海洋环流与波动重点实验室知识库

近惯性内波（Near-inertial wave, NIW）是频率接近局地科氏频率f₀的海洋内波，它们在全球海洋中无处不在，是风向海洋传递能量的重要载体。NIW由于强剪切、高垂直波数的特点，在海洋内部十分容易发生破碎，从而为深层海洋混合提供能量，进而对全球气候变化都有重要影响。目前，尽管NIW已被广泛研究，但对于菲律宾以东海域NIW的生成、传播和耗散等基本特征以及背景流、中尺度涡、背景层结等多种动力过程对NIW影响的认识尚不完善。本文基于菲律宾以东的四套潜标观测数据，研究了NIW的基本特征、动力机制、能量传播及其对跨等密度面湍流混合的影响。

首先，基于菲律宾以东[130°E, 11°N]的单点潜标观测，发现了三次强NIW事件（I、II、III），对这三次NIW事件的生成机制和动力特征进行分析，结果表明：NIW事件I是由台风“兰恩”引起，其最强近惯性流速振幅达到0.35 m/s，近惯性动能（Near-inertial kinetic energy, NIKE）极值位于表层（50 m）；NIW事件II是由中等强度的局地气旋式风场激发生成，其NIKE极值位于次表层（110 m）；在NIW事件I和II期间，气旋涡的存在导致近惯性频率发生蓝移；对于NIW事件III，表层近惯性信号是由局地弱风引起，而次表层NIW的生成机制尚不清楚；特别地，在NIW事件III期间，NIW和半日潮（D₂）之间存在非线性波—波相互作用，进一步诱导生成了强D₂±f次谐波。总体而言，三个事件中的NIW呈现不同的垂直结构，其中：事件I和II中的NIW以低模态为主，NIKE强值分别局限在250 m和270 m以浅；相反的，事件III中的NIW以高模态为主，并且受局地反气旋涡和较弱的风应力旋度影响，NIW向下传播较远，超过了360 m。此外，三次NIW事件的e折时间尺度都小于7天。

其次，基于上述[130ºE, 11ºN]的潜标观测，进一步研究了连续台风生成NIW的基本特征与传播过程，结果表明：NIW存在两个阶段，第1阶段NIW由台风“兰恩”局地生成并向下传播，而第2阶段NIW由后续台风“海葵”在其他源区生成并远距离水平传播而来。射线追踪模型结果显示，第2阶段的NIW是在台风“海葵”轨迹附近[132.2°E, 10.3°N]生成，在背景流的作用下，经历约54 h后传播到潜标站位200 m而被观测到。动力分析结果表明，第2阶段潜标站位处相对涡度的垂直梯度为负(∆ζ/∆z < 0），抑制了NIW的向下传播，因此在表层未能捕捉到局地产生的NIW；相反，第2阶段NIW生成处[132.2°E, 10.3°N]相对涡度的垂直梯度为正（∆ζ/∆z > 0），加速了NIW的向下传播，因此最终在潜标站位200 m处观测到水平传播的NIW。

最后，利用菲律宾以东[130ºE, 11ºN]，[130ºE, 12.6ºN]，[130ºE, 15ºN]，[130ºE, 17.5ºN]四个站位的潜标观测，研究了台风“兰恩”过境激发的四个强NIW波包的特征，波包分析结果表明：对于具有较大水平波长的NIW，由于其与背景流的相互作用减弱，导致其观测惯性频率接近固有频率；特别地，两个气旋涡之间的NIKE下传深度（~620 m）比负涡度区域的NIKE下传更深，射线追踪模型结果显示，这是由北面的强正涡度导致向北传播的NIW被反射，随后在层结最强处（~70 m）下传加速所导致；但是，这两个区域的NIKE下传效率相当，大约为21%-25%。能量收支分析结果表明：大约5%-25%的近惯性风功被输送到海洋上层50-200 m，其中约6%-25%可以进一步辐射到深海；平均而言，在台风“兰恩”过境后，耗散率增强了3-8倍，而对于增强的跨等密度面湍流混合，约42%的能量是由台风诱导的强NIW提供。

Near-inertial wave (NIW) is a type of ocean internal wave with a frequency near the local Coriolis frequency f₀. They are ubiquitous in the global ocean and act as an important carrier to transport energy from wind to the ocean. Due to their intense vertical shear and high vertical wave numbers, NIW are easy to trigger shear instabilities and the associated wave breaking, providing energy for deep ocean mixing and potentially affecting the climate. Although the NIWs have been extensively studied, our knowledge about their generation, propagation, and dissipation, as well as the impacts of dynamic processes like mesoscale eddies, background currents, and stratification on them, is still limited, especially in the east of the Philippines. Based on the in situ observations from four mooring stations east of the Philippines, the basic characteristics and dynamic mechanisms of NIWs were examined. At the same time, the propagation of the NIKE and its influence on the diapical mixing were also investigated.

Firstly, based on the single mooring observations at [130ºE, 11ºN] east of the Philippines, three strong NIW events (I, II, and III) were detected, and the characteristics and underlying mechanisms of these three NIW events were investigated in detail. It was found that the NIW in Event I was induced by typhoon Lan and had the strongest magnitudes of 0.35 m/s. The maximum near-inertial kinetic energy (NIKE) was shown at the ocean surface (50 m). The NIW in Event II was stimulated by a moderate cyclonic wind with the extreme NIKE located at about 110 m depth. The existence of a cyclonic eddy during Events I and II led to a blue shift of near-inertial frequencies. For NIW Event III, the surface NIW signals were also induced by local weak wind, whereas the real generation mechanisms for the subsurface NIW remain unclear. In particular, during NIW Event III, there was a nonlinear wave-wave interaction between NIW and semidiurnal (D₂) tides, which further induced strong D₂±f waves. Overall, the NIW in the three events exhibited distinct vertical structures. The NIW in Events I and II were dominated by lower modes with elevated NIKE well confined to the upper 250 m and 270 m, respectively. In contrast, the NIW Event III was dominated by higher modes and the NIW penetrated downward beyond 360 m. Such deep penetration of NIW could be attributed to the weak wind stress curl and positive sea level anomalies associated with an anticyclonic eddy. In addition, the three NIW events had e-folding timescales of less than 7 days.

Secondly, with the mooring observations at [130ºE, 11ºN], the characteristics and propagation of NIWs generated by two successive typhoons were investigated. It was found that there were two stages for the NIWs: in the first stage (Stage-1), the NIWs were excited by typhoon Lan locally and propagated downward, while in the second stage (Stage-2), the NIWs were generated by the subsequent typhoon Haikui remotely and propagated horizontally. Ray-tracing model results revealed that the NIWs in the Stage-2 were generated near the track of typhoon Haikui at [132.2°E, 10.3°N] and then propagated horizontally for about 54 h before they reached the mooring station at 200 m. Dynamic analysis further indicated that in the Stage-2, the vertical gradient of the relative vorticity (∆ζ/∆z) was negative at the mooring station and positive in the NIW source area around [132.2°E, 10.3°N]. Therefore, at the mooring station, the downward propagation of the NIWs were suppressed, and thus they were not observed at the ocean surface. On the contrary, in the NIW source area, the downward propagation of the NIWs were accelerated, and hence the NIWs were finally observed at 200 m by the mooring.

Last, utilizing observations from four moorings at [130ºE, 11ºN], [130ºE, 12.6ºN], [130ºE, 15ºN], [130ºE, 17.5ºN] in the east of the Philippines, the characteristics of four strong NIW wave packets generated by typhoon Lan were examined. The wave-packet analysis revealed that for the NIW with larger horizontal wavelengths, as their interactions with the background currents were weakened, the observed frequency was close to the intrinsic frequency. The NIKE between two cyclonic eddies penetrated deeper (~620 m) than that in a negative vorticity region. A ray-tracing model suggested that it was the strong positive vorticity to the north that caused the northward propagating NIW to be reflected, and then the reflected NIW were accelerated to propagate downward at ~70 m depth where the stratification was strongest. In these two cases, furthermore, the efficiency of the downward propagation of NIKE was at a comparable level of 21%-25%. Energy budget analysis indicated that about 5%-25% of the near-inertial wind work was injected into the upper 50-200 m, approximately 6%-25% of which could be further radiated to the deeper ocean. On average, after the passage of the typhoon Lan, the dissipation rate increased by 3-8 times, and for the enhanced diapycnal mixing, ~42% of the energy was provided by the typhoon-induced strong NIW.

第1章 绪论... 1

1.1 研究背景及意义... 1

1.2 近惯性内波的生成机制... 2

1.2.1 风生近惯性内波... 2

1.2.2 非线性波-波相互作用... 4

1.2.3 背景平均流激发近惯性内波... 6

1.3 近惯性内波传播的影响因素... 7

1.3.1 β效应... 7

1.3.2 中尺度涡旋... 7

1.3.3 背景层结... 9

1.3.4 背景平均流... 9

1.4 近惯性内波的混合效应... 10

1.5 西北太平洋近惯性内波的研究现状... 11

1.6 科学问题及章节安排... 13

第2章 数据与方法... 14

2.1 研究数据... 14

2.1.1 潜标观测数据... 14

2.1.2 高分辨率模式数据... 15

2.1.3 卫星高度计数据... 17

2.1.4 台风轨迹数据... 18

2.1.5 风场数据... 18

2.2 研究方法... 18

2.2.1 带通滤波... 18

2.2.2 旋转谱分析... 18

2.2.3 双相干分析... 19

2.2.4 动力模态分解... 20

2.2.5 混合层平板模型... 20

2.2.6 射线追踪模型... 21

2.2.7 复数解调... 21

2.2.8 波包分析... 22

2.2.9 混合估算方法... 25

第3章 菲律宾以东近惯性内波的生成机制分析... 26

3.1 单个潜标观测的流速数据分析... 26

3.2 不同近惯性内波事件的生成机制... 29

3.2.1 NIW事件I. 29

3.2.2 NIW事件II. 30

3.2.3 NIW事件III. 32

3.3 不同近惯性内波事件的相似性、差异性及独特性... 34

3.3.1 相似性... 34

3.3.2 差异性... 38

3.3.3 独特性... 42

3.4 本章小结... 43

第4章 连续台风“兰恩”和“海葵”生成近惯性内波的对比分析... 45

4.1 连续台风“兰恩”和“海葵”介绍... 45

4.2 台风“兰恩”和“海葵”期间近惯性内波的基本特征对比... 46

4.2.1 垂直传播深度... 47

4.2.2 主导垂直模态... 47

4.2.3 观测频率与垂直波长... 48

4.3 台风“海葵”期间近惯性内波的来源分析... 50

4.4 近惯性内波垂直传播的影响机制... 52

4.5 本章小结... 54

第5章 台风“兰恩”生成的近惯性内波分析... 55

5.1 四个潜标观测的流速数据分析... 55

5.2 四个近惯性波包的基本特征... 61

5.2.1 观测频率、有效科氏频率以及固有频率... 62

5.2.2 垂直波长、水平波长以及群速度... 65

5.2.3 垂直能量通量以及水平能量传播方向... 65

5.3 近惯性内波的垂直和水平传播特征及其影响因素... 67

5.4 近惯性内波对跨等密度面湍流混合的影响... 70

5.5 本章小结... 73

第6章 总结与展望... 74

6.1 主要结论... 74

6.2 本文创新点... 75

6.3 未来工作展望... 75

参考文献... 76

致谢... 89

作者简历及攻读学位期间发表的学术论文与其他相关学术成果... 90