千米；次表层强化的季节内变化信号传播速度约为-0.11m/s ，波长约为628千米。基于涡旋探测和追踪算法对北赤道流区附近表层和次表层的中尺度涡旋进行识别和追踪，发现表层强化的季节内变化与局地生成的表层中尺度涡旋有关；次表层强化的季节内变化则与局地生成的次表层中尺度涡旋有关。

       基于卫星高度计数据和OFES模式输出结果对潜标位置处表层和次表层海流的速度剪切情况进行了诊断分析。结果表明，表层的北赤道流具有较强的垂向速度剪切，但水平速度剪切较弱；次表层北赤道潜流的垂向速度剪切和气候态平均的水平速度剪切都较弱，但其水平速度剪切存在明显的年际变化，一些年份的水平速度剪切会显著地增强。进一步利用OFES模式的输出结果计算了10°N至15°N，125°E至145°E范围内表层和次表层涡旋能量转化率，分析发现北赤道流区表层涡动能主要来源于斜压不稳定；次表层的情况则相对复杂，其涡动能主要来源于水平速度剪切强相位期间显著增强的正压不稳定。

       基于两层半约化重力模式，对潜标观测发现的表层强化的季节内变化信号的动力机制进行了探讨。结果表明，潜标观测位置（13°N，130°E）表层海流中，最不稳定模态的周期约为45天，波长约为661千米，与潜标观测结果以及卫星高度计观测结果一致，表明表层强化的季节内变化信号是由斜压不稳定导致的；基于一层半约化重力模式，对潜标观测发现的次表层强化的季节内变化信号的动力机制进行了探讨。结果表明，潜标观测位置（13°N，130°E）次表层海流中，最不稳定模态的周期约为85天，波长约为590千米，与潜标观测结果以及OFES模式模拟的结果基本一致，表明次表层强化的季节内变化信号是由正压不稳定导致的。

  此外，本文还基于吕宋岛以东18°N断面上的潜标观测结果，对吕宋以东黑潮和吕宋潜流的季节内变化进行了研究，发现该区域季节内变化的核心在移动过程中从表层逐渐向次表层延伸。

  综上所述，本文分别从能量学与动力学的角度，揭示了北赤道流区，潜标观测发现的两种同时存在的季节内变化信号的生成机制。其中周期约为45天、表层强化的季节内变化信号是由表层北赤道流的垂向速度剪切所引起的斜压不稳定产生的表层中尺度涡旋导致的；而周期约为85天，次表层强化的季节内变化信号则是由次表层北赤道潜流强相位期间显著增强的水平速度剪切所引起的正压不稳定产生的次表层中尺度涡旋导致的。

    The three-dimensional circulation structure of the tropical western Pacific Ocean is complex. As the “artery” that connects different water masses and balances the oceanic material and energy circulation, the tropical western Pacific Ocean has been a hot spot for physical oceanography research. As the origin of the western boundary currents and the boundary between tropical and subtropical gyres in the North Pacific, the North Equatorial Current (NEC) plays a significant role in the water mass balance and heat budget of the North Pacific. Recent studies based on subsurface mooring measurements have revealed that the intraseasonal variability of the currents in the North Equatorial Current Region in the northwestern Pacific Ocean is characterized by surface-intensified and subsurface-intensified signals, and their distribution is latitude-dependent, with the surface-intensified intraseasonal variability (ISV) generally occurring at higher latitude (north of 13°N), while the subsurface-intensified ISV occurs at lower latitude (south of 13°N). Nevertheless, the surface-intensified and subsurface-intensified ISV that could simultaneously exist was unveiled by mooring measurements at 13°N, 130°E, and this phenomenon seems to be overlooked by previous studies. In this research, we focused on this phenomenon and investigated the properties of these two flavors of ISV by combining the satellite altimetry data and an eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES) outputs, and revealing their dynamic mechanisms.

    First, we investigated the basic characteristics of these two flavors of ISV based on the velocity profiles observed by acoustic Doppler current profiler (ADCP) mooring. It suggested that the higher frequency signal, with a period of 45 days, is surface-intensified and seems to be related to the first baroclinic mode, while the lower frequency signal, with a period of 85 days, is subsurface-intensified and seems to be related to the second baroclinic mode. Further analysis of the satellite altimeter and OFES outputs indicated that the propagation speed of the surface-intensified ISV signal is -0.15m/s, and its wavelength is about 618km, while the propagation speed of the subsurface-intensified ISV signals is -0.11m/s, and its wavelength is about 628km. the tracking results of eddy trajectory in the surface layer and subsurface layer of the NEC region in the northwestern Pacific Ocean based on an eddy detection and tracking algorithm indicated that the surface-intensified ISV signal is associated with surface eddies locally generated between 130°E and 135°E, while the subsurface-intensified ISV signal is associated with subsurface eddies locally generated between 130°E and 135°E.

    The velocity shear conditions of the surface and subsurface currents at the mooring position were diagnosed based on the satellite altimeter data and OFES outputs. The results of the diagnostic analysis suggested that the vertical shear of the NEC is relatively strong, while the horizontal shear of the NEC is relatively weak; The climatological mean vertical and meridional shear of the NEUC is weak, but the horizontal shear has significant interannual variability which would be significantly enhanced in individual years. The eddy kinetic energy (EKE) conversion rates of surface and subsurface currents in the range of 10°N to 15°N and 125°E to 145°E were calculated using the OFES outputs. The results reveal that the EKE of the surface currents in the NEC region mainly is related to the baroclinic instability, while the EKE of the subsurface currents in this region is associated with the intensified barotropic instability during the strong phase when the horizontal shear of the subsurface currents is significantly enhanced.

    Based on a 2.5-layer reduced gravity model, we explored the dynamic mechanism of the surface-intensified ISV signals revealed by mooring measurement. The results suggested that the most unstable mode has a period of 45 days and a wavelength of 661km, which is consistent with the mooring measurement and satellite altimetry. It suggests that the surface-intensified ISV signal is induced by baroclinic instability. Based on a 1.5-layer reduced gravity model, we explored the dynamic mechanism of the subsurface-intensified ISV signals revealed by mooring measurement. The results indicated that the most unstable mode has a period of 85 days and a wavelength of 590km, which is close to the mooring measurement and OFES outputs. It suggests that the subsurface-intensified ISV signal is induced by barotropic instability.

    Besides, the ISVs of the Kuroshio (KC) and the Luzon Undercurrent (LUC) east of Luzon were investigated based on the ADCP moorings along 18°N. the results suggest that the cores of those ISV signals will shift from surface to subsurface during translation.

    In summary, the generation mechanisms of the surface-intensified and subsurface-intensified ISV signals revealed by mooring measurement were solved. The surface-intensified ISV signals with 45 days are induced by the surface eddies generated locally by the baroclinic instability of the vertically sheared NEC through the eddy-current interaction, while the generation process of the subsurface-intensified ISV signals is much more complex. They are induced by the subsurface eddies generated locally by the intensified barotropic instability through the eddy-current interaction during the strong phase when the horizontal shear of the NEUC is significantly enhanced.

第1章  引言. 1

1.1 研究意义... 1

1.2 西太平洋环流多尺度变化研究现状... 2

1.3 西太平洋中尺度涡旋研究现状... 6

1.4 本文研究内容... 10

第2章  研究资料与方法... 12

2.1  主要研究资料... 12

2.1.1  潜标观测数据... 12

2.1.2 卫星高度计数据... 14

2.1.3 OFES模式数据... 15

2.2 主要研究方法... 16

2.2.1 功率谱分析... 17

2.2.2 经验正交函数分解... 18

2.2.3 垂直模态分解... 20

2.2.4 涡旋探测与追踪方法... 20

2.2.5 涡动能转化率... 23

第3章  北赤道流与北赤道潜流的季节内变化特征... 25

3.1 潜标观测的季节内变化... 25

3.2 卫星高度计观测海流的季节内变化... 39

3.3 OFES模式模拟海流的季节内变化信号... 42

3.4 两种不同季节内变化信号的来源... 46

3.4 背景流的速度剪切... 52

3.4 本章小结... 55

第4章  表层强化的季节内变化动力机制. 57

4.1 表层涡旋的能量分析... 57

4.2 两层半约化重力模式... 58

4.3 北赤道流区上层海洋的斜压不稳定... 62

4.4 斜压稳定性对参数的依赖... 64

4.5 本章小结... 67

第5章  次表层强化的季节内变化动力机制... 69

5.1 次表层涡旋的能量分析... 69

5.2 一层半约化重力模式... 71

5.3 北赤道流区次表层海洋的正压不稳定... 74

5.4 本章小结... 79

第6章 吕宋以东海流的季节内变化特征... 81

6.1 潜标观测的季节内变化... 81

6.2 卫星高度计观测海流的季节内变化... 100

6.3 本章小结... 103

第7章  总结与展望... 105

7.1 主要结论... 105

7.2 本文创新点... 107

7.3 未来工作展望... 108

参考文献... 110

致  谢... 117

作者简历及攻读学位期间发表的学术论文与研究成果... 118