海洋环流与波动重点实验室知识库

  阿古拉斯流系同时连接印度洋、大西洋和南大洋，在全球海洋环流和气候系统中扮演着重要角色，对当地的渔业资源和生态环境也具有非常重要的影响。该海域具有复杂且丰富的环流、翻转、弯曲和环状结构，是南半球中纬度海域大尺度和中尺度变率最显著的区域。其中，阿古拉斯流特殊的折返结构为全球海洋最大的中尺度涡——阿古拉斯环的产生提供了有力条件。阿古拉斯环向西运动进入大西洋，实现了跨洋盆的物质与能量交换，并对大西洋经向翻转环流（AMOC）的稳定性产生影响。因此，以阿古拉斯折返区中尺度涡的能量演变为核心命题，在该海域开展多尺度相互作用的研究，对深入掌握阿古拉斯流系多尺度变异规律，理解海洋能量循环过程，以及完善对全球海洋环流和气候变化的认识都具有非常重要的意义。

  本文基于最新发展的多尺度子空间变换（MWT）和局地多尺度能量学分析方法（MS-EVA），利用ECCO2高分辨率海洋再分析资料，对阿古拉斯流系不同时间尺度之间的相互作用进行了系统的研究，研究内容主要包括以下两方面：

一、从气候态的角度，刻画了阿古拉斯流系多尺度能量和多尺度相互作用的三维空间分布特征

  将阿古拉斯流系重构于年代际尺度低频振荡的平均流、季节到年际尺度的低频扰动和瞬变的中尺度涡三个尺度子空间，并根据平均流场和涡动能（EKE）的水平分布将阿古拉斯流系划分为四个子区域，包括折返区、漂流区、弯曲区和稳定区，对各个子区域中三个时间尺度之间的能量传输与转换分别进行了研究。结果表明，折返区的多尺度相互作用最显著，表现为南阿古拉斯流（SAC）通过混合正-斜压不稳定将能量传递给中尺度涡和低频扰动，正压不稳定是EKE生成的主导机制，其贡献约为斜压不稳定的11倍；在折返区生成的EKE又通过压强做功和平流作用向其他子区域输送，使得在下游的漂流区和弯曲区内，非局地输入成为涡旋场的主要能量来源之一，与斜压不稳定对EKE的贡献相当；但在稳定区，绝大部分EKE是通过斜压不稳定过程产生。此外，中尺度涡向低频扰动的逆向动能串级现象在阿古拉斯流系普遍存在，特别是在折返区、漂流区和稳定区，逆向动能串级是季节到年际尺度低频扰动发生的主导机制，其作用约为正向动能串级和平流作用的3–5倍；相反，在弯曲区，平均流从涡旋大量汲取动能，进而通过正压不稳定将动能传递给低频扰动，而平均流向低频扰动提供的动能约为中尺度涡逆向串级作用的5倍，是低频扰动的主要能量来源。

二、在季节时间尺度上，厘清了EKE的变异规律及其影响机制

  进一步研究了阿古拉斯流系EKE的季节变化特征，并分析了海洋内部的正压和斜压不稳定、非局地能量输送、以及局地风强迫的影响。结果表明：阿古拉斯流系EKE的季节变化具有空间不均一性：沿莫桑比克海流（MC）的EKE春季最强，秋季最弱，而北阿古拉斯流（NAC）和阿古拉斯折返区（ARF）的EKE夏季最强，冬季最弱。在以上各子区域中，平均流的正压不稳定都与EKE的季节变化规律一致；非局地作用能够对EKE进行空间再分配，其在MC子区内表现为夏强春弱的EKE输入，在ARF子区内表现为冬强秋弱的EKE输出，而在NAC子区，冬春（夏秋）两季存在EKE的强输入（输出），其中冬季输入最强，秋季输出最强，显然，该过程与各子区域中EKE的季节变化不同；此外，局地风强迫、斜压不稳定、以及低频流的正压不稳定对EKE的影响都比较弱，约为平均流正压不稳定作用的10%–20%，且他们的季节变化规律也都与EKE不同。因此，对于阿古拉斯流系EKE的季节变化，尽管存在空间差异，但在各个子区域都主要是受平均流正压不稳定的控制。

  本文从能量学的角度系统阐明了阿古拉斯流系多尺度相互作用的三维空间分布特征及EKE的季节变异规律。对于该海域EKE的演变，以往研究普遍强调平均流不稳定性的主导作用，而本文进一步指出了季节-年际尺度的低频扰动和非局地能量输送的重要贡献。这些研究结果为阿古拉斯流系涡旋运动规律的研究提供了新的视角，为今后对该海域的多尺度动力过程及其变异机理的分析奠定了基础。

    The Agulhas current system (ACS) connects the Indian, Atlantic and Southern Oceans simultaneously, playing a fundamental role in the global oceanic circulation as well as the climate system. It also has a significant impact on the local fishery resources and ecological environment. The region has complex and abundant circulations, retroflection, meanders and rings with the most significant large-scale and mesoscale variabilities in the mid-latitude of the southern hemisphere. In particular, the spacial retroflection structure of the Agulhas current provides favorable conditions for the generation of the largest mesoscale eddies, i.e. the Agulhas Rings (ARs), in the global ocean. The ARs propagate westward into the Atlantic Ocean, which contributes to the exchange of materials and energies across the ocean basins and would further affect the stability of the Atlantic meridional overturning circulation (AMOC). Thus, the study of the multiscale interactions in the Agulhas current system, including examinations of the energy evolution of the mesoscale eddies in the Agulhas retroflection region, is of great significance to understand the multiscale variabilities and the oceanic energy budget, as well as to improve the knowledge of the global ocean circulation and climate change.

    Based on the recently developed multiscale window transform (MWT) and multiscale energy and vorticity analysis (MS-EVA), this study systematically investigates the multiscale processes in the ACS using the state estimate from the “Estimating the Circulation and Climate of the Ocean” (ECCO2). Specifically, we mainly focus on the following two aspects:

1. From the climatological mean perspective, the three-dimensional  structures of the multiscale energies and interactions in the ACS are examined

    We reconstruct the ACS into three windows, namely, the decadally modulating mean flow, the low-frequency (seasonal to interannual) fluctuations, and the transient mesoscale eddies and divide the ACS into four subdomains, i.e. the retroflection, rings drift, meanders, and stable regions according to the horizontal distribution of the background currents and the eddy kinetic energy (EKE). Then we investigate the interactions among the three scale windows in each subdomain, respectively. It is found that the strongest multiscale interactions occur in the retroflection region, where mesoscale eddies are generated by mixed barotropic and baroclinic instabilities. The barotropic instability dominates the generation of EKE here, contributing power roughly 10 times larger than the baroclinic one. These locally generated eddies are transported away. In the downstream rings drift and meanders regions, the nonlocal transport serves as an important energy source for the eddy field, making a contribution comparable to that of the baroclinic instability for the EKE production. Contrarily, in the stable region, the EKE is generated mainly due to the baroclinic instability. In most of the ACS area, the kinetic energy (KE) is further transferred inversely from mesoscale eddies to other lower frequency motions. In particular, in the retroflection, rings drift, and stable regions, the inverse KE cascade plays a leading role in generating seasonal-interannual fluctuations, providing roughly 3–5 times as much power as the forward KE cascade from the mean flow and the advection effect. In the meanders region, however, the forward cascade contributes 4 times more KE to the low-frequency variabilities than the inverse one.

2. On the seasonal timescale, the variability of EKE and the underlying mechanisms are clarified

    This study further investigates the seasonal variation of the EKE and the influence of internal barotropic/baroclinic instabilities, nonlocal energy transportation, and local wind power input in the ACS. It is shown that the seasonality of EKE is spatially inhomogeneous. In the Mozambique current (MC) subdomain, the EKE level is highest in spring and lowest in autumn, whereas in the northern Agulhas current (NAC) and Agulhas retroflection (ARF) subdomains, the EKE is strongest in summer and weakest in winter. In all the three subdomains, the barotropic instability of the mean flow shows similar seasonal variation as the EKE. However, for the nonlocal transportation which helps to redistribute the EKE, it exhibits largest (smallest) EKE input in summer (spring) in the MC subdomain and shows strongest (weakest) EKE output in winter (autumn) in the ARF subdomain. In the NAC subdomain, there is a strongest EKE input (output) in winter (autumn). Obviously, the seasonality of the nonlocal advection is different from that of the EKE. Regarding the local wind forcing, baroclinic instability, and barotropic instability of the low-frequency fluctuations, they all have weak impacts on the EKE evolution, contributing power only about 10%–20% of that of the barotropic instability of the mean flow. Moreover, neither of their seasonal cycles is consistent with the EKE cycle. Therefore, in the ACS, it is the barotropic instability of the mean flow that controls the seasonal variability of the EKE.

    From the perspective of energetics, this study systematically elucidates the three-dimensional spacial distribution of the multiscale interactions and the seasonal variability of EKE in the ACS. For the evolution of EKE here, existing studies have generally emphasized the dominated role of the mean flow instabilities, while this paper further points out the important contributions of the low-frequency (seasonal to interannual) fluctuations and nonlocal energy transportation.These results provide a new perspective for the studies of the mesoscale eddies, and a model-based benchmark for future analysis of the multiscale dynamic processes and the underlying mechanisms in the ACS.

第 1章 绪论........................................................................................... 1
1.1研究背景与意义.................................................................................................1
1.2研究现状.............................................................................................................2
1.2.1阿古拉斯流系的多尺度运动......................................................................2
1.2.2阿古拉斯流系的多尺度相互作用..............................................................6
1.2.3阿古拉斯流系涡动能的季节变化..............................................................7
1.3科学问题及章节安排.........................................................................................9
第 2章 研究数据与方法.....................................................................11
2.1研究数据...........................................................................................................11
2.1.1 ECCO2模式数据 .......................................................................................11
2.1.2卫星高度计观测数据................................................................................12
2.2研究方法...........................................................................................................12
2.2.1多尺度子空间变换（MWT）..................................................................12
2.2.2局地多尺度能量学分析（MS-EVA） .................................................... 14
第 3章 阿古拉斯流系的多尺度相互作用........................................ 17
3.1引言...................................................................................................................17
3.2 ECCO2模式再分析数据的评估 ......................................................................17
3.3尺度子空间的设定...........................................................................................24
3.4多尺度能量的空间分布...................................................................................24
3.5局地多尺度相互作用和非局地能量输送的空间分布...................................29
3.5.1折返区（S1） ........................................................................................... 32
3.5.2漂流区（S2） ........................................................................................... 35
3.5.3弯曲区（S3） ........................................................................................... 37
3.5.4稳定区（S4） ........................................................................................... 38
3.6讨论...................................................................................................................39
3.7本章小结...........................................................................................................40
第 4章 阿古拉斯流系中尺度涡动能的季节变化............................ 43
4.1引言...................................................................................................................43
4.2 ECCO2再分析数据评估 ..................................................................................43
4.3阿古拉斯流系涡动能的季节变化特征...........................................................46
4.4阿古拉斯流系涡动能季节变化的影响机制...................................................50
4.4.1正压不稳定................................................................................................50
4.4.2斜压不稳定................................................................................................54

4.4.3非局地涡动能输送....................................................................................56
4.4.4局地风应力强迫........................................................................................58
4.5讨论...................................................................................................................59
4.6本章小结...........................................................................................................60
第 5章 总结与展望.............................................................................62
5.1主要结论...........................................................................................................62
5.2本文创新点.......................................................................................................63
5.3未来工作展望...................................................................................................64
参考文献................................................................................................... 66
致 谢....................................................................................................... 77
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......79